Diffuse Unilateral Subacute Neuroretinitis (DUSN) is a multifocal chorioretinitis caused by a nematode migrating in the subretinal space. It was first reported by Gass et al. in 1978 and was initially called “unilateral wipeout syndrome” 3). It commonly occurs in healthy children and young adults 1), and is typically unilateral, although bilateral cases have been reported 3).
Several causative nematodes are known, classified by the size of the worm and the presumed endemic region.
| Worm species | Body length (approximate) | Main endemic regions |
|---|---|---|
| Baylisascaris procyonis | 1500–2000 μm | Midwestern United States, North America |
| Toxocara canis | 400–1000 μm | Southeastern United States, tropical regions |
| Ancylostoma caninum | 400–1000 μm | Tropical/subtropical |
| Gnathostoma spinigerum | 400–1000 μm | Asia (Thailand, etc.) |
Endemic areas are widespread, including the Midwest and Southeast of the United States, Brazil, Caribbean islands, India, and Malaysia. In Malaysia, the prevalence of soil-transmitted helminths (STH) is reported to reach 52–76% 4).
Although typically unilateral, bilateral cases have been reported 3). Delayed treatment in one eye or the patient’s immune status may lead to involvement of the fellow eye. However, simultaneous bilateral onset is rare, and if findings in one eye are asymmetric to the other, a detailed examination considering DUSN is necessary.
The clinical findings of DUSN vary significantly depending on the stage of the disease.
Early Findings
Optic papillitis: Mild optic disc edema and hyperemia.
Vitreitis: Mild to moderate vitreous opacity (around grade 1)1).
Clusters of gray-white subretinal lesions: Multiple white spots distributed in clusters1).
Mobile subretinal track: The path of nematode migration. Observed in 91.7% of cases1)4).
Vasculitis: Inflammatory findings around retinal blood vessels1)4).
Late Findings
Optic atrophy: Pallor of the optic disc. Appears within months if early treatment is not provided1).
Retinal arteriolar narrowing: Observed in cases lasting more than 18 months1).
Diffuse RPE degeneration: Diffuse atrophy and degeneration of the retinal pigment epithelium.
Positive RAPD: Relative afferent pupillary defect. Evidence of optic nerve damage1)3).
Atypical clinical presentations include extensive exudative retinal detachment (visual acuity reduced to hand motion or no light perception)1), granulomatous anterior uveitis (mutton-fat KP, anterior chamber inflammation 4+, secondary ocular hypertension 38 mmHg)4), and cystoid macular edema (CME)4).
The route of infection for DUSN is primarily through contact with host animals or exposure to contaminated environments.
It commonly occurs in healthy children and young adults, but can also develop without any specific underlying disease 1). Risk factors include travel or residence in endemic areas (Midwest and Southeast United States, Brazil, tropical regions of Asia), contact with wild animals such as raccoons, and consumption of undercooked food.
Definitive diagnosis requires direct visualization of the nematode in the fundus, but this is possible in only about 25–39% of all cases1). Comparison of serial fundus photographs to confirm the worm’s movement pattern is useful1).
The characteristics of the main imaging tests are shown in the table below.
| Imaging modality | Key findings/features |
|---|---|
| Infrared live imaging | Enables tracking and visualization of photophobic nematodes using 900 nm light2) |
| Enface swept-source OCT | ”Wormhole” cavities along the nematode migration path (first report)3) |
| MultiColor scanning laser | GR (515 nm) and BR (488 nm) visualize the worm body3) |
| OCT (tomography) | Coiled nematode in subretinal fluid visualized1) |
| FFA | Diffuse RPE changes, leakage from the optic disc1)4) |
| FAF | Increased autofluorescence around the optic disc and macula1) |
Enface swept-source vitreous OCT reveals cavities “wormholes” at the internal limiting membrane (ILM) level along the migration path of the nematode, which were first reported as traces of the nematode moving into the vitreous 3). Infrared live imaging can track the worm in real time using 900 nm near-infrared mode, utilizing the nematode’s photophobic behavior (avoidance of visible light 450–490 nm) 2).
Differential diagnosis includes central serous chorioretinopathy, toxoplasmic retinochoroiditis, sarcoidosis, syphilitic retinitis, multiple evanescent white dot syndrome (MEWDS), and cat scratch disease (CSD). Coinfection of CSD and DUSN has been reported 4), so it is important not to adhere to a single diagnosis.
The nematode can be directly visualized in only about 25–39% of cases1). When not visible, diagnosis is made clinically based on a combination of unilateral multifocal white dot lesions, optic neuritis, vitritis, eosinophilia, travel history to endemic areas, and animal contact. It is recommended to attempt detection of the nematode using multimodal imaging such as infrared live imaging and en face OCT.
