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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


  • 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


  • Supportive

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