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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 inherited neuromuscular condition, of which there are several distinct types. 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 inherited 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 will experience reduced walking ability over time. It is a less disabling condition. Life expectancy for children diagnosed with SMA Type 3 is normal1 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.

In order to treat SMA, the underlying genetic defect needs to be addressed. At present this is not possible. The day-to-day focus is therefore on treatment of the symptoms and the delivery of the highest standards of care. This includes anticipatory care, managing symptoms and maintaining the best quality of life.

How many people are affected?

  • In the UK, approximately 100 children are born with this rare disease each year.
  • One in every 6,000 - 10,000 babies 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 genetically inherited neuromuscular disease. This means that two faulty copies of the disease 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.

What research is taking place?

There is extensive work taking place on the genetics of SMA which aims to improve our understanding of the disease mechanisms leading to damage of the nerve cells

Improved understanding of the condition has led to the development of potential treatments with the aim of restoring the production of SMN protein in the nerve cells. Some of these potential treatments are already being tested in clinical trials. 

There is considerable cutting-edge SMA research taking place both in the UK and internationally, with many effective international collaborations. Several UK Centres are involved in clinical trials and research into potential treatments.

For more information on research, go to:

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


Author: SMA Support UK, Muscular Dystrophy UK, SMA Trust
Version: 2.1
First published: July 2014
Last reviewed: July 2016
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: