AUDITORY SYSTEM DISORDERS

Published on 10/04/2015 by admin

Filed under Neurology

Last modified 10/04/2015

Print this page

This article have been viewed 3310 times

CHAPTER 26 AUDITORY SYSTEM DISORDERS

Thomas O. Willcox, Gregory J. Artz

Hearing loss can be defined as an increase in the threshold of sound perception. Understanding speech and the general world around us depends on the accurate perception and processing of complex, multifrequency sounds. There are multiple areas along the auditory pathway for pathology to occur that can cause distorted, inefficient, or unperceived sound, resulting in hearing loss. Hearing loss affects nearly 28 million Americans, including 30% of adults over the age of 65 and 50% over the age of 85. Hearing loss is one of the most common chronic illnesses, and as the population ages and lives longer, it will increasingly affect the morbidity and quality of life of patients. Hearing loss is not just a disorder of adults. It also significantly affects children and their daily lives, as well as the lives of their parents and caregivers. Otitis media can cause a conductive hearing loss due to fluid accumulation that can take months to resolve. Otitis media is the most common reason for a child to visit the pediatrician. By 3 years of age, three of every four children will have had at least one episode of otitis media. This has a significant financial impact on the health care system, in addition to the financial impact on parents, who lose income when they miss work to care for their sick children. Children can also be afflicted with congenital causes of hearing loss; two or three of every 1000 children born today will be either deaf or hard of hearing.

Hearing loss is a broad topic and first can be subdivided into hereditary and nonhereditary causes. Hereditary causes can be isolated genetic defects or associated with several childhood syndromes. Hereditary hearing impairment is not covered in depth in this chapter. Briefly, it is the cause of hearing impairment in more than 50% of children born with moderate to profound hearing loss. Of these children with genetic causes of their hearing loss, more than 75% will be determined to be nonsyndromic. More than 100 genetic loci have been implicated in causing nonsyndromic hereditary hearing loss, with defects in connexin 26 and connexin 30 being the most common. These causes are overwhelmingly autosomal recessive in approximately 75% of cases but also can be autosomal dominant or X-linked or even due to mitochondrial inheritance. Hereditary hearing loss usually manifests at birth, but some hearing disorders manifest as delayed onset or even adult onset in a nonprogressive or progressive fashion.

Hearing loss can be subdivided into categories based on the site of pathology: conductive, sensorineural, or central hearing loss. Conductive and sensorineural are the most common, whereas with central hearing loss is quite rare.

CONDUCTIVE HEARING LOSS

Conductive hearing loss is caused by impairment in air transmission of sound waves to the inner ear. The impairment of function is due to pathology at the level of the external auditory canal, the tympanic membrane, or the ossicular chain, resulting in inefficient conversion of sound waves from air to the fluid medium of the endolymph in the membranous labyrinth. A rare cause of conductive hearing loss is the “third mobile window” of the inner ear, which is attributed to a dehiscent superior semicircular canal or an enlarged vestibular aqueduct. On bedside physical examination, a patient with a conductive hearing loss has a negative Rinne test, which means that the hearing threshold for bone conduction is less than air conduction in the tested ear. This test is performed by placing the 512-Hz tuning fork first on the mastoid tip, then approximately 2 to 3 inches from the entrance to the external auditory canal. Bone conduction surpasses air conduction when a conductive hearing loss reaches 25 to 30 decibels (dB). When performing the Weber test, the end of the tuning fork is placed in the middle of the patient’s forehead. If the patient has unilateral hearing loss, then the sound will lateralize to the bad ear, or the ear with the conductive hearing loss. The sensitivity of the Weber test can be increased by alternatively placing the tuning fork on the patient’s upper incisors.

SENSORINEURAL HEARING LOSS

Sensorineural hearing loss is caused by damage to the cochlear sensory epithelium or, less commonly, the peripheral auditory neurons. The type of hearing loss a patient has can be quite different depending on whether the cochlea or the auditory nerve fibers are involved. When cochlear hair cell loss is the main reason for hearing difficulties, it often manifests as a sound threshold shift only. When the lesion involves the auditory nerve, or is called retrocochlear, a patient often has significant sound distortion that manifests as difficulty with word discrimination out of proportion to the associated hearing loss. On bedside evaluation, the Rinne test is positive, and if the hearing loss is unilateral, the Weber test lateralizes away from the side with the hearing loss, or toward the good ear. Sensorineural hearing loss is a challenge to physicians, as it progresses with age and causes significant reductions in quality of life and there are no treatments to reverse its effects, other than sound amplification with the use of hearing aids or direct auditory nerve stimulation via cochlear implantation.

CENTRAL HEARING LOSS

Central hearing loss is caused by a lesion in the central auditory pathway or in the auditory cortex. The auditory cortex processes and interprets the sounds amplified and received by the ossicles and cochlear hair cells. The auditory cortex is located on the transverse temporal gyri of Heschl. It is divided into the primary auditory cortex (Brodmann’s areas 41 and 42) and the auditory association cortex (Brodmann’s areas 22 and 52). Lesions such as brain tumors and ischemic or embolic strokes in the region of the transverse temporal gyri affect speech discrimination more than they affect hearing threshold levels, as patients with central hearing loss often have normal audiograms. Types of central hearing loss include aphasia (Wernicke’s aphasia), pure word deafness, auditory agnosia, cortical deafness, and auditory hallucinations. Another more common form of central hearing loss is associated with presbycusis, or age-related hearing loss. In the elderly, speech discrimination is often worse than would be expected based on their pure-tone hearing thresholds. It is believed that this may be due to a combination of cochlear hair cell loss, neuronal loss, and age-related central auditory processing changes.

