Spinal muscular atrophy (SMA) type 2 is a genetic condition that causes muscle weakness (hypotonia) and eventual loss of motor function due to a reduction of nerve cells in the spinal cord. Approximately 20 percent of people diagnosed with SMA have SMA type 2, also called intermediate SMA or Dubowitz disease.
SMA type 2 is called intermediate SMA because both the disease’s severity and its age of onset fall between the more severe forms (types 0 and 1) and less severe forms (types 3 and 4) of the condition. SMA type 2 is typically diagnosed in children between the ages of 6 months and 2 years.
Understanding the causes, symptoms, and treatment options for SMA type 2 can be invaluable in providing support to a loved one affected by the condition. Be sure to consult with a health care provider before trying any new treatments.
People diagnosed with SMA can experience symptoms including:
Although most infants with SMA type 2 will be able to sit independently at a normal age, most do not learn to stand or walk. SMA muscle weakness is proximal in nature, meaning that muscles closer to the center of the body (like the shoulders) are weaker than muscles that are farther away (like the fingers). Children with SMA type 2 are more likely to experience weakness in the lower body than the upper body, requiring the use of crutches or a wheelchair.
Scoliosis occurs in nearly all cases of SMA type 2. The curvature of the spine in scoliosis can create breathing problems that can lead to serious health issues.
The current life expectancy for people living with SMA type 2 is around 25. However, research is in progress to determine how the newest therapies and treatments from current clinical trials will impact life span and quality of life. With aggressive supportive care and novel therapeutics, many people with SMA type 2 may live well past their 25th birthday.
SMA type 2, like most other types of SMA, is caused by a recessive mutation in the survival motor neuron 1 gene, or the SMN1 gene. Recessive means that a child must inherit a copy of the gene mutation from each parent to be diagnosed with SMA. With two mutated genes, the body can’t make enough of an important protein called SMN protein. Not having this protein causes the motor neuron cells (responsible for voluntary movement) in the spinal cord to die, leading to the symptoms of SMA.
A second gene, the SMN2 gene, plays a noteworthy role in SMA type 2. It’s a “backup” gene that also produces SMN protein — but at a significantly lower rate than a normal SMN1 gene typically makes. The number of SMN2 genes a person has varies from person to person. That number also determines the type of SMA they will develop and its severity. A person can have up to eight copies of the SMN2 gene; children with SMA type 2 typically have only three copies.
SMA type 2 is typically diagnosed between the ages of 6 months and 2 years after a child does not meet normal milestones for movement development. Genetic testing for the SMN1 gene mutation can confirm the diagnosis. However, SMA type 2 can sometimes be confused with other neuromuscular disorders, as it lacks some of the hallmark characteristics of more severe forms of the condition. Rarely, the doctor may evaluate a muscle biopsy from the leg to determine a diagnosis.
Fortunately, prenatal genetic testing for conditions such as SMA has become more standardized in the U.S., leading to quicker and more accurate diagnoses. Genetic testing can determine the type of SMA that a baby will have before they are even born or in the first few months of life.
Supportive care and new drug treatments have greatly improved and delayed the symptoms of SMA in many people.
Recently, the U.S. Food and Drug Administration approved new disease-modifying treatments that have improved the outcomes for people living with SMA.
Spinraza (nusinersen) was the first drug approved for SMA, in 2016. The drug works by promoting the production of SMN protein. Spinraza is approved to treat SMA type 2 in people of all ages, but is most effective when given at a younger age. Spinraza is delivered by intrathecal injection (injection into the spinal canal).
The second drug to be approved for SMA was Zolgensma (onasemnogene abeparvovec-xioi), which was made available in 2019. This drug works differently from Spinraza: It’s known as a gene therapy drug because it replaces the mutated SMN1 gene with a working copy. Zolgensma is delivered by a one-time intravenous (into a vein) injection and can be given to children younger than 2 years old.
In 2020, the SMA drug Evrysdi (risdiplam) hit the market. This drug works by blocking a mechanism on the SMN1 gene, similar to how Spinraza works. Evrysdi has an advantage over Spinraza because it is taken as an oral liquid and does not require surgery or a hospital visit to be administered. This drug is approved for use in SMA types 1 through 3 and can be taken by children 2 months of age or older.
Young children with SMA commonly experience trouble breathing. These issues can often worsen due to the scoliosis that occurs in most people with SMA type 2. Treatments that can help reduce the risks of serious breathing problems include:
Occupational therapy in combination with physical therapy can help people with SMA perform normal daily activities by providing tools that may make common tasks easier. These devices can include wheelchair lifts in the home or even hand-operated devices to allow people to perform tasks such as driving.
If you’re living with SMA or supporting a loved one with SMA, having a community of people who understand your challenges and triumphs is essential. On mySMAteam — the social network for people with SMA and their loved ones — members come together to ask questions, give advice, and share their stories with others who understand.
Do you or your child have SMA type 2? Share your experience in the comments below, or start a conversation by posting on mySMAteam.