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 5q SMA which are caused by a fault in a gene called Survival Motor Neuron 1 (SMN1). This gene carries the information required for the production of an important protein called SMN. When there is not enough of this protein, the nerve cells that help control the muscles for moving and breathing become damaged.

It’s important to remember that:

The 4 main types of SMA vary greatly in severity:

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.

What causes SMA?

Usually, electrical signals from our brain are sent down our spinal cord along our nerve cells and through to our muscles. This makes it possible for us to consciously contract our muscles and to make them move.

SMA affects a particular set of nerve cells called the lower motor neurones1 which run from the spinal cord out to our muscles. The lower motor neurones carry messages that make it possible for us to move the muscles we use to crawl and walk, to move our arms, hands, head, and neck, and to breathe and swallow.

For our lower motor neurones to be healthy, we need to produce an important protein called the Survival Motor Neuron (SMN) protein. Our ability to do this is controlled by a gene called Survival Motor Neuron 1 (SMN1) 2.

We all have two copies of this SMN1 gene, one from each parent. People who have two faulty copies of the SMN1 gene have SMA. People who have one faulty copy of the SMN1 gene are carriers of SMA. Carriers do not have SMA or any symptoms of SMA.  People who have two healthy copies of the SMN1 gene do not have SMA and are not carriers.

SMA is passed from parents to their children through their SMN1genes. When two people who are carriers have a child together, their child may inherit two faulty SMN1 genes, one from each parent. If this happens, then their child will have SMA.

Having two faulty SMN1 genes means that a child is only able to produce very low amounts of the SMN protein. This causes their lower motor neurones in their spinal cord to deteriorate. Messages from their spinal cord do not efficiently get through to their muscles, which makes movement difficult. Their muscles waste due to lack of use and this is known as muscular atrophy.

How many people are affected?

How do people get SMA?

What research is taking place?

There is extensive work taking place on the genetics of SMA aimed at improving our understanding of the disease mechanisms leading to damage of the nerve cells.

Improved understanding of the condition has led to the development of treatments which restore the production of SMN protein in the nerve cells.  Potential treatments which address other factors of the condition are also being tested in clinical trials.

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


  1. 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.
  2. Standards of Care for SMA: A Family Guide.
  3. 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.