Mechanism of Hearing😊

Mechanism of Hearing

• Involves three main parts and several steps.
1. Mechanical Conduction of sound.
• Occurs in the conductive apparatus.
• External ear and Middle ear.
• (Steps 1 and 2).
2. Transduction of mechanical energy.
• Into electrical impulses.
• By the cochlea's sensory system.
• (Steps 3-5).
3. Conduction of electrical impulses to the brain.
• Inner ear to the brain.
• Sensorineural pathway.

Steps involved in Sound Transmission

1. Sound waves enter from the environment.

2. Go to External Auditory Canal (EAC).

3. Hit the Tympanic Membrane.

4. Travel through the Malleus.

5. Then the Incus.

6. To the Stapes and its Footplate.

7. Pass to the Oval window.

8. Enter the Scala vestibuli of the cochlea.

9. Proceed to the Scala tympani.

10. Reach the Round window.

11. Impulses carried from cochlea to brain via 8th cranial nerve.

Impedance Matching Mechanism

• Definition:
• Sound travels from air (middle ear) to fluid (inner ear).
• Amplitude is decreased by fluid impedance.
• Middle ear amplifies sound intensity to compensate.
• Converts sound of low pressure, high amplitude to high pressure, low amplitude vibration.
• Increases sound power to penetrate the inner ear.

• Only 0.1% of sound energy enters the inner ear.

Mechanisms by which sound becomes more powerful

• Mechanism I - Area Ratio:
• Sound travels from tympanic membrane (larger area) to stapes footplate (smaller area).
• Tympanic membrane effective vibrating area = 45-55 mm².
• Stapes footplate surface area = 3.2 mm².
• Condensation of energy makes sound
• Area ratio = 45mm² / 3.2mm² ≈ 14:1 ~14 times stronger.

• Mechanism II - Lever Ratio:
• Malleus handle is 1.3 times longer than the incus.
• M:I = 1.3:1.
• Sound travels from bigger to smaller ossicle.
• Contributes a "submissive effect".

• Total transformer ratio:
• Product of Areal ratio and Lever ratio.
• 14 x 1.3 = 18:1 ~18 times more powerful.
• (or 17 x 1.3 = 22.1, based on 55mm² area).

What Happens to The Inner Ear?

• Organ of Corti is on the Basilar membrane.
• Contains numerous hair cells (hearing receptors).

• Basilar membrane vibrates.
• Hair cells vibrate against the tectorial membrane.

• This friction converts mechanical to electrical energy.

• Type 1 hair cells on Organ of Corti secrete glutamate
• These are afferent hair cells.
• Glutamate stimulates the 8th nerve.

• This conversion is called Transduction.

Auditory Pathway

• Entire pathway from cochlea to 8th nerve to brain.

Order of Auditory Pathway (Mnemonic: ECOLIMA):

• E - Eight nerve (8th) → Axon of Spiral ganglion

• C - Cochlear Nucleus

• O - Superior olivary complex

• L - Lateral lemniscus

• I - Inferior colliculus

• M - Medial geniculate body

• A - Auditory cortex

Spiral Ganglion

• Located in Rosenthal’s canal (in modiolus of cochlea).

• Contains bipolar cells.

• Dendrites: receive input from inner & outer hair cells of organ of Corti.

• Axons: converge to form the cochlear nerve (branch of CN VIII).

• → cochlear nuclei (dorsal & ventral) in the brainstem.

Functions of Auditory Pathway parts:

• Pitch or Frequency: Cochlea
• Mnemonic: Pichunnath Cokkachi

• Amplitude or intensity: Cochlear nerve

• Feature detection: Higher auditory centres

• Localization of sound: Higher auditory centres

Assessment of Hearing

• Note
• Hearing range: 20–20,000 Hz
• Speech frequencies: 500, 1000, 2000 Hz

Order of examination:

• Tuning Fork Tests (Primarily 512 Hz).
• Weber Test.
• Rinne Test.
• Absolute bone conduction test.
• Tuning Fork Test is the first test for hearing loss patients.

• Pure tone audiometry.

• Impedance audiometry.

