Appearance
11.12 — The Ear, Hearing and Balance
Sound arrives at your eardrum as vibrations in air and must be delivered to the sensory cells suspended in fluid. Sound crossing directly from air into water loses about 99.9 percent of its energy — around 30 decibels — because the two media resist motion so differently.
The middle ear exists to solve that one problem, and it recovers almost exactly the amount that would otherwise be lost.
The same organ also, in a completely separate compartment, tells you which way up you are.
The three parts
Outer ear — the visible pinna and the ear canal, about 2.5 centimetres long.
The pinna's folds are not decorative. They modify sound differently depending on the direction it arrives from, and the brain uses those changes to locate sounds vertically — which is the one dimension that cannot be worked out by comparing the two ears.
The canal's length gives it a resonance around 3,000 Hz, amplifying that range by 10 to 15 decibels — which is squarely within the frequency range of speech consonants.
Earwax is protective, being acidic, antibacterial and water-repellent, and the canal is self-cleaning: skin migrates outward carrying wax with it. Cotton buds push wax inward and are the commonest cause of impacted wax and of perforated eardrums, which is why the standard advice is to put nothing smaller than your elbow in your ear.
Middle ear — an air-filled cavity containing the three smallest bones in the body: malleus, incus and stapes — hammer, anvil and stirrup. The stapes is about 3 millimetres long and weighs a few milligrams.
Inner ear — the fluid-filled labyrinth, containing the cochlea for hearing and the vestibular apparatus for balance.
Solving the impedance problem
Two mechanisms together recover the 30 decibels that would be lost.
Area ratio — the larger contribution. The eardrum is about 55 square millimetres; the stapes footplate about 3.2 square millimetres. The same force concentrated onto a seventeenth of the area gives about seventeen times the pressure.
Lever ratio — the smaller contribution. The ossicles act as a lever with a mechanical advantage of about 1.3.
17 \times 1.3 \approx 22\text{-fold pressure gain, about 27 dB}
Which is very close to what would otherwise be lost.
The acoustic reflex protects against loud sound. Two tiny muscles contract and stiffen the chain, reducing transmission by 15 to 20 decibels.
And its limitation matters: it takes 50 to 100 milliseconds to act, so it cannot protect against sudden impulse noise — a gunshot, an explosion, a firework. This is exactly why impulse noise causes hearing damage so readily, and why hearing protection is essential in those settings rather than optional.
The Eustachian tube connects the middle ear to the back of the nose, equalising pressure.
It is normally closed and opens on swallowing or yawning, which is why swallowing clears the ears on a descending aircraft.
In children it is shorter, wider and more horizontal, which is why middle ear infections are so much commoner in childhood — organisms travel up more easily, and drainage is poorer. It becomes more vertical with growth, and most children grow out of recurrent ear infections for that reason alone.
Glue ear — persistent fluid behind an intact eardrum without acute infection — is very common in young children and causes a conductive hearing loss. Most resolve spontaneously within three months. Persistent cases affecting speech and learning are treated with grommets, small tubes placed through the eardrum to ventilate the middle ear.
The cochlea
A fluid-filled spiral of about 2.75 turns, 35 millimetres long uncoiled.
The stapes pushes on the oval window, sending a pressure wave through the fluid. The round window bulges outward to accommodate it — necessary because fluid is incompressible, so the wave needs somewhere to go.
The wave travels along the basilar membrane, and this is where frequency is separated.
The basilar membrane is not uniform. At the base it is narrow and stiff; at the apex it is wide and floppy.
So each frequency produces maximum displacement at a specific place: high frequencies at the base, low frequencies at the apex.
This is tonotopic organisation, and it is preserved all the way to the auditory cortex. The cochlea is a mechanical frequency analyser — it performs a Fourier decomposition of the incoming sound by physical means alone, before any nerve is involved (Volume II, Chapter 9).
Georg von Békésy worked this out and received the Nobel Prize in 1961.
Hair cells sit on the basilar membrane, with fine projections — stereocilia — touching an overlying membrane.
When the membrane moves, the stereocilia bend, and bending physically pulls open ion channels via fine protein tip links between them. This is a direct mechanical gate — no second messenger, no delay — which is why hearing can follow sound at thousands of cycles per second.
