How Common is Spinal Muscular Atrophy?
Reviewed by: HU Medical Review Board | Last reviewed: August 2021 | Last updated: September 2021
Spinal muscular atrophy (SMA) is one of the most common genetic conditions that affect children. One in every 6,000 to 10,000 babies worldwide are born with SMA. Type 1 SMA is the most severe type and accounts for more than half of all cases.1
It is hard to estimate the number of people with SMA. Most studies are small and relied on symptoms for diagnosis, rather than genetic screening. Also, the classifications and outcomes of SMA continue to change.1
We need newer and larger studies to more accurately know the rates of people with SMA. However, we do have an idea of SMA statistics.
Who is affected by spinal muscular atrophy?
The incidence of SMA is about 1 out of 6,000 to 10,000 newborns worldwide. Because SMA is present at birth, the incidence of SMA is the number of newborns diagnosed with the condition. Incidence is the number of new cases within a certain time period.2,3
Some studies show a slightly lower or higher incidence in certain regions. This may reflect actual regional differences or differences in study design.2,3
What is the prevalence?
Prevalence is the number of people currently living with the disease. The prevalence of SMA is about 1 person in 100,000 people worldwide. The prevalence is lower than the incidence because of the shorter lifespan of common SMA types. About 25,000 Americans currently live with SMA.2,4
Some studies have shown a higher prevalence in certain regions. We do not know if this reflects actual regional differences or differences in study design.2
How common are the different types?
Type 1 SMA is the most common type, accounting for more than half of all new SMA cases. However, people with type 1 SMA have the shortest life expectancy. This means type 1 SMA has the lowest prevalence because fewer people with type 1 are alive when any given study is performed.2,3
Estimates of the incidence of each SMA type vary between studies. This is because of regional differences and difficulty identifying people with certain types. An analysis of multiple studies has calculated incidence rates of:2
- Type 1 SMA – 5.5 out of every 100,000 newborns (about 60 percent of all SMA cases)
- Type 2 SMA – 1.9 out of every 100,000 newborns (about 20 percent of all SMA cases)
- Type 3 SMA – 1.7 out of every 100,000 newborns (about 20 percent of all SMA cases)
Type 4 is not included in many studies. It is estimated to account for less than 5 percent of all cases.5
How many people are carriers of SMA?
People with SMA must have alterations in both copies of their SMN1 gene. People who have alterations in only 1 copy are called “carriers." They can pass on the altered SMN1 gene to their children but do not show any signs of SMA.2
About 1 in every 50 people in the United States is a carrier of SMA. This is about 6 to 7 million Americans. Asian and white people are slightly more likely to be carriers than Black and Hispanic people. This may mean that more Asian and white people have SMA than Black and Hispanic people.2,4