Laser treatment of macular diseases

Abstract

In their article, “The Treatment of Macular Diseases Using a Micropulsed and Continuous Wave 810-nm Diode Laser” (Ophthalmology 1997;104:2030–38), Friberg and Karatza verified the efficacy of a diode laser emitting in the near-infrared spectrum in the treatment of retinal and choroidal vascular diseases. The authors state that if direct closure of retinal microaneurysms is the treatment goal, the near- infrared wavelength of the diode laser would not be a logical choice. Therefore the authors did not specifically target microaneurysms but administered a grid pattern to the areas of retinal thickening. On the contrary, in a study on diabetic macular edema, Ulbig et al1Ulbig M.W. McHugh D.A. Hamilton A.M. Diode laser photocoagulation for diabetic macular oedema.Br J Ophthalmol. 1995; 79: 318-321Crossref PubMed Scopus (54) Google Scholar directly treated retinal microaneurysms with the near-infrared diode. In both studies the resolution of the retinal edema and the reduction of retinal microaneurysms were clearly noted. We recently reported similar positive results in the treatment of choroidal neovascular membranes with the same wavelength.2Lanzetta P. Virgili G. Menchini U. Diode laser photocoagulation of choroidal neovascular membranes.Int Ophthalmol. 1996; 19: 347-354Crossref Scopus (8) Google Scholar These experiences with the diode laser raise an important point with respect to the mechanism responsible for the closure of vascular lesions of the retina and choroid after laser thermal photocoagulation. Direct absorption of radiation by hemoglobin within focal vascular lesions responsible for diabetic macular edema is thought to play a major role in the efficacy of the treatment.3Wallow I.H.L. Bindley C.D. Focal photocoagulation of diabetic macular edema a clinicopathologic case report.Retina. 1988; 8: 261-269Crossref PubMed Scopus (25) Google Scholar This mechanism is certainly valid in nonpigmented tissues, such as the mesentery, where hemoglobin is the primary absorption target. In this case blood flow stasis does not follow the classic hemostatic pathway but occurs by heat coagulation of fibrinogen and appears to be less stable. In the eye, retinal and choroidal pigmented cells are the most important targets during photocoagulation with the near-infrared wavelength. Retinal pigment epithelium and choroid melanocytes and retinal pigment epithelium alone absorb, respectively, 35% and 7% of the radiation, whereas oxyhemoglobin or reduced hemoglobin practically do not absorb the radiation at threshold energies. Therefore the near-infrared wavelength has unfavorable characteristics for the occlusion of retinal vascular lesions through the direct absorption of the radiation by hemoglobin. Despite this evidence, we recently showed that the diode laser was effective in inducing the closure of retinal macroaneurysms after photocoagulation placing one to two rows of burns around the lesion. The macroaneurysms were not directly covered with photocoagulation and no signs of blood flow stasis were evident during treatment [Invest Ophthalmol Vis Sci 38(Suppl):86, 1997]. A rationale other than that of direct coagulation has to explain the closure or obliteration of vascular lesions after irradiation with the 810-nm wavelength. They may close as a result of indirect treatment effects. Thus the mechanisms underpinning the efficacy of photocoagulation could be reconsidered according to the cited biophysical characteristics of the near-infrared radiation. Recent experiments demonstrated that the retinal pigment epithelium plays a role in the occlusion of vascular lesions of the retina and choroid. Laser burns in diabetic humans that involved only the outer retinal half but spared the inner nuclear layer, showed proliferation of pigment epithelial and glial cells that resulted in the closure of the leakage sites.3Wallow I.H.L. Bindley C.D. Focal photocoagulation of diabetic macular edema a clinicopathologic case report.Retina. 1988; 8: 261-269Crossref PubMed Scopus (25) Google Scholar The involution of subretinal neovascular membranes has been associated with retinal pigment epithelium proliferation that envelopes the new vessels.4Miller H. Miller B. Ryan S.J. The role of retinal pigment epithelium in the involution of subretinal neovascularization.Invest Ophthalmol Vis Sci. 1986; 27: 1644-1652PubMed Google Scholar Finally, there is clear evidence that the beneficial effect of retinal photocoagulation is mediated by factors derived from the retinal pigment epithelium, such as transforming growth factor-β2, which is known to modulate both fibrinogenesis and vascular proliferation in vivo and transforming growth factor-β1, which has an effect on the permeability of retinal capillary endothelial cells [Invest Ophthalmol Vis Sci 38 (Suppl):754, 1997]. Laser treatment of macular diseases: Authors’ replyOphthalmologyVol. 105Issue 9Preview Full-Text PDF