eMedicine Specialties > Ophthalmology > Retina

Neuroretinitis, Diffuse Unilateral Subacute

Author: Lakshmana M Kooragayala, MD, Vitreo-retinal Surgeon, Marietta Eye Clinic
Contributor Information and Disclosures

Updated: May 26, 2009

Introduction

Background

Diffuse unilateral subacute neuroretinitis (DUSN) is a progressive parasitic disease affecting the outer retina and retinal pigment epithelium (RPE). This syndrome is primarily unilateral, although bilateral cases have occurred. The ocular findings include visual loss, vitreous cells, optic disc inflammation and leakage, and transient recurrent crops of gray-white outer retinal lesions.

Stationary or migrating nematodes have been identified deep in the retina or in the subretinal space. Later in the course of the disease, slowly progressive RPE changes and optic atrophy may be observed, as well as narrowing of the retinal vessels.

Pathophysiology

The exact pathophysiology is uncertain, but the local inflammatory changes may be related to toxic effects or immunologic stimulation from excretory products of the larva or from release of unknown soluble tissue toxins. The fleeting gray-white lesions in the outer retina appear to be a local reaction to noxious stimulation. The loss of vision and progressive optic atrophy secondary to death of ganglion cells and neural fibers may be a remote reaction to soluble toxins.

Frequency

United States

The southeast and the upper Midwest are known endemic areas for the disease.

International

DUSN has been reported initially in the Caribbean islands, Brazil, Ghana, and Germany. In the last few years, DUSN has been reported in many other countries, including China, India, South Africa, and Spain.

Mortality/Morbidity

• No cases of mortality have been reported. Four cases of severe neurologic degeneration with DUSN have been reported in children.
• The natural history of untreated DUSN involves multiple recurrent episodes of diffuse and focal inflammation of the retina and RPE with secondary progressive visual loss and optic atrophy in the affected eye.
• It rarely affects the fellow eye. Only 2 cases with bilateral involvement have been reported.

Race

DUSN does not show any particular racial preference.

Sex

This condition occurs more frequently in males than in females.

Age

It occurs most frequently in the second and third decades. Young children and older adults also may be affected.

Clinical

History

• Early stage
  • Mild-to-moderate visual loss
  • Paracentral or central scotomas
  • Floaters
  • Ocular discomfort
  • Conjunctival injection of affected eye
• Late stage
  • Severe unilateral visual loss
  • Paracentral or central scotomas
• In some patients, the disease may be asymptomatic with the characteristic changes found only on routine eye examination.

Physical

Patients should undergo a complete eye examination, including visual acuity, pupillary reactions, visual fields, slit lamp examination of the anterior and posterior segments, indirect ophthalmoscopy, and detailed examination of the retina using a fundus contact lens.

• Early stage
  • Visual acuity - Range is from 20/30 to 20/200 or less.
  • Visual field - Paracentral or central scotoma may be detected.
  • Pupils - A relative afferent pupillary defect may be noted.
  • Anterior segment findings reveal normal conjunctiva or conjunctival injection, ciliary flush, anterior chamber cells and flare, fine keratic precipitates, and small hypopyon.
  • Posterior segment examination findings reveal mild-to-moderate vitritis, optic disc swelling, narrowing of the retinal arterioles, retinitis, and nematodes.
    • Retinitis is the most characteristic feature of this syndrome. Transient, multiple, focal, gray-white lesions of the deep retina or RPE vary in size from 0.25-1 disc diameter and tend to develop in clusters over wide areas of the retina at various time periods. The active evanescent gray-white lesions fade within a period of 7-10 days as the nematode moves elsewhere in the eye, only to recur in an adjacent area or distant site over the ensuing weeks. Lesions typically resolve without any ophthalmoscopic or angiographic evidence of damage.
    • Nematode: Identification of the subretinal worm is the pathognomonic finding in DUSN. To localize the worm, careful and repeated examination with a fundus contact lens is required. The worm can be present in all layers of the retina, but it most frequently is found in the subretinal or outer retinal layers. The motile worm is more likely to be observed in the neighborhood of the active grayish-white retinal lesions. The worms appear smooth in outline, tapered on both ends, and often assume an S-shaped, coiled, or figure "8" configuration. These organisms propel themselves by a coiling and uncoiling motion and sometimes move in a snakelike fashion in the subretinal space. They may be noted to move under direct observation in an apparent aversion to bright light, and a white glistening sheen may be noted over the region.
    • Other less frequently encountered clinical signs include the following: focal retinal and subretinal hemorrhages, perivenous exudates and vascular sheathing, localized serous detachments of the neurosensory retina, cystoid macular edema, retinal striae, and choroidal neovascularization.
• Late stage
  • Visual acuity - Typically less than 20/400
  • Visual fields - Dense central or paracentral scotoma may be seen.
  • Pupils - Relative afferent pupillary defect possible
  • Posterior segment examination reveals the following findings:
    • Focal and diffuse loss or mottling of the RPE, most typically seen in the paramacular region, sparing the center of the macula in most patients
    • Generalized narrowing of the retinal arterioles
    • Marked optic disc pallor
    • Choroidal neovascularization and/or disciform scarring
    • Peripapillary arteriolar sheathing

Causes

• Precise identification of the worm has not been accomplished, but two different types of worms have been recognized in endemic areas.
• In the southeastern United States, Caribbean, and Latin America, a larval worm measuring about 400-700 µm has been recognized. It is presumed to be Ancylostoma canium, which is a known frequent cause of cutaneous larval migrans.
• In the north midwestern United States, a larger worm measuring 1000-2000 µm has been observed. It is proposed by some authors to be Baylisascaris procyonis and is a rare cause of visceral and ocular larval migrans.

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