ICD-11

Spinal muscular atrophy (SMA) is a progressive disorder with loss of anterior horn cells leading to muscle weakness and wasting. The weakness is typically symmetrical. Typically, upper motor neuron signs are absent and there is no sensory deficit. Feeding and swallowing can be affected, and involvement of respiratory muscles may occur. SMA is an autosomal recessive disorder linked to chromosome 5q13 and the disorder is caused by deletion or mutation of SMN 1 (spinal motor neuron 1) gene. The four types of SMA I, II, III and IV are categorised based on the age of onset of the disease and the ability to achieve motor milestones.

In SMA type 4 weakness, most commonly develops after 35 years of age (less commonly between 18 to 35 years old). Weakness of proximal muscles is more prominent in the legs than in the arms. Insidious onset and very slow progression are the characteristic features, and life span is normal.