Decreased circulation in the feline choriocapillaris underlying retinal photocoagulation lesions

To investigate the effects of argon laser photocoagulation on the choroidal circulation in cats.

Three sizes of argon laser lesions designed to damage the outer retina were created in six cats: larger than 1 mm, 500 μm, and 200 μm. At least 1 month after the lesions, damage to the choroidal vasculature was studied in two ways. First, scanning laser ophthalmoscopy was used to obtain infrared reflectance (IR) photographs and indocyanine green (ICG) angiograms. Second, fluorescent microspheres (15 μm) were injected into the left ventricle. The globes were fixed, the choroid was flat mounted, and images were taken with a fluorescence microscope. Retinal histology was assessed in comparable lesions.

Histology showed that the inner retina was preserved, but the choroid, tapetum, and outer retina were damaged. ICG angiograms revealed choriocapillaris loss in large lesions and in some 500-μm lesions, whereas the larger vessels were preserved; in 200 μm lesions, choriocapillaris loss was not detectable. However, in all lesions, the distribution of microspheres revealed little if any choriocapillaris flow. In larger lesions, the damaged region was surrounded by an area in which the number of microspheres was higher than in the lesion but lower than in the normal retina.

Under lesions that destroyed photoreceptors, the choriocapillaris was also compromised, even when no change could be detected with ICG angiography. Panretinal photocoagulation is designed to increase retinal PO2 by allowing choroidal oxygen to reach the inner retina, but its effectiveness may be limited by damage to the choriocapillaris.