Spinal Muscular Atrophy

Spinal muscular atrophy (SMA) is a genetically based (inherited) neuromuscular disease that disrupts the ability of nerves to communicate with muscles. A child with SMA gradually loses control of most core muscle groups, which grow weak from lack of use. Although there’s no cure for SMA, Nemours neuromuscular specialists offer many treatments and therapies to slow its progression and maximize function.

 
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Four Types of Spinal Muscular Atrophy

There are four types of SMA, based on how severe the symptoms are and when they begin. Generally, the longer it takes for symptoms to start appearing, the less severe the disease will be.

  • SMA type 1 (also called Werdnig-Hoffmann disease) is the most severe, with symptoms starting between birth and 6 months of age. These babies fail to sit or walk.
  • SMA type 2 (or chronic infantile SMA), has more moderate symptoms, which start when a child is six to 18 months old. These infants can sit but not walk.
  • SMA type 3 (Kugelberg-Welander disease or juvenile spinal muscular atrophy) has the mildest symptoms, which can start between 18 months and the teen years. These children do walk but have limitation and may lose skills over time.
  • SMA type 4 develops more slowly, with symptoms usually starting after age 35, with slowly progressive limitation in walking.
How Spinal Muscular Atrophy Develops

Healthy muscles depend on nerve cells called motor neurons to tell them what to do. The motor neurons are located in the brain and spinal cord and carry these instructions from the brain, down the spine, to the muscle. The motor neurons depend on a special protein called SMN (“Survival of Motor Neurons”) to function properly.

People diagnosed with spinal muscular atrophy lack the ability to make this essential SMN protein, because the gene for SMA that makes it possible to produce SMN is either defective or missing. Without sufficient SMN protein the motor neurons lose the ability to carry the brain’s instructions to the muscles. The muscles grow weak from lack of use and waste away (muscular atrophy). Thus the term “spinal muscular atrophy.”

Diagnosing Spinal Muscular Atrophy

Doctors who think a child might have SMA usually rely on various tests and imaging procedures to narrow down the diagnosis. These may include an electromyogram (EMG) and nerve conduction study.

In most cases, when SMA is suspected, the physician goes right to a genetic test (blood test) that checks to see if the gene responsible for producing the SMN protein is defective or missing.