Diffuse unilateral subacute neuroretinitis (DUSN) is a rare form of ocular larva migrans associated with damage to outer retinal layers and retinal pigment epithelium (RPE) by motile parasitic worms such as Ancylostoma caninum, Toxocara canis, Ascaris lumbricoides, Baylisacaris procyonis, and Dirofilaria spp. We report a case of DUSN in a 37-year-old male, who presented with sudden onset of decreased vision in OS since one week. His best corrected visual acuity (BCVA) was 6/6, N6 (OD) and 4/60, N36 (OS). The OD examination was unremarkable. Fundus examination of OS revealed mild vitritis, multiple RPE de-pigmented patches, multifocal evanescent gray-white lesions in outer retina along with multifocal neurosensory detachment (NSD) involving macula. A glistening white, non-segmented motile worm approximately 2 disc diameter (~3000 μm) with tapered ends was located in supero-temporal quadrant outside temporal arcade [Fig. 1a] with track marks in inferior quadrant [Fig. 1b]. Snake-like wriggling movements of worm increased on light stimulation by fundus photography [Fig. 1c–d]. Fluorescein fundus angiography (FFA) revealed multiple RPE leaks with window defects and pooling of dye in NSD's [Fig. 2]. Spectral domain-optical coherence tomography (SD-OCT) demonstrated NSD's over macula and surrounding area with unhealthy RPE [Fig 3a].
Argon laser barrage (Zeiss Inc.; 45 spots, 200 mW, 200 μ, 350 ms) of 532 nm was performed in an attempt to restrict mobility of worm [Fig. 4a]. The worm showed increased movements in an attempt to come out of barraged area [Fig. 4b–d]. When it succeeded [Fig. 4e], it was killed directly by laser (15 spots, same settings; Fig. 4f). Although, the effect of laser barrage may take 1-2 weeks, continuous evaluation on experimental basis was done to gauge its effectiveness by limiting further worm migration, for which we had restricted favorable result, as the worm could migrate out of the barraged area only after 15 minutes of coiling and uncoiling effort. Pre-laser 3D SD-OCT revealed the worm as subretinal coiled hyper-echogenic structure [Fig. 3b] while immediately post-laser, it showed intense laser reaction in form of hypoechogenic areas [Fig. 3c]. Single dose of oral albendazole 400 mg along with tapered dose of oral prednisolone (starting from 60 mg/day) and diethylcarbazine 100 mg three times/day were given for 3 weeks.
Next day, a small streak of blood was seen along the vessel in region of worm [Fig. 4g]. SD-OCT findings showed minimal increase in OCT thickness compared to the previous day, which could be due to inflammatory reaction [Fig. 3d]. Though investigation including blood and stool was negative.
On day 10 of post-laser, BCVA was stagnant at 4/60. Fundus examination demonstrated minimal vitritis, faded laser spots and dead worm (Interrupted white line; Black arrowhead in Fig. 4h). 3D SD-OCT revealed sub retinal fibrosis in lasered region [Fig. 3e].
On day 25, BCVA was unchanged. The vitritis resolved with complete disintegration of worm [Fig. 4i]. SD-OCT shows fibrotic scar [Fig. 3f–g] along with normal foveal contour with damaged photoreceptor and RPE [Fig. 3h]. The patient was lost to follow-up following this visit.
Direct killing of worm with laser has been associated with significant increase in intraocular inflammation. In other parts of body, systemic treatment may be associated with fever, myalgia and headache.
There has only been a single correspondence regarding post-laser OCT analysis of worm. We have utilized 3D SD-OCT imaging technique to analyze post-laser worm degradation along with retinal inflammation secondary to worm toxins and laser.
In conclusion, our report adds to an existing data for laser photocoagulation being used as first line of management for cases of DUSN, in which live worm can be identified. OCT is an additional tool other than fundus photograph, which may be helpful in understanding the amount of photoreceptor and RPE destruction caused by worm, which may not be visible on fundus photographs. It can be proposed as another vital non-invasive modality in our armamentarium along with fundus photographs to image the eyes with DUSN.
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