Hearing and balance

Sound

• Human hearing ranges from about 20 to 20,000 Hz and is most sensitive at 1,000–4,000 Hz, the speech range.
• Loudness is measured in decibels (logarithmic; each 10 dB is a tenfold rise in intensity). Conversation is about 60 dB; sustained exposure above 85 dB damages hearing; pain begins around 130 dB.

External and middle ear

• The tympanic membrane is about 17 times larger than the stapes footplate, and the ossicular lever adds about 1.3 times, giving about 22-fold pressure gain. This impedance matching overcomes the loss of energy when sound passes from air into cochlear fluid; without it about 99.9% would be reflected.
• Tensor tympani (mandibular nerve) and stapedius (facial nerve) contract to loud sound (attenuation reflex), damping low-frequency transmission. Stapedius paralysis in Bell palsy causes hyperacusis.
• The Eustachian tube equalises middle ear pressure. In young children it is short, horizontal and easily blocked by adenoids and colds; air in the middle ear is absorbed, pressure falls, and fluid transudes: otitis media with effusion (glue ear), with a dull retracted drum, flat tympanogram, conductive loss and speech delay. A persistent one-sided effusion in an adult needs nasopharyngeal examination.
• Barotrauma in divers and fliers occurs when the tube fails to equalise; unequal middle ear pressures on the two sides can cause alternobaric vertigo.
• Otosclerosis: new bone fixes the stapes footplate, causing progressive conductive loss in young adults, often worse in pregnancy. Patients may hear better in noise (paracusis), because others raise their voices over background noise that the patient cannot hear.

Cochlea

• Scala vestibuli and scala tympani contain perilymph (high Na⁺, like extracellular fluid). Scala media contains endolymph: high K⁺, low Na⁺, secreted by the stria vascularis, which also creates the endocochlear potential of about +80 mV.
• The organ of Corti sits on the basilar membrane. Inner hair cells (about 3,500 in one row) carry about 95% of afferent signals. Outer hair cells (about 12,000 in three rows) change length when stimulated (electromotility), amplifying basilar membrane movement and producing otoacoustic emissions, used for newborn screening.
• Transduction: bending of stereocilia towards the tallest row pulls tip links that open mechanically gated channels; K⁺ flows in from endolymph (driven by the large electrical gradient), depolarising the cell. Ca²⁺ entry triggers glutamate release onto cochlear nerve fibres.
• Tonotopy (travelling wave): the basilar membrane is narrow and stiff at the base (near the oval window), where high frequencies peak, and wide and floppy at the apex, where low frequencies peak.

Causes of sensorineural loss

• Paget disease of bone (thick skull, raised ALP, normal calcium) causes mixed hearing loss by compressing the cochlear nerve and affecting the ossicles.
• Sudden sensorineural hearing loss with a normal ear canal and drum is an emergency: give urgent high-dose oral steroids and arrange audiometry and MRI.

Hearing tests

• Rinne negative on the right with Weber to the left is a dead (severely sensorineural) right ear: bone-conducted sound is heard by the left cochlea, giving a false-negative Rinne.
• Other tests: pure tone audiometry, tympanometry (flat in effusion), otoacoustic emissions, auditory brainstem responses.

Auditory pathway

Cochlear nerve, cochlear nuclei (ipsilateral only), superior olivary complex (first binaural comparison for localisation by timing and intensity), lateral lemniscus, inferior colliculus, medial geniculate body, primary auditory cortex (Heschl gyrus, areas 41 and 42). Above the cochlear nuclei each ear is represented on both sides, so a unilateral central lesion rarely causes one-sided deafness; one-sided deafness implies the ear, cochlear nerve or cochlear nucleus.

Vestibular system

• Each hair cell has stereocilia and one kinocilium; bending towards the kinocilium depolarises it, away hyperpolarises.
• Vestibulo-ocular reflex keeps gaze stable as the head turns (doll's eye movements show an intact brainstem in coma).
• Nystagmus is a slow drift followed by a fast corrective phase, named by the fast phase. Caloric test: cold water produces fast phase to the opposite side, warm to the same side.
• Motion sickness arises from mismatch between vestibular and visual input (hyoscine, antihistamines).

Vestibular disorders

• BPPV: loose otoconia from the utricle drift into a semicircular canal (usually posterior), so the canal responds to gravity. Brief vertigo (under a minute) on rolling in bed or looking up; Dix–Hallpike reproduces torsional nystagmus; cured by the Epley repositioning manoeuvre, not sedatives.
• Ménière disease: episodes of vertigo lasting hours with fluctuating hearing loss.
• Vestibular neuritis: vertigo for days without hearing loss.

Remember Base hears high, apex hears low; endolymph is rich in potassium and K⁺ entry excites hair cells.

Exam trap Semicircular canals sense angular acceleration; utricle and saccule sense linear acceleration and gravity.

Exam trap In sensorineural loss Weber goes to the better ear; in conductive loss it goes to the affected ear.

How FCPS asks about this

• Where high frequencies are detected: base of the cochlea, near the oval window.
• Endolymph composition: high K⁺, low Na⁺, made by the stria vascularis; hair cells depolarise by K⁺ entry.
• Noise-induced deafness: outer hair cell damage with a 4 kHz notch; presbycusis begins at the basal turn.
• Rinne negative and Weber to the affected ear in a child with retracted drum: conductive loss from glue ear (Eustachian tube dysfunction).
• Deafness after meningitis: infection spreading into the cochlea.
• Brief positional vertigo: BPPV from loose otoconia; treat with Epley.
• Sudden unilateral sensorineural loss: urgent oral steroids.