Mednotes

L27 Nervous System Dizziness and Hearing

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tagsnervoussystem, lecture
aliasesL27 Nervous System Dizziness and Hearing

37 min read

Overview

The lecture revises the anatomy and physiology of the middle and inner ear, works through the symptoms produced when disease affects them, sets out how hearing is assessed (voice tests, tuning forks, pure tone audiogram, speech audiometry, tympanometry, evoked response audiometry), and then applies all of this to four clinical cases: a chronic otitis media with active perforation, an attic cholesteatoma, surfers’ ear canal exostoses, and a progressive severe to profound sensorineural loss treated with a cochlear implant. The unifying idea is that the middle ear is an impedance matching amplifier and the cochlea a tonotopic transducer, so the site of the lesion predicts the pattern on the audiogram and the pattern dictates rehabilitation.

Tympanic membrane and middle ear anatomy

  • Landmarks of the normal drum: annulus, pars flaccida, pars tensa, short process of malleus, malleus handle, umbo, light reflex.
  • The most useful landmarks are the handle of malleus and the umbo, which divide the drum into four quadrants. The light reflex is typically in the antero-inferior quadrant but is unreliable when pathology alters the drumhead, so it is not recommended as a landmark.
  • Side is determined from the orientation of the malleus and the light reflex (the lecture’s example was a left ear).
  • Documenting an otoscopic finding by drawing what you see, with labels, remains a useful clinical habit.
  • Ossicular chain: malleus, incus, stapes. The head of malleus and body of incus are not visible through a normal drum.

Impedance matching by the middle ear

The middle ear allows air conducted sound to be transferred into fluid conducted sound in the cochlea, overcoming the transmission loss that normally occurs at an air to water interface (as when listening with your head under water). Amplification arises from two factors:

  1. The tympanic membrane has a surface area 20 times greater than the stapes footplate.
  2. The relative lengths of the handle of malleus and the long process of incus give an additional lever effect.

Steps of transmission:

  1. Sound moves the tympanic membrane and ossicular chain.
  2. Vibration passes through the stapes footplate into the perilymph of the scala vestibuli at the oval window.
  3. Because the cochlea is entirely fluid filled and relatively incompressible, the round window moves to accommodate the pressure change at the oval window.
  4. Movement of perilymph in the scala tympani is transmitted across the basilar membrane to the scala media.

Cochlear compartments and the organ of Corti

  • Three fluid filled compartments: scala vestibuli (perilymph), scala media (endolymph), scala tympani (perilymph).
  • Scala vestibuli and scala tympani are both perilymph filled and communicate at the apex of the cochlea, the helicotrema.
  • The endolymph filled scala media sits between them and contains the organ of Corti, with inner and outer hair cells and their supporting structures; the cilia of the hair cells are embedded in the tectorial membrane, and the cells sit on the basilar membrane.

Tonotopicity

  • Different parts of the cochlea respond to different sound frequencies, determined in part by the width of the membranous basilar membrane supporting the hair cells.
  • Where the basilar membrane is narrow, higher frequencies cause maximal displacement; where it is wider, lower frequencies do.
  • High frequency sound is appreciated in the basal turn, low frequency sound in the apical turn.
  • Slide values for the cochlea: length 32 mm; width 0.04 mm at the base rising to 0.50 mm at the apex; average widths 0.21 mm basal turn, 0.34 mm middle turn, 0.36 mm apical turn. Frequency map runs from 20,000 Hz at the base down through 7000, 5000, 3000, 2000, 1000, 500, 300 to 200 Hz at the apex.
  • Tonotopicity is exploited in the design of the cochlear implant.

What goes wrong, and the symptoms produced

Causative categories affecting the middle and inner ear: infection (acute or chronic), trauma (injury, noise), tumour (benign, malignant), toxins (chemicals, drugs), congenital, inherited, and age. Symptoms overlap heavily between conditions, so history narrows the differential while knowledge of anatomy and physiology directs the examination.

