Congenital Night-Blinding Disorders With Normal Fundi
Congenital stationary night blindness (CSNB) is a nonprogressive disorder of night vision. CSNB has 3 genetic subtypes:
X-linked (most common)
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).
Figure 12-1 Dark-adaptometry curve in congenital stationary night blindness (CSNB). The dark-adaptometry curve of this patient (dotted curve) with CSNB shows no rod adaptation. Dashed curve indicates normal response.
(Used with permission from Ripps H. Night blindness revisited: from man to molecules. Proctor lecture. Invest Ophthalmol Vis Sci. 1982;23(5):588–609.)
Figure 12-2 Electroretinography (ERG) patterns of on- and off-responses in CSNB. The stimulus has a 200-millisecond (ms) duration to enable independent recording of the ERG responses to onset and offset. A, The pattern of a patient with “complete” CSNB shows a negative-waveform on-response but a normal off-response. B, The pattern of a patient with “incomplete” CSNB shows both on- and off-response abnormalities. C, Pattern of a subject with normal responses.
(Courtesy of Graham E. Holder, PhD.)
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.