X-Linked Congenital Stationary Night Blindness

CLINICAL CHARACTERISTICS

X-linked congenital stationary night blindness (CSNB) is characterized by non-progressive retinal findings of reduced visual acuity ranging from 20/30 to 20/200; defective dark adaptation; refractive error, most typically myopia ranging from low (-0.25 diopters [D] to -4.75 D) to high (≥-10.00 D) but occasionally hyperopia; nystagmus; strabismus; normal color vision; and normal fundus examination. Characteristic ERG findings can help distinguish between complete X-linked CSNB and incomplete X-linked CSNB.

DIAGNOSIS/TESTING: The diagnosis of X-linked CSNB is established in a male proband with characteristic clinical and electroretinogram (ERG) findings and a family history consistent with X-linked inheritance. Identification of a hemizygous pathogenic variant in or by molecular genetic testing can confirm the diagnosis if clinical features are inconclusive. The diagnosis of X-linked CSNB may be established in a female proband with ERG findings suggestive of X-linked CSNB and identification of a heterozygous or biallelic pathogenic variant in or by molecular genetic testing.

MANAGEMENT

Glasses or contact lenses to treat refractive error (myopia or hyperopia); conventional strabismus surgery may be required to improve binocularity or head posture. At a young age yearly eye examinations with refraction to identify and treat myopia as early as possible. Reduced visual acuity and difficulties seeing at night may preclude driving a car or restrict the class of driving license.

GENETIC COUNSELING

By definition, X-linked CSNB is inherited in an X-linked manner. The father of an affected male will not have X-linked CSNB nor will he be hemizygous for the pathogenic variant. If the mother of the proband is a carrier, the chance of transmitting the pathogenic variant in each pregnancy is 50%. Males who inherit the pathogenic variant will be affected; females who inherit the pathogenic variant will be carriers and will usually not be affected. Males with X-linked CSNB will pass the pathogenic variant to all of their daughters and none of their sons. Carrier testing for at-risk relatives and prenatal testing for a pregnancy at increased risk are possible for families in which the pathogenic variant has been identified.