The Basics of Spinal Muscular Atrophy

Spinal Muscular Atrophy, also referred to as SMA, is the name given to a group of different disorders that all share a genetic mutation that manifests in the weakness caused by the loss of motor neurons in the spinal cord and brainstem. The affected motor neurons are responsible for the control of voluntary muscles that are used in activities such as crawling, walking, head/neck control, and swallowing. Muscles throughout the body are affected by SMA; however, the proximal muscles - those located to the trunk of the body, such as the shoulders, hips, and back - are the most severely damaged.

The cause of SMA is understood to be a missing or mutated SMN1, or surival motor neuron 1, gene. The disease itself is classified as an autosomal recessive gene disease, meaning that both parents must be a carrier of the mutated gene and both must pass it on to their child. (This makes the chances of two carrier parents having a child with SMA 1 in 4 or 25%.) The severity and onset of Spinal Muscular Atrophy is almost purely dictated by the ability of the SMN2 gene to make up for the loss of protein production. Symptoms may surface as early as the first few months of life, or they may not arise until after the age of two.

Diagnosis of SMA generally does not occur until after symptoms have begun to present themselves. The most common form of diagnosis is done in a genetic blood test that determines the lack or mutation of the SMN1 gene. In some rare cases, the results of such a blood test are inconclusive or unable to definitively diagnose SMA. It is in these cases that other tests, such as an EMG (electromyography) or muscle biopsy are used.

Taking in all forms of SMN1 related SMA, the occurrence of the disease is 1 in every 6000, and about 50% of those babies born with SMA will die before their second birthday. This also makes it the number one genetic killer in children under the age of two. It is a disease that does not have regard for age, race, or gender. It can strike anyone at anytime in their life. One out of every forty persons is a carrier for SMA, and because there are no known health risks associated with being a carrier, they often do not know until they have a child born with the disease.

Statistics and information in this post were taken from:
Wikipedia: Spinal Muscular Atrophy
Families of Spinal Muscular Atrophy