What is Spinal Muscular Atrophy (SMA)?

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Spinal Muscular Atrophy (SMA) refers to a group of diseases which affect the motor neurons of the spinal cord and brain stem. These critically important cells are responsible for supplying electrical and chemical messages to muscle cells. Without the proper input from the motor neurons, muscle cells can not function properly. The muscle cells will, therefore, become much smaller (atrophy) and will produce symptoms of muscle weakness. There are dozens of diseases which affect the motor neuron.

*Spinal Muscular Atrophy kills more babies than any other genetic disease.

Degeneration and death of the motor neurons (also called Anterior Horn Cells) in the brain stem and spinal cord produces weakness in the muscles of swallowing, breathing, and limbs. This disease afflicts infants, children, and adults worldwide. It is estimated that spinal muscular atrophy occurs in between one-in-6,000 and one-in-20,000 births. Advances in our understanding of the genetics of this disorder confirm that the majority of children and adults afflicted with SMA, have inherited this disorder by receiving one gene from both their mother and their father. This is termed “autosomal recessive genetic transmission.”

Between one-in-40 and one-in-80 “normal” men and women carry the gene for spinal muscular atrophy. If both a man and woman carry the gene, the chances are 25% that any of their children will manifest SMA.

Despite the fact that SMA was described many decades ago, there is still a great deal of confusion among patients, parents, and physicians as to the diagnosis, treatment, and genetic counseling which should be provided for those affected with this disease. Some of these questions have been answered by advances in the science of molecular genetics. We now know that the common forms of SMA are the result in a change in a gene located on chromosome #5. Even though we refer to “different types of SMA,” most of these are the result of a number of alterations (mutations) in the gene which are ultimately responsible for the degeneration or premature death of the anterior horn cells. It is still useful to think in terms of several different types of SMA to assist in guiding appropriate treatment, depending on whether the disease is severe or relatively benign. In virtually all cases of SMA, the symptoms are dominated by muscle weakness. There are no problems with sensation of the face, arms, or legs. Intelligence is unaffected, and in fact many physicians who have worked with hundreds of children with SMA are impressed that these children tend to be unusually alert, interactive, and socially gifted.

The above was written for Fight SMA by: Robert T.
Leshner, M.D., Professor, Neurology and Pediatrics,
Children’s National Medical Center