RPE response to subclinical diode laser treatment

Purpose. To determine the effects on the RPE and retina of subclinical diode iaser treatment. Methods. The retinas of pigmented rabbits were lasered with a diode laser, using continuous wave or long duration (100 microsec) micropulse delivery, to produce rows of 200 micron spots that were undetectable or just detectable (pale grey) by ophthalmoscopy at the time of treatment. At time = 2 hours and at time = 5 days post-treatment, the retina and RPE were examined by light and electron microscopy, or flat mounts of the RPE were examined by fluorescence microscopy after staining with rhodamine phalloidin to show circumferential actin. Results. At t = 2hr some but not all RPE cells within the laser spots showed pigment clumping or changes in intensity of actin staining. By LM and TEM even subclinical lesions produced compaction and/or swelling of the outer retina. By t = 5d, lesions that were originally subclinical were detectable as zones of pigment mottling. The RPE showed evidence of tissue remodeling in the lasered region as indicated by variations in cell size, shape and pigmentation. Centripetally oriented rosettes of RPE cells surrounding a degenerating cell were common near but not within the laser sites suggesting ongoing cell loss and monolayer restoration. Conclusions. The response of RPE cells was heterogeneous within a subclinical laser spot suggesting cell-to-cell variation in lethality with low energy laser treatment. The outer retina was affected even adjacent to regions where all RPE were not lethally damaged. We conclude that subclinical laser treatment leads to significant but perhaps recoverable tissue injury.