About SMA

What is SMA?

Spinal Muscular Atrophy (SMA) is a genetic disease causing degeneration of motor neurons in the spinal cord. Motor neurons are nerve cells that carry information from our brain to the muscle resulting in muscle movement. In patients with SMA these motor neurons are not fully functional resulting in muscle weakness and wasting. Weakness is often more severe in the legs than in the arms. The gene defect of SMA has been identified and diagnostic tests exist. The different types of SMA are described below.

Type I (Werdnig-Hoffman, Infantile SMA)

This is the most severe form of SMA. These children present with significant weakness and generalized hypotonia which manifests in difficulty moving, swallowing, feeding and breathing. The neck, shoulder girdle, chest and pelvis demonstrate the greatest weakness. These muscles close to your chest are called proximal muscles. Wrist, hands, ankles and feet are less involved. These muscles are called distal muscles, because they are further away from your trunk. Children with SMA type I often face respiratory problems. Fasciculation's (twitching/writhing) in the tongue may be present.

Type II (Chronic Werdnig-Hoffmann, Intermediate SMA) 

Symptoms of this form of SMA are most typically first seen in children seven to eighteen months of age. Involvement varies in this type. These children are able to sit independently at some point in their lives. Weakness is noted proximally and centrally (as in the chest and torso musculature.) A fine tremor may also be seen on outstretched fingers. Mobility is usually enhanced by using a wheelchair. Many children use adaptive devices to improve their independence and maintain an increased quality of life.

Type III (Kugelberg-Welander, Juvenile SMA)

These children are typically affected after eighteen months of age. They are able to walk at some point in their life. Often they have proximal weakness, poor endurance and poor balance. They may have difficulties with climbing stairs, keeping up with peers during play or getting up from a bent over position. Adaptive devices can help with independence with their activities of daily living. Physical labor as a vocation is not recommended.

The Genetics of SMA

SMA is an autosomal recessive genetic disorder, caused by mutations in the SMN1 (Survival Motor Neuron) gene that is found on chromosome 5q13. To develop SMA, an individual must inherit two defective SMN1 genes, one from each parent.
For more information about the genetics of SMA including patterns of inheritance, gene mutations in SMA, the role of the SMN2 gene in modifying disease severity, and more.

Learn more about the genetics of SMA


Typically, SMA can be diagnosed by clinical examination and genetic testing of a blood sample. In rare cases, additional tests to evaluate for a possible neuromuscular disorder are needed. These may include:

  •  Muscle biopsy: the removal and analysis of a small piece of muscle in order to confirm/determine diagnosis.
  •  Needle biopsy: alternative to muscle biopsy, instead of a two to three inch incision, only a small nick of skin is necessary.
  •  Genetic testing/genetic studies: diagnostic tests that evaluate for conditions that have a tendency to run in families.
  •  EMG: tests that measure the electrical activity of muscle and the conduction properties of nerves. This test can help differentiate between diseases of the muscle or nerve.