Causes of spinal muscular atrophy 

Spinal muscular atrophy (SMA) is caused by faulty genes, usually passed on to a child by their parents.

The most common types of spinal muscular atrophy – types I, II and III – are caused by a fault with a gene called SMN1.

The SMN1 gene produces a protein needed by certain cells in the spinal cord (motor neurones), which connect the brain and spinal cord to the muscles. If there's a problem with the gene, this protein is only produced in very low levels.

This causes the motor neurone cells in the spinal cord to deteriorate. Messages sent through the spinal cord to the nerves and muscles are reduced or, in the most severe forms, disappear, and the muscles can't work effectively. The muscles eventually start to shrink from a lack of use (muscular atrophy).

How it's inherited

People who have the faulty gene that causes SMA but don't have SMA themselves are known as carriers. Both parents must be carriers of the faulty gene to pass it on to their child.

If two people who are SMA carriers have a child, there's a:

  • 50% chance the child will be a carrier of SMA
  • 25% chance the child will develop SMA
  • 25% chance the child will be completely unaffected by SMA

Approximately 1 in every 40-60 people is a carrier of a defective SMN1 gene.

Other types of SMA

The causes of some rarer types of SMA can be different from those of types I, II and III.

Spinal muscular atrophy with respiratory distress

A type of SMA called spinal muscular atrophy with respiratory distress (SMARD) is inherited in the same way as types I, II and III, but it's not related to a fault with the SMN1 gene. Instead, a fault with a gene called IGHMBP2 is responsible for the condition.

Adult-onset SMA

SMA that develops in adults (type IV) is linked to the SMN1 gene in some cases, although not all cases are thought to be inherited.

In cases where adult-onset SMA is passed on, the way it's inherited can be different from types I, II and III. For example, it's sometimes possible for someone to develop adult-onset SMA if only one parent has the faulty gene.

In Kennedy's syndrome, the condition is passed on by the mother and only affects male children, although female children can become carriers.

Read more about genetic inheritance.

Testing for SMA

If there are concerns a child may inherit SMA, tests can be carried out before and during pregnancy to check for the defective gene that causes the condition.

See diagnosing spinal muscular atrophy for more information.

Page last reviewed: 09/03/2015

Next review due: 01/04/2017