SMA is divided into different types according to the age of onset and functional ability. There is also a range of severity within each type and up to 25% of individuals diagnosed with SMA cannot be clearly assigned to a specific type.1

The characters shown are real patients and the required consent to use their stories has been obtained from the patients and families. Photographs are for illustrative purposes only.


Each person experience symptoms differently. Symptoms may include progressive muscle weakness, floppiness, and muscle wasting (atrophy). Muscle weakness is usually the same on both sides of the body.2


“No one knew to think about these [signs] as being red flags.” 

– Morgan’s mum

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Click through the tabs to see additional details about each type.

Infant-onset SMA (also known as Werdnig-Hoffmann disease or Type I SMA)3,4

Infant-onset is the most severe form of SMA, comprising 60% of all cases of the disease. It is often diagnosed during an infant’s first 6 months of life. Affected individuals are unable to sit and are also referred to as “non-sitters”.5

Age of onset:

Life expectancy:

0-6 months ≤ 2 years

Highest motor milestone achieved:

“NON-SITTERS” - Unable to sit


  • Poor head control
  • Weak cough
  • Weak cry
  • Progressive weakness of muscles used to chew and swallow
  • Poor muscle tone
  • “Frog-leg” posture when lying
  • Severe muscle weakness on both sides of body
  • Progressive weakness of muscles that help in breathing (intercostal muscles), resulting in the characteristic “bell-shaped” chest

Intermediate SMA (also known as Dubowitz disease or Type II SMA)3,4

Intermediate SMA is usually diagnosed between 7 and 18 months. Affected individuals can typically sit up (also known as “sitters”) without help, although they may need assistance getting into a seated position. However, they are typically unable to walk and may require a wheelchair.

Age of onset:

Life expectancy:

7-18 months
> 2 years
  • 70% still living at age 25

Highest motor milestone achieved:

“SITTERS” - Able to sit independently


  • Muscle weakness
  • Swallowing, coughing, and breathing problems may occur but are typically less common
  • Muscle aching and joint stiffness symptoms
  • Children may develop spinal problems such as scoliosis (curvature of the spine), which may require bracing or surgery

Juvenile-onset SMA (also known as Kugelberg-Welander disease or Type III SMA)3,4

Juvenile-onset SMA is usually diagnosed after 18 months of age, but before the child is aged 3 years. Individuals affected by Type III SMA are initially able to walk (also known as “walkers”), but may lose mobility as they grow, and may eventually need to use a wheelchair.

Age of onset: 

Life expectancy:

18 months+ (juvenile‑onset) Normal

Highest motor milestone achieved:

“WALKERS” - Able to walk independently, although they may progressively lose this ability


  • Scoliosis
  • Chewing and swallowing difficulty
  • Muscles in the legs are generally more severely affected than the arms
  • Progressive weakness of muscles used to chew and swallow
  • Muscle aching
  • Joint overuse symptoms

Adult-onset SMA (also known as Type IV SMA)3,4

This type of SMA is very rare. Mild motor impairment is seen in adulthood. Symptoms can begin as early as age 18, though they often begin after age 35.

Age of onset:

Life expectancy:

Late adolescence / adulthood (adult-onset) Normal

Highest motor milestone achieved:

All milestones achieved


Physical symptoms are similar to juvenile-onset spinal muscular atrophy, with the gradual onset of weakness, tremors, and muscle twitching first noted in late teens or early adulthood.


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The characters shown are real patients and the required consent to use their stories has been obtained from the patients and families. Photographs are for illustrative purposes only.


1. Kolb SJ, Kissel JT. Spinal muscular atrophy: a timely review. Arch Neurol 2011;68(8):979-984.

2. Mercuri E, et al. Diagnosis and management of spinal muscular atrophy: Part 1: Recommendations for diagnosis, rehabilitation, orthopedic and nutritional care. Neuromuscl Disord 2018;28(2):103-115.

3. Prior TW, Russman BS. Spinal muscular atrophy. NCBI Bookshelf Website. Available at: http://www.ncbi.nlm.nih.gov/books/NBK1352/. Updated November 14, 2013. Accessed April 15, 2016.

4. Cure SMA. Understanding Spinal Muscular Atrophy (SMA). Available at: https://www.curesma.org/types-of-sma/. Accessed January 9, 2017.

5. Markowitz JA, Singh P, Darras BT. Spinal muscular atrophy: a clinical and research update. Pediatr Neurol 2012;46(1):1-12.

6. Oskoui M, Levy G, Garland CJ, et al. The changing natural history of spinal muscular atrophy type 1. Neurology 2007;69(20):1931-1936.