What is spinal muscular atrophy (SMA)?
Spinal muscular atrophy is a genetic disorder that affects the control of muscle movement. It is caused by a loss of specialized nerve cells, called motor neurons, in the spinal cord and the part of the brain that is connected to the spinal cord. The loss of motor neurons leads to weakness of muscles used for activities such as crawling, walking, sitting up and controlling head movement.
What causes SMA?
In each of the cells that make up our bodies, people have their inherited information on structures called chromosomes. The normal number of chromosomes is 46, which are present in 23 pairs.
We receive 23 chromosomes, one member of each pair, from each of our parents at the time of conception. Each chromosome may have hundreds or thousands of genes. Since chromosomes are paired, we subsequently also have two copies of each gene, one from each parent. The 23rd pair of chromosomes are the sex chromosomes and they determine gender. Males have an X and a Y chromosome while females have two X chromosomes.
Autosomal recessive conditions, such as SMA, require that a person inherit two nonworking copies of the same gene (SMN1), one from each parent. Studies have shown that approximately one person in 50, or 2% of the population, are carriers of SMA. Carriers of recessive conditions do not have medical problems associated with having one nonworking gene, since their one functioning copy of the gene prevents the deleterious effects of the other copy.
Symptoms of SMA only manifest themselves when a person inherits two nonworking copies of this gene. This condition may affect males or females since the responsible gene is not on the sex chromosomes.
Who gets SMA?
SMA affects about 1/10,000 individuals. It is seen across all genders and ethnicity.