Treatment for DUSN differs depending on whether the nematode is visible. Combination therapy with laser photocoagulation, oral albendazole, and systemic steroids is associated with the best prognosis1).
This is the first-line treatment when the nematode is visible1).
The standard dosage and duration of albendazole have not been established1)2), but several regimens have been reported.
| Dosing Regimen | Notes |
|---|---|
| 400 mg/day × 30 days | Standard simple regimen2) |
| 400 mg twice/day × 30 days | High-dose regimen2) |
| 200 mg × 2/day × 6 weeks | Divided dose1) |
| 400 mg × 2/day × 6 weeks | High dose, long term1) |
| 400 mg/day × 6 weeks | Single dose, long term4) |
In eyes with vitritis, albendazole shows improved intraocular penetration due to disruption of the blood-retinal barrier (BRB), leading to higher therapeutic efficacy1).
Steroids (corticosteroids): For inflammation control, prednisone 40–60 mg/day is tapered over 2–4 weeks (e.g., schedule: 30 mg → 20 mg → 10 mg → 5 mg)2).
Treatment for coexisting infection: For cases complicated by CSD, doxycycline 100 mg twice daily for 6 weeks4). For secondary ocular hypertension, timolol, brimonidine, and acetazolamide are used4).
Laser Photocoagulation
Indication: When the nematode is directly visible in the fundus.
Principle: Irradiate immediately after visualization. Surround the nematode from 1–2 DD behind its location.
Caution: In cases complicated by exudative retinal detachment, laser may not reach the target1).
Albendazole
Indication: When the nematode is not visible, or as an adjunct to laser therapy.
Dosage: 400 mg/day to 400 mg twice daily for 30 days to 6 weeks1)2).
Note: Intraocular penetration is improved in cases complicated by vitreitis 1).
Steroids
Indication: Adjunctive therapy when inflammation is severe.
Dosage: Prednisone 40–60 mg/day, tapered gradually 2).
Purpose: Suppression of vitreitis, vasculitis, and optic neuritis.
If laser photocoagulation is performed early, visual recovery can be expected. However, in late-stage cases with optic atrophy or extensive RPE degeneration, irreversible visual impairment remains1). Early consultation within one month in patients aged 20 years or older has been shown to be associated with better final visual acuity1), highlighting the importance of early intervention.
Retinal damage in DUSN is caused by a combination of local mechanical and toxic stimuli from the nematode migrating in the subretinal space and the host immune response (inflammatory reaction). This mechanism is described as “toxic autoimmune nematode retinopathy”1).
The nematode initially migrates confined to the subretinal space but may also move into the vitreous cavity. “Wormholes” observed on enface swept-source vitreous OCT have been reported as cavities representing traces of nematode migration within the vitreous3), suggesting that B. procyonis may randomly migrate through all layers of the retina3).
The timeline of the pathology is as follows.
Both toxins produced by the worm and the host immune response cause retinal damage1). In the early stage, only reversible photoreceptor dysfunction occurs, but in the late stage, optic atrophy, diffuse RPE degeneration, and retinal arteriolar narrowing progress, leading to irreversible vision loss. Therefore, the longer the worm remains in the eye, the worse the visual prognosis.
Hänsli et al. (2024) performed real-time tracking of a nematode using infrared live imaging (900 nm) and administered navigated laser photocoagulation (Navilas 577s) in a 14-year-old boy (best corrected visual acuity 20/100) who developed DUSN after traveling to Thailand2).
By combining nematode visualization in infrared mode immediately before laser irradiation with a 4×4 pattern (100 μm, 400 mW, 20 ms, 41 spots), photocoagulation was accurately completed while suppressing the photophobic nematode’s avoidance behavior2). After treatment, albendazole (400 mg/day × 30 days) and prednisone (30 mg → tapered) were added, and serum Toxocara antibody positivity was confirmed.
Sodhi et al. (2021) performed enface swept-source vitreous OCT in a 37-year-old woman (best corrected visual acuity CF, vitreous cells 1+) with DUSN and recorded for the first time worldwide a cavity (“wormhole”) at the ILM level corresponding to the nematode’s migration path3).
Conventional tomographic OCT confirmed nematodes at the ILM level, but enface vitreous OCT images continuously depicted the migration path itself as a cavity 3). This finding is evidence that the nematode had invaded the vitreous and supports the hypothesis that B. procyonis randomly migrates through the full thickness of the retina.
In the same case, using MultiColor scanning laser imaging (green reflectance 515 nm, blue reflectance 488 nm) revealed the entire image of the nematode, which is difficult to see on white-light fundus photographs 3). These multimodal approaches are expected to contribute to improved diagnostic rates in cases where the nematode is not directly visible 1)2)3).
