Spinal muscular atrophy

Last updated: May 15, 2023

Summarytoggle arrow icon

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-Hoffmann 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.

Epidemiologytoggle arrow icon

Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon

Pathophysiologytoggle arrow icon

Clinical featurestoggle arrow icon

Overview of spinal muscular atrophy types


Type of SMA Type 0 SMA Type I SMA (Werdnig-Hoffmann disease) Type II SMA Type III SMA (Kugelberg-Welander disease) Type IV SMA (adult SMA)
Proportion of all SMA cases [2]
  • < 1%
  • ∼ 55%
  • ∼ 30%
  • ∼ 15%
  • < 1%
  • Very severe
  • Severe
  • Intermediate
  • Mild
  • Mild
Age of onset
  • Prenatal
  • 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
  • Typically, none are achieved
  • Never sits
  • Able to sit independently, but cannot stand without support
  • Able to stand and walk independently
Life expectancy
  • < 6 months
  • < 2 years
  • Approx. 30 years

The most common causes of death in individuals with SMA are respiratory insufficiency (due to respiratory muscle weakness) and aspiration pneumonia (due to bulbar weakness).

The later the age of onset, the better the prognosis.

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

Diagnosticstoggle arrow icon

Differential diagnosestoggle arrow icon

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

Referencestoggle arrow icon

  1. 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
  2. Belter L, Cook SF, Crawford TO, et al. An overview of the Cure SMA membership database: Highlights of key demographic and clinical characteristics of SMA members. Journal of Neuromuscular Diseases. 2018; 5 (2): p.167-176.doi: 10.3233/jnd-170292 . | Open in Read by QxMD
  3. Singh A, Dalal P, Singh J, Tripathi P. Type 0 Spinal Muscular Atrophy in rare association with congenital Contracture and generalized osteopenia.. Iranian journal of child neurology. 2018; 12 (1): p.105-108.
  4. 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

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