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Part B: Hearing - Complete Notes with Clinical Correlations, Diagrams, Drugs & Treatment

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CHAPTER 149: Functional Anatomy and Functions of the Ear

External Ear (Pinna + External Auditory Canal)

• Pinna (auricle): Collects and funnels sound waves; shape is slightly forward-facing, which helps with vertical sound localization. Reflections from the pinna surface change as sounds move up/down - the primary cue for locating sounds in the vertical plane.
• External Auditory Canal (EAC): ~2.5 cm long; S-shaped canal lined with ceruminous glands (wax) that has antibacterial properties and prevents foreign body entry.
• Tympanic Membrane: Translucent fibrous membrane separating the external from the middle ear. Responds to pressure changes by moving in and out, functioning as a resonator that reproduces vibrations of the sound source.

Middle Ear

1. Lever action of malleus + incus: multiplies force 1.3×
2. Area ratio: The tympanic membrane (~55 mm²) is ~17× larger than the oval window footplate, concentrating pressure. Combined amplification ≈ 22× (about 25 dB).

• Tensor tympani (CN V3): pulls manubrium of malleus inward
• Stapedius (CN VII): pulls stapes footplate outward

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The Cochlea (Inner Ear)

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Organ of Corti

• Hair cell cilia (stereocilia): Tip links connect adjacent stereocilia. Deflection toward taller stereocilia opens mechanically gated K⁺/Ca²⁺ channels.
• Tectorial membrane: Overlies and contacts the tallest stereocilia of OHCs (IHCs may be stimulated by fluid movement).
• Basilar membrane: Wider and more flexible at apex; narrower and stiffer at base.
  • Base (near oval window): best for high-frequency sounds
  • Apex: best for low-frequency sounds
• Spiral ganglion: Cell bodies of bipolar neurons of CN VIII, located in the modiolus.

Outer Hair Cell Motor Protein: Prestin

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Clinical Correlation - Meniere's Disease: Endolymphatic hydrops (excess endolymph) distorts the scala media, damaging both IHCs and OHCs. Presents with episodic vertigo, tinnitus, low-frequency sensorineural hearing loss, and aural fullness. The high K⁺ in endolymph leaking into perilymph (K⁺ intoxication) poisons hair cells.

Clinical Correlation - Otosclerosis: Sclerotic bone fixates the stapes footplate at the oval window → conductive hearing loss. Treated by stapedectomy/stapedotomy with prosthesis implant.

Clinical Correlation - Otitis Media: Fluid accumulation in middle ear reduces ossicular mobility → conductive hearing loss. Treated with antibiotics (amoxicillin first-line); myringotomy ± tympanostomy tubes for recurrent cases.

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CHAPTER 150: Auditory Pathways

Ascending Auditory Pathway

1. Spiral ganglion (CN VIII) - bipolar neurons; cell bodies in the modiolus of cochlea
2. Cochlear nuclei (medulla, at pontomedullary junction) - dorsal and ventral divisions; ALL fibers synapse here first
3. Superior olivary complex (pons) - first site of binaural convergence; important for sound localization (interaural time differences <20 μs detected here)
4. Nucleus of lateral lemniscus (pons)
5. Inferior colliculus (midbrain tectum) - auditory reflex center (startle reflex to sound)
6. Medial geniculate body (MGB) (thalamus) - last subcortical relay
7. Primary auditory cortex (A1) - Heschl's gyri on superior temporal plane (Brodmann areas 41 & 42)

• Pathway is predominantly contralateral (most fibers cross at the trapezoid body)
• However, bilateral representation exists because fibers cross at multiple levels
• Result: unilateral cortical lesions cause minimal hearing loss (both ears still relay to surviving cortex)
• Tonotopic organization is preserved throughout the entire pathway (place principle)

Efferent (Descending) Pathway

• Olivocochlear bundle (bundle of Rasmussen): Efferent fibers from superior olivary complex → organ of Corti (primarily to OHC bases)
• Neurotransmitter: Acetylcholine (inhibitory on OHCs)
• Function: Modulates OHC sensitivity - blocks background noise, improves signal-to-noise ratio

Auditory Cortex Functions

• Tonotopic map: Low frequencies processed anterolaterally; high frequencies posteromedially
• Sound pattern recognition: Musical melodies, speech processing (Wernicke's area overlaps)
• Sound localization: Binaural comparison (lesions severely disrupt localization)
• Auditory association areas (Brodmann 22): Interpret meaning of sounds

Clinical Correlation - Cortical deafness: Bilateral lesions of primary auditory cortex cause cortical deafness. Patient can have normal audiogram but cannot process meaning of sound. Wernicke's aphasia (dominant hemisphere area 22 lesion) = inability to understand spoken language despite hearing it.

