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Sir,

The early treatment diabetic retinopathy study (ETDRS)1 showed that macular grid and focal laser therapy reduces the risk of moderate visual loss in diabetic eyes with clinically significant macular oedema (CSME). Although this treatment may not benefit every patient, side effects are fortunately uncommon. Reduction of visual acuity following treatment infrequently occurs because of subfoveal choroidal neovascularisation, subretinal fibrosis, macular haemorrhage and areas of capillary nonperfusion in continuity with the foveal avascular zone (FAZ).

A case is presented where loss of visual acuity occurred following focal laser treatment (in accordance with ETDRS guidelines) adjacent to an area of nonperfusion away from the fovea.

Case report

A 65-year-old man with type II diabetes mellitus reported a gradual decrease in right visual acuity over 6 months. He was known to have moderate nonproliferative diabetic retinopathy in both eyes and had presented 2 years earlier with CSME in both eyes. He had received macular laser treatment once to his right eye and once to his left. Fluorescein angiography was performed prior to further treatment and showed leakage temporal to the fovea, superotemporal nonperfusion and drop-out of perifoveal capillaries temporal to the macula (Figure 1). On examination, corrected visual acuity was 6/60 right and 6/36 left. There was a right posterior subcapsular lens opacity and moderate nonproliferative diabetic retinopathy in both eyes but no CSME. Uneventful phacoemulsification was performed. Two weeks postoperatively, right visual acuity was 6/9 corrected but CSME was identified temporal to the fovea. This was treated with uneventful focal laser. One month later the patient returned with a right visual acuity of 6/24. Clinical findings were unchanged and fluorescein angiography showed no foveal burn (Figure 2).

Figure 1
figure 1

Photographs taken prior to temporal focal laser. (a) Red-free fundus photograph of right eye showing the area of superotemporal ischaemia (arrows). (b) Fluorescein angiogram showing area of superotemporal ischaemia (large arrow) and area of temporal ischaemia not continuous with FAZ (small arrows).

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Figure 2
figure 2

Photographs after temporal focal laser and visual deterioration. (a) Colour fundus photograph of the right macular region with the temporal area of retinal thickening (outlined) and the pre-existing area of superotemporal ischaemia (arrow). (b) Fluorescein angiogram of the right macular region showing an enlarged and irregular FAZ (arrows). (c) Fluorescein angiogram showing the treated area (outlined) and how this overlaps the temporal area of macular ischaemia (arrow).

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Comment

The ETDRS recommended macular laser treatment for eyes with CSME to reduce moderate visual loss and is supported by the Guidelines of The Royal College of Ophthalmologists on the management of diabetic retinopathy.2 In particular, the ETDRS recommends, in areas of retinal thickening, focal laser treatment to areas of focal leakage and grid laser to areas of diffuse leakage and nonperfusion.

Rare complications can occur following routine treatment and include direct foveal burns, exacerbation of macular oedema with heavy extensive grid treatments, retinal haemorrhage owing to rupture of microaneurysms, development of choroidal neovascular membranes3 and subretinal fibrosis.4 Finally, laser treatment to the margin of an enlarged or irregular perifoveal capillary network should be specifically avoided1 as it can further disrupt an already compromised FAZ and have an adverse effect on visual acuity.

This patient had evidence of nonperfusion to the superior macular prior to laser but the FAZ was intact. Fluorescein angiographic appearances were unchanged post-treatment. The aetiology of the visual loss in this case remains unclear but may be related to unrecognised disruption of macular capillaries supplying the perifoveal capillary network. These capillaries were at least 400 μm from the edge of the FAZ and adjacent to pre-existing ischaemia. It is possible that their function was compromised due to the adjacent ischaemia and that the focal laser treatment exacerbated this. Fluorescein angiography should be performed in all patients undergoing macular laser for diabetic macular oedema. Special consideration should be given to the treatment of areas of leakage adjacent to capillary network non-perfusion because of the risk of indirectly compromising the capillary network perfusion.