Retinal Damage From Repeated Subthreshold Exposures Using a Ruby Laser Photocoagulator

摘要

A ruby laser photocoagulator was used to deliver single and multiple subthreshold exposures to the retina of Macaca mulatta. The subexposures parameters were 0.5-msec. pulse, 1.3-mm.-diameter focal spot, and energy density of about 375 mJ/sq cm. Single exposures produced no microscopic changes on sacrifice at 1 to 30 days following exposure. Repetitive exposures (7 to 17 exposures) at the same energy level invariably caused characteristic damage in the outer retinal layers. These cumulative effects are similar to and more widespread than the minimal-damage lesions produced by single suprathreshold exposures using higher energy density (900 to 1000 mJ/sq cm). Current safety criteria are based on the assumption that laser damage occurs on an all-or-none basis so that damage is not cumulative if a single exposure causes no visible lesion. This work indicates that single subthreshold exposures which are less than half of a threshold dose are cumulative and therefore each 'subthreshold' exposure must damage or in some manner increase the retinal susceptibility to subsequent exposures. (Author)