Congenital Night-Blinding Disorders With Normal Fundi
Congenital stationary night blindness (CSNB) is a nonprogressive disorder of night vision. CSNB has 3 genetic subtypes:
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X-linked (most common)
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autosomal recessive
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autosomal dominant (rare)
Snellen visual acuities of patients with CSNB range from normal to occasionally as poor as 20/200, but most cases of decreased vision are associated with significant myopia. The appearance of the fundus is usually normal, except for myopic changes in some cases. Patients commonly present with difficulty with night vision; nystagmus and reduced visual acuity may also be presenting symptoms. Some CSNB patients never report nyctalopia (night blindness), perhaps because they are accustomed to it. Some patients may have a paradoxical pupillary response, in which the pupil dilates when the ambient light dims. Dark-adaptometry curves reveal markedly reduced responses (Fig 12-1).
Electroretinography is important in the diagnosis of CSNB. The most common ERG pattern seen in patients with CSNB is the negative ERG (the Schubert-Bornschein form of CSNB), in which the bright-flash, dark-adapted ERG has a normal (or near-normal) a-wave but a markedly reduced b-wave. The normal a-wave excludes significant rod photo-receptor dysfunction, and the result thus facilitates the differentiation of CSNB from the potentially blinding disorder retinitis pigmentosa (RP; see Chapter 3).
X-linked CSNB has been categorized into 2 types: “complete” and “incomplete.” Patients with complete CSNB (cCSNB) have an undetectable rod-specific, dim-flash, dark-adapted ERG and psychophysical thresholds that are mediated by cones. Patients with incomplete CSNB (iCSNB) have some detectable rod function on ERG and an elevated dark-adaptation final threshold (Fig 12-2). The differential diagnosis of CSNB includes melanoma-associated retinopathy (MAR), which demonstrates an identical ERG pattern to CSNB, but which usually presents with acquired night blindness and shimmering photopsias.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.