Clinical Correlation - Acoustic neuroma (Vestibular schwannoma): Benign tumor of CN VIII Schwann cells at cerebellopontine angle. Presents with unilateral progressive SNHL, tinnitus, then facial numbness and imbalance. Diagnosed with MRI gadolinium. Treatment: microsurgery (translabyrinthine, retrosigmoid, or middle fossa approach) or stereotactic radiosurgery (Gamma Knife).

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CHAPTER 151: Mechanism of Hearing

Sound Waves and Properties

• Loudness = amplitude of pressure wave → measured in decibels (dB)
  • 0 dB = threshold reference (0.000204 dyne/cm² or 20 μPa)
  • 0–140 dB range = 10⁷-fold variation in pressure
  • 0 dB does not mean no sound; it means equal to the reference standard
• Pitch = frequency (Hz) of the wave

• Frequency range of human hearing: 20 to 20,000 Hz
• Best hearing sensitivity: 1,000 to 4,000 Hz range (speech frequencies)
• Male conversational voice: ~120 Hz; Female: ~250 Hz
• Masking: One sound reduces ability to hear others by causing relative refractoriness in stimulated fibers

Human Audibility Curve

Sound Transmission - Step-by-Step

• High frequencies (>4000 Hz): maximum at base (narrow, stiff basilar membrane)
• Low frequencies (<500 Hz): maximum at apex (wide, compliant basilar membrane)
• ~1000 Hz: ~30 mm from base (~midway along membrane)

1. Organ of Corti moves relative to tectorial membrane
2. Stereocilia deflect toward the tallest cilia (toward modiolus)
3. Tip links pull open mechanically-gated K⁺ channels in stereocilia tips
4. K⁺ flows IN from endolymph (driven by +80 mV endocochlear potential + high K⁺ in endolymph)
5. Hair cell depolarizes → voltage-gated Ca²⁺ channels open at base
6. Ca²⁺ triggers glutamate release onto afferent CN VIII dendrites
7. Action potentials travel to cochlear nuclei

K⁺ Recycling (Cochlear Battery)

Sound Localization

• Below 3000 Hz: Detected by interaural time difference (as little as 20 μs, detected by superior olivary neurons)
• Above 3000 Hz: Detected by interaural intensity (loudness) difference
• Vertical plane: Detected by pinna shape and sound reflections
• Cortical lesions markedly disrupt localization

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CHAPTER 152: Hearing Defects and Hearing Tests

Classification of Hearing Loss

Causes of Conductive Hearing Loss

• Cerumen impaction (ear wax)
• Otitis externa ("swimmer's ear" - Pseudomonas, S. aureus)
• Otitis media - fluid accumulation/scarring/tympanic perforation
• Otosclerosis: Abnormal sclerotic bone fixates stapes footplate at oval window
• Ossicular chain disruption (trauma, cholesteatoma)
• Exostoses/osteoma of EAC

Causes of Sensorineural Hearing Loss (SNHL)

• Noise-induced: Most common preventable cause; OHCs most vulnerable; 4000 Hz "notch" on audiogram is characteristic
• Presbycusis: Age-related, gradual cumulative hair cell loss; bilateral, symmetric, high-frequency
• Ototoxic drugs (see below)
• Acoustic neuroma/vestibular schwannoma
• Viral labyrinthitis (mumps, measles, CMV)
• Sudden SNHL (idiopathic - treat urgently with corticosteroids)
• Genetic mutations - connexin 26 (most common congenital), prestin, myosin mutations
• Autoimmune inner ear disease
• Meniere's disease (endolymphatic hydrops)

Ototoxic Drugs - Key Table

Clinical Tip: The combination of aminoglycoside + loop diuretic + cisplatin is extremely ototoxic. Always check renal function and use trough monitoring for aminoglycosides.

Tuning Fork Tests (512 Hz tuning fork preferred)

• Positive Rinne (AC > BC) = normal OR sensorineural loss
• Negative Rinne (BC > AC) = conductive loss or false-negative (dead ear - sound heard by contralateral ear via skull transmission)
• A gap of >20 dB between BC and AC thresholds on audiometry suggests conductive component

• Conductive loss: Affected ear is "quieter" from outside, but bone-conducted sound through skull reaches it better = lateralizes to worse ear
• SNHL: Cochlea on affected side is damaged; normal cochlea receives the signal = lateralizes to better ear

Audiometry

• Air conduction (AC) tests entire pathway (external + middle + inner + neural)
• Bone conduction (BC) tests cochlea + neural pathway only (bypasses external and middle ear)
• Air-bone gap: BC threshold better than AC threshold by ≥10 dB → conductive component
• Speech audiometry: Tests word recognition score (WRS/SDS) - poor in SNHL

