Type 2 is an intermediate form of spinal muscular atrophy (SMA), a rare genetic disease characterized by the progressive loss of motor neurons, or the nerve cells that control voluntary movement, which leads to muscle weakness and wasting. This type of SMA usually is detected in infancy.
Similar to the other four main types of SMA, type 2 is caused by mutations in both copies of the SMN1 gene — one inherited from the mother and one from the father. That gene provides instructions to produce SMN, a protein essential for motor neuron and muscle health.
SMA type 2, also called chronic infantile SMA, results in muscle weakness, floppiness (low muscle tone), and delayed motor milestones, all of which usually are detected between 6 and 18 months of age.
Muscle weakness usually affects more the legs than the arms, and children with this type may be able to sit independently at some point in their lives. However, most likely will never be able to stand or walk unaided.
Progressive muscle weakness of the intercostal muscles, which are those supporting the chest wall, also can cause varying degrees of breathing and coughing difficulties, which may increase the risk of lung infections.
Children often develop swallowing and chewing problems, as well as scoliosis, an abnormal and progressive sideways curvature of the spine. Scoliosis is caused by weakness in the muscles of the back that support the spine’s position. The spine forms an “S” shape when viewed from behind.
Tremors in the fingers, hip dislocation, and joint contractures — which are characterized by a limited range of motion caused by the shortening of the muscles or tendons around a joint — also are common, as are deformities of the hands, feet, and chest. Involuntary tongue movements also may occur.
About 70% of children with SMA type 2 do not have deep tendon reflexes, such as the reflex that occurs when a person’s knee is tapped. This lack of reflexes can affect joints and reduce limb movement.
Gastrointestinal and metabolic problems often are present.
The disease does not affect cognitive function.
While the life span of children with SMA type 2 varies, most live into early adulthood.
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.
DNA testing, needing only 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, and most type 2 patients carry three copies, an extra copy relative to healthy people.
Combining these genetic results with the age of symptom onset and symptom severity helps to confirm the type of disease.
Genetic testing also can be used to identify SMA carriers as well as affected babies, either while still in the womb (prenatal screening), or shortly after birth (newborn screening). SMA carriers are individuals who will not develop SMA themselves, but who carry one mutated SMN1 copy and could pass the disease on to their children.
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 the muscles and motor neurons that control them by measuring muscle 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.
No cure currently exists for SMA, but three disease-modifying therapies have become available since 2016 that have the potential to slow or even prevent progression of the main types of the disease.
These include Biogen’s Spinraza (nusinersen), given directly into the spinal canal three times a year, Roche’s Evrysdi (risdiplam), taken daily as an oral solution, and Novartis’ gene therapy Zolgensma (onasemnogene abeparvovec-xioi), administered through a single into-the-vein (intravenous) injection.
Spinraza and Evrysdi, approved for all SMA types and most ages in the U.S., Europe, and several other countries, work by increasing the production of the SMN protein from the SMN2 gene.
Zolgensma uses a harmless virus to deliver a healthy copy of the SMN1 gene to cells, thereby restoring SMN levels. The gene therapy is available for use in children up to age 2 in the U.S., Japan, and Canada. In Europe, it is approved for SMA patients with a clinical diagnosis of type 1 or carrying up to three SMN2 copies who weigh up to 21 kilograms (about 46 pounds), which could cover children up to age 5.
A multidisciplinary team of healthcare professionals is recommended to manage the symptoms and care for an infant with SMA type 2. The team may include specialists in neurology, genetics, respiratory medicine, physiotherapy, occupational therapy, orthopedics, speech and language therapy, and gastrointestinal tract/dietetics.
Last updated: Aug. 11, 2021
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