Electrophysiologic Characteristics of Various Degrees of Selective Outer Hair Cell Loss in Rats

The characteristics of auditory brainstem response (ABR), electrocochleogram (ECochG), and distortion product otoacoustic emissions (DPOAE) of different degrees of selective outer hair cells (OHCs) loss may be helpful for clinicians to evaluate the pathogeny, diagnosis, and rehabilitation of individuals' hearing loss.

How many OHCs are necessary to maintain cochlear amplifier function remains unknown. The electrophysiologic characteristics may indicate different degrees of OHCs loss.

Electrophysiological characteristics were tested using 8-kHz pure-tone stimulus and OHCs counted specifically in the region of the cochlea corresponding to 8-kHz. Rat models of selective OHCs loss were established by injecting kanamycin (KM) at various dosages, and the region of 8-kHz was obtained by 8-kHz pure-tone exposure.

The ABR thresholds were affected slightly with OHCs loss < 30%, and were increased dramatically with OHCs loss ranging from 30 to 70%, but the thresholds did not increase further when OHCs loss exceeded 70%. As OHCs loss increased, the compound action potential (CAP) amplitude decreased. The CAP amplitude and OHCs loss were negatively correlated. Moreover, the summating potential (SP)/action potential (AP) increased as OHCs loss increased. DPOAE and cochlear microphonics (CM) exhibited reduced amplitudes when OHCs loss < 30%.

Electrophysiologic characteristics may indicate different degrees of OHCs loss. While OHCs loss > 70%, the cochlear amplification may lose completely, but it is difficult to detect OHCs loss < 30%, because the ABR or DPOAE may reveal "normal" at this level. Moreover, the decreased CAP amplitude or increased SP/AP may be indicators for OHCs loss.