The external ear

The external ear includes the pinna, external auditory meatus and tympanic membrane. The outer third has a cartilage skeleton, and the rest is bony. Both are covered by skin. The skin on the outer part contains hair follicles and wax glands, but these elements are absent in the bony meatus. Canal skin migrates outwards from the deep meatus, but does not desquamate until it reaches the junction with the cartilaginous meatus. This normal mechanism may be disturbed by injudicious use of cotton buds.

The eardrum is the window of the middle ear and is divided into the pars tensa and pars flaccida. The main landmark on the drum is the malleus handle (Fig. 1.2).

Fig. 1.2 A normal right eardrum.

The middle ear

The middle ear is an air-containing space connected to the nasopharynx via the Eustachian tube. It acts as an impedance matching device to transfer sound energy efficiently from air to a fluid medium in the cochlea (Fig. 1.3). The middle ear space, including the mastoid air cells, is closely related to the temporal lobe, cerebellum, jugular bulb and labyrinth of the inner ear. The space contains three ossicles (the malleus, incus and stapes) which transmit sound vibrations from the eardrum to the cochlea. The middle ear also contains two small muscles and is traversed by the facial nerve before it exits the skull.

Fig. 1.3 Section through the cochlea. Hair cells in the organ of Corti transform mechanical energy (vibration) into electrical impulses.

The inner ear

The inner ear comprises a dense bony capsule containing a membranous labyrinth which forms the cochlea, vestibule and semicircular canals.

The membranous part is surrounded by fluid, called perilymph, and is sealed from the middle ear by the stapes footplate and round window membrane.

The membranous part itself contains fluid called endolymph. The cochlea contains the organ of hearing which is connected by the auditory nerve to the brainstem (Fig. 1.3).

Normal hearing depends on transmission of sound via a vibrating tympanic membrane through a mobile ossicular chain to the cochlea. Displacement of the basilar membrane and movement of the hair cells cause an organized volley of electrical discharge in the eighth nerve, perceived as sound in the cerebral cortex.

The vestibule and semicircular canals form the peripheral balance organ. These have connections to the cerebellum and the eyes, and are important in the maintenance of posture and the ability to keep the eyes fixed when the head is moving.

Symptoms

History taking in ear complaints should be brief but thorough. Table 1.1 provides a reference guide of the major points that should be covered. The otological symptoms are discussed in greater detail later in this section, but it is important to establish the predominant complaint and whether it affects one or both ears.

Table 1.1 Major points in history taking in patients with an otological complaint

Signs

Satisfactory examination cannot be undertaken without adequate lighting. Battery auriscopes with a fibre or glass ring light give a coaxial beam with bright uniform illumination. A pneumatic attachment tests the mobility of the eardrum (Fig. 1.4).

Fig. 1.4 Inserting the auriscope. The pinna is elevated upwards and backwards. The instrument should be gripped like a pencil.

The pinna should be examined for scars and signs of crusting or weeping. Before introducing the auriscope, the ear canal must be straightened by elevating the pinna upwards and backwards (Fig. 1.4). The condition of the ear canal should be noted before the eardrum is examined. If an adequate seal of the canal is achieved by the speculum, gentle pressure on the pneumatic bulb will move the eardrum if the middle ear contains air. This is helpful in distinguishing perforations from thin or translucent segments.

A complete examination also involves viewing the Eustachian tube orifice in the nasopharynx.

Tuning fork tests

Tuning fork tests distinguish between conductive and sensorineural hearing loss, but are of limited value in children. Two tests are usually employed using a 512 Hz fork. The fork is sounded by striking the tines against the patella or elbow.

Rinne test

The Rinne test compares air conduction (AC – hearing via ear canal and middle ear) with bone conduction (BC – direct transmission to the inner ear via the mastoid process). The examiner holds the fork by the ear canal and then places it on the mastoid process using gentle counterpressure with the other hand. The patient is asked which position of the fork sounds louder: in front of the ear or touching the mastoid (Fig. 1.5).

Fig. 1.5 Interpretation of tuning fork tests. These tests are invalidated if the ear canals are not free of wax debris.

Sound is normally heard better by air conduction than by bone conduction (Rinne-positive). Disease in the external or middle ear, producing a conductive deafness, will reverse the test result (Rinne-negative).

Subjective tests

Pure tone audiograms are a standard means of recording hearing levels. Using headphones, each ear is tested individually for air conduction and, if necessary, bone conduction thresholds. The results are usually plotted as a graph. A result of 0 dB (decibels) is the average normal threshold for hearing in young adults (Fig. 1.6).

