Effects of Middle Ear Pressure on Cervical Vestibular Myogenic Potentials

Abstract

Cervical vestibular evoked myogenic potentials (cVEMPs) are small vestibular-dependent responses used to test the function of the saccule. They are commonly evoked using loud air-conducted (AC) sounds, such as clicks or tone-bursts, Pathology that limits middle ear sound transmission can, therefore, reduce cVEMP responses. However, the effects of changes in middle ear pressure (MEP) on cVEMPs have not been systematically investigated. In a novel approach, this study used continuous positive airway pressure (CPAP) to alter MEP systematically to investigate the effects of MEP on cVEMPs and hearing. Validation of CPAP as a reliable method was first undertaken in a series of preliminary studies. Eleven healthy participants were then recruited to investigate the effects of MEP on cVEMPs and hearing thresholds. The MEP was manipulated with CPAP to four predetermined levels: ± 50, +50-99, +100-149 and > 150 daPa that were verified using tympanometry. cVEMP amplitudes were recorded in response to AC and bone-conducted (BC) tone-bursts. Amplitude asymmetry ratios (AR) were determined through comparison with results from the non-test ear (without CPAP) and compared with air-bone gaps on audiometry. Preliminary investigations confirmed CPAP was a reliable method for inducing steady state changes in MEP. MEP remained stable at a set CPAP level, and the measured CPAP vibrations were too low to affect cVEMP recordings. AC cVEMP amplitudes decreased steadily with increasing MEP, starting with the lowest level. The effect was greatest for the 500 Hz stimulus, revealing an 52.2% decline in amplitudes by the final MEP (>150 daPa). When amplitudes were compared with the non-test ear, abnormal ARs were detected in 66.6% of participants. There was some evidence of an effect of MEP on 500 Hz AC hearing thresholds, but only at the highest MEP. None of the participants with an abnormal 500 Hz AC cVEMP AR had a significant 500 Hz air-bone gap on audiometry. BC cVEMP amplitudes to 1000 Hz stimuli were unaffected by MEP, whereas 500 Hz BC cVEMP amplitudes increased slightly at the highest MEP. This research confirms an effect of MEP on cVEMP amplitudes. To avoid errors in interpretation, tympanometry should be performed prior to recording cVEMPs.