Night Blindness, Congenital Stationary, CSNB2A

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.  However, the photopic ERG can be 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. 

CSNB2A, or type 2A, is associated with myopia which ranges from mild to severe.  Residual rod function is diminished but not completely absent as suggested by the presence of small b-waves.  Cone function is impacted to some degree as well.  Nystagmus and strabismus are inconsistent findings.  Retinal pigmentation is usually normal in the X-linked forms. Visual acuity ranges from 20/30 to 20/200.  Night blindness is less severe in this form than in another X-linked CSNB (CSNB1A; 310500).  Mild dyschromatopsia is present in some patients but this is primarily a disease of rods.

Systemic Features: 

No systemic disease is associated with congenital stationary night blindness.

Genetics

Congenital stationary night blindness type 2A is an X-linked disorder caused by a mutation in the CACNA1F gene located at Xp11.23.  Only males are affected and carrier females do not have clinical disease.

This disorder is allelic to Aland Island Eye Disease (300600) from which it differs by an apparent lack of progressive myopia and the presence of a normal fovea.  Aland Island Eye Disease has foveal hypoplasia as well as iris and fundus hypopigmentation.

Another allelic disorder with mutations in CACNA1F is CORDX3 (300476), a cone-rod dystrophy.

Approximately 55% of X-linked CSNB are of this type while about 45% have another X-linked form known as CSNB1A, or type 1A (310500) secondary to a mutation at Xp11.4. 

Treatment
Treatment Options: 

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

References
Article Title: 

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.

PubMedID: 20362068

Boycott KM, Pearce WG, Musarella MA, Weleber RG, Maybaum TA, Birch DG, Miyake Y, Young RS, Bech-Hansen NT. Evidence for genetic heterogeneity in X-linked congenital stationary night blindness. Am J Hum Genet. 1998 Apr;62(4):865-75.

PubMedID: 9529339

Strom TM, Nyakatura G, Apfelstedt-Sylla E, Hellebrand H, Lorenz B, Weber BH, Wutz K, Gutwillinger N, R?other K, Drescher B, Sauer C, Zrenner E, Meitinger T, Rosenthal A, Meindl A. An L-type calcium-channel gene mutated in incomplete X-linked congenital stationary night blindness. Nat Genet. 1998 Jul;19(3):260-3.

PubMedID: 9662399