Embryology and Anatomy

The inner ear

The inner ear comprises two sensory organs encased in a common fluid-filled membranous sac: the 2½ turn coiled cochlea (ventral), and the vestibule (dorsal), made up of the three semicircular canals, utricle, and saccule (Figure 7-1). The inner ear arises from the otic vesicle, the development of which begins at the end of the first month of gestation and is essentially complete in 50 days. It is encased in the otic capsule or bony labyrinth, which is adult-size at birth, lest growth alter frequency tuning of the cochlear basilar membrane. The bony labyrinth contains two separate fluid-filled compartments. The bony outer perilymphatic compartment completely surrounds the inner membranous endolymphatic compartment. The perilymph is a high-sodium plasma ultrafiltrate which, at least in infancy, communicates with the cerebrospinal fluid (CSF) through the cochlear aqueduct (see below). The auditory and vestibular receptors and first-order afferent neurons are located within the endolymphatic compartment. The endolymph is a high-potassium fluid which is maintained at +80 mV relative to the perilymph. Maintenance of this and other cellular ionic gradients is an energy-consuming process dependent on mitochondria, in particular in the highly vascularized, metabolically active stria vascularis of the scala media. These gradients are essential for generating the electrical currents that arise from the bending of the stereocilia on the endolabyrinthine vestibular and cochlear hair cells, and for reversing the ionic fluxes that bending generates. Hearing and vestibular function are dependent on the function of multiple intercellular ionic gap and tight junctions, pumps, and transporters, and on multiple neurotransmitters’ synaptic release, transporters, and receptors. Mouse models of genetic hearing losses are enormously accelerating progress in understanding the cellular and molecular biology and physiology of hearing and hearing loss [Friedman and Griffith, 2003; Frolenkov et al., 2004; Belyantseva et al., 2003; Petersen et al., 2008; Petersen and Willems, 2006; Petersen, 2002; Dror and Avraham, 2009].

Fig. 7-1 The intricate relationship of the semicircular canals, vestibule, and cochlea.

The membranous labyrinth contains endolymph and is separated from the bony labyrinth by perilymph. The ampullae at the terminus of each semicircular canal open into the vestibule. A crista is contained within each ampulla. The superior portion of the vestibule is the utricle. The inferior portion is the saccule. The stapes footplate fits in the oval window of the scala vestibuli. Direction of flow of perilymph is indicated by the arrows.

(Courtesy of the Division of Pediatric Neurology, University of Minnesota Medical School.)

In the cochlea, the perilymph-filled scala vestibuli and scala tympani, which communicate at the apex of the cochlea through the helicotrema, surround the scala media, which contains the organ of Corti bathed in the endolymph. The footplate of the stapes is encased in the oval window of the scala vestibuli. The round window of the scala tympani is covered by a compliant membrane that bulges outward into the middle ear cavity whenever the stapes footplate compresses the perilymph. The resultant traveling wave in the perilymph sets off a radial movement of the basilar membrane, starting at the base of the cochlea and ending at the apex. The frequency and intensity of the sound determine which areas of the basilar membrane sustain maximum displacement. High-pitched sounds produce maximum displacement of the basilar membrane near the base of the cochlea, whereas low-pitched sounds do so near the apex.

Both the perilymphatic and endolymphatic compartments have clinically significant intracranial extensions. The perilymphatic space close to the base of the scala tympani is continuous with the subarachnoid space of the posterior fossa through the cochlear aqueduct, patent in neonates but barely detected or obliterated in adults. Patency of the cochlear aqueduct in early life may be a factor in the higher prevalence of purulent endolabyrinthitis – which often results in profound deafness with vestibular impairment [Merchant and Gopen, 1996] – among young children than among adults. In children (and adults) fistulas around or through the stapes footplate may result in gushing of perilymph or even CSF into the middle ear, fluctuating, often progressive hearing loss, or episodic vertigo, and require surgical repair to prevent potential bacterial meningitis [Reilly, 1989; Weber et al., 2003]. The endolymphatic space of the vestibule, via the endolymphatic duct encased in the bony vestibular aqueduct of the otic capsule, opens into the endolymphatic sac located within the dura on the dorsal surface of the petrous bone, close to the internal acoustic meatus. Two disorders characterized by an excess of endolymphatic fluid production/pressure result in enlargement of these structures. The better-known is Ménière’s syndrome, notorious in adults but uncommon in children, which is characterized by episodes of severe vertigo and tinnitus and by a fluctuating, later progressive, sensorineural hearing loss. It may respond to diuretics, but if it does not, it may require drainage of the endolabyrinthine sac or other surgical approaches to decompress the endolabyrinthine compartment [Huang, 2002]. The second is Pendred’s syndrome, characterized by deficient transmembrane transport of chloride and iodide anions, causing a severe prelingual hearing loss and adolescent or adult euthyroid goiter. Knockout mice deficient in pendrin have a decreased endolymphatic potential, the pH is lowered, and Ca⁺⁺ is increased [Nakaya et al., 2007]. Fetal enlargement of the endolabyrinthic duct and sac can be presumed to be the reason for congenital enlargement of the vestibular aqueduct (EVA) or other malformations of the otic capsule (Mondini malformation) frequent in Pendred syndrome but also seen in other syndromes [Petersen and Willems, 2006; Gonzalez-Garcia et al., 2006].

The scala media, in which the organ of Corti resides, is triangular in cross-section, with its apex directed medially toward the modiolus (axis) of the cochlea, to which it is anchored. Its vertical side, mostly lined by the stria vascularis, is apposed to the osseous outer wall of the cochlea (Figure 7-2). The floor of the scala media is formed by the basilar membrane and osseous spiral lamina, which separate the scala media from the scala tympani. The roof of the scala media is formed by Reissner’s membrane, partitioning it from the scala vestibuli. The organ of Corti sits on the basilar membrane, a coiled, mobile, truncated triangular membrane whose stiffer, narrower end is located at the base of the cochlea and whose wider, more pliable end is located at the apex. The organ of Corti consists of a long, orderly, longitudinal file of radial (transverse) rows. Each row is made up of four hair cells surrounded by specialized supporting cells located on either side of the hollow tunnel of Corti. Adjacent rows are stimulated by maxima of the traveling wave and are thus narrowly tuned to particular sound frequencies. Two pillar cells meeting at the apex of the triangular tunnel make up its lateral walls and the basilar membrane its floor; the tunnel is filled by the cortilymph, which has its own ionic composition. Each radial row comprises a single inner hair cell situated on the modiolar side of the tunnel and three efferent outer hair cells. The inner hair cell is the auditory afferent receptor, of which there are about 3500 [Spoendlin, 1988].

Fig. 7-2 Structural components of the organ of Corti.

The flow of the perilymph results in movement of the scala media (cochlear partition), which vibrates around its attachment to the modiolus and osseous spiral lamina. The complex traveling wave in the basilar membrane produces unequal shearing forces on the stereocilia of groups of hair cells, which are the transducers of sound.

(Courtesy of the Division of Pediatric Neurology, University of Minnesota Medical School.)

There are about 12,000 efferent outer hair cells, which receive their inputs from the olivocochlear bundle. Their role is to modulate acoustic reception by the inner hair cells and spiral ganglion cells [Hunter-Duvar and Harrison, 1988]. On its apical surface, each inner hair cell has a straight palissade of some 60 stereocilia, whereas the palissade is arranged in a V shape on each outer hair cell. Three stereocilia of graded length, linked at their tips, make up each “stick” of the palissades, an arrangement that ensures that only the longest cilium of the outer hair cells reaches the gelatinous tectorial membrane attached to the modiolus. Stereocilia contain actin filaments and myosins and other motor molecules [Dror and Avraham, 2009]. The hair cells and supporting cells thus tightly couple movements of the basilar and tectorial membranes, ensuring synchronized bending of the stereocilia in response to the traveling waves induced by sound along the basilar membrane [Frolenkov et al., 2004]. Cell adhesion proteins (cadherins) keep hair cells, cilia, and supporting cells tightly packed, and ionic channels provide fast communication among them. The base of each inner hair cell synapses with the dendrites of about 20 type I spiral ganglion neurons, which number about 30,000, whereas the dendrites of a single smaller type II spiral ganglion neuron contact about 10 outer hair cells. Axons of the efferent olivocochlear bundle divide widely, each synapsing on many outer hair cells; in contrast, they terminate sparingly on the inner hair cells and dendrites of the type I spiral ganglion neurons [Hunter-Duvar and Harrison, 1988]. Glutamate is the main excitatory neurotransmitter between the inner hair cells and ionotropic receptors on the dendrites of type I spiral auditory neurons [Oestreicher et al., 2002]. Olivocochlear synapses of the efferent pathway with the outer hair cells and type II spiral ganglion neurons are mainly inhibitory through the release of gamma-aminobutyric acid (GABA) and dopamine, although its modulation also involves release of acetylcholine and other excitatory neurotransmitters.

The afferent first-order type I auditory neurons constitute the spiral ganglion of Corti located in the modiolus of the cochlea. Both their dendrites and their axons are myelinated. As discussed later, documenting their preserved function and that of their processes is crucial when evaluating a hearing-impaired individual for a cochlear implant because the prosthesis stimulates these neurons, not the hair cells. First-order vestibular neurons form the ganglion of Scarpa, which is located at the base of the internal auditory meatus. The meatus contains the cochlear and vestibular divisions of cranial nerve VIII, the efferent fibers of the inhibitory olivocochlear bundle, the facial nerve VII, chorda tympani, and internal auditory artery and vein.

Central auditory pathway

The axons of the spiral ganglion neurons project to the cochlear nuclei of the medulla. They are of quite uniform caliber, which means that their conduction velocities are uniform, crucial for sound transmission. Axons from the base of the cochlea, which carry high-frequency sounds, project to the ventral cochlear nucleus and ventral portion of the dorsal cochlear nucleus; those from the apex of the cochlea, which carry low-frequency sounds, project to the dorsal portion of the dorsal cochlear nucleus (Figure 7-3). Each spiral ganglion neuron is connected to five different types of neurons in the cochlear nuclei. Most axons of the cochlear nuclei cross the midline, forming the acoustic striae on the floor of the fourth ventricle as they course through the trapezoid body to form the ascending lateral lemniscus. Relays in the central auditory pathway include the cochlear nuclei, superior olivary complex, nucleus of the lateral lemniscus, inferior colliculus, and medial geniculate body of the thalamus where all fibers synapse before reaching the primary and secondary auditory cortices of the superior temporal gyrus. Tremendous multiplication characterizes the auditory pathway of the rhesus monkey, which goes from the 3,500 inner hair cells to 30,000 spiral ganglion neurons and to about 10 million neurons in the medial geniculate body, which projects to both primary (area 41) and secondary (area 42) auditory cortices in the superior temporal gyrus. Neurons of area 41 project to area 42 situated behind it. The secondary auditory areas of the two hemispheres are connected through fibers traveling in the body of the corpus callosum. They are also widely connected to other cortical areas, including frontal and temporoparietal language areas, the cingulate gyrus, and frontolateral cortices.

Fig. 7-3 Peripheral and central auditory pathway, which is both crossed and uncrossed, and projects to the horizontally oriented auditory cortex of the planum temporale.