Cardinal ear symptoms: hearing loss, tinnitus, discharge, pain, vertigo.

Hearing loss symptoms in moderate to severe loss:

  • Sound is muffled.
  • Loss of discrimination, particularly of consonants.
  • Worse with competing noise, which masks the speech signal; two simultaneous conversations may mean neither can be followed.
  • Need to watch the speaker (mask wearing in COVID removed lip cues and was a major problem).
  • The TV needs to be louder than for other family members; different voice qualities affect intelligibility.
  • Recruitment: small increments in sound level suddenly go from not heard well to much too loud, hence “don’t shout, I’m not deaf”.

Tinnitus:

  • An auditory experience associated with hearing impairment, usually a high pitched ringing or cicada-like sound.
  • Thought to arise from the absence of sound at certain frequencies: the brain generates an internal message mimicking the missing tone(s), which passes to the auditory cortex. There is no context to the noise, so it is heard as a tone.
  • Can be intensely frustrating and may become a focus for the individual, which only increases awareness of the internal noise.
  • Pulsatile tinnitus matches the heartbeat and raises the possibility of a vascular cause.

Psychosocial effects of hearing loss: embarrassment, mishearing others, a complaining spouse, giving up responsibilities, and social isolation. The impaired person avoids embarrassing situations, sits on the fringe of groups or stays at home, and home life may be strained if a partner is intolerant of repeated requests to repeat themselves.

Other ear disease related symptoms and what they point to:

  • Discharge and odour: infective origin.
  • Pain: acute conditions of the ear canal and middle ear.
  • Balance/vertigo: involvement of the semicircular canals in the disease process.
  • Facial nerve: the nerve runs through the temporal bone, so trauma, infection and tumour may cause a lower motor neurone facial palsy; injury confined to the chorda tympani branch presents as altered taste.
  • Headache with a discharging ear should prompt careful ear and CNS examination, as it may indicate intracranial pathology.

Hearing assessment: principles

  • Subjective tests need a compliant, cooperating subject; they are relatively easy. The pure tone audiogram is the commonest.
  • Objective tests need no cooperation; they are more difficult and time consuming. Examples are brainstem evoked response audiometry and cortical evoked response audiometry, in which an evoked potential is elicited and measured using a summation and averaging technique rather than a volunteered response.
  • Voice tests start with a whisper and progress through low conversational, normal conversational, raised conversational voice, to a shout. Normal hearing means hearing a quiet whisper at two to three feet. Conversational voice is about 50 to 60 dB; failure to hear it indicates significant loss in the tested ear. The non-test ear may hear the sound and confuse the result, so masking is needed unless the tested ear is the better hearing one. These tests are not often used in practice.

Tuning fork tests

Use a 256 Hz or 512 Hz fork. Do not use 128 Hz: the low frequency is also felt as vibration sense and gives false results.

Weber. Strike the fork and place it on the vertex, forehead or bridge of the nose; ask whether the sound is heard in the midline or to one side.

  • Normal, or symmetrical bilateral loss: heard in the midline.
  • Asymmetrical sensorineural loss: heard in the better hearing ear.
  • Unilateral conductive loss: heard in the worse hearing ear.
  • Can be demonstrated on yourself by striking the fork on the vertex and occluding one ear canal with a finger.
  • Weber alone cannot distinguish conductive from sensorineural loss without Rinne.

Rinne. Compares air conduction with bone conduction in the test ear. Hold the lightly struck fork about 2 cm from the external auditory meatus and ask if it is heard, then press it firmly on the mastoid process of the same ear and ask whether it is louder or quieter than before.

  • Rinne negative: louder on the mastoid, so bone conduction better than air conduction. Not normal; suggests inadequate conduction through the ear canal, tympanic membrane and middle ear (conductive loss).
  • Rinne positive: louder next to the ear, so air conduction better than bone conduction. Expected in normal hearing and in sensorineural loss.
  • With only a small conductive loss the Rinne may still be positive; an air-bone gap of about 20 dB is needed to produce a Rinne negative response.
  • These tests indicate the type of hearing loss, not the hearing level.