Fig. 1.6 A series of typical pure tone audiograms of the right ear. Hearing level (dBISO) is plotted on the vertical axis; frequency (Hz) is plotted on the horizontal axis. Air conduction (AC) signals are fed through headphones. Bone conduction (BC) signals are transmitted via a vibrator placed on the mastoid process.

Speech audiometry is a more sophisticated test. Phonetically balanced words are presented at different sound intensities and the number of correct answers is expressed as a percentage score. This is a useful test for evaluating hearing aids.

Objective tests

Impedance audiometry is an extremely useful test in the diagnosis of middle ear disease and some types of sensorineural hearing loss. By varying the pressure in the external ear canal, the compliance (i.e. mobility) of the eardrum may be calculated by the degree of sound reflected from a probe tone. This is very useful when screening for middle ear effusions, particularly in children, and for assessing Eustachian tube function (p. 7). It can also test the integrity of the middle ear mechanism and the auditory reflex arc (stapedius reflex).

Electric response audiometry (ERA) (Fig. 1.7) is another objective test. An evoked potential in the eighth nerve, brainstem or auditory cortex may be recorded using skin electrodes following acoustic stimulation of the cochlea. This principle is used to objectively assess hearing thresholds where the standard tests are not applicable, e.g. babies, disabled people and suspected malingerers.

Fig. 1.7 Evoked response audiometry. The response evoked by a sound stimulus can be recorded at a specific site along the auditory pathway, e.g. eighth nerve, brainstem or cortex.

Otoacoustic emissions testing is a useful screening test for neonatal sensorineural loss where the middle ear function is normal.

Hearing assessment in young children

From birth to about 6 months, the ‘gold standard’ method of testing is by electric response audiometry (see above). From 6 months to about 18 months, a child will turn to a noise, e.g. a rattle. This distraction test is performed by two observers and is a basic screening test performed on all children. From 2 years, various conditioning or cooperation tests are employed using free field noises, e.g. placing a peg in a basket after hearing the noise. It is not until the age of 3–4 years that headphones can be used to test each ear independently.

Tests

The functional status of the peripheral vestibular system can be tested in a variety of ways. These include:

Positional test

From an erect sitting position on a couch, the patient lies flat with the head turned to one side and below horizontal (Fig. 1.8). The onset of any vertigo is noted and the eyes are observed for nystagmus. The feeling of movement and the nystagmus, if present, are allowed to settle before the patient sits upright. The manoeuvre is repeated with the head to the opposite side. This test may help to distinguish vertigo caused by peripheral (otological) as opposed to central pathologies.

Fig. 1.8 ‘Positional testing’ in cases of vertigo.

Otitis media with effusion (glue ear)

Otitis media with effusion (OME) is the most common cause of acquired conductive hearing loss in children. The true incidence is unknown, but up to 60% of children in their first year may have middle ear effusions which are clinically asymptomatic. The peak clinical age group is 2–6 years, where about 30% of children suffer effusions. By the age of 11 the incidence has dropped to about 2%. There is a seasonal variation in the disease, associated with upper respiratory tract infections which are more common in October to March in the northern hemisphere.

The effusion in the middle ear may be serous, mucoid or thick (glue). The aetiology is usually Eustachian tube dysfunction, where normal ventilation of the middle ear is disturbed (Fig. 1.12). A diagnosis of chronic otitis media with effusion is made when fluid is present behind the eardrum for 12 weeks or more.

Fig. 1.12 Eustachian tube dysfunction resulting in defective middle ear ventilation. This can be due to a variety of causes and results in a middle ear effusion.

Clinical features

Children with OME usually present with hearing loss or recurrent otalgia. Children with a cleft palate or Down’s syndrome have a higher incidence of middle ear effusions. The otoscopic features of OME are characteristic (Fig. 1.13).

Fig. 1.13 Otoscopic appearance in otitis media with effusion (OME). The handle and short process of the malleus are brought into relief due to retraction of the eardrum. There is a slightly yellow appearance to the eardrum due to the middle ear effusion.

The hearing loss is conductive and may fluctuate down to as much as 40 dB. Tympanometry produces a flat trace, indicating an immobile drum (Fig. 1.14).

Fig. 1.14 Impedance tympanometry. This is an excellent method for detecting the presence of middle ear effusions. A flat compliance is seen in cases of otitis media with effusion.

Treatment

There is as yet no effective long-term medical treatment for established OME. Short-term improvements with antibiotics are not sustained. Decongestant mixtures are ineffective. If the effusion persists, surgery may be required to restore hearing. Removal of the adenoids reduces the incidence of recurrent effusions. Myringotomy, aspiration of fluid and insertion of a ventilation tube (grommet) immediately restores the hearing (Fig. 1.15). Grommets may remain in the drum for up to 12 months before being extruded. After grommet extrusion some children require reinsertion due to recurrent or persisting middle ear effusions.