There are about 300 times more fibers in the central auditory pathway than in cranial nerve VIII, and there are extensive connections between the left and right brainstem relays.

(Courtesy of the Division of Pediatric Neurology, University of Minnesota Medical School.)

Physiology

The relevant dimensions of sound are intensity, which is expressed in decibels (dB), and frequency, which is measured in cycles per second or Hertz (Hz) [Katz, 2002]. Units of intensity and frequency are logarithmic. The decibel scale displayed on audiograms appears to be linear, but the units themselves are logarithmic. An increase of about 10 dB results approximately in doubling of subjective loudness. Audiometers are calibrated so that audiometric 0 (i.e., normal auditory sensitivity or hearing level, or 0 dB of hearing loss) corresponds to the internationally accepted average threshold of normal hearing at each frequency. The lower limit of normal hearing has been set at 20 dB below audiometric 0 across the frequency range. The ear is most sensitive at about 2000 Hz and very sensitive between 1000 and 4000 Hz, with sensitivity decreasing rapidly below 500 Hz (low frequencies/pitches) and above 4000 Hz (high frequencies/pitches). At the extremes of the audible frequency band, more sound pressure is required to reach threshold than in the middle of the band. Therefore, the sensitivity curve across frequencies of the human ear is U-shaped; for the sake of convenience, this was transformed to a straight line representing mean normal hearing (sound pressure associated with 0 dB of hearing loss). Localization of sound in space is in part mediated by differences in intensity and lag in phase of sounds reaching each of the ears. Processing of sound frequency, intensity, location, onset, and cessation takes place largely in brainstem auditory relays.

The primary role of the pinna is in sound localization. The middle ear acts as a sound amplifier that compensates for loss of sound energy as it is transmitted from air to fluid because of the much greater area of the tympanic membrane compared to that of the oval window, the vibration pattern of the eardrum, and the leverage of the ossicular chain. Sound in air is also transmitted to the cochlea through the skull, but it occurs at threshold intensities that exceed air conduction by 40–60 dB. Bone conduction is important in children with prolonged or frequent middle ear effusions, whose thresholds may be raised up to 40–60 dB at times. Middle ear effusion may be detrimental for language acquisition if associated with other social or medical handicaps [Wallace et al., 1996], but it never renders children profoundly deaf. Bone conduction prostheses are critical for children with anotia, canal atresia, and other malformations of the external and middle ear who may have normally functioning cochleas and can therefore acquire language competently if provided promptly with appropriate bone conduction hearing aids or other prostheses.

The hair cells are the transducers from mechanical energy to sensory stimulation; their action depends on radial and longitudinal displacements of the basilar membrane by sound and on the ionic gradients of the various compartments and cells of the cochlea. The afferent inner hair cells are the sensory receptors for sound. The role of the efferent outer hair cells, which are contractile and tightly coupled to the basilar and tectorial membranes, is to modulate the activity of the inner hair cells and type I spiral ganglion neurons. These receive both excitatory and inhibitory inputs from the olivocochlear bundle. The roles of the outer hair cells are enhancement of sensitivity and frequency selectivity, but also overall damping, depending on olivocochlear inputs. Together with contraction of the middle ear muscles mentioned earlier, the olivocochlear/outer hair cells damp the speaker’s perception of what he/she is saying.

The auditory system is tonotopically selective throughout. The basilar membrane is a highly sensitive and very sharply tuned resonator. Also narrowly tuned, cells in the cochlea and higher relays of the auditory system, including the auditory cortex, respond selectively to certain frequencies and only at much higher intensities or not at all to others. At low sound intensity, each spiral ganglion neuron near the apex of the cochlea follows the periodicity of a particular low-pitched tone, whereas at frequencies greater than 1000 Hz, it is groups of neurons which follow the periodicity of particular high-pitched tones. Tuning is less selective at high sound intensities.

Physiologic tests of hearing, including acoustic immittance and middle ear reflexes, otoacoustic emissions (OAEs), electrocochleography, brainstem auditory-evoked responses/auditory brainstem responses (BAER/ABR), are discussed later in the section on testing.

Classification of Hearing Loss by Clinical Features

Classification by Severity and Shape of Hearing Loss

The lower limit of normal hearing has been set at 20 dB. Hearing loss can be classified by its severity (Table 7-1). Mild flat losses of up to 40 dB do not preclude the acquisition of speech but may significantly impoverish it, the degree of language handicap depending on the age of the child and the severity and configuration of the hearing loss. If the loss is present from early infancy, it is likely to delay speech [Abraham et al., 1996; Gravel et al., 1996], especially if the child is socially deprived or otherwise handicapped [Wallace et al., 1996]. Middle ear effusion in an older child with well-established verbal skills constitutes a minor handicap. Children with even mild losses who are having academic difficulty may benefit from amplification and preferential seating in school and from a mild-gain individual or classroom FM system.

Table 7-1 Severity of Hearing Loss

* Severity is measured in decibels of hearing loss (dB HL)/pure tone average. Each category of severity is recorded in dB HL or decibels above the average normal hearing threshold. Pure tone average refers to the mean of thresholds at 500, 1000, and 2000 Hz.

† Not all consequences of hearing loss of designated severities are listed. These characteristics vary among individual children within a category.

In most children, moderate loss of 45–60 dB interferes severely with, but does not preclude altogether, the acquisition of speech. Children with moderate and severe (65–80 dB) hearing loss gain the most from hearing aids [Nozza, 1996; Roeser et al., 1995]. Virtually all children with severe hearing loss require some type of special education to learn language adequately. Profound loss (>85 dB) and total loss preclude the acquisition of language without intensive specialized education. Strictly speaking, the term deafness applies only to these last two groups, but it is often used loosely in referring to all children who cannot learn language without special assistance.

Children with very limited or no hearing benefit little from hearing aids, except, perhaps, for the awareness that a sound has occurred. Most profoundly deaf adolescents and adults do not wear hearing aids, although, as children, they should be encouraged to wear aids because it is difficult to predict who will benefit from them. Children with profound but incomplete hearing losses gain more from their cochlear implants than those with pure tone average thresholds exceeding 100 dB [Pulsifer et al., 2003], and toddlers implanted early (from 6 months to 2 years) benefit much more from their implants than children implanted in middle childhood [Govaerts et al., 2002], [Sharma et al., 2009]. As stressed earlier, functional spiral ganglion cells are required for cochlear implantation.

The shape of the audiometric threshold provides another classification criterion for hearing loss and, with the severity of impairment, determines its effects on language acquisition. The two most common types of auditory threshold pattern in hearing loss are the flat and the sloping high-tone loss threshold curves, with U (dish)-shaped and inverted U (dome)-shaped and rising thresholds being less frequent. Sloping high-tone losses are particularly treacherous clinically. They are common and often overlooked unless systematic testing across the entire frequency range is performed; this is because children may respond normally to environmental sounds and voices that contain energy in the low frequencies, which the children can hear, but not in the mid- and high frequencies critical for consonant discrimination.

Classification by Age at Onset of the Hearing Loss

Age at deafening has a profound effect on its consequences. Schein and Delk, in their outstanding sociologic study of the deaf in the United States [Schein and Delk, 1974], list three main types of age-related childhood hearing loss: congenital or prelingual, perilingual, and postlingual. Prelingual means prenatal or infantile onset. Perilingual means onset during the acquisition of language and its grammatical rules, arbitrarily set by age 3 years. Most preschoolers who lose their hearing soon after age 3, i.e., technically postlingually, retain little expressive speech or language comprehension, even if great effort is made to preserve it, although they may have somewhat less difficulty reacquiring speech than children who never had hearing. Thus, age 3 years seems too young to mark the division between the prelingually/perilingually and the postlingually deaf. Children who do the worst are those who are essentially totally deaf – for example, from purulent meningitis – in whom pathology reveals total collapse of the entire endolymphatic organs, both vestibular and cochlear. Very young children need to hear to continue speaking. Prompt loss of speech is also typical of children with acquired epileptic aphasia with word deafness. Even if they are 5 years or older, they are likely to be mute and to start communicating with gestures or writing, not speech.

Pediatric deafness is mostly infantile. Schein and Delk found that the great majority of the prevocationally deaf – those whose deafness began before the age of 19 years, before they started working in a job or profession – are prelingually or perilingually deaf, proof that severe hearing loss acquired after infancy is uncommon. Any young child with severe loss of hearing from an ostensibly irreversible acquired cause like purulent meningitis (not epilepsy!), who shows no sign of improvement after several weeks, needs urgent electrophysiologic testing to find out whether at least some of the spiral ganglion cells have survived so the child can undergo prompt cochlear implantation in order to spare as much speech as possible. Another reason for early implantation is that ossification of the cochlea [Isaacson et al., 2009] and at least some trans-synaptic degeneration of the spiral ganglion neurons are long-term sequelae of purulent endolabyrinthitis which jeopardize successful implantation [Papsin and Gordon, 2007].

Classification by the Course of the Hearing Loss

Most severe sensorineural hearing loss is static, but about 10 percent, notably that caused by cytomegalovirus infection [Fowler et al., 1997] or a neurologic disease such as an acoustic tumor, and a significant proportion of genetic hearing loss (Table 7-2), is progressive [Davis et al., 1995]. Hearing loss that worsens rapidly during infancy is unlikely to be recognized as progressive, but it probably accounts for some of the children who pass neonatal screening and later turn out to be deaf. Genetic hearing loss that does not become apparent until later childhood or adult life is progressive by definition. It is now clear that both genetics and deleterious environmental influences, including acoustic trauma, contribute to the ubiquitous presbyacusis of old age. Genetic information about progressive hearing loss is generally insufficient to predict the course of a child’s hearing loss because different mutations of a given gene can result in disparate phenotypes, depending on the amount of protein synthesized. Unless the child belongs to a large family with many affected members with similar courses, who inherited an identical mutation from a distant founder, early prediction of outcome is not fully reliable. Slowly progressive hearing loss tends to denote less detrimental molecular effects or, perhaps, greater susceptibility to environmental influences than those causing early deafness. Other hearing loss, such as that due to middle ear effusion, episodic acoustic trauma, salicylate intoxication, and Ménière’s disease, fluctuates. Reasons for fluctuation are not always apparent.

Syndromic Versus Nonsyndromic Hearing Loss

This classification would seem straightforward; nonsyndromic implies an isolated hearing loss with or without vestibular impairment, whereas syndromic means that it is associated with involvement of other organ systems. Classification is not always straightforward, however, because systemic manifestations associated with early-onset, apparently nonsyndromic hearing losses may be quite delayed. For example, in classic Usher’s syndrome, retinitis pigmentosa worsens slowly during childhood and total blindness may be delayed until young adulthood, whereas deafness and vestibular impairment are typically prenatal; in Pendred’s syndrome, goiter, and in Alport’s syndrome, progressive renal disease may lag for several years; and in the Jervell and Lange–Nielsen syndrome (prolonged Q–T interval), the first occurrence of fainting is quite unpredictable. Thus the syndromic vs. nonsyndromic classification does not have strict etiologic implications. Genetic or nongenetic hearing loss with overt malformations or obvious systemic manifestations or cognitive deficits is that most likely to be considered syndromic. Two of the most easily recognizable types are Treacher–Collins and Waardenburg’s syndromes, but there are many dozens of others, and the list keeps growing (see later).