Pure tone audiogram

  • Tones are presented at variable volume; the threshold is the level at which the tone is just heard. A graph plots intensity against test frequency.
  • Sound intensity is measured in decibels; for the pure tone audiogram, decibels hearing level (dBHL). Zero dBHL is the sound pressure level at which eighteen year olds with normal hearing just hear the sound, that is the threshold of normal hearing. The actual sound pressure level of zero differs at each frequency.
  • The decibel scale is logarithmic. As stated on the slide: 10 dBHL is 10 times louder than zero dBHL, 20 dBHL is 100 times louder, and a 3 decibel increase roughly represents a doubling of sound intensity.
  • Air conduction is tested with headphones over the ears or insert phones in the external auditory meatus, each ear in turn, at each frequency from 250 Hz to 8000 Hz (8 kHz).
  • Bone conduction uses a bone conductor over the mastoid process, stimulating the mastoid bone and in turn the cochlea directly, bypassing the middle ear. Its zero scale differs from air conduction but is fixed so that normal hearing for eighteen year olds by bone conduction is also zero dBHL.
  • In normal hearing there should be no difference between air and bone conduction thresholds, allowing for a minor air/bone difference of up to 10 dB from ordinary variability.
  • Symbols for recording thresholds are given in the key beside the audiogram; the key is what makes the graph interpretable.
  • Children under four cannot do pure tone audiometry. Distraction or co-operative play audiometry is used, in which the child is conditioned to make a response when they hear the sound.

Masking

White noise is presented to the non-test ear while the bone conduction threshold is measured in the test ear, so the test tone is only heard in the test ear. Without masking, a bone conduction tone can be heard by the opposite cochlea through transcranial conduction, and the nature of the hearing loss can be misinterpreted (for example, an unmasked bone conduction threshold sitting near the better ear’s air conduction threshold).

Reading the patterns

  • Normal hearing: air conduction 0 to 10 dBHL with bone conduction matching. Unmasked bone conduction does not matter here, because a matching threshold cannot be spuriously better than the air conduction result.
  • Conductive loss (lecture example, left ear): air conduction raised to about 40 dBHL with normal masked bone conduction. Tuning forks would give Weber to the left (worse) ear, Rinne positive right, Rinne negative left.
  • Sensorineural loss (lecture example, right ear): masked bone conduction at the same threshold as air conduction. Tuning forks would give Weber to the left (better) ear and Rinne positive in both ears.
  • The gap between air and bone conduction thresholds is the air-bone gap.
  • Tuning fork tests are therefore useful in confirming the nature of a hearing loss found on the audiogram.

Warning

The audiogram examples on slides 21 and 22 carry transcript flags: patient identifying fields and the handwritten condition labels were only partly legible in the source images, so the plotted values are as described in the accompanying text rather than read off the charts.

Speech audiometry

  • Words one or two syllables long from a standardised list are presented at different volumes; the score is the percentage correctly identified.
  • The graph plots proportion of words correctly identified (Y axis) against presentation loudness in dBHL (X axis).
  • Normal curve: recognition rises steeply with intensity, with about 50% of words correct at about 25 dBHL (quiet speech).
  • The lecture’s abnormal curve: at best 35% of words identified, and only at shouting level.
  • Useful when hearing ability seems worse than the pure tone audiogram would suggest, and for audiologists planning hearing aid fitting.

Tympanometry

Measures middle ear function by reflected sound. Sound is presented into a sealed ear canal; part is absorbed by the middle ear and part reflected from the drum, and the volume of reflected sound indicates middle ear efficiency. Canal pressure can be varied from -400 to +200 mm of water, with 0 as atmospheric pressure.

