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This page is for families, friends and healthcare professionals who want to know more about SMA Type 3.

SMA Type 3 is a form of 5q SMA. Please see our guide What is 5q SMA? for information about the cause, diagnosis, inheritance and how many people are affected.

Before drug treatments (also sometimes known as ‘disease-modifying’ treatments) started to be developed and tested for SMA, clinicians and researchers gathered information about what impact parents / carers might expect the condition to have on their child as they grow into adulthood, and what young people and adults with the condition may expect. This was called the ‘natural history’ of the condition and was different for each ‘Type’ of SMA.

SMA Type 3 is sometimes further divided into:

  • SMA Type 3a – symptoms of muscle weakness usually begin between 18 months and 3 years of age.
  • SMA Type 3b – symptoms usually begin after 3 years, but before adulthood.

Your clinician will advise you if they would expect your child’s SMA to follow the ‘natural history’ of SMA Type 3 if they do not have drug treatment. They will also advise you of the latest clinical trial and real-world evidence for these drug treatments and what you might expect if your child is treated.

Our guide to drug treatments covers questions and topics that you may wish to consider in your discussions with your child’s clinical team – see this page.

In the natural history of SMA Type 3, each child is affected differently but, in general, children are bright and engaging. However, their SMA causes:

  • muscle weakness on both sides of the body
  • muscle weakness closest to the centre of the body as these muscles are more severely affected than muscles furthest away
  • legs that are weaker than arms

Intellectual and sexual development1 is not affected, but as children get older, their SMA usually causes them to have:

  • difficulties with standing and walking. This usually happens later for children with SMA Type 3b than for children who develop symptoms at an earlier age.
  • difficulties keeping up with daily activities. For example, if they have been able to walk or climb stairs, they may lose this ability. Some children may fall more easily because of their muscle weakness. If they are sitting on the floor, they may need help to get up.
  • muscles supporting the spinal column that are weakened. This means that some children develop a sideways curvature of their spine2 (scoliosis).
  • a reduced ability to move due to some joints becoming tight (contractures), restricting their range of movement.
  • a tendency to become weaker after infections and at times of major growth, such as puberty.

Some children, young people and adults will need help with daily tasks like washing, dressing and undressing. Some may need to use a manual or powered wheelchair for independent mobility. Though their bladder and bowel control is not affected, some may need help with getting to and from, and sitting on, the toilet.

Fewer people with SMA Type 3 have swallowing or breathing difficulties. Life expectancy is not usually affected3.

Global clinical trials and follow-up studies of possible drug treatments for those who have SMA Type 3 had tended to focus on their impact on motor milestones gained in childhood. There is more understanding now of the value and importance of respiratory and other outcomes that impact on quality of life for older children, young people and adults, such as fine motor skills and fatigue levels, which enable or maintain independence in daily living.

  • Changing outcomes

In the UK, some children with SMA Type 3 will have started ongoing treatment with either Spinraza™ / Nusinersen (from late 2019). For adults, this possibility came later. More recently, ongoing treatment with Evrysdi™ / Risdiplam also became a possibility for children and adults. Many children with SMA Type 3 have gained strength, movement and better health with treatment. Symptoms have stabilised for many adults, with some seeing improvements.

How SMA will impact the health and daily life of a treated person living with SMA Type 3 today – and how the natural history of their condition may change – is very individual. It will be influenced by factors including the age at which symptoms first appeared, the severity of symptoms, how early any treatment was started and their individual response to treatment.

Though they can alleviate some symptoms, none of the drug treatments are a cure and – for adults and children – must be combined with the best supportive care and management of symptoms to ensure the best possible outcomes for each individual.

It is important that you feel able to discuss any questions you have with your child’s clinical team.

  1. Montes J et al. (2009) Clinical outcome measures in spinal muscular atrophy, J Child Neurol 24: 968-978.
  2. Mercuri E, et al. (2012) Childhood spinal muscular atrophy: controversies and challenges. Lancet Neurol 11: 443-452.
  3. Zerres K et al. (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

Image shows the Patient Information Forum logo.Version 4
Author: SMA UK Information Production Team
Last updated: February 2024
Next full review due: September 2024

Links last checked: June 2023

The information provided in this guide, on our website, and through links to other websites, is designed to complement not be a substitute for clinical and professional care and advice.

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