6.3 Immittance Audiometry, Tympanometry & Acoustic Reflexes

6.3 Immittance Audiometry, Tympanometry & Acoustic Reflexes

[!NOTE] While pure-tone and speech audiometry provide subjective, behavioral measures of hearing sensitivity, acoustic immittance audiometry provides an objective, non-invasive physiological assessment of the outer ear, tympanic membrane, middle ear ossicular chain, Eustachian tube, and brainstem neural reflex pathways. It requires zero active behavioral participation from the patient, making it an indispensable objective tool in the hearing assessment battery. On the NBC-HIS examination, mastering tympanogram classification, equivalent ear canal volume differential diagnosis, and acoustic reflex arc mechanics is essential.

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Biomechanical Principles of Acoustic Immittance

Acoustic Admittance vs. Acoustic Impedance

Acoustic immittance is an umbrella term encompassing two reciprocal physical phenomena governing how acoustic sound energy interacts with a mechanical system:

                               ACOUSTIC IMMITTANCE
                                        │
                 ┌──────────────────────┴──────────────────────┐
                 ▼                                             ▼
      ACOUSTIC ADMITTANCE (Ya)                      ACOUSTIC IMPEDANCE (Za)
   The EASE with which sound energy             The OPPOSITION or resistance to
   flows into the middle ear system             the flow of acoustic energy
   Unit: acoustic mmho or cm³ (mL)              Unit: acoustic ohms
   (High Admittance = High Compliance)          (Za = 1 / Ya)

• Acoustic Impedance ($Z_a$): The total opposition to the flow of acoustic sound energy through the middle ear. Expressed in acoustic ohms. Impedance is determined by three interacting mechanical vectors: friction/resistance ($R_a$), mass reactance ($X_m$), and stiffness reactance ($X_s$).
• Acoustic Admittance ($Y_a$): The ease with which acoustic energy transfers through the tympanic membrane into the ossicular chain. Admittance is the mathematical reciprocal of impedance ($Y_a = 1 / Z_a$). In clinical tympanometry, admittance is measured in acoustic millimhos (mmho) or converted directly to equivalent volume of air in cubic centimeters ($\text{cm}^3$) or milliliters ($\text{mL}$), because at $226\text{ Hz}$ under standard atmospheric conditions, $1\text{ mmho} \approx 1\text{ cm}^3$.
• Stiffness Domination: The human middle ear is a stiffness-dominated mechanical system at low frequencies. Therefore, low-frequency probe tones ($226\text{ Hz}$) primarily reflect stiffness alterations caused by middle ear pathologies (e.g., fluid accumulation, otosclerotic fixation, or ossicular discontinuity).

Electroacoustic Probe Architecture

To measure acoustic immittance, an electroacoustic probe tip encased in a hermetically sealing soft silicone or rubber cuff is inserted snugly into the patient's external auditory canal. The probe housing contains three integrated components, supplemented by an acoustic reflex delivery channel:

                    ELECTROACOUSTIC IMMITTANCE PROBE ARCHITECTURE

     ┌─────────────────────────────────────────────────────────────┐
     │ 1. Probe Tone Generator (Loudspeaker)                       │
     │    Delivers constant 226 Hz tone at 85 dB SPL ─────────────┼──┐
     ├─────────────────────────────────────────────────────────────┤  │
     │ 2. Probe Microphone                                         │  │ Sealed
     │    Measures reflected sound pressure level in canal ────────┼──┼─ Ear
     ├─────────────────────────────────────────────────────────────┤  │ Canal
     │ 3. Pneumatic Manometer Pump                                 │  │ (Hermetic
     │    Varies canal air pressure from +200 to -400 daPa ────────┼──┤  Seal)
     ├─────────────────────────────────────────────────────────────┤  │
     │ 4. Acoustic Reflex Stimulus Transducer                      │  │
     │    Presents high-intensity reflex activating tones ─────────┼──┘
     └─────────────────────────────────────────────────────────────┘

