Spinal muscular atrophy (SMA) is a genetic disease that causes muscle weakness and wasting, known as atrophy. People with SMA often have difficulties moving, swallowing, sitting up, and sometimes breathing.
People with SMA usually start showing symptoms a few months after birth, but some people begin to have symptoms later in life. The severity of this disease can vary tremendously, and many different types of SMA exist.
That’s why no two people with SMA will have the exact same symptoms and disease course. For this reason, understanding the statistics and facts about SMA can be challenging.
Read on for the answers to some of the most common questions about SMA, from disease frequency to what it means to be a carrier of SMA to how SMA affects life expectancy.
SMA is considered a rare disease. Only 1 in every 6,000 to 10,000 people are born with the disease.
According to the SMA Foundation, 10,000 to 25,000 children and adults are believed to be affected by SMA in the United States.
This is comparable to other rare diseases, like:
SMA is a genetic disease, meaning that it’s passed from parents to children through their genes.
Our genes come in pairs. One copy of a gene is inherited from each parent. Sometimes a gene has a mutation (mistake) and doesn’t work properly. SMA is caused by a mutation in a gene called survival motor neuron 1 gene (SMN1).
In most cases, a child can get SMA only if both parents are carriers and pass on the SMN1 mutation. This is referred to as an autosomal recessive disease.
According to the National Organization for Rare Disorders, if both parents are carriers of a mutated SMN1 gene, there is:
- a 25 percent chance their child will have SMA
- a 50 percent chance their child will be a carrier
- a 25 percent chance their child will be unaffected
If only one parent is a carrier, it’s very unlikely that their child will be born with SMA.
In very rare cases, a mutation in SMN1 can happen during the production of sperm or eggs. In this case, a person can end up with SMA even if only one parent is a carrier.
Prenatal genetic testing is available to find out whether you’re a carrier of the mutated SMN1 gene. This is done through a simple blood test. Carrier testing can detect roughly 90 percent of carriers.
Life expectancy for someone with SMA will vary significantly depending on the type of SMA they are diagnosed with.
There are four main types of SMA. The most common type is called Type 1 SMA. About
Type 1 SMA is the most severe form of SMA. Babies born with Type 1 SMA typically die before the age of 2.
However, new treatments are helping to extend life expectancy. Some Type 1 patients may even live into adulthood.
Children with Type 2 SMA may start showing symptoms between 6 and 18 months old. Life expectancy is often reduced, depending on treatments and other factors.
The majority of people with Type 2 SMA live into early adulthood. With proper care, like respiratory support, physical therapy exercises, and nutritional support, many with Type 2 disease are even living well into adulthood.
Those born with Type 3 SMA tend to have a milder form of the disease and often have an average life expectancy, depending on their symptoms and therapies received.
Type 4 SMA usually starts affecting people in adulthood, typically after they turn 35 years old. Symptoms of this type of SMA are considered mild. Type 4 SMA doesn’t usually lower life expectancy.
Three new treatments for children and adults with SMA have been approved by the Food and Drug Administration (FDA) recently:
- nusinersen (Spinraza), approved to treat children and adults with SMA
- onasemnogene abeparvovec-xioi (Zolgensma), approved to treat children under 2 years old
- risdiplam (Evrysdi), approved for children 2 months of age and older
Several more potential treatments for SMA are also making their way through clinical trials.
If you’re interested in learning more about you or your child taking part in a clinical trial, talk with a healthcare professional.
Most types of SMA equally affect males and females. Most of the mutations that can cause SMA are autosomal recessive. The ability to pass on these mutations is unaffected by gender.
However, a very small percentage of people with SMA have a different type caused by a mutation in an X chromosome gene called UBA1.
This type of SMA, known as X-linked SMA, is much more likely to occur in males than females, according to the Muscular Dystrophy Association.
This is because males only have one X chromosome. Unlike females, they have no second X chromosome to protect them from the full effects of a mutation on the X chromosome.
SMA is considered a rare disease. It’s passed from parents to children through their genes. Only
The disease course of SMA can vary significantly depending on the type. Though people with less severe types of SMA can expect to live just as long as someone without SMA, people with the most common form of the disease – Type 1 – may not live past age 2.
Fortunately, several new treatments for SMA have been approved by the FDA recently, and several other potential treatments are making their way through clinical trials.