Acoustic Reflex Testing
۲۰ اسفند ۱۳۹۶ تستها و ارزیابیهای شنوایی و تعادلی
Acoustic Reflex Testing
Acoustic Reflex (AR) testing utilizes Tympanometry in a unique way. Instead of stiffening the middle ear system with positive or negative air pressures, AR testing stiffens the middle ear system with loud, low-frequency pure tones. When the loud tone causes an AR, the result is a temporary decrease in middle ear compliance. The AR is read as a decrease in static compliance. One could think of continuing education … cont’d. Figure . Physical Volume (PV) of ear canal is normally between 1.0 to 1.5 cc. A large PV might indicate a perforated TM. True Type BTympanogram has normal PV. If Type B with tiny PV, then probe tip is against Outer ear canal wall. Figure 8. Physical Volume (PV) of ear canal is normally between 1.0 to 1.5 cc. A large PV might indicate a perforated TM. True Type B Tympanogram has normal PV. If Type B with tiny PV, then probe tip is against Outer ear canal wall.
Figure. Physical Volume (PV) of ear canal is normally between 1.0 to 1.5 cc. A large PV might indicate a perforated TM. True Type B Tympanogram has normal PV. If Type B with tiny PV, then probe tip is against Outer ear canal wall.
the AR as causing a temporary decrease in the height of the Tympanogram. The AR test can be best appreciated with an understanding of the AR itself, and its anatomy and physiology. As an arc, the AR has a loop or circuitous route, with an ear-to-Brain Stem going (afferent) section and a Brain Stem back-to-ear(efferent) section. If a loud (85 to 110 dBHL) low-frequency sound hits the TM, the normal reaction is to have an AR. What
is the AR? It is a reflex which is always an involuntary reaction to something. In the case of an AR, a loud low frequency sound causes the reaction of two middle ear muscles that pull on the ossicles. The smaller but stronger of the two muscles is the stapedius. It pulls outward on the neck of the stapes to keep it from going in and out of the oval window. The weaker and yet larger of the two is the tensor tympani. It pulls inward on the malleus to reduce
the vibration of the TM. The AR thus works to momentarily tense the whole middle ear system. For a split second, the AR thus renders the middle ear more stiff (less compliant and thus less efficient) at
conducting its mechanical energy to the cochlea.The AR involves nearly all parts of the ear; namely, the outer, middle, inner, VIII nerve, and brain stem. Note that three cranial nerves are involved in the AR: the V, VII, and VIII. As we know, the VIII nerve is a sensory afferent nerve, sending neural information of sound to the brain. It takes cochlear information from the afferent Inner Hair Cells (IHCs)
and sends this information to the cochlear nucleus of the brain stem. From there, neural information goes to the Superior Olivary Complex (SOC) of the same side (ipsilateral) and also to the opposite SOC (contralateral). This crossover is called “decussation,” and it explains why a loud sound to one ear normally causes an AR to occur in both ears. From the brain stem SOC’s, an efferent message is sent to the V and VII cranial nerves. The V nerve is
partially sensory (afferent) for feeling in the face, and partly motor (efferent) for activating muscles, one of them being the Tensor Tympani. The VII nerve is a totally efferent motor nerve activating the cheek muscles as well as the stapedius muscle.
Incidentally, Bell’s palsy is a compromise of the VII nerve. At any rate, this whole afferent/efferent loop is known as the AR arc.
The AR is a low-frequency phenomenon, which helps to explain why we have ARs in the first place. The AR is elicited or caused by loud low-frequency tones, such as 500 or 1000 Hz. Many clinicians believe that the AR works as a natural protection against loud sounds and that it helps to reduce noise induced hearing loss. Actually, the AR helps to reduce what is known as the “upward spread of masking.” Low frequencies mask high frequencies better than highs mask lows.
This is why background noise which is mostly low in frequency content, serves as an unfortunately effective masker for the high-frequency consonants of speech.
Have you ever noticed when you hear a recording of your own voice, you are the only one who thinks you sound so weird?
Others however, think the recording sounds just fine. This is because when you hear yourself in a recording, you
hear yourself in the way that others hear you. While you talk, you hear yourself by air conduction and also by bone conduction. Others hear you only by way of air conduction. The intensity of normal ongoing speech by air conduction is about 65 dB SPL. You hear the intensity of your own air plus bone-conducted voice however, closer to 85 dB SPL, and this is enough to cause an AR. The vowels of speech are the loudest, and these are mainly low in frequency. We basically have AR’s to help reduce the upward spread of masking from the vowel sound of our own voices.