• Speech Audiometry.

Special tests.

• OAE (Otoacoustic Emission).

• BERA (Brainstem Evoked Response Audiometry).

• ECog (Electrocochleography).

Understanding Air Conduction and Bone Conduction

• Conductive pathway (CP):
• Pinna → footplate of stapes.

• Sensory neural pathway (SNP):
• Cochlea → brain.

• Air conduction (AC):
• Tuning fork in air.
• AC = CP + SNP.

• Bone conduction (BC):
• Tuning fork on mastoid.
• Bypasses EAC and middle ear.
• BC = SNP.

Positive in Normal people & SNHL

• Mnemonic: RGB
• Rinne
• Gelle
• Bing

Rinne Test

• Rinne positive is seen in:
• Normal ear.
• SNHL.

Weber Test

• Procedure: Vibrated tuning fork on midline of head.

• Normal: Sound felt in the center.
• Requires >5 dB difference to lateralize.

• Conductive hearing loss:
• Lateralized to the diseased ear.

• Sensorineural hearing loss:
• Lateralized to the normal ear.

• Mnemonic: SOCS

Absolute Bone Conducting Test

• Prerequisites:
• Examiner's hearing is normal.
• Air conduction pathway is occluded.

• Process:
• Fork on patient's mastoid, occlude air conduction.
• Patient signals when sound stops.
• Examiner checks if they still hear it.

• Inference:
• Patient = Examiner:
• Normal.
• Patient < Examiner:
• SNHL.

Schwabach Test

• Assesses both conductive and sensorineural pathway
• Tragus not occluded

• Inference: ↑/Lengthened → CHL

• Inference: ↓/Shortened → SNHL

Gelle's Test

• Method:
• Place vibrating tuning fork on mastoid.
• Pressurize ear canal using Siegel’s speculum.
• Gel - siegel

• Results:
• Positive test:
• Normal hearing or sensorineural hearing loss.
• Negative test:
• Fixed/disconnected ossicular chain.

Bing Test

• Principle: Test of bone conduction; checks occlusion effect.

• Method:
• Vibrating tuning fork on mastoid
• examiner alternately opens/closes ear canal by pressing tragus.

• Bing positive:
• Sound louder with canal occluded.
• Seen in normal or sensorineural loss.

• Bing negative:
• No change in loudness.
• Seen in conductive loss.

Cochlear vs retrochoclear

• Recruit SISI for Cock

• Roll over() and decay() in step abnormally (Stapedial reflex) → Retro

Pure Tone Audiometry

• Subjective test.

• Tests both AC and BC with pure tones.

• Intensity adjusted in 5dB increments.

• Charted graph is an audiogram.

• Confirms type and measures degree of hearing loss.

• Patient tested in a soundproof room.

• Symbols:
• Right ear → red;
• left ear → blue.

Audiogram Findings

Normal Audiogram:

• AC and BC curves are between 0 to 25 dB.

• No significant air-bone (A-B) gap (< 20dB).

Conductive Hearing Loss:

• BC curve → normal.

• AC curve → abnormal.

• Significant A-B gap (> 20dB).

Sensorineural Hearing Loss:

• Both AC and BC curves are abnormal.

• No significant A-B gap.

Mixed hearing loss

High frequency Audiometry

• Downsloping audiogram :
• Mnemonic: PONA
• Presbycusis
• Ototoxicity
• NIHL
• Acoustic Neuroma

Presbycusis:

• Age-related SNHL.

• Both AC and BC are abnormal, no A-B gap.

• Typically a down-sloping graph.

• Mnemonic: Old people → adakkam parayunnath keellkum → thala thirinjavan

Meniere's Disease:

• Donaldson line → Horizontal SCC

• Cochlear type SNHL.

• Both AC and BC are abnormal, no A-B gap.

• Up-sloping graph is specific.
• More hearing loss at low frequencies.

Otosclerosis:

• Stapes fixation to oval window → earliest at Fissula ante fenestrum

• AC curve abnormal, BC normal.

• Typical dip at 2000Hz (Carhart's Notch).
• Specific to Otosclerosis.

