Spinal Muscular Atrophy (SMA)

The age of onset, severity of symptoms and the level at which functional ability is affected, determine the type of SMA of each patient. Every person with SMA is affected differently, and up to 25% of those diagnosed with SMA cannot be clearly assigned to a specific type.1


Click on the tabs below to learn more about the different types of SMA.

Highest motor milestone achieved

Unable to sit

Life expectancy

≤ 2 years

SMA Type

Type 1 (also known as Werdnig-Hoffmann disease)


  • Hypotonia and impaired head control

  • “Frog leg” presentation

  • Weak cry

  • Weak cough

  • Swallowing, feeding, and handling of oral secretion are affected before 1 year of age

  • Atrophy and fasciculation of the tongue

  • Weakness and hypotonia in the limbs and trunk

  • Intercostal muscle weakness (note, the diaphragm is relatively spared)

  • Paradoxical breathing

  • Bell-shaped trunk with chest wall collapse and abdominal protrusion

Highest motor milestone achieved

Able to sit independently (“sitters”)

Life expectancy

> 2 years
70% still living at age 25

SMA Type

Type 2 (also known as Dubowitz disease)


  • Bulbar weakness with chewing and swallowing difficulties that may lead to poor weight gain

  • Weak intercostal muscles

  • Diaphragmatic breathing

  • Difficulty coughing and clearing tracheal secretion

  • Fine tremors with extended fingers or when attempting hand grips

  • Kyphoscoliosis, or scoliosis requiring bracing or spinal surgery

  • Joint contractures

Highest motor milestone achieved

Able to walk independently (“walkers” - although they
may progressively lose this ability)

Life expectancy


SMA Type

Type 3 (also known as Kugelberg-Welander disease)


  • Scoliosis

  • Swallowing difficulty

  • Cough, and nocturnal hypoventilation

  • Muscle aching

  • Joint overuse symptoms

Highest motor milestone achieved


Life expectancy


SMA Type

Type 4


  • Physical symptoms are similar to juvenile-onset SMA, with the gradual onset of weakness, tremor, and muscle twitching first noted in late teens or adulthood


You may notice that these characteristics of SMA are more commonly grouped by Type I-IV in internet articles or clinical research.

Learn more on how different aspects of care may relate to the signs and symptoms of SMA


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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. Arch Neurol 2011;68(8):979-984.

2. Prior TW, Russman BS. Spinal muscular atrophy. NCBI Bookshelf [online] 2013 Nov 14 [cited 2016 Apr 15]. Available from: URL: http://www.ncbi.nlm.nih.gov/books/NBK1352/?report=printable.

3. 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.

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

5. Darras BT, Royden Jones H Jr, Ryan MM, De Vivo DC, eds. Neuromuscular Disorders of Infancy, Childhood, and Adolescence: A Clinician’s Approach. 2nd ed. London, UK: Elsevier; 2015.

6. Lunn MR, Wang CH. Spinal muscular atrophy. Lancet 2008;371(9630):21‌20-21‌33.

7. Online Mendelian Inheritance in Man. Spinal muscular atrophy, Type III; SMA3. [online] 2013 Feb 7 [cited 2016 Apr 26]. Available from: URL: http://www.omim.org/entry/253400.

8. Kolb SJ, Kissel JT. Neurol Clin. 2015;33:831-846.

9. D’Amico A, Mercuri E, Tiziano FD, Bertini E. Spinal muscular atrophy. Orphanet J Rare Dis 2011;6:71.