Decreased Circulation in the Feline Choriocapillaris Underlying Retinal Photocoagulation Lesions

Proliferative diabetic retinopathy is found in about half of type 1 diabetics and in approximately 10% of type 2 diabetics who have had the disease for 15 years.1 Panretinal photocoagulation (PRP) has been the most effective treatment,2,3 but the disease may continue or worsen in a substantial fraction of patients,4,5 and in one study, retreatment was effective in only approximately half the patients.5 Although PRP damages the retinal pigment epithelium and photoreceptor cell layer, it leaves the inner retina relatively unaffected6–9 and helps preserve vision. The leading hypothesis for the effectiveness of PRP is that the destruction of photoreceptors reduces the oxygen consumption of the lesioned area, allowing an increased rate of choroidal oxygen delivery to the inner retina.10 There is considerable indirect evidence supporting this hypothesis,11–13 as well as direct evidence from microelectrode measurements that show increased vitreal Po26–7,13–16 and intraretinal Po217 after photocoagulation in the retinas of animals. Choroidal Po2 is normally higher than inner retinal Po2.18–22 In order for the choroidal circulation to provide oxygen to the inner retina, the Po2 at the choroid must remain high after photocoagulation. However, we found that photocoagulation reduces choroidal Po2,17 and we hypothesized that the choroidal circulation is damaged by the lesion. Several investigators have provided histologic evidence that the choriocapillaris is damaged and does not heal completely after photocoagulation in humans,9,23,24 but the functional implications of this are rarely discussed. There have been only two studies of choroidal blood flow after photocoagulation. Chandra et al.25 measured blood flow with radioactive microspheres after photocoagulation. The flow in the retina and choroid of rabbits was reduced at times ranging from a few hours to several weeks after photocoagulation, and in monkeys, flow was reduced immediately, the only time point measured. However, their technique did not separate retinal and choroidal blood flow, and they did not determine whether the changes were local or global. In contrast, a recent study with laser Doppler flowmetry reported increased foveal choroidal blood flow 1 month after photocoagulation for proliferative diabetic retinopathy.26 We therefore reinvestigated the hypothesis that the choroid is damaged by photocoagulation, after both the large lesions used in our earlier study of retinal Po2,17 and smaller lesions more similar to those used clinically. We used in vivo scanning laser ophthalmoscopy (SLO) and microsphere counts in choroidal flat mounts in cats, techniques that could localize changes in choroidal blood flow.