Spinal Muscular Atrophy (SMA)

Key Features

• An autosomal recessive neuromuscular disorder

• Degeneration of anterior horn cells in the spinal cord, leading to progressive atrophy of skeletal muscle

• Incidence is approximately 1 in 12,000, with the majority of the cases presenting in infancy

• Carrier frequencies approach 1 in 40 in populations with European ancestry

Clinical Findings

• Three clinical subtypes are recognized based on age of onset and rate of progression

  • SMA I is the most devastating

    • Mild weakness may be present at birth but is clearly evident by 3 months and is accompanied by loss of reflexes and fasciculations in affected muscles

    • Progression of the disorder leads to eventual respiratory failure, usually by age 1 year

  • Symptoms of SMA II begin later, with weakness and decreased reflexes generally apparent by age 2 years.

  • Children affected with SMA III begin to become weak as they approach adolescence

Diagnosis

• Prenatal diagnosis is available through genetic testing

• Careful molecular analysis of the proband and demonstration of carrier status in parents is advised

• Homozygous deletion of exon 7 of SMN1 is detectable in approximately 95–98% of cases of all types of SMA and confirms the diagnosis

Treatment

• Supportive