Spinal muscular atrophy type 2

Last updated Feb. 2, 2023, by Marisa Wexler, MS
Fact-checked by Ana de Barros, PhD

Type 2 is an intermediate form of spinal muscular atrophy (SMA), a rare genetic disease characterized by the progressive loss of motor neurons — the nerve cells controlling voluntary movement — that leads to muscle weakness and wasting. This SMA form, one of five main types, manifests in infancy.

Similar to the other four types of SMA, type 2 is caused by mutations in both copies of the SMN1 gene — one inherited from each biological parent. That gene provides instructions to produce SMN, a protein essential for motor neuron and muscle health.

What are the symptoms of SMA type 2?

Type 2 SMA is defined by the appearance of symptoms between 6 and 18 months of age. The hallmark symptom is progressive muscle weakness, which generally affects the legs more than the arms. This can result in:

• low muscle tone causing floppy limbs
• lack of deep tendon reflexes, such as the reflex that occurs when a person’s knee is tapped
• contractures, when the muscle and connective tissue around a joint becomes shortened and hard
• delayed motor development and failure to hit motor milestones — without treatment, most children with SMA type 2 will be able to sit up independently, but will never stand or walk unaided.

Muscle weakness also can affect muscles in the chest, throat, and mouth, giving rise to other SMA type 2 symptoms that affect daily living. Such symptoms include:

• problems eating, due to weakness in the muscles used to chew and swallow
• breathing and coughing difficulties resulting from chest muscle weakness, which can increase the risk of lung infections
• scoliosis, a sideways curvature of the spine caused by weakness in the muscles of the back that support the spine’s position.

How is SMA type 2 diagnosed?

When symptoms are present and/or there is a history of SMA in the family, a diagnosis can be made through genetic testing that looks for disease-causing mutations in the SMN1 gene. Genetic testing can be done through prenatal screening on the fetus or shortly after birth via newborn screening.

DNA testing, which typically is done on a blood sample, also can be used to measure the number of copies that exist of a “backup” SMN2 gene that influences disease severity. Typically, a higher number of SMN2 copies is associated with less severe SMA, with most type 2 patients carrying three copies. Combining these genetic results with the age of symptom onset and symptom severity helps to confirm the type of the disease.

Genetic testing also can be used to identify SMA carriers — people who carry one mutated SMN1 copy. These individuals will not develop SMA, but could pass the disease-causing mutation to their offspring. If both parents carry one mutated copy of SMN1, they will have a 25% chance of having a biological child with any of the most common SMA types, and a 50% risk of each of their biological children being a carrier like them.

Further tests, such as an electromyogram or muscle biopsy, may be considered if there is any uncertainty about the diagnosis. These usually are not needed to confirm an SMA diagnosis, however.

Electromyography assesses the health of muscles and motor neurons that control them; the technique measures muscles’ responses to electrical stimulations by motor neurons. In a muscle biopsy, a small section of muscle tissue is removed, usually from the upper thigh, and examined to see whether it has SMA-associated features.

What are the treatment options for SMA type 2?

Three disease-modifying therapies are approved for SMA type 2 and other forms of SMA in the U.S. These medications also are available in many other countries, though specific indications may differ.

All three — Spinraza, marketed by Biogen, Evrysdi, from Roche and Genentech, and Zolgensma, by Novartis Gene Therapies — have been approved in the last 10 years. None is a specific SMA type 2 treatment, but all are approved for use in patients with this disease type.

• Spinraza (nusinersen) is administered via injections into the spine, given every four months after an initial loading dose. It is approved to treat all ages and all main types of SMA.
• Evrysdi (risdiplam) is a daily therapy that can be taken orally or through a feeding tube. It is approved for all main types of SMA in individuals who are at least 2 months old.
• Zolgensma (onasemnogene abeparvovec-xioi) is a one-time gene therapy for SMA. It is approved to treat infants and toddlers up to age 2 with all main types of SMA.

All of these medications have been proven to slow the progression of type 2 SMA in clinical trials. Importantly, while these therapies can slow the progression of SMA, their ability to reverse damage that’s already happened is limited. Consequently, clinical outcomes are generally better when treatment is initiated as early as possible, for type 2 and all other forms of SMA.

Supportive care and assistive devices

In addition to disease-modifying treatments targeting the underlying cause of SMA, many people living with type 2 SMA commonly benefit from other kinds of supportive care. There are many options for such supportive care, such as:

• physical therapy and exercises to improve motor function and support posture
• occupational therapy, which can help create strategies for navigating daily life
• speech and language therapy, which may help with chewing and swallowing in addition to speaking
• respiratory therapy to support a patient’s ability to breathe
• dietary counseling to support good nutrition and digestion
• palliative care, which is a broad type of supportive care that works to improve quality of life for people affected by chronic diseases.

Many people with type 2 SMA use specialized equipment to help them manage their health and maintain independence. These may include:

• mobility equipment, such as adaptive strollers or wheelchairs, to help patients get around
• specialized seats or chairs that can make bathing and feeding more comfortable
• devices including braces or standers that can provide structural support to the body
• ventilators, cough assist machines, and other equipment to support lung function
• gastrostomy tubes and similar interventions to get good nutrition for patients who have difficulty chewing and swallowing.

What is the life expectancy for people with SMA type 2?

While the life span of children with SMA type 2 varies, most live into early adulthood even without disease-modifying treatment. In the absence of treatment, SMA type 2 life expectancy is longer for patients than it is for children with SMA type 1. However, life expectancy is shorter for these patients than for those with type 3 SMA, for whom life span is not affected compared with the general population.

There remains no cure for SMA type 2 or any other form of the genetic disease. But the new disease-modifying therapies developed in the last decade have been shown in clinical trials to improve outcomes for SMA patients with all types, as compared with the disease’s natural progression.

In these trials, the now-available disease-modifying therapies were found to slow the progression of SMA and lead to better motor function for patients. Many children with type 2 SMA who started treatment in early life are now walking, often within developmentally normal windows. Because the first clinical trials of these therapies started in the 2010s, the long-term effect of these treatments on survival is not yet known.

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