A clinical and genetic study of chronic proximal spinal muscular atrophy.

A study of chronic proximal spinal muscular atrophy was undertaken with the main aim of obtaining empirical recurrence risks for genetic counselling. Thirty-eight patients and their families were studied. Of these, 33 had similar clinical features and onset of disease in infancy or childhood. A division of these 33 patients by onset before or after 2 years (which was equivalent to whether or not they ever walked normally) gave recurrence risks for sibs which were higher with early onset. Among the sibs of patients with onset before 2 years, the incidence of disease was 1 in 5, due to most patients having an autosomal recessive disorder. A few patients, however, were thought to represent new dominant mutations. Among the families of index patients with onset after 2 years, the incidence of disease in sibs was only 1 in 15, but among their children it as 1 in 8. Both autosomal recessive and autosomal dominant forms therefore occurred in this age group, but it was concluded that nearly half the patients with onset after 2 had non-genetic motor neuron disease. The autosomal recessive form of chronic spinal muscular atrophy generally had onset before 2 years, but occasionally after 2. About a third of the patients never walked, and about half were in wheelchairs by age 10. No genetic heterogeneity within this form was demonstrated. Three remaining patients had distinctive clinical features associated with their proximal weakness, external ophthalmoplegia in one, dysarthria in another, and joint contractures in a third. Only 2 patients had onset in adult life, one of a probable recessive disorder and the other a probable dominant disorder.