X-linked infantile spinal muscular atrophy.

Four male infants from three sibships in an extended family were noted to have hypotonia, areflexia, and congenital joint contractures. The findings of electromyography and muscle histology were consistent with infantile spinal muscular atrophy (SMA). Pedigree analysis suggests that this disorder represents an X-linked, recessive form of SMA. Findings in similar kindreds may explain the previously reported increased male-female ratio in infantile SMA.