Classification of Hearing Loss

Abstract

The acoustic reflex, also known as the stapedius reflex refers to an involuntary muscle contraction of the stapedius muscle in response to a high-intensity sound stimulus. Due to ease of administration and information yielded, the acoustic reflex is considered one of the most powerful differential diagnostic audiological procedures. The acoustic reflex and the tympanogram (see tympanometry chapter) are commonly used to assess middle ear function and are considered best practice. Each test yields invaluable information based on the delivery of acoustic energy (sound) to the ear canal. Thanks to microprocessors, the instruments used are now capable of rapid middle ear function assessment (typically <1min/ear). Acoustic reflexes do not measure hearing threshold. Rather, they measure reflected energy which is a function of stapedius muscle contraction; it allows one to indirectly assess the middle ear, cochlea and neural innervation of the stapedius muscle. As the acoustic reflex is involuntary and bilateral, it is replicable and provides valuable diagnostic information when comparing the amount of reflected energy according to signal intensity, as well as the presence of ipsilateral and bilateral acoustic reflexes. Premise behind instrumentation An acoustic stimulus is presented to the external auditory canal; energy is transferred from the ear canal through the middle ear, some of which is reflected and some absorbed by the tympanic membrane and external ear structures. The premise behind the immittance instrumentation used to measure the acoustic reflex is the ability to accurately measure changes in reflected energy occurring from stiffening of the tympanic membrane as a result of contraction of the stapedius muscle (For the premise behind tympanometry see tympanometry chapter). The amount of reflected and absorbed energy varies depending on the admit-tance/impedance (flow/resistance) of energy within the system and the complex interaction between the ear structures. The presence of reflexes within normal intensity limits is consistent with normal middle ear and brainstem function and suggests that auditory sensitivity is not significantly impaired. An elevated or absent acoustic reflex threshold is consistent with a middle ear disorder, hearing loss in the stimulated ear, and/or interruption of neural innervation of the stapedius muscle.