Spinal muscular atrophy (SMA) is an inherited neuromuscular disease characterized by degeneration of the motor nerves in the spinal cord’s anterior horn. Most types of SMA are caused by a specific genetic mutation in the SMN1 gene, which is the gene responsible for making the survival motor neuron (SMN) proteins. The SMN proteins are essential for motor nerve survival, growth, and function. The SMN2 gene also helps make SMN proteins, but fewer of them. Motor nerves help control muscles. The loss of motor nerves due to loss of SMN protein causes progressive muscle weakness, a characteristic symptom of SMA that can lead to complications such as respiratory failure.
SMA primarily affects infants and children. Although it’s a rare disease, SMA is the leading genetic cause of infant deaths. It is also one of the most common genetic disorders to be diagnosed among infants and children, second only to cystic fibrosis. But just how common is SMA? Understanding the epidemiology — the causes, prevalence, and incidence — of SMA can be helpful in learning about the disease.
Experts use the terms incidence and prevalence to describe how common diseases are. Incidence refers to how many people develop a disease. Prevalence is a measure of how many people are living with the disease.
Across all types of SMA, it has an incidence rate of between 1 in every 10,000 live births and 1 in every 6,000 births. The prevalence rates are much lower, with 1 in every 100,000 people having SMA. Because of the disease severity, the mortality rate of SMA among infants can be high, which helps explain the discrepancy between the incidence and prevalence of SMA.
The rates of SMA differ by type. Research indicates that SMA type 1 has the highest incidence rates and represents 60 percent of SMA cases. However, infants with the type 1 form (the most severe form) have high mortality rates and a life expectancy of 2 years of age or less. Because of this, types 2 and 3 have overall higher prevalence rates, although they develop less frequently than type 1.
SMA is an autosomal recessive genetic disorder, which means that a child will only develop SMA if they inherit mutant copies of the SMN1 gene from both of their parents.
Carrier frequency studies show that within the United States, there is a slight difference between SMN1 deletion carriers based on race and ethnicity. A carrier is someone who has part of the genotype (genetic code) for a condition but doesn’t have the phenotype (physical expression or manifestation) of the disease. If a carrier of SMA has a biological child with a noncarrier, there is zero chance the child will have SMA. If two SMA carriers have a biological child, the chance of the child having SMA is still just 25 percent. There’s a 50 percent chance of the child also being a carrier, with the potential to later pass the genotype or phenotype on to their biological children — depending on their partner.
In one study conducted in the United States, white people had a carrier frequency of 1 in every 47 people (2.1 percent), whereas Black people had a carrier frequency of 1 in every 72 people (1.4 percent). However, the study’s researchers suggest that the racial disparity may be explained by the accuracy of tests. It appears that carrier frequency tests are 95 percent accurate in white people, but only 70 percent accurate in Black individuals.
Another North American study found the following carrier frequency rates:
Note that these rates refer to carriers only, not people who have SMA.
Although much research has been limited to European countries or populations within the United States, a recent study conducted in China estimated that the carrier frequency of SMA is about 1 in 50 individuals (2 percent). Another study conducted in a Venezuelan population also demonstrated that the carrier frequency of SMA was 1 in 49 individuals (2.04 percent). Both rates are comparable to the rates observed within the United States.
An additional epidemiological study of SMA in sub-Saharan Africa found that Malians had a carrier frequency of 1 in 209 individuals (0.48 percent). Again, it’s hard to know how accurate this number is based on testing sensitivity. This same study did note that, unlike Eurasian individuals, sub‐Saharan Africans are more likely to have more than three copies of the SMN1 gene (normally there are two), which may explain why carrier tests among Black people are less accurate. This is also why genetic testing and counseling for SMA carriers is so important in families with at least one Black or African American parent.
If you’re concerned about the possibility of carrying, passing on, or having SMA, talk to your health care provider, who can refer you to the right resources. It may also help to have some social support. On mySMAteam, the social network for people with spinal muscular atrophy and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with SMA.
Are you living with spinal muscular atrophy? Share your experience in the comments below, or start a conversation by posting on mySMAteam.