1. Probe Loudspeaker (Oscillator): Emits a continuous, unvarying probe tone calibrated to 226 Hz at 85 dB SPL.
   • Pediatric Mandate (Infants < 6 months): Clinicians must use a 1000 Hz probe tone rather than 226 Hz. In neonates and young infants under 6 months, the external auditory canal walls are highly cartilaginous and compliant. At 226 Hz, the canal walls vibrate and expand with the probe tone, yielding a false-normal Type A tympanogram even in the presence of severe middle ear effusion. A 1000 Hz probe tone is mass-dominated and accurately reveals middle ear status in young infants.
2. Probe Microphone: Monitors the sound pressure level (SPL) within the enclosed canal space. Because the loudspeaker puts out a constant sound energy, any change in middle ear compliance changes the sound absorbed versus reflected:
   • When the middle ear is stiff (low admittance), less sound enters the middle ear and more sound reflects back, creating a higher SPL at the microphone.
   • When the middle ear is compliant (high admittance), sound easily enters the ossicular chain, creating a lower SPL at the microphone.
3. Pneumatic Pump and Manometer: Mechanically alters air pressure within the sealed canal, sweeping continuously from positive pressure (+200 daPa) through atmospheric zero (0 daPa) down to negative pressure (-400 daPa).
4. Reflex Actuator / Contralateral Earphone: Delivers pure-tone stimuli (500, 1000, 2000, 4000 Hz) at elevated intensities to elicit the acoustic stapedial reflex.

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Tympanometric Metrics & Normative Parameters

A tympanogram is a dynamic graphical recording showing changes in acoustic admittance (y-axis, in $\text{cm}^3$ or mmho) as air pressure in the external canal is varied across a pressure gradient (x-axis, in decapascals, daPa).

       Admittance (cm³)
          ▲
      2.0 ┼
      1.6 ┼ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─  Normal Compliance Ceiling (1.6)
      1.2 ┼                     ╭─╮
      0.8 ┼                    ╭╯ │╰╮
      0.4 ┼                   ╭╯  │  ╰╮
      0.3 ┼ ─ ─ ─ ─ ─ ─ ─ ─ ─ ╭╯─ ┼ ─ ╰╮─ ─ ─ ─ ─ ─ ─ ─  Normal Compliance Floor (0.3)
        0 ┴───────┬───────────┴───┼────┴──────────┬────► Pressure (daPa)
                -400            -100    0        +50   +200
                                  └─────┬─────┘
                                   Normal Peak Pressure (-100 to +50 daPa)

Four objective physical parameters are extracted from every tympanometric tracing:

1. Tympanometric Peak Pressure (TPP)

• Definition: The canal air pressure (in daPa) at which maximum acoustic admittance occurs (the apex of the tympanometric peak). At this precise pressure point, the air pressure in the external canal exactly equals the air pressure inside the middle ear cavity, maximizing tympanic membrane mobility.
• Normal Range: -100 daPa to +50 daPa in adults (down to -150 daPa in children).
• Diagnostic Significance: Peak pressure outside normal limits indicates Eustachian tube dysfunction.

2. Static Acoustic Admittance / Peak Compliance ($Y_{\text{tm}}$ or $C_s$)

• Definition: The height of the admittance peak above the baseline tail value measured at $+200\text{ daPa}$. Represents the peak mobility of the tympanic membrane and ossicular chain.
• Normal Adult Range: 0.3 to 1.6 cm³ (or mmho).
• Normal Pediatric Range: 0.2 to 0.9 cm³ (or mmho).
• Diagnostic Significance: $< 0.3\text{ cm}^3$ indicates hypomobility/stiffness (otosclerosis); $> 1.6\text{ cm}^3$ indicates hypermobility/flaccidity (ossicular disruption).

3. Equivalent Ear Canal Volume ($V_{\text{ea}}$ or ECV)

• Definition: An estimate of the physical volume of air between the probe tip and the tympanic membrane. It is calculated by introducing $+200\text{ daPa}$ of pressure, which artificially stiffens the tympanic membrane, clamping it shut so that all acoustic admittance measured represents strictly the volume of the external canal.
• Normal Adult Range: 0.6 to 1.5 cm³ (up to $2.0\text{ cm}^3$ in large adult male canals).
• Normal Pediatric Range: 0.3 to 0.9 cm³.
• Diagnostic Significance: Crucial for the differential diagnosis of flat (Type B) tympanograms.

