Operating Instructions MI 24, MI 24h
2.2 Acoustic Reflex
An acoustic reflex, or contraction of the Stapedial muscle, occurs under normal
conditions when a sufficiently intense sound is presented to the auditory pathway.
This contraction of the muscle causes a stiffening of the ossicular chain which
changes the compliance of the middle ear system. As in tympanometry, a probe tone
is used to measure this change in compliance.
When the stimulus presentation and measurement are made in the same ear by
means of the probe, this acoustical reflex is referred to as an ipsilateral acoustic
reflex. When the stimulus presentation and measurement are made in opposite ears,
the reflex is referred to as a contralateral acoustic reflex.
For best results, this reflex measurement is automatically conducted at the air
pressure value where the compliance peak occurred during the tympanometric test.
Stimulus tones of varying intensities at 500, 1000, 2000 or 4000 Hz are presented as
short bursts. If a change in compliance greater than 0.05 ml is detected, a reflex is
considered present. Because this is an extremely small compliance change, any
movement of the probe during the test may produce an artifact (false response). The
test result is recorded as Pass/No Response
,
and in graphical form.
If the tympanometric results display any abnormal findings, the results of the acoustic
reflex testing may be inconclusive and should be interpreted with care. Theoretically,
a compliance peak is necessary to observe a reflex at peak pressure.
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