Spinal muscular atrophy

Last updated: March 24, 2022

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Spinal muscular atrophy (SMA) refers to a group of autosomal recessive motor neuron diseases that are caused by apoptosis of lower motor neurons. Patients typically present during infancy or early childhood with progressive weakness, hypotonia, muscle atrophy, hyporeflexia/areflexia, and varying degrees of bulbar weakness. The severity of disease is related to the age of onset; type 1 SMA (Werdnig-Hoffman disease) is associated with death within the first two years of life as a result of respiratory muscle paralysis or aspiration pneumonia. With other types of SMA, children usually survive to adulthood, but motor milestones may be delayed and joint contractures and/or deformities of the spine can occur. The diagnosis is confirmed by genetic testing. In 2016, the FDA approved the use of nusinersen, which is highly effective in halting the progression of SMA; in 2019, onasemnogene abeparvovec, a potentially curative genetic therapy, was approved. Supportive therapy is aimed at preventing respiratory and orthopedic complications.


Epidemiological data refers to the US, unless otherwise specified.



Type of SMA Type I (Werdnig-Hoffman disease) Type II Type III (Kugelberg-Welander disease) Type IV (adult SMA)
  • 50%
  • 20%
  • 30%
  • < 5%
  • Severe
  • Intermediate
  • Mild
  • Mild
Age of onset
  • 0–6 months
  • 7–18 months
  • > 18 months
  • 10–30 years
Typical features
  • Variable degree of muscle weakness
  • Cramps, muscle aches
  • Joint pain
Motor milestones
  • Never sits
  • Able to sit independently, but cannot stand without support
  • Able to stand and walk independently

The most common causes of death among patients with SMA are respiratory insufficiency (due to respiratory muscle weakness) and aspiration pneumonia (due to bulbar weakness)!

The older the age of onset, the better the prognosis!

Type Inon-sitters, type II → sitters, type III → walkers


  1. Genetic testing: best initial and confirmatory test
  2. Further tests



The differential diagnoses listed here are not exhaustive.

  • Definitive therapy [4]
    • Nusinersen
      • An antisense nucleotide that alters differential splicing of the transcript of the SMN2 gene, so it produces a functional SMN1 protein
      • Requires annual intrathecal injections
    • Risdiplam
    • Onasemnogene abeparvovec
  • Supportive therapy
  1. Tsao B. Spinal Muscular Atrophy. Spinal Muscular Atrophy. New York, NY: WebMD. http://emedicine.medscape.com/article/1181436-overview. Updated: January 4, 2017. Accessed: March 31, 2017.
  2. Prior TW, Nagan N, Sugarman EA, Batish SD, Braastad C. Technical standards and guidelines for spinal muscular atrophy testing. Genet Med. 2011; 13 (7): p.686-694. doi: 10.1097/gim.0b013e318220d523 . | Open in Read by QxMD
  3. Bodamer OA. Spinal muscular atrophy. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/spinal-muscular-atrophy.Last updated: February 26, 2018. Accessed: April 11, 2018.
  4. Wang CH, Finkel RS, Bertini ES, et al. Consensus statement for standard of care in spinal muscular atrophy. J Child Neurol. 2007; 22 (8): p.1027-1049. doi: 10.1177/0883073807305788 . | Open in Read by QxMD
  5. Migita R. Etiology and evaluation of the child with weakness. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/etiology-and-evaluation-of-the-child-with-weakness.Last updated: August 21, 2017. Accessed: April 11, 2018.

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