Principle: the drum is most mobile when pressure is equal on both sides, which gives optimal conduction to the cochlea and least reflected sound. A pressure differential displaces and tenses the drum, so more sound is reflected; reflection is maximal where the pressure difference is greatest. The curve plots reflected sound (admittance) against pressure, though in practice the inverse of admittance, impedance, is used. The peak represents optimal drum movement and indicates the middle ear pressure.

Curve types:

  • Type A: normal, peak between +100 and -100 mm water.
  • Type B: flat, no peak. Reflects failure to seal the ear canal, wax occluding the canal, or a middle ear effusion (glue ear, where the drum is immobile).
  • Type C1: peak between -100 and -200 mm water, representing negative middle ear pressure (as with reduced Eustachian tube function).
  • Type C2: peak beyond -200 mm water but still discernible. A proportion of these people have a middle ear effusion.

The tympanogram does not measure hearing level accurately, but recent evidence suggests a type B tympanogram carries a 90 to 95% chance of an air conduction threshold of 25 dB or worse. Some type B individuals still have thresholds of 20 dB or better, which is clinically within the normal range.

Evoked response audiometry

An objective test: essentially an EEG of the cochlear nerve, giving a threshold for 2 to 4 kHz, and diagnostic in use.

  1. A square wave sound impulse is used; the click contains sounds in the 2 to 4 kHz range. Other tones can be used, for example pulsed 0.5 kHz and 1 kHz.
  2. The stimulus is presented rapidly on/off, about a thousand repetitions, quickly stimulating cochlear nerve fibres.
  3. Electrical activity between the cochlea and brainstem is recorded with scalp electrodes over the mastoid process plus one centrally at the vertex or, more commonly, the forehead.
  4. Summation and averaging remove background neural noise: with only a few presentations the background activity obscures the response, but as more stimuli are averaged the noise flattens while the repeating response is enhanced.
  5. A typical normal waveform of sequential peaks (I to VII) is generated, and wave latencies are measurable and comparable with normal responses.

Brainstem evoked response audiometry can test hearing in neonates and detects those with a moderate hearing loss; it is one of the testing modalities for universal neonatal hearing screening.

Cortical evoked response audiometry uses a similar technique over a longer time frame, can use pure tone stimuli, and records responses from the auditory cortex. It is more time consuming and not often used in clinical practice.

Rehabilitation of hearing loss

Options: hearing tactics, lip reading, hearing aid, reconstruction of the ossicular chain, cochlear implant.

Hearing tactics are the simplest measures:

  • Look at the speaker: even without formal lip-reading training this improves interpretation and gives facial expression cues to the drift of a conversation.
  • Reduce environmental noise to improve signal reception, for example moving away from the television when on the phone. Not always practicable.
  • Tell people about the loss so they can accommodate it, though many are reluctant to admit to impairment.

Hearing aids raise the signal above the noise to a level matching the impairment, primarily by amplification. Because the audiogram is often not flat, the aid must preferentially amplify some frequencies more than others without becoming painfully loud; internal circuitry modifies captured sound and decreases background noise. Frequency output is adjustable. Limitations and problems:

  • Not real hearing; extraneous noise such as car noise, sudden bangs or wind can cause discomfort.
  • Background noise remains a problem.
  • Irritation or discomfort from something in the ear canal, secondary otitis externa, and feedback (whistling) from the proximity of microphone and speaker.
  • Compliance is variable: some people cannot tolerate aids or dislike the sound quality.

Surgical management of conductive loss

A conductive loss implies a fault in the sound transforming apparatus of the middle ear: a perforated tympanic membrane, a cholesteatoma, or an ossicular chain defect. Ossicular chain defects result from trauma, infection and sometimes congenital disorders. The two common defects shown are loss of the long process of the incus and loss of the stapes superstructure (arch).

Ossiculoplasty replaces damaged ossicles with a prosthesis, to restore sound transmission from an intact tympanic membrane to the cochlea. Success depends on an efficient, good contact join between the malleus/tympanic membrane and either the stapes arch or the stapes footplate. The intraoperative photograph shows a prosthesis joining the malleus handle to the stapes head, with chorda tympani and the rim for tympanic membrane attachment also visible.

