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November/December 2006

 
Morning Rounds

The Case of the Young Farmhand Whose Picture Was Worth a Thousand Words
By Christopher L. Haupert, MD
Edited by Thomas A. Oetting, MD
 
 

James Waring* was 19 years old when he noticed that the vision in his right eye had become blurry. The young Iowan chose not to make a fuss—it was, after all, busy season on the farm, and everybody had plenty to do. But two weeks later, one of the other farmhands realized how bad Mr. Waring’s vision had become. She insisted on driving him into town to see an ophthalmologist. The visual acuity in his right eye was found to be strikingly poor and he was referred to us.


We Get a Look

When we saw Mr. Waring, he told us that, vision aside, he felt fine. His past ocular and medical histories were negative and he was taking no medications. He was unable to tell us his family ocular history because he was adopted.

On examination, his visual acuity was counting fingers at four feet in the right eye and 20/20 in the left. His intraocular pressure was 11 mmHg in the right eye and 12 mmHg in the left. Ocular motility and external examination were normal bilaterally. The slit-lamp examination revealed cells in the anterior vitreous of his right eye, but the left eye was normal. On dilated fundus examination of the right eye, the optic disc was pale, the retinal arterioles were attenuated and numerous areas of pigment clumping and hypopigmentation were scattered throughout the posterior pole and midperiphery. Dilated fundus examination of the left eye was normal. A fluorescein angiogram showed window defects scattered throughout the macula, numerous focal areas of hypofluorescence due to pigment clumping, and staining of the optic disc—all in the right eye. The attenuation of the retinal vessels that we had seen earlier was also evident on this study. Views of the left eye showed a normal fluorescence pattern.

A Discovery

Is Made At this point, the cause of the severe visual loss and striking fundus appearance was still unclear. We took numerous photographs of the fundus of Mr. Waring’s right eye, thoroughly scanning the periphery as well as the posterior pole, and then asked him to go home for the day.

We then studied these photographs very carefully. On one of them, we discovered a subtle but distinct white “figure eight.” It was nasal to the optic disc, in the subretinal space (Fig. 1). We believed we had found the culprit.

Mr. Waring returned to our office the next day and we performed another careful examination of the fundus to confirm our diagnosis. As expected, a tiny worm was identified. It was continually changing shape and migrating about the subretinal space. At this point, it was located in the inferior macula (Fig. 2).

We diagnosed Mr. Waring with diffuse unilateral subacute neuroretinits (DUSN). Interestingly, by the time he was ready for laser treatment, the nematode had migrated to the fovea. After patiently waiting for it to migrate well away from the fovea, we and we used laser photocoagulation to destroy the worm without difficulty. Unfortunately, Mr. Waring had significant scarring and his visual acuity in the right eye remained unchanged in the months following treatment.


What's Your Diagnosis?
AM
(Fig 1) Mr. Waring’s fundus exam revealed retinal arteriolar attenuation and pigment clumping in his right eye. After studying his fundus photos very carefully, we noticed a subtle “figure eight” nasal to the optic disc.





What's Your Diagnosis?
AM 2
(Fig 2) When Mr. Waring returned the following day, the subretinal nematode has changed its shape and migrated to the inferior macula. Optic disc pallor is also evident in this photograph.


Discussion

DUSN describes a clinical syndrome in which a subretinal nematode incites inflammation that initially causes papillitis, retinal vasculitis and multiple grayish lesions in the outer retina. These findings eventually give way to optic disc pallor, arteriolar attenuation and diffuse degeneration of the retinal pigment epithelium.

The nematode migrates very slowly about the fundus and continually changes its configuration. It is typically identified on careful ophthalmoscopic examination.

Severe visual loss can occur early in the course of the disease and uniformly occurs in the later stages. The disease is almost always unilateral, but bilateral involvement has been reported.1

Making the diagnosis. In its early stages, the differential diagnosis of DUSN includes diseases that cause focal or multifocal chorioretinitis, such as histoplasmosis, multifocal choroiditis, sarcoidosis and West Nile chorioretinitis.

In its later stages, the differential diagnosis includes conditions that cause optic atrophy, arteriolar attenuation and diffuse pigmentary abnormalities, such as sarcoidosis, post-traumatic chorioretinopathy, vascular occlusive disease, syphilitic chorioretinitis and unilateral retinitis pigmentosa.

DUSN is a clinical diagnosis, based on a careful dilated fundus examination, although it can be extremely difficult to identify the tiny nematode with ophthalmoscopy alone. Careful study of numerous fundus photographs can greatly facilitate the identification of the causative organism, as in this case.

The likely cause. Multiple species have been suspected to cause DUSN. In the United States, DUSN is endemic to the southeast and midwest.2 Baylisascaris procyonis, a roundworm associated with raccoons, has been implicated as a cause of DUSN2 and was the likely causative agent in this case, as our patient was often working in close proximity to raccoons on the farm. In fact, on further questioning, he recalled that he had been working in a shed, when its ceiling—along with a large amount of raccoon feces—had caved in on top of him.

Treatment. The treatment of choice for DUSN is laser photocoagulation of the nematode. In cases when the disease has been identified early in its course, visual improvement has been reported following laser treatment. But in advanced cases such as this one, improvement is highly unlikely despite laser treatment. The main goal of treatment in most cases is to prevent further visual loss. Systemic anthelminthics are sometimes used, but their efficacy has not been clearly established.3,4,5** Pars plana vitrectomy with transvitreal extraction of the worm has been described6 but is not typically indicated.

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*Patient name is fictitious.

**Online addendum: One study in particular found that the use of an antihelminthic, albendazole, for DUSN was quite effective in a series of patients—Souza, E. C, et al, Am J Ophthalmol 2005 Sep;140(3):437–445.

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Dr. Haupert is in private practice with Iowa Retina Consultants in West Des Moines, and is an adjunct clinical professor at the University of Iowa.

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1 de Souza, E. C. et al. Arch Ophthalmol 1999;117(10):1349–1351.
2 Goldberg, M. A. et al. Ophthalmology 1993;100:1695.
3 Gass, J. D. and P. A. Braunstein. Arch Ophthalmol 1983;101:1689.
4 Casella, A. M. et al. Am J Ophthalmol 1998;125:109–111.
5 Myint et al. Br J Ophthalmol 2006;90:1125–1127.
6 de Souza, E. C. and Y. Nakashima. Ophthalmology 1995;102:1183.


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