Diffuse Unilateral Subacute Neuroretinitis

Diffuse unilateral subacute neuroretinitis (DUSN) [8] is caused by an unidentified species of nematodes that may wander in the subreti-nal space for 4 years or longer and cause progressive ocular damage. DUSN is characterized by early visual loss, vitritis, papillitis, retinal vasculitis, and recurrent crops of evanescent, grayish white outer retinal lesions that may develop in one eye of an otherwise healthy individual. At a late stage, there is severely reduced vision, optic atrophy, retinal vessel narrowing, and diffuse RPE degeneration. The appearance of the fundus can resemble that in several other diseases at different stages, such as optic neuritis, sarcoidosis, MEWDS, unilateral optic atrophy, unilateral retinitis pigmentosa, among others. Ancylostoma caninum, a hookworm of dogs, is a common cause of DUSN.

One of the reasons this syndrome is included here is because diagnosing DUSN is not always easy and the ERG findings can be helpful especially at the early stage. Full-field ERGs from the affected eye are usually reduced at all stages of the disease and are often moderately or severely reduced, with the b-wave affected more than the a-wave during the later stages of the disease [8-10].

The pathogenesis of DUSN appears to result from a local toxic reaction of the outer retina to the by-products of the worm, which are left in its wake, as well as a more diffuse toxic reaction affecting both the inner and outer retinal tissues. This latter reaction is manifested initially by rapid loss of visual function and alterations of the ERGs, suggesting inner retinal abnormalities. At a more advanced stage, the loss of ganglion cells (optic atrophy) and narrowing of the retinal vessels indicate damage to the inner retina.

There are at least two endemic areas for the nematode in the United States. It is also found in the Caribbean islands and Latin America, and individual cases have been reported from other countries including Japan. The patient to be described [9,10] was a 30-year-old man who had noticed reduced visual acuity 3 years earlier while living in Sao Paulo, Brazil. He was referred to our clinic for a diagnosis and possible treatment. His visual acuity was 1.5 (OD) and 0.07 (OS), and he had a central scotoma in the left eye with a defect in the nasal field.

The rod and cone components of the full-field ERGs were moderately reduced, and the mixed rod-cone ERG had a negative configuration (Fig. 3.25). His multifocal ERG amplitudes in the nasal retina including the macula were reduced, which roughly corresponded with the defect of visual field (Fig. 3.26).As shown in Fig. 3.27, the nematode was not detected initially in our case even with a detailed examination of the fundus (Fig. 3.27A). The blood vessels in the posterior pole of the fundus were attenuated, and mild optic atrophy was present (Fig. 3.27A). Fluorescein angiography showed diffuse RPE mottling (Fig. 3.27B). Twelve days later, a nematode was detected ophthalmoscopically and was photographed by scanning laser ophthal-moscopy (Fig. 3.27C). The nematode was killed by laser photocoagulation (Fig. 3.27D), and examination 2 years later showed no further progression of the visual loss or depression of the ERGs.

Fig. 3.25. Full-field ERGs recorded from a patient with diffuse unilateral subacute neuroretinitis (DUSN) comparing the affected eye and the normal fellow eye. Note that the mixed rod-cone (bright) ERG shows a negative configuration. (From Tanikawa et al. [3])

Fig. 3.25. Full-field ERGs recorded from a patient with diffuse unilateral subacute neuroretinitis (DUSN) comparing the affected eye and the normal fellow eye. Note that the mixed rod-cone (bright) ERG shows a negative configuration. (From Tanikawa et al. [3])

Acute Macular Neuroretinitis

Fig. 3.26. Visual fields of the affected left eye and multifocal ERGs for both eyes in a patient with DUSN. (From Ito et al. [2])

Fig.3.27. A Fundus photograph. B Composite fluorescein angiogram.C Fundus photograph obtained using a scanning laser ophthalmoscope (SLO). D Fundus photograph after killing the nematode by laser photocoagulation.At the initial examination, the nematode was not detected in the fundus (A), but thereafter it was photographed clearly by SLO (C, arrow)

Fig.3.27. A Fundus photograph. B Composite fluorescein angiogram.C Fundus photograph obtained using a scanning laser ophthalmoscope (SLO). D Fundus photograph after killing the nematode by laser photocoagulation.At the initial examination, the nematode was not detected in the fundus (A), but thereafter it was photographed clearly by SLO (C, arrow)

3.4.4 Rubella Retinitis

Rubella retinitis is the most common ocular manifestation of congenital rubella, with the appearance of the fundus taking several forms [11]. Salt-and-pepper mottling of the RPE may be localized only in the posterior pole, or it may extend farther into the peripheral retina (Fig. 3.28). Both eyes are affected in 80% of the patients. These alterations of the RPE may occur alone or may be associated with other ocular abnormalities (e.g., cataracts and microphthalmos) and systemic abnormalities, including deafness and congenital heart disease. Pathologically, the salt-and-pepper changes in the fundus are caused by altered pigmentation and some atrophy of the RPE [12].

Because the retina and choroid are unaffected, full-field ERGs, focal macular ERGs, EOG, visual acuity, and color vision are normal (Fig. 3.29). It should be noted that the results of electrophysiological tests are all normal despite the ophthalmoscopic appearance of a severely affected retina.

Rubella retinopathy may mimic inherited dystrophies of the RPE, the carrier state of X-linked ocular albinism (see Section 2.9, Fig. 2.53), X-linked choroideremia and its carrier state (see Section 2.5, Fig. 2.23), and some toxic diseases of the RPE.

Fig. 3.28. Fundus photograph (top) and fluorescein angiogram (bottom) obtained from a 20-year-old man with rubella retinitis.The visual acuity was 1.5 for both eyes

Fig.3.29. Full-field ERGs and focal macular ERGs recorded from a normal control and the patient shown in Fig.3.28. Despite the marked change in the retinal pigment epithelium, the ERGs are normal

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