The Chiari type I malformation in two monozygotic twins and first-degree relatives.

The presence of the Chiari type I malformation in 2 adult monozygotic female twins, their mother, and possibly in 2 of their 4 daughters is reported. The diagnosis was made by magnetic resonance imaging and confirmed at the time of surgery in 1 twin. Monozygosity of the twins was proved by DNA typing. The disorder in the present family should probably be classified together with the autosomal dominant craniocervical malformations. Nongenetic factors also seem to be important because the twins were discordant for the extent of herniation of the cerebellar tonsils and the presence of syringomyelia.