Familial idiopathic West syndrome.

Two families, each with occurrence of West syndrome in two siblings, are presented. Monozygotic twins in family 1 developed infantile spasms at the age of 4 months. Two female siblings in family 2 started to have seizures at the age of 6 months, but 2 years apart. The family history; development prior to West syndrome; clinical, electroencephalographic, and neuroradiologic findings; diagnostic work-up; and treatment are described. The outcome in family 1 (follow-up after 2 years) showed no conspicuous findings on physical and neurologic examination, and psychomotor development appropriate to cognitive, motor, and language developmental age in both twins. In family 2 (follow-up after 3 and 5 years), the older sister only was one standard deviation below mean in intellectual developmental age. Simultaneous occurrence of infantile spasms in both siblings from these two families but with variable clinical expression suggests there is a genetic susceptibility and variable phenotypic expression. Long-term follow-up will demonstrate whether these cases may be classified as "familial idiopathic West syndrome."