[Fundus hypothermia at 29 degrees C prevents ischemic injury of the outer retina]

We evaluated quantitatively the protective effect of local fundus hypothermia under pressure-induced ischemia using morphometric analysis. Retinochoroidal ischemia was produced in albino rabbit eyes by increasing the intraocular pressure for 60 minutes. During the ischemic procedure, a copper plate was inserted behind the eyeball. The retinal temperature in the posterior pole was thus reduced to 29 degrees C by placing solid carbon dioxide, and to 32 degrees C by placing an ice cube at the anterior end of the plate. Histopathological changes in the group with ischemia alone were obvious in visual cells and retinal pigment epithelial cells (RPE), but the retina treated with additional hypothermia was well preserved. In the retina with hypothermia at 29 degrees C, there was no significant difference from the controls in the mean thickness of the photoreceptor layer (PRL) and the RPE, and the average count of nuclei in the outer nuclear layer (ONL). In the retina with hypothermia at 32 degrees C, there was also no significant difference from the controls in the thickness of the PRL and the RPE. Otherwise, the count of nuclei in the ONL decreased significantly when compared to that of controls (p < 0.001). These findings indicate that even mild hypothermia at 29 degrees C preserves the outer retina from ischemic damage and that the protective effect of hypothermia at 32 degrees C is insufficient.