Spinal Muscular Atrophy (SMA)

Case Study A
A 3-year-old boy was able to stand and cruise at age 13-months, but lost this ability at age 18-months. There was a strong suspicion of mental retardation due to delayed speech and social development. Exam showed diffuse muscle weakness and atrophy, diminished reflexes, and absence of spasticity. EMG and muscle biopsy were typical for Spinal Muscular Atrophy (SMA) type II.

A SMA DNA test demonstrated absent PCR reaction product for exon 8, while that for exon 7 was present. Nerve conduction studies demonstrated normal sensory nerve action potential amplitudes and normal motor nerve conduction velocities with a diminished compound motor action potential amplitude. EMG demonstrated delayed recruitment and fast firing of high amplitude, as well as long duration motor unit potentials. Muscle biopsy demonstrated severe atrophy of both type I and type II muscle fibers, with a single fascicle with enlarged fibers having type I histochemistry.

While the presence of other neurological problems had precluded diagnosis by formal criteria, the DNA test resulted in a diagnosis of SMA type II/III, Idiopathic Mental Retardation. Mental retardation, cerebral palsy, seizures, or other relatively common disorders of the CNS may co-exist with SMA, or may be a sign of more global neurological disease.