SMA prevalence

The global birth prevalence of spinal muscular atrophy (SMA), which is the number of babies born with the disease, is about 1 in 10,000 live births. In the U.S., the rate is lower, at approximately 1 in 14,000 newborns.

SMA is a rare genetic neuromuscular disorder that causes progressive muscle weakness and wasting.

The most common types of the disease are caused by mutations in the SMN1 gene, which provides instructions for making the survival motor neuron protein, known as SMN.

When there isn’t enough of the SMN protein, specialized nerve cells in the spinal cord, called motor neurons, progressively die off, leading to the symptoms of SMA.

A better understanding of SMA prevalence may make a difference in healthcare planning, awareness of the disease, and research, including the development of new treatments.

Global prevalence of SMA

Globally, about 1 in 10,000 babies are born with SMA.

The exact prevalence varies across world regions and specific populations, depending on genetic makeup, as well as access to healthcare and diagnostic methods such as genetic testing.

In regions with greater access to testing, including North America and Europe, it is easier for researchers to accurately estimate the prevalence of SMA than in places with limited availability.

Estimates for the birth prevalence of SMA in some countries are:

• 1 in 13,862 or 1 in 14,694 in the U.S.
• 0.51 per 10,000 in Japan
• about 1 in 8.554 in Germany
• about 1 in 10,000 in Australia

Prevalence of different types of SMA

Different types of SMA have varying prevalence rates.

There are five types of SMA: 0, 1, 2, 3, and 4.

Type 0, which is the rarest kind, and type 1 are the most severe. People with these types have the shortest life expectancy.

Adult-onset type 4 SMA is the mildest form and does not tend to affect life expectancy.

The most common type of SMA is type 1, with an estimated prevalence of 0.04 to 0.28 per 100,000.

SMA types 2 and 3 have an approximate prevalence of 1.5 per 100,000 when grouped together.

According to other sources, SMA type 1 accounts for about 60% of cases, type 2 for about 20% to 27% of patients, and type for 12% to 20%. SMA type 4 represents fewer than 5% of SMA patients.

Factors influencing SMA prevalence

The genetic makeup of a population can influence the prevalence of SMA. So can the carrier rate, which is the proportion of people who do not develop the disease but have a single copy of a specific recessive genetic mutation.

Each person inherits two copies of the SMN1 gene — one from each biological parent. SMA is an autosomal recessive disorder, which means that you can only develop the disease if both copies of the gene carry a mutation that causes SMA.

People who have one mutated SMN1 copy are carriers of SMA. Although they will not develop the condition, they can still pass the disease-causing mutation to their biological children.

If two carriers have a child, there is a 25% chance the child will have SMA; a 50% chance the child will be a carrier; and a 25% chance the child will not have SMA and will not be a carrier. This implies that the carrier rate in a population can also influence SMA prevalence.

Genetic carrier testing can help to determine whether someone is a carrier of an SMN1 mutation.

Another gene that plays a role in SMA is the SMN2 gene, which is a backup gene that also provides instructions for making SMN protein. However, it produces only about 10% as much functional SMN protein as the SMN1 gene.

Typically, those with SMA type 1 have one copy or two of SMN2. A greater SMN2 copy number tends to correspond to less severe disease.

Carrier rates for SMA in the general population are estimated to be around 1 in 40 to 1 in 60. Caucasian and Asian populations have generally higher rates than populations with African or Hispanic ancestry.

Considering SMA’s high overall carrier rate, its effect on the prevalence of the disease is significant. For example, it increases the probability that two carriers have a baby together who has both faulty copies of the SMN1 gene. The child will be more likely to develop the disease, potentially increasing its prevalence.

Challenges in determining SMA prevalence

Determining the prevalence of SMA worldwide is challenging due to a lack of sufficient data in certain regions and populations. Access to screening and other diagnostic tools can also impact prevalence.

Similar to many other rare diseases, SMA is often underdiagnosed or misdiagnosed, which makes it difficult to track.

Limited access to screening in some regions may contribute to underdiagnosis, further complicating the determination of its prevalence.

Conversely, greater access to screening programs in other regions may lead to a potentially inaccurate impression that the prevalence in those regions is higher when that may not be the case.

The overlap of SMA symptoms with those of other neuromuscular disorders, such as limb-girdle muscular dystrophy, may also lead to misdiagnosis.

Changes in prevalence over time

SMA prevalence reporting has evolved with advancements in genetic screening, increased awareness, and the impact of new, life-extending treatments.

While underdiagnosis remains a challenge, more cases are being detected thanks to newborn screening programs, which help researchers estimate prevalence.

Greater awareness of SMA and other rare diseases has also led to increased carrier screening.

Researchers believe that increased carrier screening may have contributed to the lower prevalence data for SMA reported in the U.S. in 2024, compared with global rates.

For example, as of 2017, both the American College of Obstetricians and Gynecologists and the American College of Medical Genetics and Genomics have recommended carrier screening for SMA to all people who are considering starting a family. These recommendations may have impacted people’s reproductive choices and the prevalence of the disease.

Finally, new, disease-modifying treatments, including gene therapy, may also affect SMA prevalence by improving survival rates.

Current research and future directions

Researchers continue to study and refine estimates of SMA prevalence.

In the U.S., the extensive implementation of newborn screening for SMA in the years since 2018 has provided scientists with new sources of robust and specific data on birth prevalence for SMA, including to calculate the most recent U.S. estimates published in 2024.

As more information becomes available, scientists can continue to gain a better understanding of how common SMA is.

Further developments in gene therapies and emerging treatments may also impact SMA life expectancy, potentially affecting disease prevalence for the better.