Warning

The transcript flags that the slide text for ossiculoplasty reads “malleaud?tympanic membrane”, an apparent typo or artefact in the original slide, most likely intended as “malleus/tympanic membrane”.

In the lecture’s worked example (mild bilateral high frequency sensorineural loss plus a left conductive loss), the person hears quite well through the right ear and would rely on it, functioning less well in competing noise. Rehabilitation could be a left sided hearing aid, but one should also consider the underlying cause and how it may be managed.

Case 1: chronic otitis media with perforation, active

Presentation: discharge from the ear. Otoscopy of the left ear shows a large inferiorly positioned perforation of the pars tensa, with inflamed tissue around the rim, inflamed middle ear mucosa and some debris inferiorly in the canal. The malleus handle is hard to see but the short process of the malleus provides the landmark. Classed active because there is active infection; a dry perforation with healthy middle ear mucosa would be inactive.

Warning

Slide 39 shows a discharging ear on otoscopy but gives no diagnosis; the transcript records the image findings only and does not interpret them.

Expected symptoms: hearing loss and discharge as the primary ones, possibly tinnitus which may be more intrusive since the discharge began, often a history of childhood ear infections with hearing loss in adulthood. Pain would be unusual, because the perforation prevents pus being locked into the middle ear as it is in acute otitis media; pain suggests spread of disease beyond the temporal bone.

Audiogram: normal hearing right ear, plus a conductive loss on the left with normal bone conduction and an air-bone gap, air conduction at 35 to 40 dBHL. The most likely cause is the perforation, which has disrupted sound capture and impaired transmission to the cochlea.

Management:

  • Treat the infection. Topical antibiotic ear drops often work well. Failing that, aural toilet (dry mopping or suction) plus topical antibiotics with or without oral antibiotics usually resolves the episode. Prefer a non-ototoxic preparation such as Locorten Vioform or Ciprofloxacin HC. Sofradex and Kencomb contain aminoglycosides and are potentially ototoxic, but in an infected ear the risk of ototoxicity is estimated at 1 in 10,000 to 1 in 100,000, so use for up to two weeks in an acutely inflamed ear is considered reasonable. The risk is greatest when used prophylactically on a dry perforation to prevent discharge.
  • Address the hearing: hearing aid or repair of the perforation. After a single episode some people are content to leave it alone, especially if they do not feel significantly impaired.
  • With normal hearing in the other ear, repairing the drum may be expected to restore normal hearing on the affected side. With bilateral loss (for example age related loss as well as the perforation), repair corrects only part of the loss and bilateral aiding is still needed, though a repaired drum makes it easier to use an aid on that side.

Why operate: restore the eardrum, give a waterproof ear (reducing further infection and allowing swimming), and restore sound transforming capacity by restoring the drum’s surface area and integrity, the first component of the middle ear amplifier. Provided the remaining mechanism is intact and functional (no fibrosis from infection), there should be no conductive component left.

Myringoplasty is repair of a perforated tympanic membrane. Skin of the ear canal is lifted in continuity with the drum, usually its posterior half, allowing graft tissue to be placed medial to the edges of the perforation, on the middle ear side. Over the following 4 to 6 weeks the graft is incorporated into the drum by healing by secondary intention. In this patient the pre-op to post-op audiograms showed a very good outcome: with an intact drum and a normally mobile ossicular chain, transmission to the cochlea was fully functional again.

Case 2: cholesteatoma

An attic cholesteatoma of the right ear, arising from the pars flaccida and opening into the superior middle ear, the epitympanum or attic. Cholesteatoma may originate in either pars tensa or pars flaccida.

What it is. Despite the “-oma” suffix, given historically because of its invasive behaviour, it is not a tumour. It is an expansion of the tympanic membrane into the middle ear cleft, forming a sac of stretched, thinned drum lined with squamous epithelium, in which squamous debris collects and acts as a nidus for infection.

