Welch Allyn TM286 Auto Tymp - User Manual - Manual (Page 10)

6 Introduction Welch Allyn TM286 Auto Tymp
measurements by helping to differentiate between tympanograms with similar peak
values.
The instrument uses tympanometric width to determine the gradient by measuring the
pressure interval at one-half of the tympanogram peak height. Differing tympanogram
peak widths can point to different middle-ear conditions, even when peak height and
pressure are within normal range. For example, middle-ear effusion caused by secretory
otitis media many result in an increased tympanogram width and, therefore, an increased
gradient value. This would occur because the ossicular chain cannot react to the change in
pressure introduced during the tympanogram in the same way that it would if the middle
ear were properly aerated. The continued presence of effusion, leading eventually to a
completely fluid filled middle-ear cavity, will reduce the magnitude of the tympanogram to
the point where no change in compliance is detectable across the pressure range. Under
this condition, no gradient measurement is possible.
On the TM286, gradient measures are calculated for the 226 Hz probe tone conditions.
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 a 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.
When the stimulus is presented to the same ear as the measurement, the test is referred
to as an ipsilateral (same side) acoustic reflex test.
During ipsilateral acoustic reflex testing, both the stimulus and the probe tone are
presented via the hand-held probe. In both cases, the measurement is made from the ear
where the probe is positioned. For 226 Hz probe tone reflex measurements, 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 (or +20 daPa for a positive pressure peak).
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, no
reflex will be observed. 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, this
screening instrument automatically presents the stimulus in a very definite stimulus
intensity sequence. This preset intensity sequence may start at a level above an