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Part B: Hearing - Complete Study Notes

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149. Functional Anatomy and Functions of the Ear

Overview of Ear Divisions

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External Ear

• Cartilaginous funnel that collects and localizes sound
• Directs sound waves into the external auditory meatus (EAM)
• Contains lobule (earlobe) - only fibrofatty tissue without cartilage

• Length: ~2.5 cm, S-shaped canal
• Outer 1/3: cartilaginous (contains ceruminous glands, hair follicles)
• Inner 2/3: bony
• Resonant frequency: ~3400 Hz - explains why this frequency is most efficiently transmitted

• Area: ~85 mm²; effective vibrating area: ~55 mm²
• Three layers: outer (squamous epithelium), middle (fibrous), inner (mucosal)
• Pars tensa (large, tight) and pars flaccida (Shrapnell's membrane, small, loose)
• Attached to malleus via umbo at its center
• Acts as a pressure transducer converting sound waves to mechanical vibrations

Clinical Correlation - Otitis Media with Effusion ("Glue Ear"): Eustachian tube dysfunction blocks middle ear ventilation, producing negative pressure and fluid accumulation. Seen commonly in children. Treatment: grommets (ventilation tubes) for persistent cases >3 months with bilateral hearing loss >25 dB.

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Middle Ear (Tympanic Cavity)

1. Hydraulic lever ratio - TM area (55 mm²) vs. stapes footplate (3.2 mm²) = 17:1 pressure gain
2. Ossicular lever ratio - malleus arm longer than incus arm = 1.3:1
3. Combined gain = 17 × 1.3 = ~22:1 (≈27 dB)

Clinical Correlation - Acoustic Reflex: Absent stapedius reflex = facial nerve lesion (Bell's palsy) distal to stapedius branch. Reflex decay in 10 seconds = retrocochlear lesion (acoustic neuroma). Used in tympanometry testing.

Clinical Correlation - Hyperacusis in Bell's Palsy: When the facial nerve is paralyzed, stapedius cannot contract. Ordinary sounds seem painfully loud (hyperacusis), confirming the stapedius branch is above the lesion level.

• Connects middle ear to nasopharynx
• Normally closed; opens with swallowing/yawning
• Function: pressure equalization, drainage, protection
• Length in adults: 3.5 cm; in children: shorter and more horizontal (higher infection risk)

Clinical Correlation - Barotrauma: Rapid altitude change prevents Eustachian tube from equilibrating pressure. Results in TM rupture, haemotympanum, or perilymph fistula. Prevention: Valsalva maneuver. Treatment: decongestants (pseudoephedrine), analgesics, surgical repair for persistent fistula.

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Inner Ear - Cochlea

Cochlear Cross-Sectional Anatomy

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The Organ of Corti - The Receptor Organ

Clinical Correlation - Ototoxicity: Aminoglycoside antibiotics (gentamicin, tobramycin, neomycin) and loop diuretics (furosemide) preferentially destroy outer hair cells starting at the basal turn (high-frequency damage first). The basal OHC are most vulnerable because they have the lowest antioxidant capacity. This produces the classic high-frequency notch (4000 Hz) on audiogram. Cisplatin causes irreversible SNHL by generating reactive oxygen species. Monitoring: serial audiograms for at-risk patients.

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150. Auditory Pathways

Peripheral Pathway

• ~30,000 auditory neurons with cell bodies in the spiral ganglion (located in Rosenthal's canal within the modiolus)
• Type I fibers (95%): Bipolar, myelinated, synapse 1:1 on a single IHC
• Type II fibers (5%): Smaller, synapse on multiple OHCs
• The cochlear nerve runs through the internal auditory meatus (IAM) with the vestibular nerve as CN VIII

Clinical Correlation - Acoustic Neuroma (Vestibular Schwannoma): Benign schwannoma arising from the vestibular portion of CN VIII within the IAM. Presents as unilateral SNHL + tinnitus + vertigo. ABR shows prolonged wave I-V latency. MRI with gadolinium is the gold standard. Treatment: microsurgical excision, stereotactic radiosurgery (Gamma Knife), or watchful waiting for small tumors.

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Central Auditory Pathway

Step-by-Step Relay Stations

• All cochlear nerve fibers synapse here first (obligatory relay)
• Three subdivisions: Dorsal cochlear nucleus, Anterior ventral, Posterior ventral
• Tonotopically organized
• Axons project via three acoustic striae:
  • Ventral (trapezoid body) - most fibers, cross to contralateral side
  • Intermediate - octopus cells
  • Dorsal - contralateral inferior colliculus

Clinical Importance: Lesion here causes ipsilateral deafness. Unilateral cortical lesion does NOT cause complete deafness because of bilateral representation above the cochlear nuclei.

