Tympanometry and Acoustic Reflexes

Tympanometry Basics - Ear Pressure Play

• Objective test assessing middle ear status; non-invasive.

  ⭐ Tympanometry measures middle ear compliance (mobility) by varying air pressure in the external auditory canal.

• Probe: Emits tone (adults: 226 Hz; infants <6 months: 1000 Hz), varies pressure, measures reflected sound.
• Key Parameters:
  • Compliance (Peak $Y_{tm}$): Max sound energy absorption. Normal: 0.3-1.7 ml.
  • Middle Ear Pressure (MEP/TPP): Pressure at peak $Y_{tm}$. Normal: +50 to -150 daPa.
  • Ear Canal Volume (ECV/$V_{ea}$): Estimates outer ear canal volume. Normal (adults): 0.6-2.0 ml.

Tympanogram Types - Peak Peaks & Valleys

Tympanometry graphically represents middle ear compliance (mobility) at different air pressures in the external auditory canal. Key parameters: Tympanometric Peak Pressure (TPP) in daPa and Static Acoustic Admittance (Peak $Y_{tm}$) in ml or mmho.

• Jerger's Classification of Tympanograms:

  Type	TPP (daPa)	Peak $Y_{tm}$ (ml/mmho)	Common Causes
  A	0 to -100	Normal (0.3-1.6)	Normal middle ear
  As	0 to -100	↓ Stiff (< 0.3)	Otosclerosis, Tympanosclerosis, Malleus fixation
  Ad	0 to -100	↑ Deep (> 1.6)	Ossicular chain disarticulation, Monomeric/Flaccid TM
  B	No Peak (Flat)	Not recordable / Very Low	Otitis Media with Effusion (OME), TM perforation (if ECV large), Cholesteatoma
  C	< -100 (often to -200 or more)	Normal or ↓	Eustachian Tube Dysfunction (ETD), Early/Resolving OME

  📌 Mnemonic:

  • A: All good, normal pressure & compliance.
  • As: Stiff system, normal pressure, shallow peak.
  • Ad: Disarticulated / Deep, normal pressure, high peak.
  • B: Blocked (fluid) / Big hole (perforation with large ECV) - flat.
  • C: Canal pressure negative (ETD) - peak shifted left.

⭐ A Type B tympanogram with a large ear canal volume (ECV > 2.0-2.5 ml) is highly suggestive of a tympanic membrane perforation.

Acoustic Reflexes - Stapedius Sound Shield

• Involuntary stapedius muscle contraction to loud sounds (>70 dB HL); protects inner ear.
• Mechanism: Stiffens ossicular chain, ↓ sound transmission (mainly low frequencies).
• Threshold: Lowest intensity eliciting reflex.
  • Normal: 70-100 dB HL above pure tone threshold for activating stimulus.
  • Probe tone (measures compliance change): 226 Hz (adults), 1000 Hz (infants < 6-9 months).
• Nerve Pathway: 📌 8 in, 7 out (CN VIII afferent, CN VII efferent).
• Reflex Pathways (see flowchart below):

• Significance: Assesses integrity of CN VII, CN VIII, and lower brainstem pathways.
• Absent/elevated thresholds: Suggest conductive loss, cochlear damage, CN VIII/VII lesions, or brainstem pathology.

⭐ Acoustic reflex decay testing is particularly useful in detecting VIIIth nerve (retrocochlear) lesions.

Clinical Correlation - Diagnostic Duo

Combining Tympanometry and Acoustic Reflex Testing (ART) enhances diagnostic accuracy for various middle ear and retrocochlear pathologies.

High‑Yield Points - ⚡ Biggest Takeaways

• Tympanometry assesses middle ear function, not hearing thresholds.
• Type A: Normal; Type As: Stiff (e.g., otosclerosis); Type Ad: Flaccid (e.g., ossicular discontinuity).
• Type B (flat): Indicates middle ear effusion (OME) or TM perforation.
• Type C: Suggests negative middle ear pressure (e.g., Eustachian tube dysfunction).
• Acoustic reflex tests the integrity of the stapedial reflex pathway (CN VII, CN VIII, brainstem).
• Absent acoustic reflex can indicate conductive hearing loss, severe SNHL, or facial nerve palsy.
• Acoustic reflex decay is a key sign of retrocochlear pathology (e.g., acoustic neuroma).