Fig. 1.15 Types of grommets (middle ear ventilation tubes). The designs are numerous in variety in an attempt to prevent too rapid an extrusion from the eardrum and to assist in insertion.

Following grommet extrusion, it is common to see tympanosclerosis (white patches) in the eardrum. This type of white patch does not impair hearing.

Chronic failure of the Eustachian tube to function normally results in persisting middle ear effusion and thinning of the tympanic membrane. The eardrum may collapse onto the ossicles, producing a retraction pocket. Long-term ventilation may be necessary to prevent progressive retraction from the low middle ear pressure and subsequent development of chronic suppurative otitis media with cholesteatoma.

Otorrhoea after grommets

Grommets may become infected, producing a mucoid discharge. This should be mopped away and anti-inflammatory drops instilled. Massaging the tragus will allow the drops to penetrate the grommet lumen. Antibiotic eardrops are not recommended except under specialist supervision. Oral antibiotics may also be necessary if the discharge follows an upper respiratory tract infection. Persistent otorrhoea will necessitate removal of the grommet.

Children with grommets should not be prevented from participating in swimming activities. Infection is uncommon and usually readily treated.

Ear canal

Wax production varies between individuals and races. Blind attempts to remove wax with cotton buds usually result in impaction. Wax may be properly removed by syringing the ear or with a blunt hook (p. 25). Preliminary softening can be achieved with sodium bicarbonate eardrops three times a day, or hydrogen peroxide. Rarely, excessive accumulations of desquamated skin and wax in the deepest part of the external meatus can expand and erode the ear canal. This is termed keratosis obturans, and an anaesthetic may be required to remove it.

The external canal may be narrowed by bony exostoses predisposing to keratin accumulation (Fig. 1.16). These exostoses often occur in swimmers and require no treatment unless they cause external otitis or hearing deficits.

Fig. 1.16 Multiple exostoses may lead to narrowing of the external meati. They are sited at the osseocartilaginous junction of the ear canal.

Eardrum and middle ear

Perforations of the eardrum can occur from trauma and acute or chronic otitis media (Fig. 1.17). The degree of hearing loss depends on the site of the perforation and the extent of middle ear disease.

Fig. 1.17 A subtotal perforation of the left eardrum. The round window niche (RWN), the inudostapedial joint (ISJ) and promontory (P) are clearly visible.

Perforations from simple chronic otitis media where the mastoid is not diseased may be repaired by a tympanoplasty procedure using a graft (temporalis fascia). Ossicular discontinuity may also be treated surgically. Traumatic perforations, e.g. blow to the ear, invariably heal spontaneously if the ear is kept dry.

Adults may suffer with middle ear effusions, although less commonly than children. Investigations should rule out sinusitis, or nasopharyngeal tumours blocking the Eustachian tube (see Fig. 1.12, p. 7).

Otosclerosis is a disease where new bone growth occurs in the capsule of the inner ear. This may fix the footplate of the stapes. Hearing loss characteristically develops in the young adult and is usually conductive (p. 3), although the otoscopic appearance of the eardrum is normal. Pregnancy can accelerate the symptoms, suggesting a hormonal association with the disease. A family history is frequently elicited. Tinnitus may also be present.

Surgery for otosclerosis may restore normal hearing but also carries a small risk of total hearing loss. Use of a hearing aid has no complications, but is often refused (Fig. 1.18).

Fig. 1.18 The pathology and management of otosclerosis.

Presbyacusis (common)

Presbyacusis is a progressive loss of hair cells in the cochlea with age. Roughly 1% of cells are lost each year, and this affects the high-frequency part of the inner ear first (Fig. 1.19). It becomes clinically noticeable from the age of about 60–65 years. The degree of loss varies, as does the age of onset. Some patients with presbyacusis have recruitment (reduced dynamic range of hearing) which reduces effective amplification. The threshold for hearing and the uncomfortable level of sound are abnormally close (e.g. ‘Speak up, I can’t hear you … don’t shout so loud!’). Discrimination may also be affected (‘I hear you but can’t understand you’). There is no treatment to prevent this loss. When a significant social or work handicap is present, a hearing aid may be prescribed. This should be digital so that the pattern of amplification is tailored to the pattern of the individual’s hearing loss. Two hearing aids are better than one, because of binaural hearing.

Fig. 1.19 A typical audiogram seen in a patient with presbyacusis. There is a ski-slope-type high-frequency loss. The low frequencies may remain within the normal range.

