SMA Type 3 Information

The information on this page gives a general overview of SMA Type 3, the International Standards of Care for SMA and how they relate to children who have SMA, and signposts to the other most relevant areas of the website for families of children with this clinical diagnosis.

What is SMA Type 3?

  • SMA Type 3a: symptoms usually begin between 18 months and 3 years of age. Children can stand and walk, though this will become more difficult with age and they will need more support over time.
  • SMA Type 3b: symptoms usually begin after 3 years. Difficulties with standing and walking usually occur later than they do for children with SMA Type 3a.

Depending upon the individual impact of their condition, children and adults with SMA Type 3 may be described as ‘sitters’ or ‘walkers’. Very few people with SMA Type 3 have breathing problems and their life expectancy isn’t usually affected1. Most can live long, fulfilling lives.

It’s important to remember that there are no distinct lines that distinguish the Types of 5qSMA from each other and that the impact varies greatly between individuals. Additionally, now that some are receiving drug treatments, there may be even greater variation in the impact of the SMA. 


When a clinical diagnosis is made, this is usually given as a Type of SMA. In general, this is based on the age at which symptoms first appear and the physical milestones reached:

SMA Type Usual age of symptoms onset Impact of muscle weakness on sitting / walking 
Type 1 Younger than 6 months Unable to sit or roll independently
Type 2 7 months – 18 months Able to sit but not walk independently
Type 3a 18 months – 36 months  Able to walk, though may lose this ability over time
Type 3b 3 years – 18 years Able to walk, though may lose this ability over time
Type 4 Over 18 years Mild walking (motor) difficulties

Clinical classification for SMA (adapted from RS Finkel et al, 2017, p.597)

When discussing best care and management, it’s therefore important to think not only about ‘clinical classification’, but also what physical milestones a child, young person or adult has reached. For simplicity these terms are used:

  • non-sitters - those who are unable to sit 
  • sitters - those who are able to sit but not walk 
  • walkers - those who are able to walk 

Signposting to other relevant information:

Please bear all the above information in mind when you’re  looking at these guides:
You may also find the section of our website Living with SMA helpful. This is being expanded from 1st August to include much more information, such as: equipment; transport; housing; leisure and holidays; financial, education, employment and social support, with information sorted separately for adults, teenagers, and parents of children.
1. Zerres K, Rudnik-Schöneborn S, Forrest E, Lusakowska A, Borkowska J, Hausmanowa-Petrusewicz I (1997) A collaborative study on the natural history of childhood and juvenile onset proximal spinal muscular atrophy (type II and III SMA): 569 patients. J Neurol Sci 146: 67-72.