Histologic effect of semiconductor diode laser transscleral cyclophotocoagulation on the normal equine eye

To determine the acute histologic effects of semiconductor diode laser transscleral cyclophotocoagulation (TSCP) on the normal equine eye.

Part 1: eight eyes of four horses. Part 2: 10 eyes of five horses.

Part 1: TSCP was performed on four eyes at 4 mm and four eyes at 6 mm posterior to the limbus with 15 sites treated in four quadrants at 1800 mW for 1500 ms. The globes were sectioned transversely or sagitally to examine all quadrants and histologic sections were taken every 1 mm for the entire globe. Part 2: Based on the results from Part 1, TSCP was performed at 20 sites 4 mm posterior to the dorsotemporal limbus with a constant energy varying from 0.75 to 4 J/site. Histologic sections were taken every 1 mm for a total of 10 sections per eye and 20 sections per energy level group.

Part 1: At 4 mm posterior to the limbus, coagulation of the nonpigmented epithelium (NPE) of the pars plicata was observed in the temporal (14%) and dorsal quadrants (12%). Retinal detachment was observed in the nasal quadrant (12%). Hemorrhage was common in the nasal (19%) and temporal (12%) quadrants. At 6 mm posterior to the limbus, coagulation of the NPE of the pars plicata was observed in the dorsal (14%), ventral (16%), nasal (2%), and temporal (2%) quadrants. Retinal detachment was observed in the dorsal (8%), ventral (18%), nasal (20%) and temporal (2%) quadrants. Part 2: Settings of 0.75 J/site were ineffective; 1.5, 2.25 and 3 J/site damaged the pars plicata without disruption of anatomy; and 4 J/site caused disruption of normal architecture.

The most appropriate site for equine TSCP appears to be 4 mm posterior to the dorso- and ventrotemporal limbus avoiding the 3 and 9 o'clock positions and using an initial energy setting of 2.25 J/site. This results in effective damage to the pars plicata while minimizing surgical complications such as retinal detachment and hemorrhage.