• Mnemonic: Oto → Car

NIHL (Noise-Induced Hearing Loss):

• Acoustic dip : Dip in AC & BC at 4000Hz
• Earliest feature
• Boiler's notch

• Both AC and BC curves are abnormal (SNHL).

• Specific for NIHL.

Mid frequency HL

• Congenital SNHL

• Mnemonic: Children eat cookie

Impedance Audiometry (Objective test)

• Comprises acoustic reflex and tympanometry.

• Done using 226 Hz, 220 Hz frequencies.

• Measure middle ear pathology (CHL + Intact TM)
• Tympano → Middle Ear
• Metry → Measurement

• Component
• Tympanometry
• Best investigation to assess ET function
• Stapedial reflex
• Protective mechanism of inner ear against noise trauma.

• Compliance ∝ Ease of mobility of TM

• A type graph:
• Normal.
• Peak near 0, occupies ~half Y-axis.

• As type graph:
• Otosclerosis, tympanosclerosis.
• Peak at 0, covers < half Y-axis.

• Ad type graph:
• Ossicular discontinuity.
• Ascending/descending curves, no peak.

• B type graph:
• Fluid in the middle ear.
• Single flat line.
• Mnemonic: Fluid → Flat

• C type graph:
• Eustachian tube dysfunction.
• A-type graph on the negative side.

Most effective for ET function

• Tympanometry > Politzer test

Stapedial Reflex

• Also known as acoustic reflex.

• Loud sound on one side causes bilateral stapedius muscle contraction.

• It is an objective test.

• Afferent : I/L 8th nerve.

• Centre : Superior olivary complex.

• Efferent : B/L 7th nerve (B/L reflex).

Pathologies

• 8th nerve or cochlear pathology -> bilateral loss of reflex.

• 7th nerve pathology -> unilateral loss of reflex.

Speech Audiometry (Subjective test)

• Cochlear Pathology → Plateau

• Retrocochlear → Roll over

 

• Speech Reception Threshold:
• Threshold where 50% of words are repeated correctly.

• Speech Discrimination Score (SDS):
• Ability to identify phonetically balanced words.
• Score <50% suggests retro cochlear hearing loss.

Electrocochleography (ECog) (Objective test)

• IOC to diagnose Meniere's disease.

• Measures potentials in cochlea and 8th nerve.

• Electrode placed transtympanically on the promontory.

• Normal
• SP/AP <30% or <0.3

• Meniere's disease
• SP/AP >30% or >0.3

• Meniers → munnu → 3 il kuduthal

• Mnemonic:
• Sum → Pitch → Cochlea (Sum cheyyan arinjudathond kochina pichi)
• Action → Loudness → Nerve (Takes Nerve → Action speaks louder than words)

Brainstem Evoked Response Audiometry (BERA)
AKA Auditory brainstem response.

• Assesses auditory pathway from 8th nerve to brainstem.

• Non-invasive and objective test.

• Uses clicks and tone bursts.

• Normal person generates 5-7 electrical waves.

Latency Response :

Wave Representation:

Uses of BERA:

• Objective test for hearing.

• Non-organic hearing loss.

• Neonates:
1. Confirm hearing loss in neonates & infants.
2. Screen hearing loss in neonates in ICU.
3. Determine hearing threshold

• Best audiometric test for:
• Differentiating cochlear vs retro cochlear hearing loss.
• Retrocochlear hearing loss (Acoustic neuroma)
• delay in Wave V
• Mnemonic: Vestibular Schwannoma → V wave
• NOTE: Best Ix for acoustic neuroma → Gadolinium enhanced MRI
• Detecting malingering

Approach in Neonates

Otoacoustic Emission (OAE)

• Emissions from outer hair cells of the cochlea.
• If emissions are recorded,
• patient "passed the test".
• Absence warrants further testing,
• not confirmed hearing loss.

• Uses:
• Best screening test for hearing loss in neonates, infants, children.
• Differentiates cochlear from retro cochlear hearing loss.
• Early detection of noise-induced hearing loss.

• Outer hair cells:
• generally efferent.

