۲۲ اسفند ۱۳۹۶ تستها و ارزیابیهای شنوایی و تعادلی
Tympanometry is a non-behavioral test of middle ear function, which means it requires no voluntary response on the part of the client being tested. It can be routinely utilized by the HIS clinician in private practice, and should become a regular part of a client test battery. As health care professionals know, one cannot base conclusions on one single test. As math teachers always say, “It takes two dots to make a line.” In our field, air-bone gaps seen in Pure Tone Testing can be backed up by a quick, five-minute assessment of Tympanometry.
The purpose of this article is to describe the principles behind commonly used Tympanometry, how it is done, and how to interpret the results. The general thrust here is to familiarize clinical practitioners, in the clearest way possible, with generally known and widely accepted procedures of Tympanometry.
inner ear is filled with fluids called perilymph and endolymph. Perilymph, which fills the outer two boney labyrinths
is similar to the fluid that surrounds the brain; namely, cerebral-spinal fluid. The inner membranous labyrinth is filled
with endolymph, which has the opposite chemical composition. The job of the cochlea is to transduce fluid motion
energy into electrical energy, because this is the “language” the brain understands.
Outer and Middle Ear Resonances and Speech
shows that our outer and middle ears actually improve our hearing for the high-frequency consonants of speech. The middle ear ossicles resonate 24 best at around 2000 Hz, and the middle ear space has two other resonances of 750-900 Hz and 1200 Hz. The outer ear canal resonance falls roughly between 1500 and 4000 Hz. Together, the outer and middle ears thus serve to create the human hearing sensitivity curve, which shows our very best hearing sensitivity to be between 1000-4000 Hz. This all contributes to better hearing for speech.
Tympanometry and the Middle Ear
Tympanometry involves the use of a probe inserted into the ear canal with a tight seal, so that no air can leak out. The assumption behind Tympanometry is that in order for the middle ear to be most efficient atpassing incoming sounds through it, air pressure should be even on both sides of the TM. Contrary to common belief, Tympanometry does not determine “how much the eardrum wiggles.” The probe has three holes in it to provide: 1) a tiny speaker, 2) a tiny microphone, and 3) a way to change air pressure. The client can feel these air pressure changes during
the test. During the air pressure changes, a steady low-frequency tone at 70 dB sound pressure level (SPL) is presented through the probe speaker, and the probe microphone picks up whatever sound bounces back off from the TM. If the least amount of sound bounces back off the TM when the air pressure in the outer ear canal is at regular room air pressure, this means. the air pressure behind the TM is the same. In this way, Tympanometry measurement in the outer ear canal tells us about the middle ear air pressure behind the TM!
Why Does Tympanometry Typically Use a Low-Frequency Tone?
With Tympanometry, we test the com-pliance of the middle ear by measuring the amount of low-frequency tone reflecting off the stiff middle ear as a function of air pressure changes. Compliance is the opposite or inverse of stiffness. Tympanometry uses a low-frequency tone because the middle ear is a “stiffness dominated system.” The middle ear system, which involves the TM and ossicular chain, is always stiff, but it is least stiff when
the air pressure is even on both sides of the TM. The middle ear ossicles are tiny and therefore, do not have much mass. Stiffness is therefore the main source of opposition to the passage of sound through the middle ear. Stiffness opposes the passage of low frequencies and resonates with high frequencies, while mass opposes the passage of high frequencies and resonates with low frequencies. A low- frequency tone is used so that some sound will bounce off from the TM, even when the middle ear is least stiff. If it didn’t, the sound would pass through the TM and there would be nothing left for us to measure! Now consider the normal situation, when the air pressures inside the outer ear canal and the middle ear space are both at regular room air pressure. When the low- frequency Hz tone is presented at 70 dB SPL, some of it will pass through the stiff middle ear system, but because the middle ear is a stiffness dominated system, some of it will bounce back off the TM. With positive or negative air pressure in the outer ear canal however, the air pressure is made to be different from that inside the middle ear space, and this makes the normally stiff middle ear system become stiffer yet. In these situations, even more sound bounces off the TM and less goes through it. In other words, with uneven air pressure on both sides of the TM, the Middle ear is made temporarily more stiff than it usually is and therefore, less efficient. Consequently, more of the low- frequency sound bounces off the TM!