Some static characteristics of the stapedial muscle reflex

Abstract

The stapedial muscle reflex was investigated in a group of normal and sensorineural ears. Pure tones of 250, 500, 1 000, 2 000, and 4 000 Hz, 600 Hz bandwidth narrow-band noise centered at 500, 1 000, 2 000, and 4 000 Hz and modified wide-band noise were utilized as reflex producing stimuli. All stimuli were presented through either of 2 prototype reflex-indicator systems generating probe tones of 625 and 800 Hz developed in the Audiology Department of Sahlgren's Hospital, or a Madsen ZO-70 electro-acoustic impedance meter generating a probe tone of 220 Hz. Reflex thresholds for pure tones and noise stimuli were determined with the prototype system delivering a probe tone of 800 Hz for normal and sensorineural ears. the average reflex threshold for pure tone stimuli approximated 85 dB hearing level for normal ears and 86 dB for the ears showing a sensorineural hearing loss. No differences were found for an ascending or descending approah to threshold or by measuring reflex response for changes in amplitude or amplitude phase. The hearing level limits for normal reflex response found in this study for pure tones between 250 and 4 000 Hz ranged from a lower limit of 75 dB to an upper limit of 95 dB. Comparison of reflex thresholds for 500 and 4 000 Hz for the 3 reflex-indicator systems resulted in most sensitive thresholds for the 220 Hz probe tone system (82.5 dB) and least sensitive thresholds for the 625 Hz probe tone system (88.0 dB). Narrowband and white noise stimuli produced reflex thresholds approximately 15 dB more sensitive than for pure tones. As was true for pure tones, aspproaching threshold from above or below, or measuring reflex response as change in amplitude or amplitude phase showed no difference. Altered middle ear pressure by Valsalva and Toynbee maneuvers in normal ears elevated reflex thresholds as much as 20 dB for pressure changes exceeding ±50 mm water pressure. Studies of reflex growth for pure tone stimuli increasing or decreasing in intensity shows a steeper response pattern for normal ears than those with sensorineural hearing loss and more shallow response for both grops at 4 000 Hz than for lower frequencies. An abnormally flat reflex growth rate may be of diagnostic significance.