The therapeutic effects of retinal laser treatment and vitrectomy. A theory based on oxygen and vascular physiology

The physiologic mechanism of photocoagulation can been seen in the followin g steps. The physical light energy is absorbed in the melanin of the retina l pigment epithelium. The adjacent photoreceptors are destroyed and are rep laced by a glial sear and the oxygen consumption of the outer retina is red uced. Oxygen that normally diffuses from the choriocapillaris into the reti na can now diffuse through the laser scars in the photoreceptor layer witho ut being consumed in the mitochondria of the photoreceptors. This oxygen fl ux reaches the inner retina to relieve inner retinal hypoxia and raise the oxygen tension. As a result, the retinal arteries constrict and the bloodfl ow decreases. Hypoxia relief reduces production of growth factors such as V EGF and neovascularization is reduced or stopped. Vasoconstriction increase s arteriolar resistance, decreases hydrostatic pressure in capillaries and venules and reduces edema formation according to Starling's law. Vitrectomy also improves retinal oxygenation by allowing oxygen and other n utrients to be transported in water currents in the vitreous cavity from we ll oxygenated to ischemic areas of the retina. Vitrectomy and retinal photo coagulation both improve retinal oxygenation and both reduce diabetic macul ar edema and retinal neovascularization.