Spinal muscular atrophy (SMA) is caused by one or more faulty genes. It is usually inherited, which means the genes can be passed on from parents to children.
The genes that are affected normally produce a protein that is needed by the motor neurones. Motor neurones are nerves that connect the brain and spinal cord to the muscles. The protein is needed to send electrical impulses to the muscles, enabling them to work effectively.
Without this crucial protein, the motor neurone cells in the spinal cord will deteriorate and die.
The deterioration and death of the motor neurones in the spinal cord means that the muscles they control will be unable to receive vital nerve impulses. In turn, the muscles will not be able to work effectively, leading to weakness and impaired movement. Because of a lack of use, the muscles will eventually start to shrink. This is known as muscular atrophy.
How it is inherited
For a child to develop SMA, both parents must have the defective gene and pass it on to their child. These parents are known as carriers, and don't actually have SMA.
If both parents are SMA carriers and have a child, there is a:
- 50% chance that the child will be a carrier of SMA
- 25% chance that the child will develop SMA
Approximately one in every 40 people is a carrier of the defective gene that causes SMA.
Adult forms of SMA may be inherited in various ways.
