Potential Injuries To The Inner Ear Following CO₂Laser-assisted Cochleostomy

Objective To investigate the potential injuries to the inner ear followingcochleostomy with superpulsed CO2laser in SD rat.Methods Twenty-four SD rats were randomly assigned to laser2W group, laser5W group, sham-operated group, and normal control group (n=7,7,5,5). Alaser-assisted cochleostomy was performed using superpulsed CO2laser with a powerenergy of2W or5W. Auditory brainstem responses (ABR) thresholds, includingclick-ABR and tone-burst ABR, were measured pre-and postoperatively. Afternormalizing the basilar membrane, the ribbon synapses at apical and middle cochlearturns were observed under laser scanning confocal microscope and were thenquantified with3ds Max software. Ultrastructure of the cochlea was further examinedwith transmission electron microscope.Results The postoperative click-ABR thresholds increased significantly in the2W group and the5W group (P <0.01in both). A statistical difference was foundbetween the thresholds in both groups (P <0.05). The values of the tone burst-ABRthreshold shifts in both laser groups were greater with increasing frequency. Theirradiated spot was situated at32.7%basilar membrane length (apex=0%; basal end=100%), which corresponded to the middle cochlear turn. The synapse number at themiddle turn decreased significantly in animals from5W group (P <0.01), while nosignificant change was noted at the apical turn. There was no statistical difference in2W group. Both the decompaction of the myelin sheath and the swollen mitochondriain axon were found in2W group. In contrast, swollen mitochondria, wrinkled axon,fused myelin sheath, and even vacuolations and necrocytosis occurred in5W group.Conclusions The superpulsed CO2laser-assisted cochleostomy with low energy isaccompanied by mild auditory nerve degeneration, although no significant change was found in synapse number. Reduced synapse number, as well as severe damage toauditory nerve, could be noted after a high-energy laser exposure, while no obviouseffects after the low-energy laser surgery. Therefore, caution should be taken and theradiation dosage should be properly controlled when performing a laser-assistedcochleostomy with such a laser system.