Spinal Muscular Atrophy

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Spinal Muscular Atrophy: General Information

Spinal muscular atrophy (SMA) is a condition that causes a loss of motor neurons, which are specific nerves in the brain and spinal cord that control movement. It presents autosomal recessive inheritance, which means both parents have to be carriers of the disease.

SMA is caused by a deficiency of the SMN protein, which is most often the result of a deletion (or loss) of part of the SMN1 gene. Without motor neurons, messages cannot be passed from the brain to the muscles of the body. In severe cases, a patient will not be able to sit independently and their breathing and swallowing may be impaired. Life expectancy is significantly reduced. In the mildest cases, symptoms begin in adulthood and independent movement such as walking may become more difficult, but still possible. There are four main subtypes of SMA, each described below. It is not always possible to predict which type of SMA an individual will have based on their genetic testing results.


Type 0 is the most severe form of SMA. Symptoms can often be seen in the later stages of pregnancy, as the fetus is less active than expected. Once born, the infant will have little ability to move and may not be able to breathe and swallow independently. Infants with SMA type 0 often die before six months of age.


Type I is another severe form of the condition. Symptoms typically develop within the first six months of life. Infants with type I SMA often have trouble breathing and swallowing. Their muscle tone and strength are extremely poor; they cannot sit without support and will not achieve any motor-skill milestones.


In children with type II SMA, muscle weakness becomes apparent between the ages of 6 and 12 months. When placed in a sitting position, children with the condition can usually maintain the position without support; however, they often lose this ability by their mid- teens. Individuals with type II SMA cannot stand or walk without assistance. They have poor muscle tone and strength, and their fingers usually tremble uncontrollably.


Type III is a milder form of the condition. Symptoms begin sometime between the age of one year and early adulthood. As young children, these individuals may fall repeatedly and have trouble walking downstairs. While their muscles are weaker than normal, individuals with type III SMA can usually stand and walk without assistance, although they may lose this ability later in life. The legs are often more severely affected than the arms.


Type IV is the mildest form of SMA. With this form of the condition, muscle weakness does not begin until one’s 20s or 30s, or potentially even later. This weakness is often mild to moderate, and the individual is generally able to walk and move independently. They may also experience mild to moderate tremors and/or twitching of the muscles.


Copy number analysis of exons 7 and 8 of SMN1 and SMN2 genes (Survival Motor Neuron 1 and 2) is carried out using the MLPA technique.

Sample type: Peripheral blood in EDTA containing tubes

Time to result: 2 weeks

* Prenatal diagnosis is also available on CVS or amniotic fluid samples

The Molecular Biology department participates successfully in the exteral quality control sheme for Spinal Muscular Atrophy provided via INSTAND.