• Conductive loss: Flat loss across frequencies; BC normal; air-bone gap present
• Presbycusis: Bilateral symmetric high-frequency (>2 kHz) sloping SNHL
• Noise-induced: Notch at 4000 Hz (4 kHz notch), with partial recovery at 8 kHz
• Meniere's disease: Low-frequency SNHL (early stages)
• Acoustic neuroma: Unilateral asymmetric SNHL, poor speech discrimination disproportionate to pure tone loss

Other Hearing Tests

• Wave I: Distal CN VIII (spiral ganglion)
• Wave II: Proximal CN VIII
• Wave III: Cochlear nucleus
• Wave IV: Superior olive
• Wave V: Lateral lemniscus / inferior colliculus
• Wave VI/VII: Medial geniculate body / auditory cortex

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Treatment of Hearing Loss

Conductive Hearing Loss

• Cerumen: Ear drops (carbamide peroxide, sodium bicarbonate, olive oil) or mechanical removal/irrigation
• Otitis media: Amoxicillin (first-line), amoxicillin-clavulanate (resistant strains); tympanostomy tubes for chronic effusion
• Otitis externa: Topical antibiotic + steroid drops (e.g., ciprofloxacin + dexamethasone)
• Otosclerosis: Stapedectomy/stapedotomy (Teflon or wire prosthesis); sodium fluoride may slow bone resorption; hearing aids
• Ossicular chain reconstruction: Prostheses (PORP/TORP) in tympanoplasty

Sensorineural Hearing Loss

• Sudden SNHL: High-dose systemic corticosteroids (prednisone 1 mg/kg/day for 10-14 days) or intratympanic steroid injection (dexamethasone or methylprednisolone) - guidelines from Kitoh et al. 2024, PMID 38968877
• Aminoglycoside-induced: Discontinue drug; no proven rescue therapy; prevention with careful dosing and monitoring
• Noise-induced: Prevention (hearing protection >85 dB); no proven reversal; N-acetylcysteine and magnesium studied as protective agents
• Autoimmune inner ear disease: Corticosteroids; methotrexate in refractory cases
• Meniere's disease:
  • Acute attacks: Vestibular suppressants (meclizine, diazepam), antiemetics (prochlorperazine)
  • Prophylaxis: Low-sodium diet, diuretics (hydrochlorothiazide/triamterene), betahistine (may improve endolymphatic circulation)
  • Surgical: Endolymphatic sac decompression; intratympanic gentamicin (chemical labyrinthectomy); labyrinthectomy; vestibular neurectomy

Rehabilitation/Devices

• Hearing aids: Amplify sound; most suitable for mild-moderate hearing loss; analog or digital (digital preferred - converts sound to numerical code for selective amplification by frequency)
• Cochlear implants: For severe-profound bilateral SNHL; approved for children as young as 12 months. Components: Microphone → speech processor → transmitter/receiver → electrode array → auditory nerve. As of 2012, >324,000 implanted worldwide. Bypass damaged hair cells, directly stimulate spiral ganglion neurons.
• Bone-anchored hearing aids (BAHA): Titanium implant in skull transmits bone-conducted sound; used in conductive/mixed loss and single-sided deafness
• Middle ear implants: Vibrate ossicles directly

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Key Clinical Correlations Summary

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Recent Evidence (PubMed 2023-2024)

• A 2024 Japanese clinical practice guideline for acute SNHL (PMID 38968877) recommends systemic corticosteroids as standard first-line, with intratympanic steroids for patients who cannot tolerate systemic therapy or as salvage.
• A 2024 meta-analysis (PMID 38506449) on bilateral sudden SNHL found prognosis is worse than unilateral cases, reinforcing the importance of early aggressive treatment.
• COVID-19 is associated with SNHL and tinnitus; an umbrella review (PMID 37093291) found moderate-quality evidence linking SARS-CoV-2 with auditory dysfunction, possibly via vascular/inflammatory mechanisms.

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Importance Summary (Exam Focus Points)

1. Endocochlear potential (+80 mV) is the driving force for hair cell transduction - maintained by stria vascularis K⁺ pump
2. Tonotopy (place principle): High frequencies → base of cochlea; Low frequencies → apex
3. Inner hair cells = primary signal generators; Outer hair cells = amplifiers (prestin motor)
4. Auditory pathway is bilateral above cochlear nucleus → unilateral central lesions cause minimal hearing loss
5. Inferior colliculus = auditory reflex center (startle); superior olive = sound localization (binaural)
6. Rinne negative (BC>AC) = conductive loss in that ear
7. Weber lateralizes to worse ear in conductive, better ear in SNHL
8. 4 kHz notch on audiogram = pathognomonic for noise-induced hearing loss
9. Connexin 26 = most common genetic cause of congenital deafness
10. Sudden SNHL = medical urgency - treat within 2 weeks with steroids for best outcomes
11. Aminoglycosides + loop diuretics = synergistic ototoxicity - avoid combination
12. Cochlear implants work by directly stimulating spiral ganglion neurons, bypassing all hair cells