Welch Allyn TM 262 Auto Tymp - User Manual - Manual (Page 14)

TM 262
1-6 Revised 4/11/08
The presence of a pathological condition which interferes with the mobility of the tympanic
membrane, the ossicular chain, or the air pressure within the middle-ear space can be detected
during tympanometry. For example:
• If the air pressure within the middle-ear space becomes negative due to a blocked
eustachian tube, tympanometry permits the measurement of this negative pressure and its
effect on middle-ear compliance.
• 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.
• In the case of a “glue-ear”, the ossicular chain is restricted in mobility but the air pressure
within the middle-ear space is at atmospheric pressure. This tympanogram would depict a
restricted compliance peak at or near 0 daPa.
Gradient
Gradient (width) measurements are used to describe the shape of a tympanogram in the vicinity
of 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 in 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 tymp and ear canal volume measurements by helping to
differentiate between tymps with similar peak values.
The instrument uses tympanometric width to determine the gradient by measuring the pressure
interval at one-half of the tymp peak height. Differing tymp peak widths can point to different
middle-ear conditions, even when peak height and pressure are within normal range. For
example, middle-ear effusion brought on by secretory otitis media might 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.