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Case ReportWait Until Dark: An Atypical Case Of Night BlindnessSherry J. Bass, O.D., F.A.A.O. Adam Rubin, O.D. New York
A 33-year-old black male with a previous diagnosis of congenital stationary night blindness (CSNB) presented for a routine eye exam. When he was 8 years old, the patient complained that it was difficult for him to get around in the dark, and that he needed the lights on in order to see anything. His older brother had been previously diagnosed with CSNB. Electroretinography (ERG) at that age had revealed a reduced B-wave under photopic and scotopic illumination, which supported a diagnosis of CSNB. Diagnostic DataAt this visit, the patient’s best-corrected visual acuities were 20/30 O.D. and 20/50 O.S., unchanged from previous records. External examination revealed pupillary responses somewhat sluggish to light, but normal to accommodation. This had also been noted in earlier examinations.Extraocular motilities and confrontation visual fields were full, and anterior segment evaluation was unremarkable. Intraocular pressures were 14mm Hg in each eye. Subjective refraction was -8.50-1.50 x 70 O.D. and -8.50 -1.75 x 105 O.S. Funduscopy revealed small, anomalous elevated optic nerve heads with
poorly defined disc borders (figures 1,2). Maculas,
vasculature and fundus grounds were otherwise normal.
B-scan ultrasonography demonstrated elevations of increased sonoreflectance overlying the optic nerve head that persisted with decreased sensitivity settings (figures 3,4). This was consistent with calcification typical of disc drusen. Visual field testing (Full Field 120) revealed a relative nasal defect in the right eye and isolated inferior relative defects in the left (figure 5). Treatment and Follow-upSince there is no treatment for CSNB or buried disc drusen, we educated the patient of the finding of an optic nerve head anomaly and its relationship to CSNB. We continue to monitor him annually.DiscussionCongenital stationary night blindness (CSNB) has varied phenotypic expressions, but they all have one thing in common: Patients usually have difficulty functioning in dim illumination. However, unlike other debilitating night vision disorders, such as retinitis pigmentosa, CSNB is non-progressive. When associated with myopia, CSNB is a documented etiology of reduced visual acuity. Optic nerve head anomalies have also been an associated finding. To our knowledge, this is the first report of small optic discs and optic disc drusen associated with CSNB and myopia.CSNB is actually a group of inherited disorders, all characterized by a non-progressive loss of night vision. Patients with CSNB fall into two categories: those having a normal fundus and those having specific fundus abnormalities, such as fundus albipunctatus. The latter is characterized by white, uniform opacities distributed throughout the retina and sparing the macula. Patients who have normal fundus grounds, like the one in this case, are further divided into autosomal dominant, autosomal recessive and X-linked modes of inheritance (see Types of CSNB). Interestingly, some patients with CSNB have an associated optic nerve anomaly. Tilted discs, missing portions of the disc, disc pallor, or misshapen discs have all been reported in patients with CSNB.7 Only one case of drusen of the optic nerve head has been reported with CSNB, and this was in a hyperopic patient (hyperopes more typically have small, crowded discs).8 No myopic patient has been previously reported to have disc drusen and CSNB. As figures 1 and 2 show, our patient’s optic nerve heads were small and elevated with blurred disc borders. Included in the differential for the reduced visual acuity is optic disc hypoplasia. Because no one has fully explained the reasons for decreased acuity in CSNB with myopia, it’s difficult to determine why this particular patient’s acuity is reduced. Optic disc drusen or hyaline bodies are deposits that present in less
than 1% of the population.9
Caucasians and patients with small, crowded discs are most likely to have
disc drusen, and younger patients up to age 20 may have buried disc drusen.11
Our patent’s optic nerve heads were elevated, consistent with buried drusen
deposits as demonstrated with B-scan. Using a normal gain or sensitivity,
sonography may display optic nerve head elevations. However, when the gain
is reduced, calcified deposits such as drusen remain sonoreflective even
after the other tissues have disappeared from view (figures 3,4). As patients
get older, optic nerve head drusen are easier to detect since they surface
with aging.
Optic nerve heads with drusen typically have no optic cup, making it difficult to diagnose glaucoma when intraocular pressures are elevated. What complicates this differential further is that while disc drusen usually don’t affect visual acuity, visual field testing most commonly demonstrates nasal field defects but can be variable.9 Eila Mustonen reported arcuate defects in 87 of 193 patients (45%) with drusen.10 Monitor glaucoma suspects with buried disc drusen with visual fields at least every 6 months. This is also why it’s important to get baseline visual fields in patients with optic disc drusen or any other disc anomaly that make it difficult to view the cup. Visual fields in CSNB generally are normal under standard testing conditions.
The nasal visual field of this patient’s right eye is constricted, while
the left field defects are scattered inferiorly. This is likely due to
disc drusen, since CSNB is not associated with visual field defects, and
the patient’s intraocular pressures were normal.
Buried drusen of the optic nerve are not related to the retinal drusen you might see in a patient with age related macular degeneration. Optic nerve head drusen are inherited in an autosomal irregular pattern, and they develop slowly. Because the disc borders are often blurred and the disc appears elevated, an important differential is papilledema. Fluorescein angiography is a useful adjunctive procedure in that papilledema will result in leakage of fluorescein (hyperfluoresence) from the optic nerve head. Drusen may autofluoresce, but will not hyperfluoresce. This myope has the relatively rare findings of CSNB and buried disc
drusen, which have been reported together only once before, but not in
a myope.8 Careful monitoring
will be necessary to differentiate the development of other ocular diseases
from his current conditions.
References
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Types of CSNBCSNB patients who have normal fundus grounds may have the autosomal dominant mode of inheritance, the autosomal recessive form or the X-linked form. The classification of CSNB hinges on the inheritance pattern, ERG results and other findings.The autosomal recessive and X-linked forms—the latter is the most common—fall into the Schubert-Bornschein type of CSNB.1 A normal or nearly normal A wave but an attenuated B wave (electronegative wave) under scotopic conditions characterizes the ERG in these patients. This may be due to a defect in neural transmission in the outer retina.2 Visual acuity can be reduced, especially when associated with myopia, as in our patient. Some patients also demonstrate nystagmus.3 The autosomal dominant form (known as the Nougaret type) differs from the Schubert-Bornschien type in that the ERG typically is not electronegative, and visual acuity is normal, unless associated with myopia.4 There is also the Riggs type, in which the ERG likewise is not electronegative, but the pattern of inheritance is unclear.5 Yozo Miyake in 1986 proposed a new classification of CSNB based on the electronegative ERG psychophysical dark adaptation tests into a complete type (patients totally lacking rod function) and an incomplete type (patients having some remaining rod function).6 |
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