Key points
- SMA is a genetic condition that weakens the muscles used for moving, swallowing and breathing.
- It is caused by a missing or faulty SMN1 gene. It is nobody's fault.
- SMA does not affect intelligence, feeling or the ability to learn.
- It varies a lot from person to person — from babies who need help to breathe, to adults who walk with mild weakness.
Spinal Muscular Atrophy (SMA) is a genetic disease that takes away physical strength by affecting the motor nerve cells (motor neurons) in the spinal cord. These nerve cells carry signals from the brain and spinal cord to the muscles.
When motor neurons are lost, the link between nerves and muscles breaks down. The muscles used for crawling, walking, sitting up, holding up the head, and even swallowing and breathing, become progressively weaker and smaller (atrophy). Muscles closest to the centre of the body — shoulders, hips, back — and the breathing muscles are usually affected first.
SMA is the most common genetic cause of death in infants. But it is also a condition many people live with into adulthood — studying, working, raising families and leading full lives.
What causes SMA?
Everyone has two copies of a gene called SMN1 (Survival Motor Neuron 1). It makes the SMN protein that motor neurons need to stay healthy. People with SMA have two faulty or missing copies of SMN1, so they cannot make enough SMN protein, and their motor neurons gradually stop working.
A second, “back-up” gene called SMN2 can make a small amount of working SMN protein. How many copies of SMN2 a person has is one reason why SMA is so much more severe in some people than in others. Read more on genetics and inheritance.
SMA is inherited from both parents, who are usually healthy carriers and have no idea they carry the gene. Nothing a parent did, or did not do, caused SMA.
What SMA does — and does not — affect
| SMA can affect | SMA does not affect |
|---|---|
| Strength in the arms, legs, neck and trunk | Intelligence and the ability to learn |
| Sitting, standing and walking | Sensation — people with SMA feel touch and pain normally |
| Swallowing, chewing and feeding | Personality, curiosity and sense of humour |
| Breathing and coughing | The ability to study, work and have relationships |
| The spine (scoliosis) and joints (contractures) |
How common is it?
Around 1 in 50 people carries one faulty copy of the SMN1 gene, usually without knowing. When both parents are carriers, each pregnancy has a 1 in 4 chance of a child with SMA. Worldwide, SMA affects roughly 1 in 10,000 babies born.
Types of SMA
SMA is usually grouped into types according to when symptoms start and the highest physical milestone a child reaches — from Type 1, where babies are never able to sit unaided, to Type 4, which begins in adulthood. See Types of SMA.
Is there a treatment?
Yes. Until 2016 there was no approved treatment for SMA. Today there are approved therapies that slow or stop the loss of motor neurons, and one has been available in India since 2021. Treatment works best when started early, which is why early diagnosis matters so much. See Treatments in India.
Alongside medicines, good supportive care — for breathing, nutrition, physiotherapy and posture — makes a huge difference to health and quality of life. Our care guides explain how.
Where to next?
- Recognising the signs and getting a diagnosis
- Just received a diagnosis? First steps
- Talk to us — our team includes parents who have been where you are.