Genetic Causes

A number of useful reviews [Petersen et al., 2008; Petersen and Willems, 2006; Petersen, 2002; Kokotas et al., 2007; Friedman and Griffith, 2003; Dror and Avraham, 2009] discuss the cell biology and pathophysiology of known genetic hearing loss, although they cannot keep up with progress in the field. Two indispensable regularly updated web resources are Online Mendelian Inheritance in Man (OMIM) (http://www.ncbi.nlm.nih.gov/omim/) and Hereditary Hearing Loss Homepage (http://hereditaryhearingloss.org). The following abbreviated review, which is limited to a few hearing loss types that child neurologists may encounter, depends heavily on these sources. The somewhat outdated book by Toriello and colleagues [Toriello et al., 2004] remains useful because of the clinical details and pictures it provides.

Genetic hearing loss is generally bilateral but may be asymmetric, and may be static or progressive. The most salient asymmetric dominant hearing loss is neurofibromatosis type 2 (NF2) [Asthagiri et al., 2009]. Mendelian gene defects can be autosomal-recessive or autosomal-dominant, X-linked recessive – even, exceptionally, Y-linked – or mitochondrial. Compound heterozygous mutations of recessive genes are generally more prevalent than homozygous mutations. Sporadic deafness may be due to unrecognized recessive disease, to new dominant mutation, cytogenetic anomaly, or single-stranded copy number variation (CNV). When insertions, deletions, translocations, or other chromosomal rearrangements are large, they may affect two or more adjacent genes, and many are detectable by high-resolution chromosome banding studies. As modern genetics keeps demonstrating, what was thought clinically to be a single disease regularly turns out to be several, resulting from mutations of distinct genes that affect a common pathophysiologic or metabolic pathway. Further, clinically distinct syndromes often arise from allelic or nonallelic mutations in different parts of the same gene, with more or less severe consequences for protein synthesis. For example, the amount of the protein pendrin determines in large measure the severity of the phenotype in Pendred’s syndrome, and in some cases whether the trait is inherited in dominant or recessive fashion. Because of these complexities, we are not giving the names of most genes we mention in the text or tables; nor do we try to mention all of the phenotypes resulting from the mutations of a particular gene. OMIM is more up to date and should be consulted for details because only super-specialists can hope to dominate the genetics of hearing loss.

Malformations

Ear malformations may be sporadic or genetic, and involve one or several parts of the ear, some isolated, some part of complex malformation syndromes. Bilateral anomalies are more likely than unilateral ones to involve the brain and other organ systems. Some, but by no means all, are associated with intellectual deficits or autism [Wiznitzer et al., 1987]. The severity of malformations of the external ear and face does not predict outcome reliably unless associated with a major brain abnormality, so that a reliable prognosis cannot be provided in infancy.

Stenosis or atresia of the external ear canal may occur in isolation or be associated with malformation of the pinna and middle ear, and, in some children, the inner ear [Toriello et al., 2004]. Atresia produces a severe conductive loss and requires neonatal referral for bone conduction aids. The pinna may be absent, reduced to a small remnant (microtia), associated with one or more preauricular tags, or be present but malformed. The pinna with its lower half missing or deformed strongly suggests the CHARGE association (i.e., coloboma, heart defects, atresia choanae, retardation of growth and development, genitourinary problems, and ear anomalies) [Stromland et al., 2005]. Malformations of the ossicular chain and cochlea, and those of the central auditory canal containing cranial nerves VII and VIII are best visualized with ultra-thin, high-resolution computed tomography (CT) of the temporal bones. Patency of the cochlear scala tympani is critical for implantation candidacy. Microtia is frequently associated with ipsilateral facial palsy or deficient growth of the face (hemifacial microsomia), and conductive or mixed hearing loss [Carvalho et al., 1999]. In Möbius’ syndrome sensorineural hearing loss and involvement of other cranial nerves besides VI and VII may occur [Griz et al., 2007]. As previously mentioned, some cochlear anomalies are associated with perilymphatic/CSF fistulas into the middle ear, usually through the stapes footplate; they may produce fluctuating or progressive hearing loss and severe intermittent labyrinthine dysfunction with vertigo, nausea, or vomiting, and they may provide a path for a purulent endolabyrinthitis or meningitis [Reilly and Weber, 1996].

Acquired Causes

See Box 7-2.

Box 7-2 Main Causes of Acquired Hearing Loss

Infection

Congenital infections

Cytomegalovirus, rubella

Syphilis (childhood-onset), toxoplasmosis

Acquired infections

Chronic purulent otitis media, mastoiditis

Middle ear effusion due to repeated upper respiratory infection

Mumps, measles, scarlet fever*

Endolabyrinthitis from bacterial meningitis

Others

Perinatal insults

Hyperbilirubinemia (especially in premature infants)

Prematurity†

Severe anoxia

Ototoxic drugs

Ototoxic drugs

Aminoglycosides (e.g., streptomycin, dihydrostreptomycin, kanamycin, neomycin)

Furosemide, ethacrynic acid

Cisplatin

Thalidomide (prenatal exposure)

Aspirin (transient)

Trauma

Temporal bone fracture

Hemorrhage into the middle ear or cochlea

Ossicular dislocation

Sound trauma

Neoplasms

Bilateral acoustic neurinoma (neurofibromatosis type 2)

Other cerebellopontine angle tumor

Tumor of the middle ear (e.g., rhabdomyosarcoma, glomus)

^* These are rare causes of deafness and their etiologic importance has been exaggerated.

^† Perinatal insults and prematurity have additive effects; if hearing loss occurs, it is generally multifactorial. Selective IHC loss in some.

Patient and Family Histories

A detailed family history and pedigree are an integral part of the evaluation of every newly identified hearing-impaired child. Parents must be questioned closely about consanguinity, infections during pregnancy, prematurity, perinatal insults, exposure to ototoxic drugs, prior meningitis, and any other event that might have resulted in hearing loss in the child. It is better to obtain birth records of mother and child, and records from any other hospital where the child might have been admitted for a serious illness, than to rely on the parents’ collective memory, because parents are prone to blame deafness on some trauma, episode of transient anoxia, or bad ear infection that had nothing to do with the hearing loss. Most parents are unaware that unexplained sporadic hearing loss is more likely to be genetic and recessive than the sole consequence of a perinatal incident.

Physical, Developmental, and Otolaryngologic Evaluations

In addition to examining the child for pathologic conditions of the external ear and tympanic membrane, such as a malformed pinna, middle ear effusion, tympanic perforation, fistula, or palatal or submucosal cleft likely responsible for eustachian tube dysfunction, it is essential to look for evidence of associated anomalies in other organ systems. Pigmentation disorders of the skin and iris, unusual facial features, preauricular tags, anhidrosis, hyperkeratosis, and anomalies of the fingers or nails can suggest a specific diagnosis. Syndromic hearing loss may be associated with retinal pathology, congenital heart disease, spinal anomalies, and genitourinary or gastrointestinal malformations or diseases. The many genetic and nongenetic syndromes associated with hearing loss are detailed, usually with pictures, in other sources [Cohen, 1997; Toriello et al., 2004] and websites (http://www.ncbi.nlm.nih.gov/omim/ Online Mendelian Inheritance in Man [OMIM] and http://hereditaryhearingloss.org Hereditary Hearing Loss Homepage).

A neurologic evaluation is indicated whenever there are associated problems. Malformations of the ear are quite often associated with malformations of some of the structures of the posterior fossa, notably nuclear agenesis [Wiznitzer et al., 1987]. Hypotonic hearing-impaired children with or without delayed motor milestones require neurologic assessment to decide whether vestibular impairment contributes to the hypotonicity or whether the child has cerebellar, lower motor neuron, or muscle involvement, or one of many conditions affecting both the brain and the ear. As mentioned, detection of a neuropathy is particularly important in children with “auditory neuropathy.”

Assessment of the child’s developmental level and mental status is essential, but keep in mind that few evaluators are qualified to disentangle the consequences of language and information deprivation from independent cognitive deficit; in addition, the evaluators may not share a common language with a deaf child. Most states mandate triennial formal psychological testing of children with severe hearing loss, but neurologists need to be aware that few competent psychologists have adequate experience of evaluating the deaf. A significant proportion of deaf children are truly intellectually deficient or autistic as well as hearing-impaired, and it is critical to provide habilitation for both deficits [Jure et al., 1991]. In older deaf children, whether or not they have an implanted cochlear prosthesis, a review of academic skills is required because hearing-impaired children are no less prone to developmental problems, including intellectual deficiency, attention-deficit hyperactivity disorder, and learning disabilities, than children with normal hearing. Children with severe hearing loss, especially those whose families do not share an adequate common language with them, are at high risk for behavioral and psychiatric problems. There are small numbers of psychiatrists, social workers, and psychologists who have adequate experience and communication skills to evaluate and provide counseling for deaf children. Primary care providers and other professionals who evaluate these children need to be alert to these problems and refer children having difficulties promptly for adequate help. No meaningful long-term prognosis can be offered without knowledge of the child’s neurologic, psychiatric, and developmental status.

Screening in Neonates and Infants

Universal screening of neonates and graduates of neonatal intensive care units before they leave the hospital is now mandated in most states and many countries. This requires availability and efficient administration of electrophysiologic testing 7 days a week, now that early discharge is the rule. As an occasional inborn infant and those home-delivered will be missed, it is essential that pediatricians check results and inform parents at the first well-baby visit. In order not to misdiagnose children with “auditory neuropathy,” it is recommended that both ABR and OAE, described below, be used in the nursery [Cone-Wesson et al., 2000; Norton et al., 2000; Robertson et al., 2009]. Prompt referral and follow-up testing within a month is crucial for all neonates who fail the neonatal screen. Referral for definitive hearing testing is also required for any neonate who passed but has an immediate family member with a significant hearing impairment; who has a malformed pinna, face, or palate; who has Down syndrome or some other syndromic condition known to be associated with hearing loss; or who manifests any condition listed in the position paper of the American Academy of Pediatrics, Joint Committee on Infant Hearing [American Academy of Pediatrics, 2007] (Box 7-3). No child, including a neonate, is too young or too handicapped to undergo quantitative assessment of the functional state of the middle ear and sensitivity of the cochlea and auditory pathway. The mere suspicion of inadequate hearing, language or other developmental delay at any age, regardless of any other plausible cause for inadequate or unintelligible speech, mandates prompt, definitive assessment of hearing and middle-ear function with physiologic testing, without wasting time on repeated and often unreliable behavioral tests. Hearing loss is a hidden handicap in infancy. Even today, there are still infants who do not receive adequate follow-up after identification, especially among immigrant and indigent families.