• First binaural processing center
• Lateral superior olive (LSO): processes interaural intensity differences (IID)
• Medial superior olive (MSO): processes interaural time differences (ITD)
• Function: Sound localization in horizontal plane
• Gives rise to the olivocochlear bundle (efferent - descends to modulate OHC function)

Clinical Importance: Damage to the SOC impairs sound localization even with normal pure-tone thresholds.

• Relay station for ascending fibers

• ALL ascending auditory fibers synapse here
• Tonotopically organized
• Integrates binaural cues, spectral shape, and timing
• Connected to superior colliculus (coordinates audio-visual reflexes)
• Commissure of the inferior colliculus allows cross-hemisphere communication

Clinical Importance: Inferior colliculus lesion = binaural deficits, impaired localization. Also important in the "orienting reflex" to sounds.

• Portal for all ascending auditory input to the cortex
• Three divisions: ventral (tonotopic, projects to primary cortex layers III/IV), dorsal, medial
• Medial division receives vestibular and somatosensory input too - role in "arousal" to sound

• Brodmann area 41 (AI - primary) and 42 (AII - secondary)
• Located in the transverse gyri within the Sylvian fissure on the superior temporal gyrus
• Tonotopically organized: low frequencies anterolateral, high frequencies posteromedial
• Adjacent Wernicke's area (area 22) - receptive speech understanding

Clinical Correlation - Central Auditory Processing Disorder (CAPD): Normal audiogram but poor understanding in noise. Due to lesions/dysfunction in the central pathways. Common in elderly, post-stroke patients, and children with learning difficulties. Assessment: speech-in-noise tests, ABR, dichotic listening. Management: FM systems, auditory training programs.

Clinical Correlation - Wernicke's Aphasia: Lesion of Wernicke's area (dominant hemisphere, posterior STG) = fluent but incomprehensible speech with poor comprehension. The patient can hear but cannot decode linguistic meaning.

Mnemonic for Auditory Pathway

• Spiral ganglion → Superior olivary complex → Medulla (cochlear nuclei) → Lateral lemniscus → Inferior colliculus → Cortex (Auditory) → Wernicke's

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Efferent (Descending) Auditory Pathway

• Medial OCB: projects to OHCs - suppresses OHC electromotility (reduces cochlear amplification, protects against noise)
• Lateral OCB: projects to afferent dendrites below IHCs

Clinical Importance: The OCB is the basis for otoacoustic emission suppression testing, used to detect brainstem lesions and auditory neuropathy. The presence of OAEs with absent ABR waves suggests auditory neuropathy spectrum disorder (ANSD).

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151. Mechanism of Hearing

Steps in Sound Transduction

Step 1 - Sound Collection and Transmission (External Ear)

Step 2 - Middle Ear Impedance Matching

Step 3 - Basilar Membrane Traveling Wave (Von Bekesy's Theory)

• Base (near oval window): Narrow (0.04 mm), stiff - responds to HIGH frequencies
• Apex (helicotrema end): Wide (0.5 mm), floppy - responds to LOW frequencies

• 20,000 Hz → maximum displacement at the base
• 20 Hz → maximum displacement at the apex
• 1000 Hz → peaks ~20 mm from base

Why this matters clinically: This explains tonotopy - the spatial frequency map of the cochlea that is preserved throughout the entire auditory pathway. Noise-induced hearing loss first damages the 4000 Hz region because the 4 kHz region of the basilar membrane is adjacent to the region exposed to the maximum stapes-induced velocity.

Step 4 - Mechanoelectric Transduction (Hair Cell Transduction)

1. Basilar membrane displacement causes a shearing motion between tectorial membrane and reticular lamina
2. Stereocilia on OHCs (longest tips contact tectorial membrane) are deflected
3. Tip links (fine filaments connecting tops of shorter stereocilia to sides of taller ones) are stretched
4. Mechanically gated K+ ion channels open at stereocilia tips
5. K+ flows INTO the hair cell (down its electrochemical gradient - from endolymph [+80 mV, high K+] into hair cell [−45 mV])
6. Depolarization → Ca2+ entry at base → neurotransmitter release (glutamate)
7. Excitatory postsynaptic potential in Type I afferent fibers → action potentials in cochlear nerve

The Endocochlear Potential (ECP): The scala media maintains a resting potential of +80 mV (relative to perilymph), maintained by the stria vascularis pumping K+ actively. The combined driving force for K+ entry is 80 mV (ECP) + ~45 mV (hair cell resting potential) = ~125 mV - an unusually large driving force that allows exquisite sensitivity. Conditions that damage the stria vascularis (Meniere's, endolymphatic hydrops) reduce the ECP and impair transduction.

Step 5 - Outer Hair Cell Amplification (Active Cochlear Mechanism)

Clinical Importance - Otoacoustic Emissions (OAEs): OHC electromotility generates sounds back into the external ear canal that can be measured (OAEs). OAEs are absent when OHCs are damaged (noise, ototoxins) but present with retrocochlear lesions. Used in newborn hearing screening.

