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RHO codes for rhodopsin found in rod photoreceptors.
RHO codes for rhodopsin found in rod photoreceptors.

Night Blindness, Congenital Stationary, CSNBAD1

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 9 mutant genes are responsible with phenotypes so similar that genotyping is usually necessary to distinguish them.  All are caused by 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.

Congenital stationary night blindness disorders are primarily rod dystrophies presenting early with symptoms of nightblindness and relative sparing of central vision.  Nystagmus and photophobia are usually not features.  Dyschromatopsia and loss of central acuity can develop later as the cones eventually become dysfunctional as well but these symptoms are much less severe than those seen in cone-rod dystrophies.  The amount of pigmentary retinopathy is highly variable. 

In this disorder (CSNBAD1), one of three autosomal dominant CSNB conditions, the b-wave responses are absent (no scotopic response) with some a-wave decrease in amplitude under dark adapted conditions.  Night vision in dim conditions may be reduced but not with bright backgrounds.  Daytime vision is normal as is color vision.  Older patients can have peripheral bone-spicule pigmentary changes with visual field restriction and narrowing of retinal arterioles.

Systemic Features

No systemic disease is associated with congenital stationary night blindness.

Genetics

CSNBAD1, or type AD1, is one of three congenital nightblindness disorders with autosomal dominant inheritance.  It results from mutations in the RHO (3q21-q24) gene coding rhodopsin.

Other autosomal dominant CSNB disorders are: CSNBAD2 (163500) and CSNBAD3 (610444).  Three CSNB disorders are transmitted in an autosomal recessive pattern and 2 as X-linked recessives.

Treatment Options

No treatment beyond correction of the refractive error is available but tinted lenses are sometimes used to enhance vision.

References

Berger W, Kloeckener-Gruissem B, Neidhardt J. The molecular basis of human retinal and vitreoretinal diseases. Prog Retin Eye Res. 2010 Sep;29(5):335-75.

PubMed ID: 
20362068

al-Jandal N, Farrar GJ, Kiang AS, Humphries MM, Bannon N, Findlay JB, Humphries P, Kenna PF. A novel mutation within the rhodopsin gene (Thr-94-Ile) causing autosomal dominant congenital stationary night blindness. Hum Mutat. 1999;13(1):75-81.

PubMed ID: 
9888392

Sieving PA, Richards JE, Naarendorp F, Bingham EL, Scott K, Alpern M. Dark-light: model for nightblindness from the human rhodopsin Gly-90-->Asp mutation. Proc Natl Acad Sci U S A. 1995 Jan 31;92(3):880-4.

PubMed ID: 
7846071