INCOMPLETE CONGENITAL STATIONARY NIGHT BLINDNESS (CSNB) - AN IMPORTANT DIFFERENTIAL-DIAGNOSIS IN CONGENITAL NYSTAGMUS

Background Classifying congenital nystagmus in the absence of biomicro scopically detectable abnormalities of the eye, and in an otherwise he althy child is difficult, especially early in life. At that age, nysta gmus and visual loss may be the predominant symptoms of congenital sta tionary night blindness. Unless night-blindness is specifically asked for or an ERG performed the correct diagnosis may be missed. Patients and methods We present the clinical data of two families with X-linked incomplete CSNB previously undiagnosed. ERG recordings in both famili es were suggestive of CSNB. The ERG of the obligate carrier was normal . In an attempt to distinguish between the complete and the incomplete type, and to identify further carrier signs, scotopic perimetry and d ark adaptation were performed in both affected males and carriers. Sco topic perimetry allows to test the rod-mediated visual pathway in its spatial distribution. Results In affected males with non-recordable ER Gs scotopic perimetry and dark adaptation disclosed residual rod funct ion indicating an incomplete type. In carriers, there was a sensitivit y loss at 600 nm, which may be a new carrier sign. Conclusions The cor rect diagnosis of the different forms of CSNB together with the identi fication of carriers is important for (1) genetic counselling and (2) linkage studies to identify the gene(s) for CSNB.