OMIM ID:
Night Blindness, Congenital Stationary, CSNB2B
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
Night blindness is a feature of many pigmentary and other retinal disorders, most of which are progressive. However, there is also a group of genetically heterogeneous disorders, with generally stable scotopic defects and without RPE changes, known as congenital stationary night blindness (CSNB). At least 10 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them. All are caused by defects in visual signal transduction within rod photoreceptors or in defective photoreceptor-to-bipolar cell signaling with common ERG findings of reduced or absent b-waves and generally normal a-waves. The photopic ERG is usually abnormal to some degree as well and visual acuity may be subnormal. In the pregenomic era, subtleties of ERG responses were frequently used in an attempt to distinguish different forms of CSNB. Genotyping now enables classification with unprecedented precision.
In this disorder (CSNB2B) the b-wave responses are deficient (little or no scotopic response) and a-waves seem to be normal. However, many if not most patients do not complain of night blindness. Nystagmus, strabismus, and restriction of visual fields may be present. Visual acuity is mildly to severely reduced.
Foveal thinning has been documented in this condition.
Systemic Features
No systemic disease is associated with congenital stationary night blindness.
Genetics
Inheritance
CSNB2B, or type 2B, is one of four congenital nightblindness disorders with autosomal recessive inheritance. It results from mutations in the CAPB4 gene (11q13.1) important in receptor to bipolar cell signaling.
Other autosomal recessive CSNB disorders are: CSNB1C (613216), CSNB (unclassified; OMIM number pending), and CSNB1B (257270).
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.