4. Tympanometric Width / Gradient (TW)

• Definition: The width of the tympanogram (in daPa) measured at half the height of the admittance peak.
• Normal Range: 50 to 110 daPa in adults; 60 to 150 daPa in children.
• Diagnostic Significance: Abnormally wide gradients ($>150\text{ daPa}$) suggest early middle ear effusion or resolving otitis media.

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The Jerger Tympanogram Classification System

In 1970, James Jerger introduced the universally recognized classification system categorizing tympanograms into distinct operational profiles:

                   THE JERGER TYMPANOGRAM CLASSIFICATION SYSTEM

         TYPE A (Normal)          TYPE As (Shallow/Stiff)      TYPE Ad (Deep/Flaccid)
            Peak: 0 daPa               Peak: 0 daPa                Peak: 0 daPa
           Comp: 0.8 cm³              Comp: 0.15 cm³               Comp: > 2.0 cm³
                 ▲                          ▲                            ▲
                / \                        / \                          / │
               /   \                      /   \                        /  │
           ───┴─────┴───              ───┴─────┴───                   /   │
                                                                  ───┴────┴───

         TYPE C (Negative)                 TYPE B (Flat - Low / Norm / High ECV)
          Peak: -240 daPa                               No Peak
           Comp: 0.7 cm³                            Comp: 0.0 cm³
                 ▲                          ─────────────────────────────────
                / \
               /   \
           ───┴─────┴───                    ECV determines underlying pathology!

1. Type A Tympanogram (Normal Middle Ear Aeration)

• Pressure: Normal ($-100\text{ to }+50\text{ daPa}$).
• Compliance: Normal ($0.3\text{ to }1.6\text{ cm}^3$).
• Ear Canal Volume: Normal ($0.6\text{ to }1.5\text{ cm}^3$).
• Clinical Interpretation: Normal tympanic membrane mobility, normal ossicular chain dynamics, and intact Eustachian tube function. Observed in normal-hearing individuals and patients with pure sensorineural hearing loss (presbycusis, noise trauma).

2. Type As Tympanogram ("Shallow" / Stiff Middle Ear)

• Pressure: Normal ($-100\text{ to }+50\text{ daPa}$).
• Compliance: Abnormally low ($< 0.3\text{ cm}^3$).
• Ear Canal Volume: Normal ($0.6\text{ to }1.5\text{ cm}^3$).
• Pathophysiology: The middle ear system is properly aerated, but the ossicular chain or tympanic membrane is mechanically stiffened.
• Associated Etiologies:
  • Otosclerosis: Ankylosis or stapes footplate fixation in the oval window.
  • Malleus Head Fixation: Congenital or acquired bony fusion of the malleus.
  • Tympanosclerosis: Extensive calcified hyaline or chalky plaques scarring the tympanic membrane.
  • Resolving Otitis Media: Highly viscous "glue ear" or thickening of middle ear mucosa.

3. Type Ad Tympanogram ("Deep" / Flaccid Middle Ear)

• Pressure: Normal ($-100\text{ to }+50\text{ daPa}$).
• Compliance: Abnormally high ($> 1.6\text{ cm}^3$, often off-scale).
• Ear Canal Volume: Normal ($0.6\text{ to }1.5\text{ cm}^3$).
• Pathophysiology: The middle ear system is hypercompliant and offers little to no mechanical resistance to sound.
• Associated Etiologies:
  • Ossicular Chain Discontinuity: Complete physical disarticulation or fracture of the ossicles (most commonly incudostapedial joint separation secondary to temporal bone trauma, blast injury, or cholesteatoma erosion).
  • Monomeric Tympanic Membrane: A previously perforated eardrum that healed with only an epithelial monolayer, lacking the dense fibrous middle layer, resulting in a hyper-elastic "mirror membrane".