Box 7-3 Indications for Hearing Screening Tests (OAE, BAER) in High-Risk Newborns, Infants, and Children*^,†

A Birth to 1 month

Family history of hereditary childhood sensorineural hearing loss

Congenital perinatal TORCH infection (i.e., cytomegalovirus, rubella, toxoplasmosis)

Craniofacial anomalies: malformation of the pinna, ear canal, face, palate, other syndromic or nonsyndromic dysmorphology; Down syndrome

Stigmata or other findings associated with syndromic hearing loss

Neonatal intensive care unit admission, or on mechanical ventilation >5 days, or received ECMO

Hyperbilirubinemia: untreated or required exchange transfusion

Exposed to ototoxic drugs: aminoglycosides, loop diuretics

Bacterial meningitis (e.g., group B streptococcus, Escherichia coli, Listeria monocytogenes, etc.)

Low birth weight (<1500 g or 3.3 lb)

B One month through 3 years: conditions requiring rescreening

Parent or caregiver concern regarding hearing, speech, language, or developmental delay

Confirmed postnatal bacterial and viral infections associated with sensorineural hearing loss

Significant head trauma, especially basal skull/temporal bone fracture

Recurrent or persistent otitis media with effusion lasting at least 3 months

Chemotherapy, ototoxic drugs, loop diuretics

Syndromes associated with progressive or late-onset syndromic hearing loss (e.g., osteopetrosis, Usher’s, Waardenburg’s, Alport’s, Pendred’s, Jervell and Lange–Nielsen syndromes)

Conditions previously listed in section A, rescreening at 24–30 months of age

C Throughout infancy and childhood: conditions requiring periodic monitoring

Parent, caregiver, or teacher concern regarding hearing, speech, language, or developmental delay

Family history of hereditary childhood-onset deafness, or earlier diagnosed genetic hearing loss

All children with new-onset, progressive, severe, or profound hearing loss

TORCH infections (except herpes simplex)

Neurofibromatosis type 2

Genetic neurodegenerative disorder (e.g., Hunter’s syndrome, sensorimotor neuropathies such as Friedreich’s ataxia and Charcot–Marie–Tooth syndrome, mitochondrial disease, leukodystrophy, etc.)

^* This list of indicators is meant to be used when universal hearing screening at birth is not available and to identify children in need of further audiometric testing beyond infancy.

^† BAER/ABR, brainstem auditory-evoked responses/auditory brainstem responses; ECMO, extracorporeal membrane oxygenation; OAE, otoacoustic emissions; TORCH, toxoplasmosis, rubella, cytomegalovirus, herpes simplex, syphilis.

(Adapted from American Acadamy of Pediatrics, Joint Committee on Infant Hearing. Most recent: Year 2007 position statement: Principles and guidelines for early hearing detection and intervention programs. Pediatrics 2007:120:898.)

Assessments of Vestibular Function and Vision

Ideally, children with sensorineural hearing loss should undergo vestibular testing, described in Chapter 8, to detect Usher’s, Pendred’s, and other deafness syndromes affecting both the vestibule and the cochlea. Lack of vestibular function may explain delayed gross motor milestones and stumbling in dim light [Kaga, 1999a]. Lack of vestibular function puts children at risk for drowning because, in the absence of gravity, they may become disoriented under water.

Detection of suboptimal vision is crucial because hearing-impaired children are exceptionally dependent on their eyes. Yearly refraction of young deaf children is recommended, inasmuch as children who require glasses rarely complain of poor vision and since most refractive errors arise in childhood. Examination of the retina by an ophthalmologist is essential because it may yield crucial etiologic evidence, like the chorioretinitis associated with an intrauterine infection or the retinitis pigmentosa of Usher’s syndrome. Children with vestibular dysfunction must be referred immediately to an ophthalmologist who is aware that Usher’s syndrome (see Table 7-2 for subtypes) is being considered. These children are prime candidates for early cochlear implantation, as their deafness is due to abnormality of the cochlear (and vestibular) cilia of the hair cells. Electroretinography is more sensitive than ophthalmoscopy in the early stages of retinitis pigmentosa [Young et al., 1996].

Types of Hearing Tests

There are two broad categories of hearing tests: behavioral tests that assess perception and behavioral verbal or nonverbal response to calibrated auditory stimuli, and physiologic tests of the sensitivity of the auditory system.

Behavioral hearing tests

Behavioral Tests of Conductive Hearing

Because sound is conducted through bone as well as through air, albeit with a threshold some 60 dB higher, presenting sound to the skull makes it possible to differentiate conductive from sensorineural hearing losses. Bone conduction is part of formal audiometry, with a vibrator placed on the mastoid of the ear being tested. Bone conduction audiometry is crucial in children with anotia (absent pinna and ear canal) or atresia of the external ear, or other middle-ear malformations, and is required in children with abnormal immittance tests and absent acoustic reflexes. Neurologists will occasionally screen older children for unilateral conductive hearing loss with the Weber test. This consists of placing a vibrating tuning fork on top of the head; the sound is heard better in the ear with a conductive loss than in the opposite ear. In the Rinne test the sound of a vibrating tuning fork is heard better and longer when it is placed on the mastoid process of an ear with a conductive hearing loss than when it is held in front of that ear.

Office Behavioral Screening Tests

In a pediatric office setting, testing is limited to behavioral screening and perhaps to tympanometry, which assesses the mechanical properties of the middle ear (see below). At best, infant screening is reassuring in normally developing children in whom there is no suspicion of auditory impairment. Lack of behavioral response to sound may not signal a peripheral hearing loss in uncooperative, autistic, or severely retarded children. Because of their pitfalls, behavioral screening tests are insufficient and they are inappropriate in multiply handicapped children, children with risk factors for deafness, or a clinical suspicion of hearing loss, all of whom must be referred promptly for formal quantitative testing.

Speaking softly to a child without risk factors, even if the response is appropriate, may be misleading if it provides nonverbal tactile or visual cues, or if speaking is louder than intended. Crumpling paper or ringing a small bell out of the child’s sight and observing whether the child turns the head and eyes promptly in the direction of the sound may exclude profound hearing loss but not a milder or high-tone loss. Consistent head turning in the horizontal plane does not occur before 6 months of age and in the vertical plane until almost 1 year, and young blind children do not turn toward a sound source. It is dangerous to rely on such subtle responses as eye widening and cessation of activity as indicators of hearing because they are subject to observer bias. In a toddler aged 1 year or older who is not suspected of deficient hearing, lack of turning toward the speaker or failure to respond verbally should also lead to suspicion of autism or attention deficit disorder, but it mandates immediate referral for audiometry because interpreting lack of response as unwillingness to cooperate rather than failure to hear or comprehend is a major error. Producing a loud sound out of an infant’s or toddler’s field of vision is uninformative because hand claps and dropped metal basins are likely to provide a falsely reassuring somatosensory cue. As with all biologic measurements, a single observation does not suffice; a reproducible one is required, with the caveat that habituation occurs after a few presentations, as soon as the stimulus has lost its novelty and alerting properties. These characteristics are exploited in the research laboratory to study speech sound discrimination in infancy [Kuhl et al., 1997]. There are screening instruments available, such as a 3000-Hz warbler of variable and calibrated intensity, but they are subject to some of the caveats just discussed.

Formal behavioral audiometry

Behavioral audiometry in a soundproof room, presenting calibrated pure tones and other acoustic stimuli though air and bone to each ear through earphones, is the gold standard against which all other tests of hearing are gauged. Behavioral audiometry is quantitative and efficient in experienced hands. It ensures that sound has been perceived and processed because it has elicited a specific response, but it does necessitate cooperation on the child’s part and adequate motivation and attention. Pure-tone behavioral audiometry yields sensitive, reliable, and reproducible threshold curves. It may be applicable to some young and handicapped infants with such refinements as visually reinforced and play audiometry. Conditioned behavioral techniques are not clinically reliable in infants whose developmental age is less than 6 months [Gravel and Traquina, 1992], despite the fact that it may be measurable in the research laboratory with experimental behavioral methods.

In older children, standard audiometry enables the comparison of air with bone thresholds in the same way as in adults. In sensorineural hearing losses, air and bone thresholds are identical, whereas in conductive hearing losses, bone thresholds are better than air-conduction thresholds. Many children have mixed hearing losses, in which bone threshold is better than air threshold, but both are elevated. Middle-ear fluid is a common cause of fluctuating hearing loss. Especially if associated with signs of vestibular dysfunction, sensorineural fluctuating loss may suggest cytomegalovirus infection, labyrinthine fistula, endolymphatic hydrops, and other disorders, some of which may require surgical intervention.

Many kinds of sensorineural hearing loss and most kinds of conductive hearing loss produce flat threshold curves (i.e., they impair hearing for sounds at all frequencies to more or less the same degree) (Figure 7-4). The average profoundly deaf child has minimal or no functionally useful hearing (Figure 7-5), despite responses to low frequencies that may represent awareness of vibration rather than hearing. Sloping hearing losses (Figure 7-6) are common and affect higher frequencies to a much greater degree than lower frequencies. This type of hearing loss is notoriously easy to overlook clinically. Such children are not deaf because they may respond to voices, especially male voices and broadband environmental noises which always contain some energy at the low end of the frequency spectrum, at nearly the same intensities as hearing children. Lack of hearing in the high frequencies has particularly deleterious consequences for language learning because consonant sounds differ in their high-pitched components. Because consonants carry most of the linguistic message, children with sharply sloping hearing losses are more handicapped than hearing-impaired children with the same mean pure tone average but a flat audiogram. Children with sloping hearing losses and other irregular thresholds may benefit from, and better tolerate, newer hearing aids that amplify certain frequencies selectively. Some hearing aids transpose frequencies into a range where the child has better preserved hearing [Auriemmo et al., 2009; Gravel and Chute, 1996]. Rarer types of audiometric threshold curves, some of them associated with particular causes of hearing losses, are U-shaped, in that they involve the middle frequencies more than the high and low, and inverted U-shaped or dome-shaped losses which involve the high and low frequencies more than the middle frequencies. Occasionally, patients have preserved hearing in the so-called ultra-high frequencies, those above 8000 Hz [Feghali et al., 1985]. Poor correlation between language ability and audiometric threshold is especially common in patients with “auditory neuropathy” and those with pathology of the central auditory pathway rather than hair cell pathology [Sininger and Starr, 2001].

Fig. 7-4 Pure tone air-conduction audiogram for moderate binaural flat hearing loss.

If it had been a mixed hearing loss, the level of bone conduction would have been indicated in brackets. O, right ear; X, left ear.

Fig. 7-5 Pure tone air-conduction audiogram for profound binaural loss at all frequencies, with slight residual hearing at 250, 500, and 1000 Hz.

No response (arrows) occurs at 2000 and 4000 Hz in the left ear. O, right ear, X, left ear.

Fig. 7-6 Pure tone air-conduction audiogram for severe binaural sloping hearing loss.

An acoustic reflex (Z) was present at 500–1000 Hz but absent (arrows) at 2000–4000 Hz, confirming the shape of the hearing loss. O, right ear; X, left ear.