• Inner hair cells:
• generally afferent.
• Mnemonic: Inneril kude keri outeril kuda varum

• Some impulses travel to EAC, recorded by a probe.

• Spontaneous emissions:
• recorded without sound stimulation.

If OAE is not absent, in adults

• Perform
• Tympanometry
• To r/o middle ear pathologies
• Speech audiometry
• To r/o cochelar vs retocochlear pathologies

Auditory Neuropathy Spectrum Disorder (ANSD)

Pathophysiology

• Damaged inner hair cells

• Demyelination of nerves

• Loss of axon
→ Leads to dyssynchrony

Clinical Features

• Hearing: Normal

• Speech intelligibility: Absent

• Often presents late (school-going age)

Investigations

Management

• Hearing aids

• Cochlear implantation (TOC)

Test to differentiate Cochlear Pathology

Recruitment

• Definition: Abnormal growth in sound loudness.

• Seen in cochlear lesions
• e.g., Meniere's, presbycusis

• Differentiates from retro cochlear pathology.

ABLB (Alternate Binaural Loudness Balance Test):

• Identifies unilateral cochlear lesions.

SISI (Short Increment Sensitivity Index):

• Cochlear pathology patients appreciate small intensity changes.

• Differentiates cochlear from retro cochlear pathology.

Threshold Tone Decay Test

• A test for retro cochlear pathology.

• Normal person hears a tone for 60 seconds

• Retro cochlear pathology patients
• Stop hearing earlier due to nerve fatigue

Summary of Tests

One Liners (Physiology and Assessment)

• High-frequency audiometry is for NIHL / Ototoxicity.

• Stapedial reflex is mediated by 8th and 7th nerves.

• High auditory centres → localization and discrimination.

• Angular movements
• sensed by SCC ducts - Cristae.

• Screening test for hearing loss is OAE.

• Confirmatory test for hearing loss is BERA.

• Objective hearing tests:
• Impedance audiometry, OAE, BERA, ECog.

• Subjective hearing tests:
• Tuning fork, PTA, Recruitment, Tone decay, Speech audiometry.

ASSESSMENT OF THE VESTIBULAR SYSTEM

Vestibular System Anatomy

• Central vestibule has elliptical and spherical recesses.
• Spherical recess → saccule.
• Elliptical recess → utricle.

• Superior vestibular nerve:
• from utricle, superior, and lateral SCCs.

• Inferior vestibular nerve:
• from saccule and posterior SCC.

• Impulses → vestibular nucleus via vestibular nerve.

• Three efferents of vestibular nucleus:
• Medial longitudinal fasciculus
• 3rd, 4th, 6th nerve nucleus
• Stimulation causes Nystagmus (vestibulo-ocular reflex)
• Spinal cord
• Cerebellum

Category	Peripheral	Central
Latency	2-20 sec	No latency
Duration	< 1 min	> 1 minute
Direction of nystagmus	Fixed, towards the undermost ear	Direction keeps changing
Fatigability intensity of Vertigo	Fatigable severe	Non-Fatigable Mild
Incidence	Common	Rare
Improves	With Fixation	-
Worsens	With Darkness	-

• Peripheral → For a minute, Fixed, Fatigue

Direction & Types of Peripheral Nystagmus

A patient presented to emergency with acute vertigo and horizontal nystagmus. The slow component of the nystagmus is towards the left side. What is the most likely diagnosis?

1. Posterior canal BPPV

2. Superior canal BPPV

3. Left hypoactive labyrinth

4. Right hypoactive labyrinth
ANS
• 3
• Slow component towards Left ⇔ Right sided Nystagmus
• Cause:
• Right sided hyperactive labyrinth
• Left sided hypoactive labyrinth /

NOTE

• Serous labyrinthitis (Hyperactive):
• Same side

• Simply Labrynthitis/Purulent labyrinthitis/Trauma (Hypoactive)
• C/L side

Head Impulse Test (HIT)

• Left vestibular neuritis/labrynthitis → head turn left → rightward saccade.

• Assesses: Vestibulo-ocular reflex (VOR).

• Normal: Eyes stay fixed on target during head rotation.

