An unusual form of spinal muscular atrophy with mental retardation occurring in an inbred population.

Three sibs are described suffering from hereditary non-progressive spinal muscular atrophy with non-progressive mental retardation. One of them had in addition signs of pyramidal tract involvement. Muscular weakness was more pronounced proximally than distally and the neck muscles were severely involved. Th.ey all had small skulls and several associated congenital malformations were observed including syndactyly of the left hand in 1 patient. The patients belong to a small inbred community in the Netherlands. Erythropoietic protoporphyria was also present in the family but segregated independently. This combination of "congenital" mental retardation with "congenital" non-progressive spinal muscular atrophy is believed to represent a new syndrome, caused by a rare recessive gene.