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If fluid builds up within the middle-ear space, this fluid will restrict the ability of the ossicular
chain to conduct sound to the cochlea. If small air pockets exist within the fluid, the
tympanogram will indicate the negative pressure where the restricted mobility occurs. With
a totally fluid-filled middle-ear space, no mobility will be measured during tympanometry at
any pressure value.
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In the case of a “glue
-
ear”, the ossicular chain is restricted in mobility. This tympanogram
would depict a flat line with no identifiable pressure peak.
Gradient
Gradient (width) measurements are used to describe the shape of a tympanogram near the peak.
Often, the presence or absence of fluid in the middle ear is not clearly indicated by otoscopy and
tympanometry alone. This evaluation is especially difficult when the peak pressure is within the
normal range.
The presence of fluid within the middle-ear space alters the shape of a tympanogram (i.e., makes the
tympanogram wider near its peak). A larger-than-normal gradient can indicate the presence of fluid
in the middle ear when other parameters are within normal limits. In this way, the gradient acts as an
adjunct to the tympanogram and ear canal volume 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 GSI 39, gradient measures are only 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)
or through an insert phone (contralateral). 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).