Behavioral Tests of Central Auditory Function

Large batteries of tests requiring responses to auditory stimuli more complex than pure tone detection are often administered to children with learning disabilities and language disorders in an effort to understand the source of their problem [Hood and Berlin, 2003]. Speech audiometry measures the ability to discriminate speech sounds and to repeat phonetically balanced sounds. The ability to comprehend words that are distorted or presented against a noisy background is often impaired in children with high-tone losses. Competing words or sounds may be presented simultaneously to both ears (dichotic test) to assess cerebral dominance for the processing of language. In virtually everyone, including most left-handed people, language is processed preferentially in the left hemisphere, whereas the right hemisphere is dominant for musical melody, the intonation of speech (prosody), and environmental sounds. In dichotic testing, words presented to the right ear of left hemisphere-dominant individuals are somewhat more likely to be reported than those presented to the left ear. Dichotic testing is helpful for detecting pathologic lateralization of the auditory brain because stimuli presented to the ear contralateral to a damaged hemisphere may be not be reported [Hugdahl and Carlsson, 1994; Nass et al., 1992].

Electrophysiologic hearing tests

The advent of electrophysiologic hearing tests has revolutionized pediatric audiometry and enabled universal neonatal screening, as well as definitive assessment, without the need for a behavioral response, even in uncooperative, multiply handicapped youngsters. Some tests necessitate sedation or general anesthesia. Most physiologic tests do not ensure that sound has been “heard” – that is, processed – to elicit a response.

Acoustic Immittance and Middle Ear Reflexes

Acoustic immittance quantifies the compliance of the eardrum and middle-ear transducer (ossicular chain, stapes footplate). It is widely used to screen for middle-ear effusions and other hearing anomalies [Katz, 2002; Nozza, 1996]. Tympanometry consists of introducing air under measured pressure into the occluded external canal and aspirating it to assess the compliance and mobility of the eardrum. Eustachian tube occlusion results in negative middle-ear pressure, which stiffens (decreases the compliance of) the tympanic membrane. Negative middle-ear pressure causes secretion of incompressible fluid (serous otitis media, glue ear if the effusion is chronic and inspissated), which immobilizes the tympanic membrane.

An immobile or perforated drum, or an open ventilating tube precludes recording of the acoustic reflex. This reflex is caused by contraction of the stapedius muscle in response to a loud sound (at least 40 dB, usually 70–90 dB above normal hearing threshold). Its presence indicates that the middle-ear transducer is intact, the middle ear is aerated, and the acoustic (cranial nerve VIII) to facial (cranial nerve VII) reflex arc is intact, including its relays in the brainstem. It is abolished in auditory and facial neuropathies and in brainstem pathologies that interrupt their connection. The acoustic reflex is not a test of auditory sensitivity. It is useful for localizing the site of pathology in patients with Bell’s palsies: present when the pathologic process responsible for the facial palsy is distal to the middle ear, absent when the process is proximal to the nerve to the stapedius muscle. It is abolished or its threshold elevated in cochlear deafness and “auditory neuropathies,” brainstem tumors, leukemic meningeal infiltration, and other more central processes.

Otoacoustic Emissions (OAEs)

Bending of the stereocilia of the outer hair cells in response to clicks or tones generates transient narrowband sounds (OAEs) detectable above background emissions. These OAEs can be measured if highly amplified with a probe in the external ear canal, provided the middle-ear transducer and outer hair cells are functional (Figure 7-7). Recording of OAEs is now automated, easy, and quick, and the results are reliable over a wide range of frequencies. OAE testing only exceptionally requires sedation; it is noninvasive and much cheaper than electrocochleography in the operating room, and faster than BAER or ABR. These qualities make OAE a suitable relatively inexpensive test for universal neonatal screening [Norton et al., 2000; Morton and Nance, 2006]. The recommendation is to test, twice within a few weeks, those infants who fail the neonatal screen because about 10 percent of results are false-positives generally caused by amniotic fluid in the middle ear. If the infant fails again, brainstem-evoked responses are needed to corroborate the result; in fact, some audiologists recommend using ABR in the neonatal nursery if no OAEs are detected. OAEs are especially useful for detecting mild-to-moderate hearing loss, such as that caused by middle or inner ear pathology, or for monitoring early signs of antibiotic or antineoplastic ototoxicity [Stavroulaki et al., 2001, 2002]. As stressed several times earlier, the presence of OAEs is deceptive when pathology is selective for the inner hair cell or type I spiral ganglion neurons: e.g., in occasional carriers of the GBJ2 gene, which codes for the gap junction connexin 26 [Santarelli et al., 2008], those with OPA1 mutations, who lack otoferlin at the mutation between inner hair cells and type I spiral ganglion cell dendrites, and others with “acoustic neuropathy,” as seems to be the case in some sick newborns with some auditory pathologies, including hyperbilirubinemia [Amatuzzi et al., 2001, 2011].

Fig. 7-7 Averaged transient evoked otoacoustic emission recording in an ear of a 7.5-year-old girl with normal hearing.

A, Amplitude scale to permit examination of the input (click) stimulus. The stimulus was an 83-dB sound pressure level and was presented repeatedly. B, Amplified view of the time course of the evoked otoacoustic emission recorded from the external ear canal, with the first 4 milliseconds of the trace blanking out the stimulus artifact. Two separate averages are superimposed to show the high level of reproducibility. C, Frequency spectrum of the response versus background noise in the ear canal (shaded blue), demonstrating an excellent signal-to-noise ratio and therefore unequivocal outer hair cell function across all the frequencies up to 6 kHz. The output displays provided by clinical instruments include the number of stimuli that entered into the average, the number and percentage of stimuli automatically rejected because of excessive noise, the reproducibility of the two recorded waves, and the elapsed test time.

(From Durant JD. Physical and physiologic bases of hearing. In: Bluestone CD, Stool SE, Kenna MA, eds. Pediatric otolaryngology, 3rd edn. Philadelphia: WB Saunders, 1996.)

Transient suppression of the OAEs by presenting a loud stimulus to the contralateral ear and recording OAEs (and compound action potential) with measurably damped amplitude denotes an intact bilateral brainstem reflex which stimulates the olivocochlear bundle, thus inhibiting the outer hair cells. Like the acoustic stapedius reflex, its input depends on functioning ipsilateral inner hair cells, type I spiral ganglion cells, eighth nerve, and brainstem cochlear nucleus, but its output, measured contralaterally, signals integrity of the superior olivary complex, olivocochlear bundle, and outer hair cells [Hood and Berlin, 2009].

Electrocochleography and Cochlear Potentials

A number of electric potentials are generated in the cochlea in response to sounds of various frequencies. They are recorded optimally with a transtympanic electrode at the round window, which requires anesthesia in children [Pickles, 1997; McMahon et al., 2009]. Electrocochleography has become increasingly important for deciding about the eligibility of hearing-impaired individuals for a cochlear implantat. It provides reasonably reliable data on the selective function of the outer hair cells (cochlear microphonic function), dendrites of the type I spiral ganglion neurons (dendritic potential), the activation of these neurons (summating potential), and the compound action potential generated in their synchronously discharging axons (auditory nerve).

Brainstem Auditory-Evoked Responses/Auditory Brainstem Responses (BAER/ABR)

ABR testing does not require cooperation from the subject; it can be performed reliably at any age, whether the child is awake, asleep, or anesthetized. It is an ideal technique to supplement behavioral audiometry in hard-to-test infants and children. Its disadvantage is that it often requires sedation. As just stated, ABR testing is widely used in neonatal screening, often in conjunction with OAEs. Presence of an ABR does not rule out pathology above the level of the superior colliculus, thus “hearing.” This limitation does not detract from ABR’s usefulness because these higher deficits are much less prevalent than more peripheral pathologies. ABRs to clicks provide data on the collective response to sounds containing energy at frequencies of 100–8000 Hz and on the function of each of the relays of the auditory pathway. For audiometric applications where frequency-specific threshold estimates are required, it is important also to obtain ABRs to either brief tone bursts or notched white noise [Stapells et al., 1995]. For example, reproducible responses to 500 and 2000 Hz at intensities of 30 and 20 dB above normal hearing level for these stimuli almost certainly indicate normal peripheral sensitivity. In infants with microtia or an extremely narrow canal, or when it is crucial to determine that hearing loss is conductive rather than sensorineural, the stimuli can be presented through a bone oscillator.

The five waves of the ABR reflect activation of each of the relays of the auditory pathway: the auditory nerve, cochlear nucleus, superior olivary complex, nucleus of the lateral lemniscus, and inferior colliculus or structures in their vicinities. Averaging techniques enable the recording from the scalp of the tiny negative compound action potential of the auditory nerve, called wave I of the ABR (Figure 7-8). In normally hearing adolescents and adults, the average latency of wave I to clicks is 1.5 msec (maximum 1.9 msec) at high intensity, and the average latency of wave V, the largest, is 5.5 msec (maximum 5.9 msec). Latency is longer at low intensities and in infants [Stapells and Oates, 1997]. Absence of the ABR may indicate peripheral sensorineural hearing loss, or dys-synchronous conduction due to an axonopathy in either the auditory nerve or the brainstem [Sininger and Starr, 2001]. Alone, absence of wave I does not discriminate between pathologies in the inner hair cells, their synapses with the spiral ganglion cells, or spiral ganglion cell/auditory nerve pathology. Absence of wave I (and later waves) with absent OAE signifies peripheral hearing loss; absence of wave I with recordable OAE (and cochlear microphonic function) suggests either selective damage to the spiral ganglion cells or a true auditory neuropathy with preservation of the hair cells, or at least of the inner hair cells. Sparing of wave I with absent later waves or prolonged interwave interval suggests intrinsic brainstem pathology such as by demyelination or a neoplasm. Increased interpeak latency between waves I and III points to a demyelinating auditory neuropathy, and between waves III and V, to a pathologic process in the brainstem. Compression of the acoustic nerve by an acoustic tumor may prolong or suppress waves I–III and affect later waves.

Fig. 7-8 Components of the scalp-recorded averaged auditory-evoked responses to 1000 clicks presented at 60 dB above the subject’s threshold.

Each panel contains four different trials superimposed to show the reliability of the responses. Notice the different time scales on the abscissas and different amplitude scales on the ordinates. The top panel shows the seven waves of the brainstem response, the middle panel shows the middle components, and the lower panel shows the early obligatory cortical waves. Negativity at the vertex electrode produces an upward deflection.

(From Durant JD. Physical and physiologic bases of hearing. In: Bluestone CD, Stool SE, Kenna MA, eds. Pediatric otolaryngology, 3rd edn. Philadelphia: WB Saunders, 1996.)

Transmission time is prolonged in premature infants and newborns, and decreases with maturity as myelination advances. It is often prolonged transiently in stressed neonates and some young children, but prolonged transmission time alone is rarely predictive of future neurologic, auditory, or cognitive deficits. To reiterate for neurologists, the audiologists’ definition of “auditory neuropathy” does not necessarily imply a neuropathy of the eighth nerve [Marsh and Kazahaya, 2009]; rather, it means absent ABR or distorted ABR with prolonged latency, preserved OAE, and more severely impaired speech discrimination and production than predictable from the behavioral audiogram, which may reveal anything from normal to severely impaired hearing threshold.

Subcortical and Cortical Tests

For clinical purposes, later cortical potentials are less useful gauges of hearing sensitivity than brainstem potentials, primarily because they are altered by changes in alertness and sleep phase. Some of their components provide physiologic correlates of the later stages of auditory perception and discrimination [Hood and Berlin, 2003]. They are therefore very useful for research into the brain basis of auditory processing and language disorders because they supplement functional imaging by providing data about hearing in the msec time domain.

