Mechanism study of laser cochleostomy-induced early hearing loss in a rat model.

Hearing loss following laser-assisted ear surgery has been reported. However, the mechanism responsible for the hearing loss remains largely speculative. The aim of this study was to investigate the correlation between laser-induced hearing loss and changes in the number of hair cell ribbon synapses and ultrastructure in the cochlea. Laser cochleostomy was performed with a superpulsed carbon dioxide (CO2) laser at 2 and 5 W in Sprague-Dawley rats. Auditory brainstem responses (ABRs) were measured preoperatively and 2 days after surgery. The synapse numbers in apical and middle cochlear turns were quantified. Transmission electron microscopy was employed to further examine the subcellular changes in the cochlea. Click and tonal ABR threshold shifts in both 2 and 5-W groups displayed a frequency-dependent loss within the frequency range measured. Laser cochleostomy induced a significant decrease of synapse numbers in the middle turn in both groups (p < 0.05). Electron microscopy data indicated varying degrees of auditory nerve degeneration in both groups. Auditory nerve degeneration might contribute to laser-caused hearing loss even under low-energy laser cochleostomy. The high-energy laser-induced hearing loss was associated with more reduction of synapse number.

[Effects of laser-assisted cochleostomy on inner hair cell ribbon synapse in rats].

OBJECTIVE: To investigate the effects on ribbon synapse of inner hair cells after superpulsed CO2 laser-assisted cochleostomy in SD rats. METHODS: Eighteen SD rats were randomly divided into laser-assisted surgery groups (2 W group and 5 W group), sham-operated group and control group. Ten of those were performed a cochleostomy using superpulsed CO2 laser with a corresponding power. Auditory brainstem responses (ABR) were measured pre-and postoperatively. The ribbon synapses at apical and middle cochlear turns were observed under laser scanning confocal microscope and then were quantified with 3ds Max software. RESULTS: The postoperative ABR thresholds of the 2 W and 5 W groups were larger than the preoperative case (t = -5.65, P < 0.01; t = -4.97, P < 0.01). The synapse number at the middle turn decreased significantly in 5 W group (F = 17.15, P < 0.01), while no significant changes were noted at the apical turn (P > 0.05). There was no statistical difference in 2 W group (P > 0.05). CONCLUSIONS: The superpulsed CO2 laser-assisted cochleostomy with high-power is accompanied by a synaptic injury, while no obvious effects after the low-power laser surgery, which might be a safe strategy to preform cochleostomy.