Sir,

Diffuse unilateral subacute neuroretinitis (DUSN) is caused by a variety of nematodes, mostly subretinal, frequently leading to panretinal degeneration and visual loss.1, 2 We report the unusual presentation of a large nematode in DUSN.

Case report

A healthy 28-year-old south Indian man presented to us with an unremitting floater in the right eye (RE) for 1 week. Snellen acuity was 6/6 in RE and 6/12 in the left eye (LE). Examination of RE was unremarkable. LE fundus revealed a hyperkinetic non-segmented, 6.8 mm worm, apparently trapped under pre-foveal internal limiting membrane (ILM) (Figure 1a). There were midperipheral tracks of pigmentary degeneration; vitreous was quiet. The patient neither had any previous history of fever, skin rashes or fits nor any treatment for filariasis. Systemic evaluation and laboratory investigations, including a nocturnal peripheral smear (for microfilaria), were negative. Optical coherence tomography (StratusOCT, Carl Zeiss Meditec, Dublin, CA, USA) confirmed the worm's sub-ILM location (Figure 1b). The patient was initially prescribed oral albendazole (400 mg o.d.) and diethylcarbamazine (100 mg t.i.d.). When status quo persisted for a month, vitrectomy was performed with patient's informed consent. Perifoveal capillaries bled during posterior-hyaloid removal. An extrafoveal tear occurred while aspirating the blood-trapped worm. ILM was removed and perfluoropropane–air tamponade used. The worm could not be subjected to parasitological evaluation because it disintegrated during the traumatic aspiration. Post-operatively, the eye remained quiet, with retained preoperative vision, intact macula and minimal juxtafoveal atrophy (Figure 1c and d) for 6 months.

Figure 1
figure 1

(a) Fundus photograph of the LE. A 6.8-mm long, non-segmented, tapered worm is seen in front of the central fovea, trapped under the locally detached ILM as indicated by arrowheads. (b) Horizontal 5 mm OCT scan through the left fovea, showing a dome-shaped, lax, undulating membrane corresponding to the clinically suspected ILM detachment; with rapidly shifting hyper-reflective sub-ILM echoes corresponding to the worm trapped underneath. (c) One month after vitrectomy, LE shows a small juxtafoveal area of retinal atrophy superotemporally, representing closed iatrogenic parafoveal retinal hole. The edges of peeled ILM are clearly visible (arrowheads). (d) Post-operative OCT scan in the ‘repeat mode’ through the LE macula shows normalized foveal contours; atrophic patch was observed only in superotemporal scans (not seen here).

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Comment

This case had many unusual attributes: While the subretinal tracks (Figure 2) pointed to trans-retinal migration of the worm as described in DUSN,1 this is the first OCT-documentation of its sub-ILM location, which facilitated the extraordinary motility of this suspected filarial nematode (endemic in patient's native area). Previously reported nematodes were smaller, slow-moving, and subretinal.1, 2, 3, 4 This worm did not produce the oft-reported intraocular inflammation, macular oedema or visual loss,1, 2, 3 probably because its pre-macular migration and sequestration prevented the deleterious effects of prolonged subretinal movements. Our patient thus became symptomatic in the usually ‘insidious’ initial stage.1 However, subclinical trauma to macula due to incessant flagellations by the worm probably contributed to the intraoperative bleeding and tear. This case presented a management dilemma: the pre-foveal parasite could neither be photocoagulated, nor killed by pharmacotherapy, which failed in absence of intraocular inflammation.2, 3, 5 In view of the poor long-term prognosis, we recommend early surgical removal in such a case if photocoagulation is not feasible; even in the presence of good vision and no inflammation.

Figure 2
figure 2

Composite fluorescein angiogram of the LE, highlighting the classic subretinal worm-tracks of DUSN, leading up to the macula. The worm itself is not visible.

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