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Prevention
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Advanced Clinical Audiology: A Comprehensive Analysis of Acoustic Immittance, Reflex Pathways, and Neuro-Otologic Diagnostics

Acoustic Immittance & Vestibular Assessment

Principles of Acoustic Immittance

Acoustic immittance testing provides objective physiological data regarding the status of the tympanic membrane (TM), ossicular chain, and middle ear space. Unlike behavioral audiometry, this does not rely on patient response.

Physics of Admittance

Modern diagnostics primarily measure Admittance (Y)—the ease with which energy flows through a system.

Components of Admittance

  • Conductance (G): The resistive component (friction). Represents energy dissipation via middle ear structures and mucous membranes.
  • Susceptance (B): The reactive component (energy storage). Influenced by:
    • Mass Reactance: Weight of ossicles (dominant at high frequencies).
    • Stiffness Reactance: Elasticity of ligaments and enclosed air (dominant at low frequencies like 226 Hz).

The Probe Assembly

The clinical tympanometer uses three transducers in a hermetically sealed cavity:

  • Pneumatic Pump: Sweeps pressure from +200 to -400 daPa.
  • Probe Tone Generator: Emits 226 Hz (adults) or 1000 Hz (infants).
  • Microphone: Measures the sound reflected back from the eardrum.

Tympanometry: Mechanisms & Interpretation

Tympanometry measures eardrum compliance in response to pressure changes. It is the primary tool for identifying conductive pathologies.

The Jerger Classification System

Type A (Normal)
+50 to -100 daPa | 0.3-1.5 ml

Normal middle ear function; intact ossicular chain; no effusion.

Type As (Shallow)
Normal Pressure | < 0.3 ml

Stiffness dominated. Otosclerosis, tympanosclerosis, or malleus fixation.

Type Ad (Deep)
Normal Pressure | > 1.5 ml

Hyper-mobile. Ossicular discontinuity or monomeric (scarred) TM.

Type C (Negative)
< -100 daPa

Eustachian Tube Dysfunction. Retracted TM or developing otitis media.

Type B (Flat): The Critical Role of Volume

When the tracing is flat (Type B), the Equivalent Ear Canal Volume (ECV) determines the diagnosis.

Normal ECV

0.6 – 1.5 ml
Indicates Otitis Media with Effusion (OME). The probe measures to the drum, but the drum won't move due to fluid.

Large ECV

> 2.0 ml
Indicates a Perforation or patent PE tube. The probe measures the canal plus the middle ear space.

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Pediatric Note: In infants <6 months, the ear canal is compliant. Standard 226 Hz tones cause "false normals." A 1000 Hz probe tone is mandatory for accurate diagnosis.

The Acoustic Reflex

The Acoustic Reflex Threshold (ART) measures the bilateral contraction of the stapedius muscle in response to high-intensity sound.

Reflex Arc Anatomy

  • Afferent (Sensory): Cochlea → CN VIII → Ventral Cochlear Nucleus (VCN).
  • Central (Decussation): VCN → Superior Olivary Complex (SOC). Crossing fibers allow the signal to reach both sides.
  • Efferent (Motor): SOC → CN VII (Facial Nerve) → Stapedius Muscle.

Interpretation of Patterns

Comparing Ipsilateral (same side) and Contralateral (opposite side) reflexes localizes the lesion.

Cochlear Pattern

Reflexes present at reduced sensation levels due to recruitment (rapid loudness growth). Common in sensory hearing loss.

Retrocochlear (Afferent)

Reflexes absent when stimulating the affected ear (both Ipsi and Contra). Suggests Acoustic Neuroma (CN VIII).

Facial Nerve (Efferent)

Reflexes absent when measuring in the affected ear. Suggests Bell's Palsy (CN VII).

Central Pattern

Bilateral Ipsi present; Bilateral Contra absent. Indicates a brainstem lesion affecting crossing fibers.

Acoustic Reflex Decay

Used to screen for retrocochlear pathology. A stimulus is played for 10 seconds. If the muscle relaxes (amplitude drops >50%) before time is up, it suggests the nerve cannot sustain firing (neural fatigue).

Advanced Electrophysiology

For complex pathologies like Meniere's or Neuropathy, immittance is insufficient. We utilize specialized potentials.

Electrocochleography (ECochG)

Records potentials from the cochlea. Primarily used for Meniere's Disease (Endolymphatic Hydrops).

Diagnostic Metrics

  • SP/AP Amplitude Ratio: >0.40 is abnormal.
  • SP/AP Area Ratio: >1.94 is abnormal. (Higher sensitivity).

Vestibular Evoked Myogenic Potentials (VEMP)

Assess the otolith organs. Essential for diagnosing Superior Canal Dehiscence (SCDS).

cVEMP (Cervical)

Inhibitory response from the SCM muscle. Tests the Saccule and Inferior Vestibular Nerve.

oVEMP (Ocular)

Excitatory response from the eye muscle. Tests the Utricle and Superior Vestibular Nerve.

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SCDS Finding: Abnormally low VEMP thresholds (<70 dB) indicate the hypersensitivity caused by the "third window" dehiscence.

Vestibular Assessment Integration

Comprehensive care often involves evaluating the vestibular division of CN VIII.

VNG (Videonystagmography)

Calorics: The gold standard for lateral canal function. A >25% asymmetry indicates peripheral weakness (e.g., Neuritis).

vHIT (Head Impulse Test)

Measures the VOR gain for all six canals. Detects "Covert Saccades"—compensatory eye movements invisible to the naked eye.

Patient Safety & Protocols

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Hyperacusis/Tinnitus: Reflex testing (up to 105 dB) can exacerbate symptoms. Proceed with extreme caution or contraindicate.
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Post-Surgery: Tympanometry is contraindicated after recent stapedectomy or tympanoplasty until cleared by a surgeon.