Sudden (idiopathic) hearing loss (rare)

Cochlear failure may occur in a previously normal ear. The patient is suddenly aware of a blockage or rapid deterioration in hearing. Tinnitus or vertigo may be present. The aetiology is thought to be either a virus infection or vascular ischaemia. Treatment is largely empirical, and changes from decade to decade. Currently there is a great deal of interest in the role of steroids and antiviral agents. Preservation of high-frequency hearing on serial audiograms indicates a good prognosis.

Noise exposure (common)

Acoustic trauma occurs from sudden exposure (impact or blast), or from prolonged exposure, e.g. noise of heavy industrial machinery. Levels of 90 dB or greater require ear protection with properly fitted ear defenders. After exposure there may be a temporary threshold shift, perceived as ‘woolly hearing’ and associated with tinnitus. Continued noise exposure will lead to permanent threshold shifts, usually affecting the higher frequencies starting at 4 kHz.

Treatment is by avoidance or employing adequate ear protection in the form of ear defenders. An established hearing loss may benefit from a hearing aid. A system of compensation is available for occupational hearing loss.

Perilymph fistula (rare)

A rupture of the labyrinthine windows (round or oval) will result in leakage of perilymph fluid and a sensorineural type hearing loss. Mild features of imbalance or even frank vertigo may also occur. The rupture is usually preceded by an event that raises the intracranial pressure, e.g. straining to lift. It may also follow a stapedectomy operation where an iatrogenic fistula is sealed but the leak persists. The middle ear should be explored in cases where the hearing is deteriorating; otherwise, there is a risk of total hearing loss. The offending rupture is then sealed with a fat plug.

Inflammatory diseases (rare)

Measles, mumps, meningitis or syphilis may cause cochlear damage and can result in permanent sensorineural hearing loss. Chronic middle ear disease is also often associated with some degree of sensorineural loss.

Ototoxicity

The inner ear has many active metabolic processes which are susceptible to drugs. The cochlea or labyrinth may be affected in isolation or in combination, and this can result in hearing loss and symptoms of imbalance. Agents that are toxic to the renal system commonly affect the ear, e.g. systemic aminoglycosides and cytotoxic agents. Salicylates and quinine have reversible toxicity.

Acoustic tumours

Acoustic tumours are rare, but treatable, tumours of the vestibular element of the eighth cranial nerve. The most common presentation is a progressive unilateral hearing loss with tinnitus. MRI scanning with gadolinium is the investigation of choice and can demonstrate small tumours (Fig. 1.20). Current treatment options include serial scanning (for small tumours), surgical excision or stereotactic radiosurgery.

Fig. 1.20 An axial view of an acoustic neuroma seen on MRI after injection of gadolinium contrast. The tumour stands out clearly.

Dysacusis

Despite having normal hearing thresholds, some patients are still unable to hear well, particularly in noisy environments. This is termed dysacusis. The aetiology is presumed to be a cochlear abnormality.

Problems with electronic hearing aids

To gain the maximum benefit from the aid, it is important to provide patients with training. It is a shock to many to learn that an aid cannot produce normal hearing. Patients with conductive hearing losses have better results with aids than those with sensorineural losses. This is due to the fact that many of the latter losses are associated with a phenomenon called ‘recruitment’, where loud sounds are heard exceptionally loudly so that the amplification from a hearing aid merely adds to the patient’s difficulties.

The common problems encountered with electronic aids are listed in Table 1.7. Probably the most frequent difficulty is with acoustic feedback. This produces the familiar high-pitched whistle and is particularly seen in patients who require high amplification, and in whom the ear mould allows sound to escape into the microphone. A similar event will occur if the mould is incorrectly inserted, as is frequently seen in elderly people suffering from arthritic joints.

Table 1.7 Common problems with electronic hearing aids

A persistent otorrhoea may occur due to allergy to the mould. Alternative non-allergenic material can be employed. In some patients this manoeuvre is simply ineffective and, in others, continued insertion of a mould produces otitis externa or a discharge from a mastoid cavity. Such cases may benefit from a bone conducting aid worn as a headband with the microphone abutting firmly onto the mastoid. However, these are cosmetically unsightly.

More recent alternatives are bone conduction aids that are anchored in the temporal bone. The external stimulator sets the aid in vibration either across the intervening skin or by a direct percutaneous attachment facility (Fig. 1.22; Fig. 1.53, p. 24). Such aids do not suffer the feedback problems of conventional air conduction aids and also have the advantage of greatly reduced background noise.

Fig. 1.22 Osseointegrated aid. This avoids many of the problems of traditional aids.