4. Type B Tympanogram (Flat / Non-Mobile)

• Pressure: No identifiable peak.
• Compliance: Flat tracing ($0.0\text{ cm}^3$ or negligible change across pressure sweep).
• Diagnostic Mandate: A Type B tympanogram cannot be interpreted without evaluating the Equivalent Ear Canal Volume ($V_{\text{ea}}$). Ear canal volume provides the critical differential diagnosis:

                               TYPE B (FLAT) TYMPANOGRAM
                                          │
               ┌──────────────────────────┼──────────────────────────┐
               ▼                          ▼                          ▼
        LOW VOLUME (ECV)           NORMAL VOLUME (ECV)        HIGH VOLUME (ECV)
        Adult: < 0.6 cm³           Adult: 0.6 - 1.5 cm³       Adult: > 2.0 cm³
        Child: < 0.3 cm³           Child: 0.3 - 0.9 cm³       Child: > 1.0 cm³
               │                          │                          │
               ▼                          ▼                          ▼
     ┌───────────────────┐      ┌───────────────────┐      ┌───────────────────┐
     │ Probe Occlusion / │      │  Middle Ear Fluid │      │    TM Perforation │
     │ Impacted Cerumen  │      │  (Otitis Media    │      │    or Patent PE   │
     │ Probe on canal wall│     │   with Effusion)  │      │    Tube (Grommet) │
     └───────────────────┘      └───────────────────┘      └───────────────────┘

1. Type B with LOW Ear Canal Volume ($< 0.6\text{ cm}^3$ in adults; $< 0.3\text{ cm}^3$ in children):
   • Physical Mechanism: The probe tone is measuring only the tiny acoustic cavity between the probe tip and an obstruction inside the canal.
   • Etiologies: Probe tip occluded by earwax, probe opening driven against the ear canal wall, or total cerumen impaction.
   • Clinical Action: Remove probe, inspect tip, perform otoscopy, clear cerumen, and re-seat probe.
2. Type B with NORMAL Ear Canal Volume ($0.6\text{ to }1.5\text{ cm}^3$ in adults; $0.3\text{ to }0.9\text{ cm}^3$ in children):
   • Physical Mechanism: The ear canal volume is anatomically correct, but the intact tympanic membrane cannot move because the space behind it is filled with non-compressible liquid rather than air.
   • Etiologies: Otitis Media with Effusion (OME), serous otitis media, acute suppurative otitis media, hemotympanum (blood behind TM), or massive middle ear tumor (cholesteatoma/glomus tumor).
   • Clinical Action: FDA Red Flag medical referral to an otolaryngologist.
3. Type B with HIGH / ABNORMALLY LARGE Ear Canal Volume ($> 2.0\text{ cm}^3$ in adults; $> 1.0\text{ to }1.2\text{ cm}^3$ in children):
   • Physical Mechanism: The tympanic membrane is open. The probe measures the combined volume of the external canal plus the middle ear cavity and mastoid air cells.
   • Etiologies: Tympanic Membrane Perforation (traumatic laceration, chronic perforation) or a patent, functioning Pressure-Equalization (PE) tube (tympanostomy tube / grommet).
   • Clinical Action: If a PE tube is known to be in place, confirms patency. If an unexpected perforation is visualized, medical referral is required.

5. Type C Tympanogram (Significant Negative Pressure)

• Pressure: Abnormally negative ($< -150\text{ to }-200\text{ daPa}$).
• Compliance: Normal ($0.3\text{ to }1.6\text{ cm}^3$).
• Ear Canal Volume: Normal ($0.6\text{ to }1.5\text{ cm}^3$).
• Pathophysiology: The Eustachian tube is obstructed or failing to open during swallowing/yawning. The mucosal blood vessels in the closed middle ear absorb trapped oxygen, creating a negative vacuum that retracts the tympanic membrane inward.
• Associated Etiologies: Eustachian Tube Dysfunction (ETD), upper respiratory viral infection, acute allergic rhinitis, early-stage developing otitis media, or otitic barotrauma (inability to clear ears during aircraft descent or scuba diving).