• Abnormal: Impaired VOR → catch-up saccade.

• Interpretation:
• Head turned towards lesion side → corrective saccade opposite.

Induced Nystagmus

• Can be induced by changes in:
• Position.
• Pressure.
• Temperature.

1. Positional Change: Dix Hallpike Test

• Diagnostic test for BPPV.

• Steps:
1. Patient sits at a table.
2. Head is rotated 45° (right or left).
3. Patient taken down with head hanging at 30° angle.

• Mechanism:
• Displacement of otolith into semicircular canal.
• Stimulates labyrinth → vestibular nucleus → MLF → nystagmus.

• Posterior semicircular canal is commonly stimulated.

• Nystagmus direction:
• towards undermost ear with torsional component.

• Inference:
• Positive test: Diagnosis is BPPV.
• Negative test: Patient is normal.

2. Thermal Change: Caloric Test

Caloric Test

• Mnemonic: COWS: Its Normal
• Cold water -> Opposite side nystagmus.
• Warm water -> Same side nystagmus.

• Check whether Labynth is working or not
• If not working → ↓↓ COWS

• Principle:
1. Thermal stimulation of EAC.
2. Convection currents in middle ear.
3. Stimulates lateral semicircular canal (LSCC).
4. Impulses to MLF → 3, 4, 5
5. Causes nystagmus.

• Patient position:
• Make 30° with horizontal
• Supine with 30° head tilt forward.
• Or sitting with 60° head tilt backward.
• Makes LSCC vertical and maximally responsive.

• Procedure:
• Canal irrigated with
• 5 ml cold or warm water (+/- 7°C from Normal Body temp)
• for 60 seconds.

• Nystagmus
• Horizontal + Torsional component
• No vertical component
• Eyes moves rapidly (Not slowly)

• Inference:

Observation	Interpretation
Nystagmus occurs	Normal
Nystagmus absent	Increase water quantity
Nystagmus at higher volume	Hypofunctional labyrinth
No nystagmus at 40 ml	Non-functional / dead labyrinth

• Variations:
• Modified Kobrak's Test:
• Uses only cold water.
• Mnemonic: Modified cobra → Likes cold water
• Fitzgerald-Hallpike test:
• Uses both cold and warm water.
• Cold water: 30°C (7° below body temp).
• Warm water: 44°C (7° above body temp).
• Mnemonic: F - H → Freezing & Hot

• Contraindications:
• Perforated tympanic membrane.
• Alternative: Dundas Grant tube test (uses air).

• Congenital absence of pinna.
• Alternative: Barany's rotational chair test.

3. Pressure Change: Fistula Test

Fistula Test

• Fistula:
• Abnormal middle ear-inner ear connection.

• Procedure:
• Siegel's speculum in EAC.
• Pressure is increased in the canal.

• Normal response:
• No nystagmus or vertigo.

• Positive Fistula Test:
• Positive when
• Erosion of horizontal SCC
• Post-stapedectomy fistula
• Fenestration surgery
• Pressure transmits to inner ear.
• Results in vertigo and/or nystagmus.

Fistula Test

• Inference:

Test Result	Interpretation / Condition
False positive fistula (Hennebert's test)	1. Seen in congenital syphilis; → due to hypermobile stapes footplate 2. Meniere disease (Tulio/Tumarkin)
False negative fistula	• Dead labyrinth • Cholesteatoma covering fistula

Preferred position for lying down

• affected ear is down → because the endolymph shift decreases stimulation.

NOTE: Hennebert's test vs Hennebert's sign

• Hennebert's test
• False positive fistula
• Seen in
• Congenital syphilis
• due to hypermobile stapes footplate
• Meniere disease

• Hennebert's sign
• Pressure-triggered vertigo + nystagmus
• NOTE: Seen in
• Meniere’s
• Perilymphatic fistula
• SCC dehiscence

Tullio Phenomenon

• Loud soud → Vertigo

• Seen in Superior SCC dehiscence syndrome

Tumarkin crisis

• Loud soud → Drop attack

Vestibular evoked myogenic potential (VEMP)