Spinal Muscular Atrophy (SMA) is a broad term which refers to numerous neuromuscular diseases which cause the progressive and irreversible deterioration of muscle tissue. These conditions fall within two categories: atrophies which refer to the deterioration arising from a disorder originating in the nerve system which causes loss of the ability to use muscles and dystrophies the wasting of the muscles from within themselves.
SMA occurs in approximately 1 in 10,000 births. SMA Type I, the most severe form of this disease, is the most common single genetic cause of death in infancy. There is currently no cure for SMA or treatment to stop its progression. Medical care and physical therapy may help prevent complications and ensure the best possible quality of life for those affected.
The shared feature of all the forms of SMA is progressive muscle weakness and wasting caused by degeneration of the anterior horn cells of the spinal cord (also called the lower motor neurons). Normally, these nerve cells relay messages from the brain to the muscles and stimulate them to contract. Without this stimulation, the muscles atrophy and will eventually no longer respond. Respiratory issues can also develop as a result of SMA. However, the brain and sensory nerves that allow us to feel sensations such as temperature, touch and pain are not affected and intelligence is normal.
For SMAs that occurs at childhood, a genetic defect on chromosome 5, which is transmitted through a pattern of recessive inheritance, is the cause. Affected children are born to parents who are both carriers and possess a normal and mutated form of the SMA gene. The mutation is harmless in carriers, but a child who inherits a “double dose” of the gene (that is, one copy of the faulty gene from each parent) is born with SMA. When both parents are carriers, there is a 25% risk in each pregnancy that their child will be born with the disease. SMA is a relatively common disease as an estimated one in 80 people in Canada is a carrier, so the chance of a carrier mating with another carrier is appreciable even if the disorder is completely unknown in previous generations of their families.