Cochlear nerve demyelination causes prolongation of wave I latency in ABR of the myelin deficient (md) rat

In this study, we examined the auditory brainstem responses (ABRs), distortion product of otoacoustic emissions (DPOAEs) and cochlear morphology of the myelin deficient (md) rat, which completely lacks central myelin but not peripheral myelin. ABRs showed a marked prolongation not only wave II-IV latencies but also wave I latency. Cochlear nerve fibers near the modiolus lost their myelin halfway into the internal auditory canal. DPOAEs also decreased at a lower frequency of the combined tone. Since nerve fibers ending at the apical turn of the cochlea passed through central portion of the cochlear nerve, wave I prolongation of ABRs and decrease of DPOAEs at a lower frequency might originate mainly from the demyelinated CNS part of the cochlear nerve and efferent olivocochlear bundle in the internal auditory canal.