Physiologic activation of higher relays of the central auditory pathway can be recorded with averaging (see Figure 7-8) [Steinschneider and Dunn, 2002]. The somewhat inconsistently recordable waves VI and VII of the brainstem response are believed to reflect activity in the vicinity of the medial geniculate body and auditory radiations. So-called middle latency potentials [Kraus and McGee, 1990], which have a latency of about 15–80 msec, may reflect activation of the acoustic radiations and initial activation of the auditory cortex. Later obligatory potentials originate in the primary and secondary auditory cortices and are recorded with maximum amplitude at the vertex of the scalp because of the geometry of the primary auditory cortex, which lies horizontally in the planum temporale of the Sylvian fissure. The mismatch negativity, a frontocentral potential with a latency range of 60–120 msec, is recorded when the response to infrequent auditory stimuli, interspersed unpredictably among frequent stimuli, is subtracted from the response to the frequent stimuli. As the mismatch negativity does not require attention or the generation of a behavioral response, it may be suitable to test auditory discrimination in infants or uncooperative children [Steinschneider and Dunn, 2002].

Later components of auditory event-related potentials (ERPs) differ by their latencies and topographies, and they are contingent on the cognitive processing of various aspects of the signal. These later components depend on what parts of the language-processing cortical areas are activated when subjects are required to respond to various phonologic, syntactic, or semantic aspects of language. For example, Steinschneider and Dunn [Steinschneider and Dunn, 2002] recorded differences in evoked responses to voiced consonants such as d, compared with unvoiced consonants such as t, generated in auditory cortex in monkeys and humans. Neville and Mills [Neville and Mills, 1997; Mills and Neville, 1997] have studied electrophysiologic correlates of grammatical maturation by examining changes in a negative complex recorded over the anterior temporal or frontal cortex of the left hemisphere (including Broca’s area) in response to closed class words such as articles, prepositions, and conjunctions. They further studied correlates of semantic maturation by examining changes in a negative complex recorded from temporoparietal loci (including Wernicke’s area) in response to open class words such as nouns, verbs, and adjectives. They also used evoked responses to study the emergence of word comprehension in normal and language-delayed toddlers. Dunn and colleagues [Dunn and Bates, 2005] were able to characterize differences in semantic processing (i.e., organization of the lexicon or repository of word meanings) in verbal children with autism compared to normal subjects. Together with imaging, including functional magnetic resonance imaging (fMRI), positron emission tomography (PET), or single photon emission computed tomography (SPECT), these electrophysiologic techniques make it possible to identify the cortical and subcortical circuitry that participates in the various stages of auditory and language processing.

Imaging of the Ear

In neonates, the bony labyrinth is seen on transorbital view of a plain skull radiograph because the otic capsule is fully grown at birth and proportionately larger and better calcified than the remainder of the skull. High-resolution, thin-cut, and tridimensional CT and MRI have added an entirely new dimension to the imaging of the ear [Westerhof et al., 2001; King et al., 2002]. Every structure from the external canal to the internal auditory meatus, and the many pathologies and malformations that can affect them, are depicted clearly, but interpretation requires expertise. The yield of radiologic examination of the temporal bones is much higher when the hearing loss is mixed, conductive, or fluctuating, and when there are malformations in other organ systems than in nonsyndromic hearing loss. Imaging, together with sophisticated electrophysiology, is critical in children who are being considered for cochlear implants because a patent scala tympani and viable spiral ganglion cells are prerequisites. Prostheses applied to the brainstem may be effective when the cochlea is unsuitable for an implant [Grayeli et al., 2003].

Miscellaneous Blood, Urine, and Other Tests

By far the most prevalent viral infection causing hearing loss today is congenital CMV, which can be recovered from the urine, in some infants for months, and be responsible for progressive or fluctuating loss [Fowler et al., 1997; Rosenthal et al., 2009]. High levels of immunoglobulin M suggest an active infection in the neonate, whereas high immunoglobulin G levels are acquired passively from the mother. Many neonates are not visibly infected and later antibody titers are of limited value because most young children acquire CMV asymptomatically. A deaf child born with a cataract or chorioretinitis almost certainly was infected with rubella or CMV because, among the other TORCH infections, toxoplasmosis rarely causes deafness, herpes simplex probably does not, and syphilis does but in later childhood [Boppana et al., 1997; Chess et al., 1971]. Further epidemiologic studies are needed to assess the relative importance of CMV infection as a cause of sporadic hearing loss.

Congenital hypothyroidism with goiter (e.g., cretinism) due to iodine deficiency used to be a prominent cause of hearing loss but only remains so in small pockets of the world [Squatrito et al., 1981]. Individuals with Pendred’s syndrome are euthyroid despite progressive or fluctuating goiters in adolescence. Diagnosis based on the perchorate discharge test is less reliable and informative than genetic testing [Campbell et al., 2001).

Children with genetic hearing losses must, at a minimum, have a urinalysis, perhaps on a yearly basis, because a literature search reveals many dozens of syndromes in which hearing loss may be associated with renal or genitourinary anomalies. The yield is highest in, but by no means limited to, those with syndromic hearing loss. Hearing loss with anomalies of the external and middle ear, including tags and fistulas, requires kidney imaging because as many as 10 percent may have serious genitourinary anomalies [Senel et al., 2009; Kochhar et al., 2007). Alport’s syndrome, with dominant, recessive, or X-linked inheritance, associates progressive life-threatening nephritis with hematuria and progressive hearing loss [Hertz, 2009]. Waardenburg’s syndrome is occasionally associated with renal anomalies [Ekinci et al., 2005].

All deaf children must have an electrocardiogram to exclude the rare but life-threatening long Q–T syndrome (Jervell and Lange–Nielsen syndrome), a potassium channelopathy responsible for both the heart dysrhythmia and the deafness with vestibular impairment [Crotti et al., 2008]. The syncopal attacks may be mistaken for seizures, a major error because many of these children have died suddenly and unexpectedly, but might otherwise have been treated effectively with a noradrenergic beta-blocker or a pacemaker. Other tests for heart disease need to be considered in congenital rubella and a number of hearing loss syndromes.

Genetic Testing and Counseling

Children suspected of having a genetic disease known to be associated with hearing loss – like Hunter-type mucopolysaccharidosis [Cho et al., 2008], for example – require genetic confirmation. How far to pursue a specific diagnosis in undiagnosed children with suspected genetic disease or sporadic hearing loss is currently in flux. The number of “deafness genes” for which there is now a definitive genetic test stands at well over 100. Gene microarrays provide the opportunity to test for hundreds of genes simultaneously, without the need for a candidate gene, but they remain expensive. Genetic testing has led to stunning progress toward clarifying the pathophysiology of deafness, but it rarely has an impact on the proband. Many centers test sporadic nonsyndromic profound congenital hearing loss for connexin 26 because it accounts for some half of the cases [Petersen and Willems, 2006]. A major reason to pursue a specific genetic diagnosis, besides pointing to potential associated problems, is genetic counseling.

Several hundred mutations are estimated to be responsible for genetic hearing losses. Carrier frequency in the general hearing population for the connexin 26 mutations, the most prevalent cause of recessive nonsyndromic hearing loss, is more than 1 in 100 and as high as 1 in 30 in some regions, with strong evidence of founder effects [Mercier et al., 2005; Gasparini et al., 2000]. There are no figures for overall carrier frequency for deafness, but given the very large number of recessive genes discovered so far, an estimate of 1 in 4 may not be unreasonable. The probability of unrelated hearing parents having a second hearing-impaired child if the cause of deafness in the older child is unknown is at least 1 in 10, a composite of the general population risk for deafness of 1 in 1000 and of 1 in 4 when the deafness is genetic and recessive, plus 1 in 2 when it is dominant but not expressed (or unrecognized) in one of the parents. The empirically derived recurrence risk of 1 in 10, applicable with minor deviations worldwide, indicates that sporadic deafness is much more likely to be genetic than acquired. An earlier survey in 1969–70 suggested that 62.8 percent of deafness was genetically caused (47.1 percent of cases recessive, 15.7 percent dominant), and the sporadic 37.2 percent remainder mainly acquired [Marazita et al., 1993]. More recent estimates are quite similar: approximately 30 percent acquired [Morton and Nance, 2006]. An unknown proportion of the sporadic cases may be isolated autosomal-recessive cases, an assumption supported by the report that, in Jerusalem, the prevalence of childhood deafness decreased in parallel with the diminishing rate of consanguinity [Feinmesser et al., 1990]. Parents must be told that autosomal-recessive inheritance cannot be excluded when the cause of their child’s hearing loss is unknown.

There are two reasons, besides research, to offer genetic testing: first, for the benefit of probands, and second, for providing interested parents, families, and, in the future, probands with genetic counseling . Clinical benefits for probands lie mainly in discovering the likelihood of involvement of other tissues, such as the retina, heart, kidney, or inner ear malformations, with potentially deleterious consequences; as pointed out above, other recommended and less expensive screening tests can identify most such involvement. Testing for specific mutations is indicated in what were previously viewed as single-gene deafness diseases, which we now know can arise from separate genes with quite different prognoses: e.g., Usher’s syndrome [Ouyang et al., 2005], Alport’s disease [Hertz, 2009], the long Q–T interval [Crotti et al., 2008], Pendred’s syndrome [Pera et al., 2008], and others. Genetic counseling regarding the risk for future pregnancies is a major reason for offering genetic testing. Prenatal diagnosis is desired by many parents of children with hearing loss [Brunger et al., 2000], especially when hearing loss is part of a major systemic illness or multiorgan malformation syndrome like the CHARGE association [Jyonouchi et al., 2009] and arguably Usher’s syndrome type I. Prenatal genetic diagnosis for the consideration of selective abortion in isolated nonsyndromic hearing loss is quite another matter, hotly debated, not only in the deafness community but by ethicists as well. How individuals in the current generation who have benefited from cochlear implants will feel about genetic counseling in the future is an open question. The number of genetic causes of hearing loss, which now include mitochondrial genetics, microRNAs, and epigenetic influences, is so large and growing so rapidly that even recent reviews are out of date. Sophisticated genetic counseling is time-consuming and requires consulting up-to-date electronic resources such as Online Mendelian Inheritance in Man (OMIM) (http://www.ncbi.nlm.nih.gov/omim/), Hereditary Hearing Loss Homepage (HHH) (http://hereditaryhearingloss.org) and syndrome-specific Connexin Deafness Homepage (http://davinci.crg.es/deafness).

Testing for research is crucial, as it has reaped countless benefits for understanding the cellular pathophysiology of deafness, but it needs to be clearly labeled as such and the way it is paid for discussed openly. Public or private research funds, not out-of-pocket expense or charges to public or private medical insurance, are the legitimate sources of support for genetic testing for research. Genetic testing and the development of animal models based on it are responsible for spectacular progress in understanding the workings of the cochlea [Dror and Avraham, 2009]. They have also revealed how much more complicated than anticipated hearing is at the molecular and cellular level. Genetic results have spilled over into the understanding of parallel pathways in other organs like the heart, muscle, and eye. The deaf community is very interested in this research, but understandably wants transparency in its purposes and manner of funding.