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Frequency Coding

1. Place coding (tonotopy): Basilar membrane location of maximum displacement encodes frequency - dominant for frequencies >200 Hz
2. Temporal/phase-locking: Neurons fire in phase with the sound waveform - dominant for low frequencies (<4000 Hz)
3. Combined: for frequencies 200-4000 Hz, both mechanisms contribute

Intensity Coding

1. Rate coding: Higher stimulus intensity → higher firing rate in individual neurons
2. Recruitment: More neurons (with higher thresholds) recruited at higher intensities
3. Spread of excitation: Higher intensities excite a broader region of basilar membrane

Clinical Correlation - Recruitment: In cochlear (not retrocochlear) damage, there is an abnormally rapid growth of loudness. Patients say "don't shout!" even though they asked you to speak louder. This occurs because loss of OHC amplification removes the compressive nonlinearity of the cochlea. Recruitment helps localize the lesion to the cochlea (vs. nerve).

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152. Hearing Defects and Hearing Tests

Classification of Hearing Loss

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Tuning Fork Tests

Weber's Test (512 Hz)

• Place tuning fork on vertex of skull, glabella, or upper incisors
• Ask: "Does the sound sound the same in both ears, or louder in one?"

Rinne's Test (512 Hz)

• Compare Air Conduction (AC): fork tines near EAM
• vs. Bone Conduction (BC): fork base on mastoid

Tuning Fork Frequencies: 256 Hz - feels vibration more than hears; 512 Hz - ideal for Rinne/Weber (best balance of tactile/auditory); 1024/2048 Hz - for very quiet environments.

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Pure-Tone Audiometry

• Air conduction: O (right, red), X (left, blue)
• Bone conduction: < (right), > (left)
• Air-bone gap >10 dB = conductive component
• Normal threshold: 0-25 dB HL

• Normal: ≤25 dB HL
• Mild: 26-40 dB (difficulty in noisy environments)
• Moderate: 41-60 dB (speech understanding impaired)
• Severe: 61-80 dB (shout not heard)
• Profound: >80 dB (candidate for cochlear implant)

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Special Hearing Tests

Tympanometry

Clinical Correlation - Otosclerosis: Abnormal bone remodeling of the otic capsule, causing stapes footplate fixation. Audiogram: flat conductive HL with Carhart's notch (dip at 2 kHz on bone conduction - pathognomonic). Tympanogram: Type As. Treatment: Surgical stapedectomy/stapedotomy (prosthesis insertion). Medical: sodium fluoride (slows disease), hearing aids.

Otoacoustic Emissions (OAEs)

• Spontaneous OAEs (SOAEs): Generated without stimulus; present in ~70% of normal ears
• Transient evoked OAEs (TEOAEs): Evoked by click stimulus; used in newborn screening (NHS)
• Distortion product OAEs (DPOAEs): Two-tone stimulus; sensitive for OHC function at specific frequencies
• OAEs present = OHC intact (cochlear function adequate); OAEs absent = cochlear damage

Clinical Application: OAEs are absent in noise-induced hearing loss even before pure-tone thresholds shift, making them an early sensitive marker of cochlear damage. In auditory neuropathy spectrum disorder (ANSD): OAEs present (OHCs intact) but ABR absent (nerve/brainstem dysfunction).

Auditory Brainstem Response (ABR)

• Wave I: Cochlear nerve (distal)
• Wave II: Cochlear nerve (proximal) or cochlear nucleus
• Wave III: Superior olivary complex
• Wave IV: Lateral lemniscus
• Wave V: Inferior colliculus

Clinical Application:

• Prolonged I-III interpeak latency = lesion between cochlear nerve and SOC (cerebellopontine angle tumor)
• Prolonged III-V interpeak latency = brainstem lesion
• Absent wave V with present wave I-III = midbrain lesion
• All waves absent = profound deafness
• ABR is used for: neonatal hearing screening, acoustic neuroma detection, intraoperative monitoring

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Major Hearing Defects - Clinical Correlations, Drugs, and Treatment

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1. Otosclerosis

• Surgery: Stapedectomy or stapedotomy (small fenestration + piston prosthesis) - curative
• Medical (slows progression): Sodium fluoride 20-60 mg/day (stabilizes abnormal bone remodeling)
• Hearing aid: For those refusing surgery or with cochlear involvement
• Estrogen accelerates progression - avoid OCP in suspected otosclerosis

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2. Meniere's Disease (Endolymphatic Hydrops)

Betahistine is the only drug specifically licensed for Meniere's disease. Recent evidence (PMID: 38808803) confirms spontaneous recovery occurs in ~50% of untreated sudden SNHL cases, highlighting the importance of controlled studies for any Meniere's intervention.

