An acoustic reflex is an involuntary reflexive action that occurs in the middle ear of all mammals. This involuntary movement or muscle contraction that occurs inside the middle ear is also known as the attenuation reflex, stapedius reflex or auditory reflex. The contraction of the muscle occurs typically as a response to high intensity sound. Any other stimulus doesn’t cause such a reflex.
There are two muscles, namely the stapedius and the tensor tympani in the ossicles. When a high intensity stimulus is received by the ear, both these muscles contract involuntarily, pulling the stirrup away from the cochlea and the hammer away from the eardrum. This is a survival reflex and occurs in order to protect the ear from high intensity sounds. This reflex action automatically decreases the intensity of the vibration caused by the high intensity sound so that they wouldn’t get converted into electrical impulses.
In normal scenarios, the acoustic reflex only occurs as a response to high intensity sounds, if it gets activated even because of quieter sounds, it is an indication of a dysfunction of the ear, neural hearing loss, or a decay of the acoustic reflexes.
Right from the 1970s, acoustic reflex decay was considered to be caused due to the presence of acoustic tumors. However, when researched, it was found that in the presence of acoustic tumors, acoustic reflexes are either normal or completely absent. A decay of the reflex most likely signifies that a tumor is not present.
An acoustic reflex decay test is usually ordered when there is a suspicion of decay of the acoustic reflex. To administer the test, a high intensity stimulus is presented to the ear, just contralateral to the tip of the probe. The stimulus is given in the form of a pure tone with a frequency of at least 10dB above the range of the acoustic reflex. This tone is played for about 10 seconds as intense stimulation can cause damage to the ear canal. A stimulus above the range of 105 dB should not be used as it can cause permanent damage to the ear drum.
If the reflex is functioning normally, the muscles stay contracted for the full ten seconds that the tone is played. Though based on the same principle, there are separate tests for measuring the threshold and latency of the acoustic reflex. The technique of administering may change a little, depending on what you are trying to measure