Welch Allyn TM 262 Auto Tymp - User Manual - Manual (Page 15)

Revised 4/11/08 1-7
Introduction
Screening acoustic reflex
An acoustic reflex occurs when a very loud sound (stimulus) is presented to the auditory
pathway. During acoustic reflex testing, the stimulus is presented to the ear canal through a
probe (ipsilateral). This stimulus then travels through the middle ear to the cochlea. From the
cochlea, frequency and intensity information is transmitted via the 8th nerve to the brain stem
where a determination is made as to whether or not the intensity of the stimulus is high enough
to elicit the reflex response. If it is, a bilateral response occurs i.e., the right and left 7th nerves
innervate their respective middle-ear muscles (stapedial muscles) causing them to contract. As
these muscles contract, they stiffen their respective ossicular chains. This stiffening of the
ossicular chain reduces the compliance of each middle-ear system. As in tympanometry, a
probe tone is used to measure this decrease in compliance.
During ipsilateral acoustic reflex testing, both the stimulus and the probe tone are presented via
the hand-held probe. For best results, the air pressure within the ear canal where the probe is
positioned is set to the pressure value measured at the point of maximum compliance for that
ear during tympanometry with an offset of -20 daPa.
Acoustic reflex measurements are useful to determine the integrity of the neuronal pathway
involving the 8th nerve, brainstem, and the 7th nerve. Since the acoustic reflex test is
performed at high intensity levels and since it involves a measurement of middle-ear mobility,
acoustic reflex testing is not a test of hearing.
The acoustic reflex also serves as a good validation of tympanometric results since an acoustic
reflex cannot be measured in the absence of a compliance peak. In other words, if the
tympanometric results indicate no mobility over the pressure range available with your
instrument, no reflex can be measured. If the test results indicate a reflex response in the
absence of a compliance peak, one has cause to question the validity of the tympanometric test
results. This indicates that the tympanogram should be repeated.
Clinical middle-ear instruments allow the measurement of the acoustic reflex threshold since
they provide the ability to manually change the intensity of the stimulus to a level where a
reflex response is just barely detectable for each patient tested. However, the instrument
automatically presents the stimulus in a very definite stimulus intensity sequence. This preset
intensity sequence may start at a level above an individual’s acoustic reflex threshold level.
Also, since the instrument uses a hand-held probe and noise from hand motion can be detected
by the instruments circuitry, the magnitude of a detectable response must be somewhat higher
than the criterion generally used during clinical acoustic reflex threshold testing to avoid
artifact caused by hand motion. Thus, the acoustic reflex measurements made with the
instrument are referred to as screening acoustic reflex testing. The purpose of these screening
reflex tests is to determine whether a reflex is detectable rather than to determine the lowest
intensity at which the reflex occurs (i.e., threshold testing).