Two kinds, and their roles are quite different.
Inner hair cells — about 3,500 — do the hearing. They carry about 95 percent of the auditory nerve fibres.
Outer hair cells — about 12,000 — are amplifiers. They change length in response to voltage, actively pushing on the basilar membrane and amplifying the vibration by 40 to 60 decibels, sharpening frequency discrimination enormously.
And this active process produces a genuinely strange and clinically useful side effect. The ear emits sound. Otoacoustic emissions — faint sounds generated by outer hair cell movement — can be detected with a sensitive microphone in the ear canal.
This is the basis of newborn hearing screening. A probe plays a click and listens for the echo. It takes minutes, needs no cooperation, and it identifies hearing loss in the first days of life — which matters enormously, because early intervention before the language-learning window makes the difference between normal language development and lifelong difficulty.
Hearing loss
Conductive — sound cannot reach the inner ear. Wax, fluid, perforation, ossicle problems.
Otosclerosis is worth knowing: abnormal bone growth fixes the stapes, causing progressive conductive loss in young adults, often with a family history, and it is surgically correctable by replacing the stapes with a prosthesis.
Sensorineural — the hair cells or nerve are damaged. Usually irreversible, because human hair cells do not regenerate.
Birds and fish regenerate theirs, and humans do not, which is one of the more frustrating facts in the field and an active research target.
Causes: age, noise, drugs, infection, genetic conditions, and acoustic neuroma.
Age-related loss (presbycusis) affects high frequencies first. And that pattern explains the characteristic complaint precisely. Consonants are high frequency and carry most of the information in speech; vowels are low frequency and carry most of the volume. So the person hears that someone is speaking and cannot make out what they said, and struggles most in background noise, where the remaining consonant information is masked.
"I can hear you, I just can't understand you" is not a contradiction — it is exactly what high-frequency loss produces.
Noise-induced hearing loss is entirely preventable and extremely common.
Damage depends on both intensity and duration, and the exchange is steep: for every 3 decibel increase, the safe exposure time halves. 85 dB is safe for 8 hours; 88 dB for 4; 91 dB for 2; 100 dB for about 15 minutes.
A rock concert is around 100 to 115 dB. Personal music players reach 100 to 110 dB. A power tool is around 100 dB.
The characteristic early sign is a notch at 4,000 Hz on an audiogram, and it appears before the person notices anything.
Temporary threshold shift — the muffled hearing and ringing after loud noise — indicates damage has occurred, even though hearing recovers over hours. Repeated episodes accumulate into permanent loss.
Ototoxic drugs damage hair cells directly. Aminoglycoside antibiotics such as gentamicin — for the mitochondrial ribosome reason in Chapter 1.5 — cisplatin chemotherapy, loop diuretics at high doses, and high-dose aspirin. Aspirin's effect is reversible; the others largely are not.
Sudden sensorineural hearing loss — hearing lost over hours to days in one ear — is an emergency and is frequently not treated as one. Steroids started within days give a substantially better chance of recovery than steroids started weeks later, and it is often dismissed as wax or as an ear infection. Sudden one-sided hearing loss should be assessed the same day.
Cochlear implants bypass the hair cells entirely, converting sound into direct electrical stimulation of the auditory nerve through an array of electrodes threaded into the cochlea — placed so that each electrode stimulates the tonotopic region matching its frequency band.
They work remarkably well, and children implanted early frequently develop normal spoken language. They are also the subject of genuine and serious disagreement within the Deaf community, where deafness is understood as a cultural and linguistic identity rather than a deficit, and where the decision to implant a deaf child is contested. That disagreement deserves to be stated rather than skipped over, because it is a real ethical question and not simply a misunderstanding of the technology.
Tinnitus
Perception of sound with no external source — ringing, buzzing, hissing. Affects 10 to 15 percent of adults; a small proportion find it severely distressing.
Usually associated with hearing loss, and the leading explanation is that the brain, deprived of input at the damaged frequencies, increases its own gain and generates activity that is perceived as sound. It is analogous to phantom limb pain (Chapter 11.8) — a perception generated centrally to fill an absence.
There is no cure, and there is genuinely effective management, which is worth stating clearly because people are frequently told nothing can be done.
