Cochlear effects of hyaluronan applied on ruptured round window membrane.

The purpose of the present experimental study was to determine whether hyaluronan (hyaluronic acid, HYA) exerts any functional or morphologic ototoxic effects on the cochlea when in contact with a ruptured round window membrane (RWM). HYA in 1% solution was applied in the round window (RW) niche of rats (n = 6) prior to perforating the RWM. In a control group (n = 6) the RWM was perforated alone. Cochlear functioning and structure were monitored by recording auditory brainstem responses (ABRs) at 2-31.5 kHz and by scanning electron microscopy. Perforation of the RWM alone resulted in immediate loss of ABR thresholds between 6 and 31.5 kHz in 2 of 6 animals. Similar results were obtained after application of HYA into the RW niche and subsequent RWM perforation. In both treatment groups the mean ABR thresholds and mean latencies for wave II at the ABR threshold returned to the pre-surgical (normal) range after 2 months. Also with respect to the cochlear morphology the results in both treatment groups were alike including minor structural changes in hair cell stereociliae but no loss of hair cells. It is concluded that HYA, when instilled into the middle ear with the inner ear opened, is free from cochlear ototoxic effects. Our present findings enhance the potential usefulness of HYA in otosurgery, even in cases when the inner ear has been opened and/or when one wishes to protect the inner ear.