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.


Presymptomatic SMA:

A newborn screening program uses a blood test to identify SMA in babies before any symptoms appear.2 Early detection and treatment of SMA during this presymptomatic time are essential. Starting a baby on disease-modifying treatments immediately after diagnosis enables them to reach major milestones of normal, healthy development.3

Learn more about newborn screening

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

Infant-onset SMA (also known as Werdnig-Hoffmann disease or Type I SMA)4,5

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”.6

Age of onset:

Life expectancy
without treatment:

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)4,5

Intermediate SMA is usually diagnosed between 7 and 18 months of the child's age, (if no newborn screening has been carried out). 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
without treatment:

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)4,5

Juvenile-onset SMA is usually diagnosed after 18 months of age, but before the child is aged 3 years (if no newborn screening has been carried out). 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
without treatment:

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
  • Muscle aching
  • Joint overuse symptoms

Adult-onset SMA (also known as Type IV SMA)4,5

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
without treatment:

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.


People diagnosed with SMA were traditionally classified into 1 of the 4 types above, depending on when they started showing SMA symptoms as well as their level of muscle weakness.
This classification was based on how the disease was expected to progress in that person.

With the development of new and effective SMA treatments, and depending on when treatment was started, the disease does not always progress according to the definitions of SMA types 1 to 4.7

For this reason, people living with SMA are now also being classified according to their functional abilities at the time of their diagnosis, namely non-sitter, sitter, or walker.7

Child has never been able to sit freely.8

  • Mainly includes children with SMA type 1 or 2
  • Have muscle weakness in their upper body and upper limbs, poor muscle tone and show no reflexes when their tendons are gently tapped
  • Muscles in their face are weak and they have difficulty swallowing (SMA 1)
  • SMA 1 patients will eventually be unable to breathe on their own, whereas SMA 2 patients may have difficulty breathing and could need ventilation (breathing support)
  • Contractures and severe scoliosis (curvature of the spine) are common and usually require surgery

Person can sit freely for more than 10 seconds and is able to stand upright without using their arms to support or balance themselves. They can be placed and remain in a sitting position.8

  • Includes people living with SMA 2 and SMA 3, as well as some children with SMA 1 who have received early treatment
  • May have weak muscles in their upper arms and legs; lower limb muscles are not as severely affected
  • Typically show no reflexes when their tendons are gently tapped
  • Over the course of their development, contractures and scoliosis (curvature of the spine) can occur
  • Muscle weakness in the chest that makes breathing difficult is less common than in a non-sitter but does occur, especially at night.
  • May need to use a ventilator at home with a mask that covers the nose and mouth to help the person breathe

Can walk freely for 10m (upright, with a straight spine and without help)8

  • Typically includes people living with SMA 3, or those treated early/presymptomatically
  • Muscle weakness is generally more noticeable in their upper limbs
  • Can show normal or slightly weaker reflexes when their tendons are gently tapped
  • May have some difficulty breathing normally
  • Muscles in their face are not usually weak
  • May have some difficulty swallowing


Each person living with SMA experiences symptoms differently. Symptoms may include gradual muscle weakness, floppiness, and muscle wasting (atrophy). Muscle weakness is usually the same on both sides of the body.1,9

Learn more about signs and symptoms of SMA in different age groups


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

– Morgan’s mum

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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. Loeber JG, Platis D, Zetterström RH, et al. Neonatal screening in Europe revisited: An ISNS perspective on the current state and developments since 2010. Int J Neonatal Screen. 2021;7(1).

3. Jędrzejowska M. Advances in Newborn Screening and presymptomatic diagnosis of spinal muscular atrophy. Degener Neurol Neuromuscul Dis. 2020;10:39-47.

4. Prior TW, Leach ME, Finanger E. Spinal muscular atrophy Seattle (WA): University of Washington, Seattle; 2020 [Accessed 5 October 2022]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1352/.

5. Cure SMA. Types of SMA [Accessed 5 October 2022]. Available from: https://www.curesma.org/types-of-sma/.

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

7. Castellano IP, Cabrera-Serrano M, Medina RC, et al. Delphi consensus on recommendations for the treatment of spinal muscular atrophy in Spain (ret-ame consensus). Neurología. 2022;37(3):216-228.

8. Kölbel H, Müller-Felber W. Spinale muskelatrophie (SMA), diagnostik und therapie AWMF online2020 [Accessed 6 October 2022]. Available from:

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