Hearing aids help substantially where there is hearing loss, by restoring input. Sound therapy and masking reduce the contrast between the tinnitus and silence. And cognitive behavioural therapy has the strongest evidence of anything — it does not reduce the sound but reliably reduces the distress and the attention paid to it, which is what determines whether tinnitus is a nuisance or a torment.
Pulsatile tinnitus — a rhythmic whooshing in time with the pulse — is different and needs investigating, because it can indicate a vascular abnormality.
Balance
The vestibular apparatus sits in the same bony labyrinth as the cochlea, sharing its fluid.
Three semicircular canals, at right angles to each other, detecting rotational acceleration in three planes. Fluid lags behind when the head turns, deflecting a sensor at the base of each canal.
Two otolith organs — the utricle and saccule — detecting linear acceleration and head tilt. They contain a gel layer with calcium carbonate crystals on top; gravity and acceleration drag the crystals, bending the hair cells beneath.
And balance is not one sense but three, combined.
Vestibular — head position and movement. Visual — where the horizon is. Proprioceptive — pressure on the feet and joint position (Chapter 11.3).
When they agree, you feel stable. When they disagree, you feel ill.
Motion sickness is that disagreement. Reading in a moving car: the vestibular system reports motion, the eyes report a stationary page. Seasickness below deck is the same conflict.
And the standard remedies follow from it. Looking at the horizon restores agreement. Being the driver helps because you anticipate the movements. Antihistamines and hyoscine work by suppressing the vestibular input (Chapter 9.2).
The vestibulo-ocular reflex is the fastest reflex in the body — about 10 milliseconds. It moves the eyes exactly opposite to head movement so the image stays fixed on the retina.
Demonstrate it two ways. Hold this page still and shake your head — the text stays readable. Now hold your head still and shake the page at the same speed — it blurs. The reflex is far faster than voluntary eye tracking, and the difference is immediately obvious.
Nystagmus — rhythmic eye oscillation — occurs when the system is disturbed, and its direction and pattern localise the problem, which is why it is examined carefully in a dizzy patient.
Vertigo
Vertigo is the illusion of movement — usually spinning — and it is not the same as light-headedness.
Separating the two is the first and most useful step, because light-headedness suggests a circulatory or systemic cause and true spinning suggests a vestibular one.
Benign paroxysmal positional vertigo (BPPV) is the commonest cause, and it is the most satisfying condition in this chapter.
Crystals from the otolith organ become dislodged and fall into a semicircular canal, where they move with gravity and stimulate it inappropriately.
The result is brief, intense spinning triggered by specific head movements — rolling over in bed, looking up, bending down — lasting under a minute each time.
And it is cured by a manoeuvre, not a drug. The Epley manoeuvre moves the head through a sequence of positions that walks the crystals back out of the canal into the chamber where they belong. It takes a few minutes and resolves symptoms in around 80 percent of people after one or two attempts.
Being able to fix a distressing condition by moving someone's head in the right sequence is unusual in medicine, and it is badly under-used — many patients are given anti-sickness tablets for months instead.
Vestibular neuritis — inflammation of the vestibular nerve, usually after a viral infection. Severe constant vertigo lasting days, with nausea and unsteadiness, and normal hearing. It settles over weeks, and vestibular rehabilitation exercises speed recovery, while prolonged use of sedating anti-sickness drugs slows it by preventing the brain from recalibrating.
Ménière's disease — episodes of vertigo lasting hours, with fluctuating hearing loss, tinnitus and a sensation of fullness in the ear. Attributed to raised pressure of the inner ear fluid. Managed with salt restriction, diuretics and, in severe cases, procedures that reduce vestibular function on that side.
And the important distinction to hold: vertigo with other neurological signs is not a benign inner ear problem. Sudden vertigo with double vision, slurred speech, weakness, numbness, or an inability to walk at all suggests a stroke in the posterior circulation, and it needs urgent assessment. The inability to stand or walk unaided is the single most useful discriminator — someone with BPPV or vestibular neuritis can usually walk, unsteadily; someone with a cerebellar stroke frequently cannot.
What the next page fixes
Three senses remain, and they turn out to be more interesting and more clinically consequential than their reputation suggests. Chapter 11.13 covers smell, taste, touch and pain — including why pain is not a simple readout of tissue damage, and what that means for treating it.