How it forms (pathway from the lecture diagram):

  1. Inflammatory processes of otitis media damage the middle fibrous layer of the drum without necessarily perforating it, leaving a thin hypermobile segment.
  2. Ongoing Eustachian tube dysfunction under-ventilates the middle ear, so that segment is drawn medially into the middle ear space (retraction from reduced middle ear pressure).
  3. It may become adherent to the underlying ossicles or the bony walls of the tympanic cavity.
  4. With time the retraction extends beyond the ring of the drum, up into the attic and mastoid. Beyond the drum rim it is called a retraction pocket or a cholesteatoma.
  5. A cholesteatoma is a retraction pocket with retained keratin, retained because of disordered/lost normal migration of skin from the drum surface.
  6. The resulting stasis lets commensals become pathogenic, giving foul smelling aural discharge from Pseudomonas aeruginosa or Proteus mirabilis, and inflammation causing bone erosion, an enlarging cholesteatoma and risk of complications.

Symptoms are the same as for a perforated drum (hearing loss, discharge, tinnitus, pain), unsurprisingly since the disease process originates in the tympanic membrane, though it is a different process.

How it differs from a perforation: a slowly growing keratin filled sac rather than a hole, giving more persistent infection; it causes bone destruction; and it risks temporal bone and intracranial complications. The sac tissue has an inflammatory action contributing to bone erosion, extension into the mastoid and ossicular chain damage, and creating routes of infection from the middle ear to the labyrinth (vertigo and hearing loss), the facial nerve (facial palsy) and the intracranial space (meningitis and brain abscess). Mastoid extension destroys mastoid air cells, visible on CT.

Audiogram in this case: bilateral mild sensorineural hearing loss plus a right sided conductive loss. The patient is older than the previous case, so the high frequency threshold elevation in both ears represents age related loss.

Tuning forks for this right sided conductive loss: Weber refers to the right ear; Rinne negative on the right (BC greater than AC) and positive on the left (AC greater than BC), the left result being consistent with a functional middle ear.

Surgery for cholesteatoma

Because of the serious potential consequences there is a greater commitment to eradication, requiring surgery to identify the limits of disease and remove it completely. The primary aim is a safe, dry ear; maintaining or improving hearing is secondary, because the disease may have damaged the ossicular chain and repair does not restore the full function of a normal mobile chain, although ossiculoplasty can sometimes fully close the air-bone gap.

Approaches:

  • Trans-mastoid through the mastoid, aiming to keep the ear canal intact.
  • Epitympanic, a more direct approach opening the attic and removing bone overlying the cholesteatoma.

Canal wall-down. Approaching from the origin of the cholesteatoma and sequentially removing bone lateral to the sac limits tissue removal and gives a direct view of the ossicular chain and of the facial nerve running through the middle ear. A large cholesteatoma requires more bone removal to see the limit of disease. The canal wall is progressively removed, leaving a defect: a small defect can be reconstructed with a cartilage graft, while extensive removal produces an exteriorised mastoid, with the mastoid cavity in direct communication with the ear canal and a graft used to seal the middle ear. Mastoid cavities are prone to infection and may preclude water exposure, which has driven a move towards the transmastoid approach.

Canal wall-up (transmastoid). Preserves the ear canal so the ear looks normal afterwards, but gives reduced visualisation of the middle ear contents and so a risk of residual cholesteatoma that can grow and cause the same problems as the original disease. Vigilant follow-up is therefore essential.

Choosing between them requires disease-related, anatomy-related and patient-related factors: if the patient is unlikely to attend follow-up, there is a stronger case for the procedure with less risk of recurrent disease. Further experience of chronic otitis media comes during ALM tuition.

