A histological comparison of micropulse and standard laser treatment in threshold and sub-threshold modes

Abstract

Purpose: Laser photocoagulation is a common treatment for diabetic macular edema but not without side effects and can damage the neurosensory retina. Sub-threshold micropulse laser produces therapeutic effects without inducing detectable clinical visible changes. However, sub-threshold standard (continuous wave) laser can produce detectable changes in optical coherent topography. We aim to compare the effects of the 577nm (yellow) micropulse laser, 577nm (yellow) standard mode laser and 532nm (green) standard mode laser on the retina by histological examination.

Methods: Twelve Dutch-belted rabbits received laser treatment in their left eyes. The 532nm and 577nm laser photocoagulation in standard mode were used in six rabbit eyes. The other six rabbits received sub-threshold 577nm micropulse laser photocoagulation at 5% and 10% duty cycle. Treatment was given at threshold and sub-threshold (approximately 50% of threshold) powers in different areas of the retina. At 1 week and 1 month post-laser photocoagulation, histology of the retinal sections was analyzed and compared.

Results: In the treated areas by threshold mode, extensive retinal damage was present with all 4 treatment modalities. In the sub-threshold-treated areas, at 1 week and 1 month post-laser treatment, retinae treated by 532nm laser photocoagulation in standard mode exhibited more retinal morphological changes than the ones treated by 577nm laser in standard mode. Increased extent of retinal fold, retinal pigment epithelium disruption and outer retinal cell death occurred. In general, the overall appearance of the retinae treated with 577nm micropulse laser in both 5% and 10% duty cycles were better preserved when compared with the ones using standard setting with either 577nm or 532nm laser. Most importantly, cellular morphology appeared best preserved in the retinae using the 5% duty cycle, with slight or minimal disruption by histological examination.

Conclusions: This study confirmed the clinical findings that no matter what modality is used, if there was a threshold treatment, extensive retinal damage occurs. However, by reducing power to 50% of threshold power, less damage incurred by using the 5% duty cycle micropulse laser.