Acoustic Reflex Decay
Acoustic Reflex Decay of a neuroma
Purpose
Acoustic reflex decay is an objective measure used to assess retrocochlear function with immittance measurement methods. Abnormal decay can be a sign of retrocochlear dysfunction.
The acoustic stapedial reflex occurs as a result of auditory stimulation. The reflex typically occurs for the duration of the stimulus. As the stimulus is presented continuously over time, however, the stapedius muscle contraction may diminish as the auditory system adapts to the stimulus. This phenomenon is known as acoustic reflex decay.
The acoustic stapedial reflex occurs as a result of auditory stimulation. The reflex typically occurs for the duration of the stimulus. As the stimulus is presented continuously over time, however, the stapedius muscle contraction may diminish as the auditory system adapts to the stimulus. This phenomenon is known as acoustic reflex decay.
Materials
Tympanometer with Acoustic Reflex Decay
Procedures/Instruction
Acoustic reflex decay can be inferred using acoustic immittance measures similar to those used to observe acoustic reflex thresholds. In clinical situations, instead of the short-duration stimulus used in acoustic reflex threshold measures, a stimulus of about 10 seconds is presented. This stimulus is presented at 10 dB above the acoustic reflex threshold level to observe a robust acoustic reflex response. The stimuli used are 500 and 1000 Hz tones. Typically, the stimulus is presented to the ear contralateral to the ear with the probe. The resulting change in admittance of sound is observed at the onset of the stimulus, over the duration of the stimulus presentation, and at the stimulus offset. Under normal circumstances, the acoustic reflex decay will be less than half of the maximum change in admittance over the 10-second recording interval. In cases of abnormal acoustic reflex decay, the decay will be greater than half the maximum change in admittance during the course of the stimulus presentation.
Interpretation/Site of Lesion
Abnormal acoustic reflex decay raises suspicion of retrochochlear pathology that causes the auditory system to adapt too readily to acoustic stimulation. A tumor of the VIIIth cranial nerve is an example of a lesion that can cause abnormal acoustic reflex decay. In many cases of retrocochlear dysfunction, acoustic reflex thresholds will be absent at equipment limits or will be too high to present the stimulus tone. In these cases, acoustic reflex decay testing cannot be performed. However, when acoustic reflex thresholds are at a level at which acoustic reflex decay testing can be performed at +10 dB SPL (re: the acoustic reflex threshold level), then this testing can be a useful addition to the diagnostic picture.
Management
Works Cited
Gelfand, Stanley A. Essentials of Audiology. Thieme, 2016.
DeRuiter, Mark, and Virginia Ramachandran. Basic Audiometry Learning Manual. Plural Publishing Inc., 2017Gelfand
DeRuiter, Mark, and Virginia Ramachandran. Basic Audiometry Learning Manual. Plural Publishing Inc., 2017Gelfand