Visual impairment in Nordic children. V. X-linked juvenile retinoschisis

In a study on 2,527 visually impaired children from four Nordic countries X-linked juvenile retinoschisis was diagnosed in 35 male children. Striking differences in frequency between the four countries were found, with 26 cases reported from Finland, 5 cases from Denmark, and 4 cases from Norway. None was reported from Iceland. The corresponding age and sex-specific prevalence rates of X-linked juvenile retinoschisis (N:1,000,000) were 44.5 in Finland, 8.8 in Denmark, and 7.9 in Norway. The uneven geographical distribution is possibly attributed to a 'founder effect' due to the settlements in Finland by European immigrants in the 17th century. The visual impairment of the registered cases was usually mild with 91.4% falling into WHO category 1. However, one child was totally blind, demonstrating large phenotypic heterogeneity. None of our cases had additional impairments. The majority were more than five years old, indicating a progressive course during childhood. Nevertheless, two children were diagnosed at the age of one. The most common age at registration was seven years, coinciding with the beginning of school attendance.