CAUSES OF CONDUCTIVE HEARING LOSS

Suboptimal air conduction of sound waves to the cochlea can cause a hearing loss. This is referred to as a conductive hearing loss. There are several places that a conductive hearing loss can occur: the external auditory canal, the tympanic membrane, the middle ear space, and the inner ear. Most of the causes of conductive hearing loss are due to anatomical obstruction or damping of the transmitted sound waves on their way toward the sensory epithelium of the cochlea. Almost all of these causes are amenable to surgical or medical correction and, by definition, all patients with purely conductive hearing losses have normal eighth nerve function and can achieve normal hearing thresholds through hearing aid amplification.

Cerumen Impaction

One of the most common causes of hearing loss is cerumen impaction. The external auditory canal is one third cartilaginous and two thirds bony. The lateral one third of the canal is cartilaginous and has overlying skin containing subcutaneous tissue, hair follicles, cerumen, and sebaceous glands. The medial two thirds of the canal are bony, has thin skin, and lacks hair follicles or glandular tissue that are adherent to the bone. A conductive hearing loss can occur if cerumen completely obstructs the lumen of the external auditory canal. This often occurs suddenly, often after cotton swab manipulation or water exposure. Symptoms can include aural fullness and tinnitus. Treatment is atraumatic cerumen removal. Irrigation should be performed with caution, as irrigation in a patient with a tympanic membrane perforation can cause vertigo and subsequent otitis media.

Cholesteatoma

Cholesteatoma is normally associated with the middle ear and mastoid, but it can occasionally occur in the external auditory canal. Through inflammation and associated infection, it can cause a conductive hearing loss. Cholesteatomas of the external canal are usually unilateral and have associated symptoms of otalgia and otorrhea. On examination, there is narrowing or occlusion of the external auditory canal, abundant keratin debris, and sometimes granulation tissue. Treatment includes resolution of the external otitis infection with topical antibiotic eardrops and occasional systemic antibiotics, followed by surgical debridement and excision of the cholesteatoma.¹

External Auditory Canal Tumors

Tumors of the external auditory canal, benign or malignant, can cause a conductive hearing loss. The two most common benign bony tumors are exostoses and osteomas.¹ Exostoses are broad-based lesions that are often multiple and bilateral. Patients usually give a long history of cold water exposure, such as swimming, diving, or surfing. Exostoses are found in the medial portion of the bony external auditory canal near the annulus and often along the tympanomastoid and tympanosquamous suture lines. Osteomas are solitary and unilateral and are not associated with any significant history such as that of patients with exostosis. They are found in the lateral portion of the external auditory canal at the bony-cartilaginous junction. Treatment of exostoses and osteomas is based on symptoms, as they are benign lesions with no known malignant conversion. Chronic or recurrent acute otitis externa is the most common reason patients undergo surgical excision. Care must be taken not to injure the mastoid segment of the facial nerve when removing these lesions, particularly when operating in the posteromedial external auditory canal. The most common malignant tumor of the external auditory canal is squamous cell cancer. Fortunately, these are rare head and neck tumors. They can arise from anywhere within the external auditory canal, and patients often have symptoms of otorrhea, otalgia, and occasionally hearing loss. Treatment is surgical excision with postoperative chemoradiation therapy depending on the stage of the tumor.

External Auditory Canal Stenosis or Absence

A rare cause of conductive hearing loss is stenosis or absence of the external auditory canal, as in congenital aural atresia. The incidence of aural atresia is 1 in 10,000 to 20,000 births.² Aural atresia is usually associated with a large conductive hearing loss or air-bone gap, assuming that the cochlear function is normal. Varying degrees of external ear malformations (microtia), temporal bone atresia, ossicular deformities, and facial nerve anomalies are seen. Surgical treatment to repair the external ear, external auditory canal, and middle ear abnormalities can restore hearing to normal levels in favorable candidates.

Tympanic Membrane

Pathology of the tympanic membrane includes perforations, atelectasis, and tympanosclerosis. Aside from its role in protecting the middle ear, the tympanic membrane is critical in receiving sound waves and efficiently transmitting them through the ossicular chain to the endolymph of the cochlea. Any pathological process that compromises the mobility or efficiency of the tympanic membrane results in a conductive hearing loss. Tympanic membrane perforations can be caused by acute and chronic infections, head trauma, or iatrogenic causes, such as after tympanostomy tube extrusion. Tympanostomy tube placement is common in infants with otitis media and is one of the most common surgical procedures performed today. The reported rate of tympanic membrane perforation depends on the type of tube placed; however, routine grommet-type tubes have a 1% to 3% incidence.³ Most perforations from tympanostomy tubes are small, causing a 10-dB hearing loss or less, and usually heal with time. However, larger perforations and total perforations of the tympanic membrane, usually seen in patients with a history of chronic otitis media, can result in a significant conductive hearing loss of 30dB or more. A thin, atrophic, atelectatic tympanic membrane can also cause a conductive hearing loss, particularly if there is associated ossicular erosion. A retracted, atelectatic tympanic membrane is caused by eustachian tube dysfunction and the resultant chronic negative middle ear pressure. Hearing loss can be further affected in these patients by chronic middle ear fluid. Initial treatments consist of tympanostomy tube placement, tympanoplasty, and medical therapy, including decongestants and nasal steroid sprays. A common finding on otoscopy during routine physical examination is tympanosclerosis, a white discoloration of the tympanic membrane. Tympanosclerosis can be due to a prior history of tympanostomy tube placement and/or an associated history of otitis media. Unless the tympanic membrane involvement is particularly severe, it is rare for tympanosclerosis to cause an appreciable conductive hearing loss.

Buy Membership for Neurology Category to continue reading. Learn more here

Related

Neurology and Clinical Neuroscience E-Book