Otolaryngology
Ear and Hearing
Outer, middle, inner — how sound becomes neural signals
Hearing converts air pressure waves into neural signals. The outer ear (pinna, canal) gathers sound to the tympanic membrane. The middle ear (malleus, incus, stapes) provides impedance matching — crucial because air-fluid transition would otherwise reflect 99% of sound. The inner ear cochlea performs frequency analysis: high frequencies stimulate the base, low frequencies the apex (tonotopic organization). Hair cells transduce vibration into action potentials in the auditory nerve (CN VIII). Normal hearing range: 20-20,000 Hz; speech mostly 250-8,000 Hz.
- Frequency range20 Hz - 20,000 Hz (young adult)
- Speech range250-8,000 Hz
- OssiclesMalleus, incus, stapes
- TonotopyHigh Hz → base; Low Hz → apex
- Hair cell typesInner (transduce); outer (amplify)
- Pain threshold~120 dB
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Why ear physiology matters
- Newborn hearing screening. Universal in many countries; OAE and ABR detect congenital loss.
- Noise-induced hearing loss. >85 dB exposure damages hair cells; preventable with protection.
- Vertigo workup. BPPV (Dix-Hallpike, Epley), Meniere's, vestibular neuritis, central causes.
- Cochlear implants. Indication has expanded; single-sided deafness and unilateral hearing loss now candidates.
- Otitis media. Most common pediatric infection; pneumococcal vaccine reduces incidence.
- Tinnitus. 15% of adults; cognitive behavioral therapy and sound therapy primary.
- Otosclerosis. Stapes fixation; treated surgically with stapedectomy.
Common misconceptions
- Q-tips clean ears. They impact cerumen and perforate TM; cerumen is self-cleaning.
- Ringing means damage is permanent. Acute tinnitus often resolves; chronic may persist.
- One loud concert is harmless. Single >115 dB exposure can damage hair cells; cumulative effect over years.
- Hearing aids restore normal hearing. They amplify; clarity in noise remains a challenge.
- Hearing loss only hurts ears. Linked to social isolation, depression, cognitive decline.
- Children outgrow ear infections without consequence. Recurrent OM with effusion can delay speech; tubes (myringotomy) often warranted.
Frequently asked questions
How does sound reach the inner ear?
Pinna funnels sound into ear canal; resonance amplifies 2-4 kHz (speech range). Tympanic membrane vibrates. Three ossicles (malleus → incus → stapes) lever amplification (~1.3×) plus the area ratio of TM to oval window (~17×) yields ~22× pressure gain. This impedance matching prevents the ~30 dB loss that would occur from air-to-fluid transition. Stapes plunges into the oval window of the cochlea.
What's inside the cochlea?
A snail-shaped, fluid-filled tube ~3 cm long (uncoiled). Three chambers: scala vestibuli, scala media, scala tympani. Basilar membrane separates middle from lower; varies in stiffness — stiff and narrow at base (high frequency resonance), wider and floppier at apex (low frequency). Organ of Corti sits on basilar membrane: ~3,500 inner hair cells (sensory) and ~12,000 outer hair cells (amplifiers).
How do hair cells transduce?
Stereocilia bend with basilar membrane motion, opening mechanotransduction channels. K+ flows in (endolymph has high K+) — depolarization. Calcium entry triggers glutamate release onto auditory nerve. Outer hair cells contract via prestin protein, sharpening frequency tuning ~1000-fold (cochlear amplifier). Damage to outer hair cells (noise, aminoglycosides, age) causes sensorineural hearing loss.
How is hearing loss classified?
Conductive: outer/middle ear problem (cerumen impaction, otitis media, otosclerosis, TM perforation). Air conduction worse than bone conduction; Rinne BC>AC, Weber to affected side. Sensorineural: cochlea or auditory nerve (presbycusis, noise, ototoxic drugs, Meniere's). Air worse than bone, both reduced. Mixed: both. Audiogram plots threshold (dB HL) vs frequency for each ear, by air and bone conduction.
What is presbycusis?
Age-related sensorineural hearing loss. Begins with high frequencies (cochlear basal turn — most metabolically active, first to fail). Affects ~30% over 65, ~50% over 75. Speech becomes muffled, especially consonants (which carry information at high frequencies). Risk factor for dementia (untreated hearing loss may double risk; hearing aids may attenuate it per ACHIEVE trial 2023).
What ototoxic drugs exist?
Aminoglycosides (gentamicin, tobramycin, streptomycin) — concentrate in cochlea, damage hair cells. Cisplatin — high-frequency loss in 30-50% of patients. Loop diuretics (furosemide) — usually reversible. High-dose aspirin, quinine — reversible. Vancomycin in renal failure. Monitor with serial audiograms in chemo and prolonged antibiotic courses.
How do cochlear implants work?
Bypass damaged hair cells. External processor digitizes sound, splits into frequency bands, transmits to internal receiver. Electrode array (12-22 contacts) inserted into cochlea stimulates auditory nerve directly at tonotopic locations. Indicated for severe-to-profound bilateral SNHL with poor benefit from hearing aids. Bilateral implantation in children before 12 months yields near-typical speech development.