A 10-year-old Hispanic female presented as an emergency case with decreased vision in her right eye that had persisted for a week. The girl's parents accompanied her and informed me that, "she bumps into things." When questioned, the girl reported that she could not see anything out of her right eye. She denied any history of illness, ocular infection or trauma. Her medical history was unremarkable. Her last eye examination was two years ago.
On examination, her visual acuity measured bare light perception (LP) O.D. and 20/20 O.S. She was unable to perform confrontation visual field testing O.D. due to her vision loss, but she was full to careful finger counting O.S. Her pupils were equally round and reactive, with a 3+ afferent pupillary defect O.D. The anterior segment examination was unremarkable.
Dilated fundus exam of the right eye showed grade 1 to grade 2 vitritis and mild optic nerve pallor. Other changes can be seen in figure 1. Optical coherence tomography (OCT), fluorescein angiography (FA) and fundus autofluoresence were also performed. Her left eye was completely normal.
1. Retinal montage image of our patients right eye. Note the peculiar lesions seen throughout the posterior pole.
Take the Retina Quiz
1. How would you classify this condition?
a. Developmental.
b. Idiopathic.
c. Infectious.
d. Inflammatory.
2. What is the underlying etiology of this condition?
a. Autoimmune.
b. Nematode.
c. Bacteria.
d. Spirochete.
3. What is the correct diagnosis?
a. Multifocal choroiditis and
panuveitis.
b. Active toxoplasmosis
infection.
c. Diffuse unilateral subacute
neuroretinitis (DUSN).
d. Toxocara canis.
4. What is the most effective treatment for this condition?
a. Laser photocoagulation.
b. Thiabendazole.
c. Bactrim (sulfamethoxazole/
trimethoprim, AR Scientific)
d. Intravitreal Avastin
(bevacizumab, Genentech).
For answers, see below.
Discussion
Our patient has a unique condition: diffuse unilateral subacute neuroretinitis. DUSN is caused by a nematode that may wander in the subretinal space for several years. The condition was first identified in 1978 by J. Donald M. Gass, M.D., who reported on 36 healthy patients with a presentation of
unilateral profound vision loss,
afferent pupillary defects in the presence of vitritis and mild papilledema, as well as peculiar crops of multiple, deep, evanescent, gray-white retinal lesions.1 In two cases, Dr. Gass was able to observe a motile, subretinal worm; he believed the worm resulted from a Toxocara canis infection.1
In 1983, Dr. Gass provided further insight when he published another report on approximately 100 patients.2 In this series, most of the patients were young (age 11 to 65, with a mean of 24 years of age) and lived in the southeastern U.S. and Caribbean. He speculated that the warmer climate might foster transmission of the parasite because its eggs thrive in the soil. All
reported cases of infection were unilateral, and patients usually demonstrated advanced vision loss at the time of presentation.2 Additionally, most patients not only had central or paracentral vision loss, but also complained of discomfort, as well as transient vision loss.
Since these initial reports, DUSN cases have been identified in northern climates as well.3 At least two worms have been identified—both intestinal round worms. The first is Ancylostoma caninum, a common hookworm parasite found in dogs. It is a smaller worm (measuring 400m to 1,000m) and is more commonly found in the southern
DUSN-related infections are acquired via contact with sand or soil, not by direct interaction with a dog or raccoon. It is believed that the eggs hatch after ingestion. Then, the larvae penetrate the stomach wall and migrate to a variety of tissues, including the eye. Once the worm finds its way into the eye, it can exist in the subretinal space for up to four years. During times of activity, the worm may travel around the eye within the subretinal space and leave byproducts, which manifest as evanescent, multifocal, gray-white lesions at the level of the outer retina. These active evanescent lesions disappear in one to two weeks as the worm moves elsewhere in the eye. Once the active lesions disappear, the retinal pigment epithelium (RPE) takes on a diffuse, mottled appearance. As a result, patients often present with a significant vitritis, papillitis and retinal vasculitis.1-3
Vision loss in patients with DUSN generally results from toxic dysfunction of the retina and optic nerve. Failure to locate and kill the worm often results in progressive optic atrophy, narrowing of the retinal arteries, and degenerative changes within the RPE and retina. Unfortunately, because DUSN is more commonly seen in young children, the condition often goes undiscovered until very late, after significant damage has occurred.
The best treatment for this condition is to find the worm and destroy it with laser photocoagulation. However, because the worm is very small and mobile, this is easier said than done. Also, when the worm is discovered in young children, you may have difficulty gaining sufficient cooperation from them to perform extended ophthalmoscopy. In these instances, the use of wide-angle photography may help to locate and identify the worm so laser can be applied.
If the worm cannot be found, anthelminthics have demonstrated some success. Thiabendazole was the first anthelminthic to be employed, but the drug does not cross the blood-retina barrier unless the patient has a moderate to severe vitritis.2 More recently, albendazole has emerged as an effective treatment (400mg/day for 30 days).3 When using albendazole, several reports document complete inactivation of the worm in as few as four weeks, without any adverse drug-induced side effects.3
DUSN should be suspected in any patient (especially a child or young adult) who presents with either unilateral optic atrophy or disc swelling in the presence of vitritis and diffuse RPE mottling throughout the fundus. In many instances, the disease likely goes unrecognized because children often do not complain of unilateral vision loss. As a result, patients may present years later, when the disease has run its course. In these cases, patients may be older with optic atrophy, quiet vitreous, attenuated vessels and diffuse atrophy of the RPE throughout the fundus.
In our patient, we identified the worm over a series of two exams administered within a one-week period. Once the worm was located, we applied laser photocoagulation. During the subsequent week, the vitritis slowly began to clear and her vision improved from LP to 20/80 O.D. one month after treatment. We hope that her vision will continue to improve.
Answers to the Retina Quiz: 1) C; 2) B; 3) C; 4) A.
1. Gass JD, Gilbert WR Jr, Guerry RK, Scelfo R. Diffuse unilateral subacute neuroretinitis. Ophthalmology 1978 May;85(5): 521-45.
2.Gass JD, Braunstein RA. Further observations concerning the diffuse unilateral subacute neuroretinitis syndrome. Arch Ophthalmol 1983 Nov;101(11):1689-97.
3. Garcia CA, Sabrosa NA,