Psychologic Evaluation

Hearing speech is a major vehicle for incidental learning that is unavailable to deaf children. Television has little meaning without sound, and signed captions are available for only a few selected programs. With the exception of the most proficient implanted or amplified children, most deaf children lag behind hearing children by several years in learning to read fluently enough to read printed captions on TV, or read for pleasure or as an efficient means for gaining information. Even today, most of the unimplanted (and an unknown fraction of the implanted) do not achieve this level of proficiency [Conrad, 1979; DiFrancesca, 1972]. Their school experience is likely to be less enriching than that of their hearing peers, especially as much time is consumed, at least in the early grades, by learning language and, later, by attempting academic catch-up. Even normally intelligent deaf children are likely to suffer severe experiential and cultural deprivation. A study performed by Furth (1966) found that they knew and experienced less than their hearing peers, and impoverished language for thought altered their cognitive experience. This is still the case for many children who risk being marginalized in the adult workplace.

Not surprisingly, unimplanted deaf children tend to do dramatically less well than hearing children on verbal portions of intelligence test batteries; however, some of them inexplicably also do less well on nonverbal tests, perhaps because they have an unrelated learning disability or are penalized by their meager language because verbal ability for many people is advantageous for carrying out ostensibly nonverbal memory or picture arrangement tasks. A few psychologic test batteries have been developed and standardized for deaf populations such as the Wechsler Scales, Central Institute for the Deaf Preschool Performance Scale [Geers and Lane, 1984], Snijders–Oomen Non-Verbal Intelligence Scale [Snijders et al., 1988], and Leiter International Performance Scale [Roid and Miller, 1997]. Others can be administered verbally or nonverbally; these include the Hiskey–Nebraska Test of Learning Aptitude [Hiskey, 1966], which has lower norms for deaf than hearing children, and the Raven Matrices [Raven, 2003]. Although there are encouraging reports on the achievements of implanted children, it is still too early to determine how many of them will match their hearing peers. While there have always been some exceptional deaf individuals who have performed at the highest level of intellectual and professional ability, as a result of frequently associated brain anomalies, the difficulty of acquiring language skills in a hearing world, and the inadequacy of conventional IQ testing in atypical populations [Lane, 1992], they represent a small minority of the congenitally deaf. It is still too early to evaluate to what extent cochlear implants will reverse this disparity in adults fortunate enough to have been implanted early and to have received the intensive training required to profit optimally from these prostheses.

Amplification and Hearing Aids

As some genetic and nongenetic hearing loss, notably that due to congenital CMV infection, is progressive, audiograms must be obtained regularly throughout childhood. Hearing aids are variable gain amplifiers inserted into the external ear canal. They must be evaluated at regular intervals so that they continue to fit snugly in the growing ear canal. Behind-the-ear miniaturized hearing aids yield as much amplification as the body-worn aids of the past. Their smallness does have disadvantages: easier loss and damage.

All infants and children with a significant hearing loss must be fitted forthwith with hearing aids by trained pediatric audiologists who select optimal amplification devices to fit each child’s particular needs so as to maximize the development of residual auditory function [Chase and Gravel, 1996; Roeser et al., 1995]. Cochlear implantation should be considered promptly as early as the age of 6 months if the loss is severe or profound (see below). Amplification is especially effective for children with conductive loss, who derive more benefit from amplification than children with sensorineural hearing loss because their inner ears are normal and the aid bypasses their defective mechanical sound transducers. Children with atresia can be fitted with bone oscillators. Some hard-of-hearing children have difficulty accepting a hearing aid inasmuch as amplification may make them perceive moderately intense sounds and ambient noise as intolerably loud (i.e., recruitment). Advanced prescriptive fitting techniques and wide dynamic range compression circuitry allow output-limiting characteristics to be determined and verified instrumentally, even in very young children. Children with high-tone and other nonflat hearing loss require hearing aids with selective frequency amplification fitted to the shape of their individual thresholds. Even profoundly deaf children may gain awareness of sound despite their having little discrimination.

It is essential to explain to parents that hearing aids do not restore hearing and only bring within the child’s narrow hearing window sounds that would otherwise be below hearing threshold. The aids do so at the expense of distortion of amplitude (and often frequency) and loss of signal-to-noise ratio. Sound cannot be amplified to the point of acoustic trauma, which occurs at equal intensity in normal and impaired ears.

Radio-frequency FM closed-loop listening systems are widely used in schools for the deaf, in some mainstream educational settings, and by some parents. The advantage of such a system is that each child receives the amplified speech signal from the speaker in his or her hearing aids, with much less amplification of background noise than with standard hearing aids which amplify unselectively. The teacher or parent can wear a transmitter and the child the FM receiver, enabling some severely hearing-impaired children to be mainstreamed into normal classrooms. Schools for the deaf insist that all children wear hearing aids in school, at least during the elementary years. Many of the profoundly deaf adolescents and adults who derive little or no benefit from aids choose not to wear them. Usually binaural aids are prescribed so that children have access to cues for sound localization and can benefit from better hearing in noisy backgrounds [Chase and Gravel, 1996].

Surprisingly, it turns out that some children with severe unilateral hearing loss are at somewhat of a disadvantage for language development and academic achievement [Lieu, 2004]. Children with unilateral (or bilateral) anotia, microtia, or canal atresia need to be fitted with a bone conduction hearing aid soon after birth; after age 4 years, when skull growth is essentially complete, they can be fitted with an implanted bone conduction hearing aid [Snik et al., 2008]. It is generally recommended that children with unilateral losses be provided with an aid, despite normal hearing contralaterally, because a significant proportion of them experience academic difficulties [Lieu, 2004]. A variety of approaches have been recommended to improve sound localization, including provision of an FM system in their classrooms [Updike, 1994].

Cochlear Implants

In developed countries, cochlear implants now dominate management of severe to profound hearing loss in children [Papsin and Gordon, 2007]. A cochlear implant is a complex electronic prosthesis with five main parts: three external and two implanted. The external components are (1) a microphone that picks up sound signals and sends them to (2) a speech processor that selects the sounds to be passed on to (3) a transmitter worn on the scalp. The transmitter transfers the signals through the scalp to (4) an implanted receiver/stimulator anchored to the skull under the scalp, sitting opposite to the external transmitter. This computerized device digitizes sound signals into electronic pulses of graded frequencies that it transmits to (5) stimulating electrodes on very thin wires of variable calibrated lengths bearing stimulating electrodes. The wires are bundled and inserted into the scala tympani under the basilar membrane, enabling the electrodes to stimulate the spiral ganglion neurons’ dendrites tonotopically, according to their positions along the basilar membrane [Wilson and Dorman, 2008]. Implant technology keeps improving and recent cochlear prostheses, which enable superior speech discrimination, are multichannel and programmable. A cochlear implant costs $40,000–$60,000, and to be optimally effective has on-going costs; the unit requires close follow-up to monitor its calibration and performance, and children require intensive long-term speech and language training if they are to achieve good speech comprehension and adequate intelligibility, given the coarseness of the device’s input compared to ear’s ability to code the richness of natural speech signals. Its cost puts it out of reach of all but a small minority of children in developing countries, although considerably cheaper instrumentation is under development [An et al., 2008].

Observations over more than a dozen years indicate that cochlear implants are reassuringly safe and maintain their effectiveness in the long term. Originally – and some still hold this view – unilateral implantation was recommended in order to reserve the other ear in case of a major complication in the implanted side, or in case of a future major technologic breakthrough. A new trend is bilateral simultaneous implantation so as to enhance sound localization and mitigate interfering ambient noise effects [Eapen and Buchman, 2009; Papsin and Gordon, 2007]. Follow-up studies indicate that bilateral implantation is superior to unilateral in that it optimizes language learning and speech intelligibility because it increases discrimination. Unilateral insertion remains widely used and quite effective.

Most complications occurred within 5 years among 500 consecutive implants at all ages in one center. The overall rate was 16 percent, minor in 5.6 percent, major in 3.2 percent, and reimplantation, usually successful, had to be done in 7.2 percent [Venail et al., 2008]. Only a handful of postmortem evaluations of long-term effects of prostheses on the cochlea have appeared so far [Kawano et al., 1998]. Granuloma in the cochlea [Nadol et al., 2008], and new bone formation and fibrosis along the electrode [Somdas et al., 2007] were described, but their prevalence awaits study of a larger number of specimens. There are very rare reports of damage upon insertion [Verbist et al., 2009] and of delayed meningitis because of the artificial communication between the middle ear and scala tympani created by the implant [Reefhuis et al., 2003; Papsin and Gordon, 2007]. This risk mandates routine immunization against Streptococcus pneumoniae for all implanted children [Wilson-Clark et al., 2006].

The etiology of the severe or profound hearing loss being considered for implantation is marginally relevant. As already stated, the prosthesis requires a patent scala tympani and a sufficient contingent of functional spiral ganglion neurons. Therefore CT and detailed electrophysiologic testing to establish neuronal viability are mandatory. Development of a prosthesis directly stimulating the brainstem auditory pathway for persons in whom cochlear implantation is impossible is at an early stage of development [Colletti et al., 2007].

Age at implantation is important [Amatuzzi et al., 2011]. As discussed earlier, neonatal screening yields an occasional false diagnosis of deafness, presumably due to a potentially reversible neonatal insult like hyperbilirubinemia or hypoxia [Attias and Raveh, 2007; Amatuzzi et al., 2001], or perhaps amniotic fluid in the middle ear, or conversely a false diagnosis of adequate hearing if only OAEs without ABRs were used for screening. Therefore, implantation is not recommended until after age 6 months, when a definitive audiometric curve will have been established, the scalp will be thicker, the skull and mastoid will have grown substantially, and the risks of anesthesia are lower. Infancy, between 6 and at most 24 months, the language-learning age, is the optimal age for implantation [Cheng et al., 1999], although later implantation still provides substantial though shrinking benefit [Beadle et al., 2005]. Superiority of the prosthesis over hearing aid amplification is especially marked for speech articulation [Flipsen, Jr., 2008; Artieres et al., 2009].

By now, several thousand prelingually deaf children have been implanted worldwide, and age at implantation and intensity of postimplantation habilitation are shown to be major factors for enabling profoundly deaf children to be mainstreamed [Geers et al., 2003]. Speech production and especially intelligibility remain low for many months after implantation, and comprehension is limited, except for words that have been taught specifically. With intensive intervention, young children eventually reach the stage of “picking up” oral language. This considerable delay after implantation has raised the issue of whether to continue the use of sign language to assist communication. Most oralist educators are vehemently opposed to Sign. Others point out that continued exposure to Sign is helpful to prelingually deaf children while intensive therapy is teaching them what hearing and speech are all about because it provides them with an effective means for communication and explanation. Deaf children at the language-learning age can “pick up” Sign without instruction if exposed to fluent signers for an adequate number of waking hours. In Sweden, commitment of the parents to becoming proficient in Sign and continuing to use it is the norm for implanted children. Follow-up several years after implantation found that good speakers were also good signers; in fact, they had become bilingual [Preisler et al., 2002]. Some members of the deaf community, especially deaf parents of deaf children, have mixed feelings about cochlear implants, fearing that the children will distance themselves and give up the rich cultural heritage of that community. Understandably, they want their children to be fluent signers as well as speakers. Some are justifiably anxious about the short- and long-term demands and potential complications of implants [Christiansen and Leigh, 2004].