Case 3: ear canal exostoses (surfers’ ear)

A man deaf since birth, wearing hearing aids since childhood, managing well with aids and lip reading. No family history, uneventful birth, no recalled childhood ear trouble, otherwise healthy and active, a keen surfer whose ears tend to wax up, which stops the aids working. Increasing trouble with the left ear, which both blocked and became infected. Examination: bony exostoses of both ear canals, the left severely narrowed with the drum barely visible. Two problems: the hearing loss and the exostoses, with the exostoses complicating management of the hearing loss.

Audiometry timeline: no childhood audiometry available; over a 6 year period (2012, 2015, 2018) the loss is fairly stable across most frequencies, moderately elevated across the main speech frequencies and worse in the high frequencies. This pattern rehabilitates well with hearing aids because sound can be amplified enough to present most speech noise to the ear, but it is not the same as normal hearing and he must concentrate and use lip cues in conversation.

Exostoses:

  • Thickenings of the bony ear canal occurring in response to cold water exposure, usually in surfers and divers but also in frequent swimmers. The mechanism of formation is not known.
  • They usually cause no problems other than water retention in the canal, but with repeated exposure the bone growth leaves very little canal diameter, and skin and wax clearance fails so the canal blocks or becomes infected.
  • Narrowing is worst inferiorly in the case shown, where water is trapped; larger exostoses narrow the mid-canal, making drainage harder.
  • Repeated infections make surgery appropriate: bony meatoplasty drills out the exostoses while retaining as much ear canal skin as possible, since preserved skin speeds healing. The aim is a patent, skin lined ear canal. Bone can regrow if the person keeps surfing without ear protection.
  • Prevention: silicone putty is often used to occlude the external auditory meatus, but surfers seem to find blu-tack more effective.
  • In this patient it was the combination of hearing loss and the need for aids that drove surgery: the aid occludes the canal and increases infection risk, and when infected or blocked the aid does not work. A wider canal after surgery gets less infected and is less likely to block.
  • Post-operatively the drum was not normal: it showed tympanosclerosis (white plaques), often seen after childhood otitis media.

Case 4: progressive sensorineural loss and cochlear implantation

A man presented to ENT in Dunedin in 1993 with hearing loss interfering with his job as a teacher. No family history; he heard well through childhood and university. Tympanic membranes normal, but the audiogram showed a severe to profound sensorineural hearing loss of uncertain cause. On the 2003 audiogram there is threshold elevation across all frequencies, with low and mid frequencies at 70 to 85 dBHL: a significant impairment in which even shouting would be hard to interpret unaided. Hearing aids gave limited benefit.

By 2005 the loss had progressed despite upgraded aids and he had to stop teaching English because he could not follow the children in class, moving to sick leave and then administrative work. He could not hear people on the phone, watched television only with subtitles, avoided group situations, and found conversation with his wife very difficult. Aided thresholds (marked “A”) still left him struggling except in a quiet environment, and his unaided speech audiogram showed only 4 out of 10 words correct even at shout level.

He received a cochlear implant and has not looked back: a world trip, conversations with people of many nationalities, and dinner parties are now manageable. His audiometry shows no hearing in the right ear with the implant off, and implant-aided thresholds around 20 to 40 dB. He does not have normal hearing, but his impairment is far less than before. He moved from increasing seclusion to interacting readily with those around him.

Important

Hearing at 25 to 30 dBHL is considered sufficient, though not perfect, for social interaction.

How a cochlear implant works

  1. An externally worn microphone and speech processor receive sound and split it into different frequencies and volumes.
  2. The signal is relayed to the internally placed part of the implant.
  3. The implant is placed in the cochlea, inserted through the round window; it is flexible and curls around the cochlear spiral.
  4. It carries 22 electrodes, seen as lighter bands, sitting against the cochlear nerve fibres in the centre of the cochlea.
  5. Electrodes are programmed to fire in response to different frequencies in the received speech, so different parts of the cochlear nerve are stimulated, recreating the tonotopic feature of a normal cochlea.
  6. Information is relayed to the auditory cortex, where it is reformatted into speech sounds.

