An electrophysiologic study of the guinea pig inner ear following low pressure barotrauma

We examined electrophysiologic and morphologic changes following low pressure barotrauma in 25 guinea pigs. Compound action potentials (CAPs), cochlear microphonics (CMs), and transiently evoked otoacoustic emissions (TEOAEs) were elicited by tone bursts (1, 2, 4 and 8 kHz) or non-linear clicks immediately following barotrauma. CAP threshold elevations were observed in 19 out of 25 cochleas, mainly at lower stimulus frequencies. Furthermore, CM and TEOAE thresholds were significantly increased, while CAP and CM amplitudes demonstrated reductions at all stimulus frequencies and intensities. CAP N1 latencies exhibited slight elongations at all stimulus frequencies and intensities. The regression coefficient between the mean CAP thresholds of four stimulus frequencies and TEOAE thresholds was statistically significant. Scanning electron microscopy (SEM) study of six electrophysiologically abnormal cochleas revealed altered stereocilia morphology in four, but no changes in two. We hypothesize that low pressure barotrauma can injure inner ear hair cells through an early threshold shift secondary to dislocation of the basement membrane.