Reconstructive Surgery

Reconstructive surgery for external and middle ear malformations is contraindicated in very young children because they are at high risk for middle-ear infections. Even in older children, middle-ear reconstruction carries with it the small danger of adding a sensorineural component to the conductive loss. Unilateral surgery is usually recommended. Bone oscillation hearing aids, particularly those anchored internally to the mastoid, invariably give good results, whereas the outcome of reconstructive surgery is somewhat problematic [Raveh et al., 2002; Siegert, 2003]. In severe microtia or anotia, exploration of the middle ear, and creation of an ear canal, tympanic membrane, and an acceptable pinna, entail several stages of plastic and otologic procedures carried out in middle childhood or the teenage years, when important cosmetic and functional benefits balance out their burden [Siegert, 2003].

Education

Since the enactment of U.S. Public Law 94-142 in 1975, a variety of infant auditory training programs have become available to any infant or preschooler younger than 3 years who is identified as hearing-impaired, and most states provide full-day, 11 months per year special education for deaf children, starting at age 3. Programs in different localities differ in detail. Most infant programs are at least partially home-based, provided by an itinerant teacher of the deaf working with parents and child and providing auditory, speech, and language training, including Sign if desired.

Most hard-of-hearing children and those with unilateral hearing loss can be mainstreamed into normal classes from the start, provided they have hearing aids and an FM system, and speak reasonably intelligibly. They need to have access to a part-time special teacher for the hearing-impaired to give them individual academic assistance and to receive on-going speech and language therapy. To attend at least some mainstream classes, those more severely impaired who have inadequate English may require a deaf interpreter and also access to notes and help with reading assignments. Many children with implants are able to function in mainstream classes, although most require on-going individual speech training [Bosco et al., 2005; Geers et al., 2008].

A much smaller proportion of the deaf than the hearing population is able to acquire a college education or enter a profession, even though there are specialized universities for the deaf, such as Gallaudet University in Washington, DC; the National Technical Institute for the Deaf at the Rochester Institute of Technology in Rochester, New York; and other smaller programs. What proportion of deaf adolescents who have been implanted early will be able to attend college or technical schools without special help remains to be seen.

How best to educate deaf children who do not have cochlear implants has, for more than a century, been the subject of raging controversy between the proponents of oralism and those who champion the use of sign language plus, at a minimum, competent literacy. One problem is that, starting in the late 19th century, the hearing have prescribed how the deaf should be educated [Lane, 1984, 1992]. Inevitably, hearing parents would like their deaf children to speak and be educated well enough to blend into the normal population. This aspiration has become much more realistic with the advent of implants. Bilingualism in Sign is highly desirable for the implanted deaf and required for the hearing children of deaf parents who are members of the signing community.

Not all parents and teachers of the deaf understand that implants are doomed to failure when there are no spiral ganglion cells or the eighth nerve is affected by a severe true auditory neuropathy. Persisting with a purely oralist approach in such children is to repeat what was a dismal failure for the majority of the prelingually deaf, and lasted a century, until the resurgence of ASL in the 1980s. Many parents overestimate the efficiency of lip reading, not realizing that many speech sounds are not visible on the lips. Fortunately, TV and education have lessened the stigma of Sign, but by no means does the public truly believe that speaking is but one form of language, and that Sign is just as rich and viable a language as English, Arabic, or Chinese [Klima and Bellugi, 1979; Lane and Philip, 1984]. Few people understand the importance of children acquiring language on their own, without instruction, at the normal language-learning-age, i.e., during infancy and the early preschool years, by being exposed to fluent users of a language. Later, languages have to be taught by a much less efficient process; taught languages, especially their phonology, are almost never acquired as well as a person’s native tongue, and taught languages can be shown to occupy cortical areas that differ, with overlaps, from those of the native language [Neville and Mills, 1997]. Because most deaf children without implants who are taught with the oralist approach do not become fully proficient in English, including written English, it is unfair to delay exposing them to Sign until they have failed oralism. We and others strongly believe this also applies to implanted children, not all of whom will profit as much as hoped from the implant.

Deaf and hearing children of deaf parents, who are exposed to the native language of their deaf community, i.e., ASL in the United States, French Sign Language in France, and other sign languages in other countries, begin to express themselves in Sign at an earlier age than hearing children start to speak. Presumably this is because gestures develop earlier than speech, and because producing signs requires less refined motor coordination than producing intelligible words [Mayberry, 2003; Schlesinger and Meadow, 1972]. Children whose native language is some form of Sign acquire at the expected age syntactic devices such as the question forms, pronouns, prepositions, etc., of Sign, and they understand that each person, thing, feeling, and action has a name; thus, from the start, they possess a rich and fully adequate vehicle for self-expression.

Like simultaneous translation between any two languages, simultaneous expression in ASL and spoken English is not literal because Sign’s syntax is encoded entirely differently from that of English. Starting in the early 1970s, many deaf educators recommended the use of Signed Exact English for deaf education. Signed Exact English is a contrived language that transliterates English word for word into signs borrowed from ASL; words are produced in their order of appearance in English, and there are signs for English grammatical markers like plurals or tense indicators. It enables speaking and signing to proceed simultaneously (i.e., total communication). It was hoped that this approach would provide deaf children with a channel to express themselves, simultaneously facilitating their learning of English. Although it was a step in the right direction, Signed Exact English is not well suited for a visually coded language; consequently, there is strong movement, that started in the 1980s, toward a return to ASL, which had been banished from many schools for the deaf by well-meaning hearing teachers [Lane, 1984; Sacks, 1989].

There is no convincing evidence that learning Sign delays the acquisition of written English. Lane [Lane, 1992], writing before the advent of implants, strongly urged the use of Sign with deaf children, to provide them with a language to serve as a foundation for learning English as a second language. In native signers, learning English at school age can begin by concentrating on proficiency in written language and the alphabetical code. Currently, unimplanted deaf children, even in schools or classes that use Sign, spend much time in speech training, yet most children are grossly deficient in their knowledge of written and spoken English, to the detriment of their educational achievement and general cognitive proficiency [Conrad, 1979; DiFrancesca, 1972; Lane, 1992]. Happily, cochlear implants are changing this state of affairs, as stressed repeatedly [Geers et al., 2008]. A consequence of this success story is that, as children sufficiently proficient in English and other subjects are increasingly mainstreamed into regular classrooms with appropriate assistance, the population in the shrinking number of schools and classes for the deaf has changed. It now includes a higher proportion of hearing-impaired, multiply handicapped children than in the past, in addition to nonimplanted children by parental choice and those whom the prosthesis did not help.

It is still debatable whether deaf children with minimal knowledge of spoken English perform better in mainstream schools with deaf interpreters and individual tutoring, or in schools for the deaf where everyone uses Sign. It is also uncertain which approach provides “the least restrictive environment” prescribed by U.S. Public Law 94-142. The small number of boarding schools for the deaf remain practical for children who live in small, isolated communities unable to provide special services for one deaf child, especially if the child has multiple needs. Residential placement is much more acceptable than in the past, because it is possible to have children go home for some or all weekends. Residential school placement offers the advantage of deaf peers, the consistent use of Sign by most teachers and caretakers, and the loss of the stigma of being considered handicapped. It may be the least restrictive environment for children from deprived backgrounds whose families are not motivated to learn Sign, cannot provide extensive auditory language training, and may not even speak English, the only language besides ASL used in deaf education in the United States. In day schools for the deaf, such children may have no opportunity to communicate with anyone except during the 6 hours of the 180 school days. Some deaf children have serious behavior problems because their parents may feel guilty over their child’s handicap and are reluctant to discipline him or her, or may not be able to communicate adequately enough with their child to impose parental control. Yet most deaf children do not have severe behavior problems, and deafness alone is not an adequate explanation for autistic behaviors.

Genetic Counseling in Hearing Loss

Providing parents with up-to-date genetic counseling, which was discussed earlier, is an important aspect of management. To reiterate, the following points should be conveyed:

Even well-educated parents are unlikely to be familiar with straightforward Mendelian genetics, let alone mitochondrial genetics, or modern genetic concepts like single-strand microdeletions or other rearrangements. Genetic counseling requires time, expertise, and the services of a professional genetic counselor to provide in-depth information.

Consequences of Hearing Loss for Overall Development

Intelligent deaf children who have a supportive family, whose hearing loss was diagnosed early, and who had the benefit of optimal habilitation, including an implant and vigorous language training, do well cognitively, socially, emotionally, and educationally, no matter their proficiency in oral language. Even today, with rare exceptions, the less privileged and those implanted late still remain significantly or severely deficient in language and overall academic proficiency. Their reading ability and other educational accomplishments are likely to lag and fall further behind those of their hearing peers [Conrad, 1979; Schein and Delk, 1974; Geers et al., 2008]. At a minimum, deafness affects the life experience of the child (and family) profoundly, with secondary consequences for “smarts”, education, and personality [Mayberry, 2003; Rapin, 1979]. Yet many deaf people do find a niche in the workplace and are able to function reasonably independently, albeit often supported in part by Social Security Disability Income (SSI).

The deaf are at risk for significant medical and emotional problems because there are few members of their families, neighbors, physicians, nurses, social workers, job counselors and coaches, psychiatrists, psychologists, and other health-care workers with whom they can communicate adequately. The profound jeopardy of even intelligent deaf-autistic, deaf-blind, and otherwise multiply handicapped deaf children is not being addressed adequately. There is a need for medical and other professionals who understand the particular problems of deaf individuals and who can communicate with them to evaluate and treat them competently.

Despite these obstacles, many nonimplanted deaf adults are independent and self-supporting [Greenberg, 1970]. Deaf communities exist in most large cities and provide informal networks of social and practical assistance to their members [Schein and Delk, 1974]. Many deaf children are not diagnosed as early as would be desirable, and grow up without the benefits of optimal habilitation. They may experience severe social deprivation and may have parents who are not able to communicate with them, except through a primitive gesture system, a few spoken nouns and verbs, and words such as “no.” However, even totally deprived deaf adults are capable of learning a language, provided they are intelligent, highly motivated, and lucky enough to meet a truly dedicated teacher [Schaller, 1991].

The great majority of the prevocationally deaf – i.e., those whose deafness began before the age of 19 years, before they started working in a job or profession – are prelingually or perilingually deaf. The impact of cochlear implants and more active efforts to accommodate hearing-impaired students into grade school and mainstream higher education will no doubt change the conclusions of the 1974 Schein and Delk study, but it remains to be seen by how much. Schein and Delk discuss the jobs the prevocationally deaf were able to secure, and their family and social circumstances. In general, because of their curtailed verbal skills, including those in written language, these deaf adults encountered severe limitations to their vocational options. Most married deaf partners, and the community of deaf persons that exists in every sizable city helped them find suitable employment and navigate the complexities of mainstream society. It remains to be seen what proportion of the severely or profoundly prelingually or perilingually deaf cochlear implants will allow to be integrated seamlessly into the society of the hearing.