Summary of Jerger Tympanogram Profiles

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Acoustic Reflex Testing (ART)

Anatomy and Neurophysiology of the Acoustic Reflex Arc

The acoustic stapedial reflex is an involuntary bilateral contraction of the stapedius muscles in response to high-intensity acoustic sound stimulation. The stapedius muscle (the smallest skeletal muscle in the human body, innervated by the stapedial branch of the seventh cranial nerve - Facial Nerve) attaches to the neck of the stapes.

                               THE ACOUSTIC REFLEX ARC

               TEST EAR (Stimulus)                     CONTRALATERAL EAR
               ┌─────────────────┐                    ┌─────────────────┐
               │     Cochlea     │                    │     Cochlea     │
               └────────┬────────┘                    └─────────────────┘
                        │ CN VIII (Afferent)
                        ▼
               ┌─────────────────┐
               │ Ventral Cochlear│
               │  Nucleus (VCN)  │
               └────────┬────────┘
                        │
         ┌──────────────┴──────────────────────────┐
         ▼                                         ▼
┌─────────────────┐                       ┌─────────────────┐
│ Ipsilateral     │                       │ Contralateral   │
│ Superior Olive  │                       │ Superior Olive  │
│     (SOC)       │                       │     (SOC)       │
└────────┬────────┘                       └────────┬────────┘
         │                                         │
         ▼                                         ▼
┌─────────────────┐                       ┌─────────────────┐
│ Facial Motor    │                       │ Facial Motor    │
│  Nucleus (VII)  │                       │  Nucleus (VII)  │
└────────┬────────┘                       └────────┬────────┘
         │ CN VII (Efferent)                       │ CN VII (Efferent)
         ▼                                         ▼
┌─────────────────┐                       ┌─────────────────┐
│ Stapedius Muscle│                       │ Stapedius Muscle│
│ (Test Ear Probe)│                       │  (Opposite Ear) │
└─────────────────┘                       └─────────────────┘
     [UNCROSSED / IPSILATERAL]                 [CROSSED / CONTRALATERAL]

When sound exceeds approximately 85 dB SPL, the neural reflex activates:

1. Afferent Pathway: The acoustic signal travels from the cochlea along the Cranial Nerve VIII (Vestibulocochlear Nerve) to the ventral cochlear nucleus (VCN) in the lower brainstem.
2. Central Interneuronal Crossing: Fibers from the VCN travel to the Superior Olivary Complex (SOC) bilaterally.
3. Efferent Pathway: From the SOC, impulses project to the Facial Motor Nuclei (Cranial Nerve VII) on both the ipsilateral and contralateral sides of the brainstem.
4. Effector Action: Motor axons travel via CN VII to the stapedius muscles. Contraction of the stapedius muscle pulls the stapes posteriorly, stiffening the ossicular chain. This stiffening sharply decreases acoustic admittance at the tympanic membrane, which is detected and measured by the probe microphone.

Ipsilateral vs. Contralateral Reflex Protocols

• Ipsilateral (Uncrossed) Reflex: The activating sound stimulus and the measuring probe are seated in the same ear.
• Contralateral (Crossed) Reflex: The activating sound stimulus is delivered to one ear via an earphone, while the measuring probe monitors compliance in the opposite ear.

Testing both ipsilateral and contralateral configurations across both ears allows the clinician to pinpoint lesions within the peripheral cochlea, 8th nerve, facial nerve, or lower brainstem (e.g., intra-axial brainstem tumors or multiple sclerosis plaques affecting the crossing trapezoid body fibers).

Normative Acoustic Reflex Thresholds

The Acoustic Reflex Threshold (ART) is defined as the lowest stimulus level (in dB HL) that elicits an observable decrease in middle ear admittance (a deflection $\ge 0.02\text{ mmho}$ or $\text{cm}^3$).

• Normal Range: 70 to 100 dB HL (typically 70 to 95 dB SL above the pure-tone threshold) at 500, 1000, and 2000 Hz.

