Spinal muscular atrophy (SMA) is an autosomal recessive progressive neuromuscular disorder leading to progressive degeneration and loss of spinal cord and brain stem motor neurons. It is most commonly caused by homozygous deletions of SMN1 (survival motor neuron) exon 7 and exists in 3 main clinical types:
Type 1 (Werdnig-Hoffman disease) is characterized by symptom onset before 6 months of age. These patients are profoundly hypotonic, have significant respiratory and nutritional difficulties, and never achieve sitting without support. Historically, this type is fatal within 2 years.
Type 2 is characterized by symptom onset between 6-12 months, weakness, tremors, and respiratory and nutritional problems. These patients are able to achieve sitting but never walk independently.
Type 3 is the least severe type and is characterized by symptom onset after 1 year of age. Many live into adulthood and achieve walking prior to losing this ability in late childhood or early adulthood. The weakness in type 3 is variable. These patients typically do not have respiratory or nutritional difficulties.
The disease is modified, in part, by the SMN2 copy number.