Role of adjunctive laser photocoagulation in a clinical setting of invisible subretinal worm

Abstract

A 16-year-old male presented to us with a history of progressive painless blurring of vision in the right eye for 6 months. On evaluation, his corrected distance visual acuity was 2/200 in the right eye and 20/20 in the left eye. Anterior segment evaluation was essentially unremarkable in both the eyes except for a mild relative afferent pupillary defect in the right eye. Left eye posterior segment evaluation was unremarkable. In the right eye fundus, yellow white outer retinal lesion with ill-defined margins was observed superotemporal to fovea (Fig. 1A). There was no vitreitis. On fluorescein angiography, there was early blocked fluorescence and late staining in the area of the retinal lesion, attenuation of arterioles, enlargement and distortion of foveal avascular zone, and multiple pinpoint areas of hypofluorescence and hyperfluorescence in the macula (Fig. 1B). On optical coherence tomography, the inner segment/outer segment junction and external limiting membrane were observed to be intact in the macula. There was loss of foveal contour and diffuse retinal thinning (Fig. 1C). Based on all the earlier findings, a diagnosis of right eye diffuse unilateral subacute neuroretinitis (DUSN) was made. Fundus photographs were taken of all retinal areas to localize any subretinal worm, but none was visualized. Patient was started on 200 mg oral albendazole twice a day and 40 mg oral prednisolone once a day in weekly tapering dose along with antacids. Over the next 6-week follow-up period, similar retinal lesions kept appearing and disappearing in different locations in the posterior pole without leaving any scar behind (Fig. 2A–C). After 6 weeks, focal laser photocoagulation was done on and around the new retinal lesion using 532 nm green laser, 400-µm spot size, 250 mW power, and exposure time of 20 milliseconds. Oral albendazole and prednisolone were continued for 2 more weeks. At 1-week follow-up after laser, intense scar appeared in the laser area (Fig. 2D), which gradually faded with minimal pigmentation, and visual acuity improved to 20/400 and stabilized. No new retinal lesions appeared in the ensuing 7 months after laser photocoagulation. DUSN was first described by Gass and Scelfo1Gass J.D. Scelfo R. Diffuse unilateral subacute neuroretinitis.J R SocMed. 1978; 71: 95-111PubMed Google Scholar in 1978 as a unilateral retinitis in a healthy child or young adult with an insidious course of progression. Gass implicated the role of nematodes as the cause after observing viable subretinal nematode. The gold standard of treatment is direct photocoagulation of the larva, but live worm can be identified in about 40% of cases only.2de Amorim Garcia Filho C.A. Gomes A.H. de A Garcia Soares A.C. de Amorim Garcia C.A. Clinical features of 121 patients with diffuse unilateral subacute neuroretinitis.Am J Ophthalmol. 2012; 153: 743-749Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar The treatment in cases of DUSN where worm is not found is not well defined. There have been contrasting results with the use of antihelminthics like thiabendazole and albendazole in various studies.3Casella A.M.B. Farah M.E. Belfrd Jr, R. Anthelminthic drugs in diffuse unilateral subacute neuroretinitis.Am J Ophthalmol. 1998; 125: 109-111Abstract Full Text PDF PubMed Scopus (35) Google Scholar, 4Gass J.D. Callanan D.G. Bowman C.B. Successful oral therapy for diffuse unilateral subacute neuroretinitis.Trans Am Ophthalmol Soc. 1991; 89 (discussion 113-6): 97-112PubMed Google Scholar Gass observed that antihelminthics worked only in those cases that were associated with moderate to marked vitreitis because this causes disruption of the blood–retinal barrier and allows better penetration of drug into ocular tissue.4Gass J.D. Callanan D.G. Bowman C.B. Successful oral therapy for diffuse unilateral subacute neuroretinitis.Trans Am Ophthalmol Soc. 1991; 89 (discussion 113-6): 97-112PubMed Google Scholar In our case, there was no vitreitis, and this could be the reason for recurrent crops of retinal lesions even on oral albendazole. Stokkermans5Stokkermans T.J. Diffuse unilateral subacute neuroretinitis.Optom Vis Sci. 1999; 76: 444-454Crossref PubMed Scopus (16) Google Scholar reported a case in which worm was not identified and laser was done to areas of inflammatory retinal lesions. Visual acuity slightly improved and no new lesions appeared in that case. Our case and its management highlight the role of laser photocoagulation as an adjunctive treatment to antihelminthics in cases of DUSN with no vitreitis and subretinal worm not visualized. It possibly can act in 2 ways. One, it will disrupt the blood retinal barrier and facilitate better penetration of antihelminthics in ocular tissue and second, it may directly kill the worm which might be obscured by the retinal lesions.