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3. Sudden Sensorineural Hearing Loss (SSNHL)

• Oral corticosteroids (1st line): Prednisolone 1 mg/kg/day (max 60 mg) × 10-14 days, then taper
• Intratympanic steroids (ITG): Dexamethasone or methylprednisolone - for patients who cannot take systemic steroids or as salvage (2nd line)
• Hyperbaric oxygen (HBO): Adjunct to steroids - 2025 meta-analysis (PMID: 40405024) confirms HBO as effective adjunct to corticosteroids
• Salvage treatment: 2025 systematic review (PMID: 40734818) reviews salvage options including HBO, IT steroids, and combination approaches for refractory SSNHL
• Antiviral therapy (acyclovir): Only if herpes zoster oticus suspected
• Vasodilators (pentoxifylline, carbogen inhalation): Weak evidence; 2025 review (PMID: 40332573) on pentoxifylline in inner ear disease

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4. Noise-Induced Hearing Loss (NIHL)

• Prevention: Ear protection (>85 dB noise levels at work - OSHA standard), monitoring programs
• Antioxidants (experimental): N-acetylcysteine (NAC), Mg2+ supplementation, D-methionine - reduce ROS-mediated OHC damage in animal models; human trials ongoing
• Established: Hearing aids for established NIHL; cochlear implant if profound

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5. Presbycusis (Age-Related Hearing Loss)

• Sensory type: OHC loss at base (high frequency)
• Neural type: Spiral ganglion neuron loss (poor speech discrimination)
• Strial/metabolic type: Stria vascularis atrophy (flat HL)
• Mechanical/cochlear conductive type: Basilar membrane stiffening

• Hearing aids: First-line for mild-moderate HL; amplification helps but does not restore normal discrimination
• Cochlear implants: For severe-profound HL; meta-analysis (PMID: 38016438) confirms excellent outcomes even in auditory neuropathy
• Hearing loops, captioned telephones, assistive listening devices
• No established pharmacological treatment; research into BDNF, NT-3 to protect spiral ganglion neurons

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6. Auditory Neuropathy Spectrum Disorder (ANSD)

• FM systems (bypass background noise)
• Cochlear implants work well because they bypass the damaged IHC/synapse and directly stimulate the nerve - PMID: 38016438 meta-analysis confirms cochlear implantation gives excellent outcomes in ANSD children

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7. Congenital Hearing Loss

• 50-60% of congenital SNHL is genetic
• GJB2 (Connexin 26) mutation - most common cause of non-syndromic autosomal recessive SNHL (DFNB1); connexin channels in supporting cells maintain K+ recycling
• Pendred syndrome: SNHL + goitre (SLC26A4/pendrin mutation) - enlarged vestibular aqueduct; second most common syndromic cause
• Usher syndrome: SNHL + retinitis pigmentosa + vestibular dysfunction
• Waardenburg syndrome: SNHL + white forelock + heterochromia iridis (PAX3/MITF mutations); 2023 meta-analysis (PMID: 37847940) supports cochlear implantation

• Transient OAEs (TEOAEs) or ABR in all neonates within 1 month
• Refer if fail; audiological diagnosis by 3 months
• Intervention by 6 months ("1-3-6 rule")

• Hearing aids for all degrees of HL from 3-4 months
• Cochlear implants: eligible from 12 months (or earlier in some guidelines) for bilateral profound SNHL
• Sign language and oral-aural habilitation

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Drug-Induced Hearing Loss (Ototoxic Drugs)

Monitoring Protocol for Ototoxic Drugs: Baseline audiogram before starting cisplatin or aminoglycosides; repeat at each cycle and 3 months post-treatment. Monitor UHF (ultra-high frequencies 8-20 kHz) - earliest sign of damage. If significant threshold shift (>10 dB at 2+ frequencies), discuss with oncology team about dose modification or alternative agent.

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Summary Table: Clinical Diagnosis at a Glance

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Key Drugs Summary for Hearing Disorders

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Recent Evidence (2023-2025)

• Natural history of untreated SSNHL (PMID: 38808803, 2024) - ~50% spontaneous recovery; underlines why controlled studies are needed
• Hyperbaric oxygen + steroids for SSNHL (PMID: 40405024, 2025) - Systematic review/meta-analysis confirms HBO is an effective adjunct to corticosteroids
• Salvage treatment for refractory SSNHL (PMID: 40734818, 2025) - Comprehensive review of IT steroids, HBO, and combination approaches with practical recommendations
• Pentoxifylline in inner ear disease (PMID: 40332573, 2025) - Reviews vasodilator use; limited evidence
• Cochlear implantation in ANSD children (PMID: 38016438, 2024) - Meta-analysis confirms excellent speech outcomes post-CI

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