Diagnostic Reflex Patterns Across Pathologies

1. Conductive Hearing Loss

• Probe in the Affected Ear: Reflexes are completely absent whenever the probe is in an ear with conductive pathology (even with an air-bone gap as small as 5 to 10 dB). Fluid, ossicular discontinuity, or otosclerosis mechanically immobilizes the ossicles, preventing the probe from detecting admittance changes.
• Stimulus in the Affected Ear (Contralateral Probe in Normal Ear): Reflexes are elevated by the degree of the air-bone gap or absent if the stimulus intensity cannot overcome the conductive attenuation.

2. Cochlear (Sensory) Hearing Loss & Loudness Recruitment (The Metz Test)

• In ears with sensory hearing loss secondary to outer hair cell destruction, the acoustic reflex threshold remains relatively constant between 75 and 95 dB HL despite pure-tone threshold elevation.
• Reduced Sensation Level: When pure-tone thresholds are elevated (e.g., $60\text{ dB HL}$) and the reflex threshold occurs at $85\text{ dB HL}$, the reflex is elicited at a sensation level of only $25\text{ dB SL}$ ($85 - 60 = 25\text{ dB SL}$). An acoustic reflex elicited at a sensation level $< 60\text{ dB SL}$ is diagnostic of abnormal loudness recruitment (The Metz Test), confirming a cochlear (sensory) site of lesion.

3. Retrocochlear Lesions (Acoustic Neuroma / CN VIII Pathology)

• Cranial nerve VIII lesions disrupt neural transmission. Consequently, acoustic reflexes are elevated ($> 100-105\text{ dB HL}$) or completely absent when the stimulus is presented to the affected ear, even when pure-tone thresholds are completely normal or only mildly elevated.

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Acoustic Reflex Decay Testing

Principle of Neural Adaptation vs. Pathological Decay

In a healthy ear, when a high-intensity activating tone is sustained continuously over time, the stapedius muscle maintains its tonic contraction with minimal adaptation. However, when the eighth cranial nerve is compressed or demyelinated by a space-occupying tumor (vestibular schwannoma), the nerve fibers suffer from severe abnormal auditory adaptation and neural exhaustion, failing to maintain the continuous neural firing required to sustain muscle contraction.

                          ACOUSTIC REFLEX DECAY RECORDING

       Stimulus: Continuous tone at ART + 10 dB for 10 seconds

       Admittance
       Deflection
       Initial 100% ┼────────╮
       Peak         │        │
                    │        │
                    │        ╰──────────╮ (Negative Decay: Normal Maintenance)
        50% Line ───┼───────────────────┼─────────────────────────────────────
                    │                   │   ╰──╮
                    │                   │      ╰────── (Positive Decay: Abnormal
                    │                   │               Drops >50% within 5 sec)
          0% Baseline ┴───────────────────┴──────────────────────────────► Time
                    0                   5                             10 sec

Clinical Test Protocol

1. Frequency Selection: Reflex decay is conducted at 500 Hz or 1000 Hz.
   • Critical Rule: 2000 Hz and 4000 Hz are never used for reflex decay testing. Normal ears frequently exhibit natural physiological adaptation at 2000 and 4000 Hz, producing false-positive decay.
2. Stimulus Level: The tone is presented at 10 dB above the Acoustic Reflex Threshold ($\text{ART} + 10\text{ dB}$).
3. Duration: The continuous tone is sustained for precisely 10 seconds.

Diagnostic Criteria

• Normal (Negative Reflex Decay): The reflex amplitude is maintained throughout the stimulation window, or decays by less than 50% over the 10-second period.
• Abnormal (Positive Reflex Decay): The amplitude of the acoustic reflex decreases by 50% or more within the first 5 seconds of the 10-second stimulus window.

[!CAUTION] A positive acoustic reflex decay is a cardinal diagnostic sign of retrocochlear pathology (vestibular schwannoma / cerebellopontine angle lesion). It demands immediate cessation of hearing aid dispensing procedures and an urgent medical referral for neuro-otologic evaluation and contrast-enhanced MRI.