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 cannot 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.
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.
Between one in 40 and one in 80 men and women carry the gene for spinal muscular atrophy. If both parents 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 the fifth chromosome. 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.
Spinal muscular atrophy is divided into subtypes based on the severity and age of onset of symptoms. