Both experimental and clinical studies have demonstrated that carbon dioxide laser is suitable for stapedotomy. The aim of this study was to investigate morphological, electrophysiological and functional changes in the inner ear after irradiation with CO2 laser set with different energy parameters.

A cochleostomy in the basal cochlear turn of guinea pig cochleae was performed with CO2 laser of 1, 2 and 3 w, respectively. The cochleae were removed three weeks after laser irradiation. The auditory evoked brainstem response (ABR) was measured before and after laser application and immediately before removal of the cochlea. Immunohistochemical methods were used to examine inducible nitric oxide synthase (iNOS/NOSII) and heat-shock protein 70(Hsp70) concentrations in the cochlea after laser application. The organ of Corti was studied by scanning electron microscopy.

Worse hearing loss was observed in animals receiving higher-power CO2 laser. These findings correlated with more intense injury of the cochlear ultrastructure and with positiveexpression of iNOS and Hsp70 in spiral ganglion cells, nerve fibres, supporting cells of the organ of Corti and cells of the spiral ligament.

The CO2 laser as a noncontact procedure is shown to be effective and safe if the total amount of energy is kept within the limits applied in this study. Nitric oxide and stressproteins play important roles in the traumatic mechanism of the inner ear, which are related tohearing loss and injury of the ultrastructure of the inner ear.