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Key Facts about Spinal Muscular Atrophy

Spinal Muscular Atrophy Support UK and Muscular Dystrophy UK, both certified members of The Information Standard, have worked alongside The SMA Trust to produce this information sheet. It is written for anyone wanting to know some basic facts about Spinal Muscular Atrophy (SMA).

What is Spinal Muscular Atrophy?

Spinal Muscular Atrophy (SMA) is a rare, genetically inherited neuromuscular condition, of which there are several distinct types. SMA causes progressive loss of movement and muscle weakness as a result of muscle wasting (atrophy). The condition may affect crawling and walking ability, arm, hand, head and neck movement, breathing and swallowing.

There are four main types of SMA which are caused by a fault in the gene called Survival Motor Neuron 1 (SMN1). This gene carries the information required for the production of an important protein called Survival Motor Neuron (SMN) protein. When there is not enough SMN protein, the nerve cells that help to control the muscles for moving and breathing become damaged.

The four main types of SMA vary greatly in severity:

  • The symptoms of SMA Type 1 appear within the first few months of life, sometimes before birth. It is the most severe form of SMA. Children are never able to sit unaided and rarely survive their second birthday.
  • The symptoms of SMA Type 2 usually appear between the ages of 7 and 18 months. The condition is severely physically disabling, with children never able to stand unaided. Though this is a serious neuromuscular condition that may shorten life expectancy, improvements in care standards mean that the majority of people can live long, fulfilling and productive lives.
  • The symptoms of SMA Type 3 appear after 18 months of age. Children are able to stand and walk, though they will experience reduced walking ability over time. It is a less disabling condition than SMA Types 1 or 2. Life expectancy for children diagnosed with SMA Type 3 is normal and most people can live long productive lives. 
  • The symptoms of SMA Type 4 appear in adulthood. It is a type of Adult Onset SMA and is not life-threatening.

The effects of each type of SMA vary greatly between individuals. Life expectancy varies between and within the different types of SMA.

How many people are affected?

  • In the UK, approximately 100 children are born with SMA each year.
  • One in every 6,000 - 10,000 babies worldwide are born with the condition.
  • At any one time it is thought that there are between 2,000 – 2,500 children and adults in the UK living with SMA.

How do people get SMA?

  • SMA is an autosomal recessive inherited neuromuscular condition. This means that two faulty copies of the SMN1 gene, one from each parent, are needed for a person to have the condition.
  • When two SMA carriers have a child together, there is a 1 in 4 (25%) chance that the child will have inherited both faulty copies of the SMN1 gene and will develop SMA. These chances are the same for each pregnancy.
  • When two SMA carriers have a child together, there is a 1 in 2 (50%) chance that the child will be a carrier.
  • Approximately one in 40 – 60 people is an SMA carrier, which equates to around 1 to 1.5 million people in the UK.

Is there a treatment or cure?

Although there is currently no cure for SMA, this does not mean that nothing can be done. There are a range of options aimed at managing symptoms, reducing complications of muscle weakness and maintaining the best quality of life.

There is a considerable amount of research taking place around the world to develop potential treatments. The UK is a significant contributor to this, with several UK centres being involved in clinical trials and international collaborations.

In order to best treat SMA, the underlying genetic fault needs to be addressed and SMN protein production increased. There are now potential treatments that work in this way and are being tested in clinical trials. One of these, nusinersen (brand name Spinraza), has gained licensing approval from the US Food and Drug Administration (FDA) and is now available to people living with SMA in the US. It is not yet licensed in the UK and Europe but is currently being considered for a licence by the European Medicines Agency (EMA).

There is also extensive research taking place into the genetics of SMA and the underlying mechanisms that lead to damage of the nerve cells. This research will not only improve our understanding of the condition but will also help to develop effective treatments.

For more information on research, go to:

For more information about the different types of SMA, visit:


Version 2.2
Authors: Spinal Muscular Atrophy Support UK and Muscular Dystrophy UK in collaboration with the SMA Trust
First published: July 2014
                                               Updated: February 2017
                                               Full review due: July 2017



Montes, J., Gordon, A.M., Pandya, S., De Vivo, D.C. and Kaufmann, P. (2009) ‘Clinical outcome measures in spinal muscular atrophy’, Journal of Child Neurology, 24(8), pp. 968-978.

Standards of Care for SMA: A Family Guide. Available at:

Wang, C.H., Finkel, R.S., Bertini, E.S., Schroth, M., Simonds, A., Wong, B., Aloysius, A., Morrison, L., Main, M., Crawford, T.O. and Trela, A. (2007) ‘Consensus statement for standard of care in spinal muscular atrophy’, Journal of Child Neurology, 22(8), pp. 1027-49. Available for free download at: