3.3 Immittance Audiometry & Middle Ear Assessment

Immittance Audiometry & Middle Ear Assessment

Principles of Immittance Testing

While pure-tone and speech audiometry require subjective behavioral responses from the patient, immittance audiometry is an objective measure of middle ear function. It does not measure hearing sensitivity directly; rather, it assesses the acoustic integrity and mechanical properties of the tympanic membrane and the middle ear space. The term "immittance" encompasses both impedance (opposition to energy flow) and admittance (ease of energy flow, also known as compliance).

An immittance probe is sealed into the ear canal, creating a closed cavity. The probe houses three components: a speaker to emit a continuous 226 Hz probe tone, a microphone to measure the sound pressure level remaining in the ear canal, and an air pump to vary the air pressure against the eardrum. By sweeping the pressure from positive (+200 daPa) to negative (-400 daPa), the equipment measures how well the middle ear transmits acoustic energy. When the air pressure in the ear canal exactly matches the pressure in the middle ear cavity, the tympanic membrane is at its most compliant, and acoustic energy flows most efficiently through the ossicular chain.

Three primary measurements are obtained during tympanometry:

1. Compliance (Admittance): The peak mobility of the eardrum.
2. Peak Pressure: The middle ear pressure at which maximum compliance occurs.
3. Ear Canal Volume (ECV): An estimate of the physical volume of the air space between the probe tip and the eardrum.

Jerger Tympanogram Classification

The results of the pressure sweep are plotted on a graph called a tympanogram. The most widely used classification system for these graphs is the Jerger Tympanogram Classification, which categorizes results into distinct types based on their shape.

Type A (Normal): Shows a distinct peak around 0 daPa (normal middle ear pressure) and normal compliance. This indicates normal middle ear function. Type As (Shallow): Shows a normal peak pressure but abnormally low compliance (a stiff system). The "s" stands for shallow or stiff. This is commonly associated with conditions that restrict ossicular movement, such as otosclerosis or advanced tympanosclerosis. Type Ad (Deep): Shows a normal peak pressure but unusually high compliance (a hypermobile system). The "d" stands for deep or discontinuous. This occurs when the eardrum is excessively flaccid, which can result from aging, a healed perforation (monomeric membrane), or ossicular discontinuity (a break in the ossicular chain).

Type B (Flat): Displays no clear peak at any pressure; compliance remains flat. A Type B tympanogram indicates that the middle ear system is not responding to pressure changes. To determine the cause, the clinician must look at the Ear Canal Volume (ECV):

• Type B with Normal ECV: Suggests fluid behind the eardrum (otitis media with effusion). The fluid physically blocks the movement of the tympanic membrane.
• Type B with Large ECV: Indicates that the probe is measuring a larger space than just the ear canal. This happens if there is a tympanic membrane perforation or a functioning patent PE (pressure equalization) tube. The sound is traveling into the middle ear space, artificially increasing the volume reading.
• Type B with Small ECV: Typically indicates an occlusion in the ear canal, such as impacted cerumen, or that the probe tip is pressed against the canal wall.

Type C (Negative Pressure): Shows a distinct peak, but it is shifted significantly to the negative side (typically <-100 daPa). This indicates negative pressure within the middle ear cavity, which is the hallmark of Eustachian tube dysfunction. The Eustachian tube is failing to equalize middle ear pressure with atmospheric pressure, often a precursor or resolving phase of otitis media.

Acoustic Reflex Thresholds (ART)

Beyond tympanometry, immittance equipment can measure the Acoustic Reflex. The acoustic reflex is an involuntary bilateral contraction of the stapedius muscle in the middle ear in response to a loud sound. This contraction stiffens the ossicular chain, momentarily decreasing the admittance of the middle ear system.

The Acoustic Reflex Threshold (ART) is the lowest intensity level that elicits this measurable contraction. Normal ARTs occur between 70 and 100 dB HL. Because the reflex is a bilateral arc, presenting a loud sound to one ear causes muscles in both ears to contract. This allows for two types of measurements:

• Ipsilateral: The stimulus is presented, and the reflex is measured in the same ear.
• Contralateral: The stimulus is presented to one ear, but the reflex is measured in the opposite ear. Analyzing the pattern of ipsilateral and contralateral reflexes across both ears helps localize pathologies along the auditory pathway, from the middle ear to the auditory nerve (VIIIth cranial nerve) and the lower brainstem (superior olivary complex and VIIth cranial nerve).

Acoustic Reflex Decay

An extension of ART testing is Acoustic Reflex Decay. In a healthy neural system, the stapedius muscle can maintain its contraction for at least 10 seconds when presented with a continuous loud tone (typically at 10 dB above the ART at 500 or 1000 Hz). If the reflex amplitude drops by more than 50% within those 10 seconds, the result is considered positive for reflex decay. A positive reflex decay is a strong retrocochlear indicator, suggesting a lesion or tumor (like an acoustic neuroma) on the auditory nerve that prevents it from sustaining continuous neural firing.