Congenital stationary night blindness, type 1A

Common Name(s)

Congenital stationary night blindness, type 1A

Congenital stationary night blindness (CSNB) is a clinically and genetically heterogeneous group of nonprogressive retinal disorders that can be characterized by impaired night vision, decreased visual acuity, nystagmus, myopia, and strabismus. CSNB can be classified into 2 groups based on electroretinography (ERG) findings: the Schubert-Bornschein type is characterized by an ERG in which the b-wave is smaller than the a-wave, whereas the Riggs type is defined by proportionally reduced a- and b-waves. In addition, Schubert-Bornschein CSNB is associated with decreased visual acuity, myopia, and nystagmus, whereas in Riggs CSNB patients have visual acuity within the normal range and no symptoms of myopia and/or nystagmus (summary by {33:Riazuddin et al., 2010}). Additionally, Schubert-Bornschein CSNB can be subdivided into 'complete' and 'incomplete' forms (summary by {33:Riazuddin et al., 2010}). {37:Van Genderen et al. (2009)} noted that standard flash ERG distinguishes a 'complete' form, also known as type 1 CSNB, from an 'incomplete' form, also known as type 2 CSNB (see CSNB2A, {300071}). The complete form is characterized by the complete absence of rod pathway function, whereas the incomplete form is due to impaired rod and cone pathway function. Complete CSNB results from postsynaptic defects in depolarizing or ON bipolar cell signaling, whereas the hyperpolarizing or OFF bipolar cell pathway is intact. {9:Bijveld et al. (2013)} noted that the term 'incomplete' CSNB refers to the less-impaired rod system function in CSNB2, whereas the more severely impaired cone system function results in a greater decrease in visual acuity, with a greater impact on a patient's daily life activities than the impairment in CSNB1. Thus, patients with so-called 'incomplete CSNB' actually experience more visual restrictions than those with 'complete CSNB,' which can be misleading to patients and their parents. Genetic Heterogeneity of Congenital Stationary Night Blindness Autosomal recessive forms of complete CSNB have been reported: CSNB1B ({257270}), caused by mutation in the GRM6 gene ({604096}); CSNB1C ({613216}), caused by mutation in the TRPM1 gene ({603576}); CSNB1D ({613830}), caused by mutation in the SLC24A1 gene ({603617}); and CSNB1E ({614565}), caused by mutation in the GPR179 gene ({614515}). Autosomal dominant forms of complete CSNB that have been reported include CSNBAD1 ({610445}), caused by mutation in the RHO gene ({180380}); CSNBAD2 ({163500}), caused by mutation in the PDE6B gene ({180072}); and CSNBAD3 ({610444}), caused by mutation in the GNAT1 gene ({139330}). In addition, an X-linked recessive form of incomplete CSNB (CSNB2A; {300071}), caused by mutation in the CACNA1F gene ({300110}), has been reported. A form of autosomal recessive CSNB in which all other visual functions are normal is designated Oguchi disease: Oguchi type 1 ({258100}) is caused by mutation in the SAG gene ({181031}), and Oguchi type 2 ({613411}) is caused by mutation in the RHOK gene (GRK1; {180381}). In 101 Dutch patients from 72 families diagnosed with CSNB, {9:Bijveld et al. (2013)} screened 6 known CSNB-associated genes and identified mutations in 94 patients. Of the 39 patients with CSNB1, 20 (51%) had mutations in the NYX gene, 10 (26%) in TRPM1, 4 in GRM6, and 2 in GPR179; no mutations were detected in 3 of these patients. Of the 62 patients diagnosed with CSNB2, 55 (89%) had mutations in the CACNA1F gene; no mutations were detected in 4 of these patients. {9:Bijveld et al. (2013)} stated that the electrophysiologic distinction between CSNB types 1 and 2 was thus confirmed by DNA analysis in 93% of the patients. In addition, 3 patients from the CSNB cohort, including 2 Dutch sibs originally reported by {22:Littink et al. (2009)}, were found to be homozygous for a nonsense mutation in the CABP4 gene and to exhibit a distinct phenotype that {22:Littink et al. (2009)} designated 'congenital cone-rod synaptic disorder' (CRSD; {610427}).
 

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