Acoustic Immittance MeasurementsOzarks Technical Community College

HIS 125

Acoustic Immittance Measurements

Objective measurements of acoustic impedance and acoustic admittance◦Impedance = opposition to flow of sound

through auditory system◦Admittance = ease with which sound flows

through the auditory system

Consist of the following:◦Tympanometry◦Acoustic reflex thresholds◦Acoustic reflex decay

Tympanometry

Objective measure of middle ear function◦ determines the amount of energy transmitted by the middle

ear system Procedure

◦ A probe is placed in the ear canal consisting of three ports: a loudspeaker, manometer pressure pump, and microphone

◦ A 226 Hz tone is introduced by the loudspeaker while the manometer pressure pump automatically and slowly varies the pressure in the ear canal from +200 to -400 daPa (decapascals).

◦ In the meantime, the microphone measures the change in intensity (in dB SPL) in the ear canal as the pressure is varied. As admittance decreases (i.e., the eardrum is stiffer and more

sound is reflected off the tympanic membrane), the measured SPL increases. As admittance increases (i.e., the eardrum is more compliant and less sound is reflected off the tympanic membrane), the measured SPL decreases.

Normal Tympanogram

Admittance decreases, Reflected SPL increases

Admittance increases, Reflected SPL decreases

Tympanometry

Three outcome measures:static admittance (SA) or maximum

compliance= point along the tympanogram where the amount of reflected SPL is the least

ear canal volume (ECV)=physical space between the end of the probe assembly and the tympanic membrane

middle ear pressure (MEP)= pressure along the x-axis of the tympanogram where the eardrum is the most compliant or where pressure is equal on both sides of the tympanic membrane

Parameter Normal Range

Ear Canal Volume 0.65-1.75 mL or cm3

Middle Ear Pressure - 100 to + 50 daPa

Static Admittance 0.3-1.4 mL

Normal Tympanometry Values for Adults

Abnormal Tympanograms

Any middle ear abnormality will result in some abnormality on the tympanogram

Tympanograms are classified by type:◦ Type A = all 3 outcome measures are normal ◦ Type B = no static admittance, the TM is immobile;

normal ECV suggestive of middle ear fluid, while an enlarged ECV suggests a TM perforation or patent tube

◦ Type C = negative MEP (i.e. eustachian tube dysfunction)◦ Type AS= stiff/hypocompliant; SA is reduced (i.e.

otosclerosis)◦ Type AD = flaccid/hypercompliant; SA is greater than

normal (i.e. ossicular discontinuity)

Abnormal Tympanograms

Type B=flat*Note: based on the enlarged ECV, this patient has a patent tube or a hole in the TM

Type C=negative pressure

Abnormal Tympanograms

Type As=stiff, hypocompliant

Type AD=flaccid, hypercompliant

Other use of tympanometry

If otoscopy reveals significant cerumen accumulation, a tympanogram will allow you to determine if an accurate audiogram can be obtained.◦If tympanometry is normal in the presence of

excess cerumen, then there is enough of an opening in the ear canal for accurate hearing testing to occur.

The Acoustic Reflex

The introduction of a loud sound to the ear canal of either ear results in the acoustic reflex, or the contraction of the stapedius muscle, which causes the tympanic membrane to stiffen.◦This results in a change in middle ear

immittance, which is measured as an increase in dB SPL by the microphone in the probe assembly.

Acoustic Reflex Thresholds

Acoustic reflex thresholds (ARTs) = the lowest level in dB HL at which the acoustic reflex can be elicited (deflection ≥0.02 ml)◦Uses the same bracketing technique that is used in

obtaining puretone thresholds ◦Measured at 500, 1000, 2000, and 4000 Hz◦Useful in determining site-of-lesion

Directly evaluates middle ear status and indirectly evaluates cochlear and retrocochlear status

◦ARTs can be measured ipsilaterally (stimulus is in the probe ear) and contralaterally (stimulus is in the non-probe ear) via a supra-aural or insert earphone

Here is a graph of acoustic reflex testing at 500, with threshold being established at 85 dBHL

Interpretation of Acoustic Reflexes

Hearing Status Expected Acoustic Reflex Threshold (dB HL)

Normal 70-100 dB HL

Conductive Loss Elevated or Absent

Cochlear Loss Normal sensation level (SL) or reduced SL

Neural Loss Normal SL, elevated, or absent

Acoustic Reflex Pathway

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reflex pathway, the measurement of ARTs not only provides information regarding the status of the middle ear system, but also of the inner ear, auditory nerve, regions of the lower auditory brainstem, and the facial nerve.

Acoustic Reflex Decay

Acoustic reflex decay is a measure of the sustainability of the acoustic reflex, or how long the stapedius muscle can remain contracted during continuous stimulation.

Procedure:◦ Reflex decay is measured with contralateral stimulation at 500 and

1000 Hz at 10 dB SL (re: contralateral acoustic reflex threshold). ◦ The tone is presented continuously for 10 seconds and the patient is

instructed to remain as still as possible. Outcomes:

◦ Negative reflex decay indicates that the magnitude of the stapedial reflex contraction did not decrease by ≥50% in the first five seconds of testing

◦ Positive reflex decay indicates that the magnitude of the stapedial reflex contraction decreased by ≥50% in the first five seconds of testing Positive acoustic reflex decay is highly suggestive of a retrocochlear

pathology and further evaluation by an otologist is strongly recommended.

Acoustic Reflex Decay

Negative (upper) and positive (lower) acoustic reflex decay at 1000 Hz.

In positive decay, the stapedius contracts initially, but then “lets go”