Indication in the lecture’s first example: profound bilateral hearing loss, worse in the left ear, with a speech audiogram showing very few words correct, so that even with conventional aiding conversation is a struggle. Such a person is an ideal candidate.

Wider role:

  • A major advance in the management of severe and profound hearing loss.
  • Essential in childhood congenital and inherited hearing loss. Universal neonatal hearing screening allows early detection of the 1 in 1000 children born with significant hearing loss, who are then more thoroughly assessed and managed by severity and progression.
  • Children born with severe to profound loss are now fast-tracked, with parental or guardian consent, for implantation. The earlier they are implanted, the earlier they have optimal hearing, and the better their hearing the better their language skills and their educational and socioeconomic outlook.
  • In severe adult onset (post-lingual) loss, an implant can reverse the isolating, work-ceasing effect of hearing loss on the individual and their family.

Self-test

  1. Explain how the middle ear achieves impedance matching, and give the two structural factors responsible for its amplification.
  2. Describe, in order, how vibration of the tympanic membrane is converted into movement across the basilar membrane.
  3. Name the three cochlear compartments, the fluid in each, and where two of them communicate.
  4. Explain what tonotopicity is and how the properties of the basilar membrane determine which frequencies are heard at the base and at the apex.
  5. Define recruitment and give the phrase a patient with it might use.
  6. Explain the proposed mechanism of tinnitus, and state what pulsatile tinnitus should raise the possibility of.
  7. A patient with a discharging ear also complains of headache. Explain why this combination matters and what you would examine.
  8. Distinguish subjective from objective hearing tests, and give one example of each.
  9. Why should a 128 Hz tuning fork not be used for Weber and Rinne testing?
  10. Predict the Weber and Rinne findings in a patient with an isolated right sided conductive hearing loss.
  11. A patient has a unilateral conductive loss with only a 10 dB air-bone gap and a Rinne positive result. Explain this apparent discrepancy, and state the air-bone gap needed to turn Rinne negative.
  12. Define zero dBHL, and state the frequency range tested by air conduction on a standard pure tone audiogram.
  13. Explain why masking is used when measuring bone conduction, and what happens to the interpretation if it is omitted in a unilateral sensorineural loss.
  14. Distinguish the audiogram pattern of a conductive loss from that of a sensorineural loss.
  15. How is hearing tested in a child under four, and why?
  16. Describe how a tympanogram is generated and why the peak occurs where it does.
  17. List the four tympanogram types with the pressure range or shape that defines each, and one clinical implication of a type B tracing.
  18. Describe the steps of brainstem evoked response audiometry, and explain why about a thousand stimulus repetitions are needed.
  19. List the limitations and complications of hearing aids.
  20. List the two common ossicular chain defects shown, and state what determines the success of an ossiculoplasty.
  21. In an active chronic otitis media with perforation, explain why pain would be unusual, and what pain should make you suspect.
  22. Distinguish the drops you would choose for a discharging perforated ear from those you would avoid, and state the estimated risk that justifies short term use of the latter.
  23. Describe the myringoplasty procedure and its healing timeline.
  24. Describe the steps by which a cholesteatoma forms from an inflamed tympanic membrane.
  25. Distinguish cholesteatoma from a simple perforation in terms of pathology and risk, and list the complications produced by its bone erosion.
  26. Compare canal wall-down and canal wall-up mastoidectomy, giving one advantage and one drawback of each.
  27. Explain why surfers’ exostoses cause problems, and what bony meatoplasty aims to achieve.
  28. Describe how a cochlear implant converts environmental sound into a signal the auditory cortex can use, and state the number of electrodes.
  29. What hearing level is considered sufficient for social interaction, and what proportion of newborns have significant hearing loss?
  30. Integrative: a patient has a right sided conductive loss on the audiogram with normal bone conduction, foul smelling discharge and a facial weakness. Explain which pathology this suggests, why the discharge smells, why the facial nerve is at risk, and how the primary surgical aim differs from that in a simple perforation.

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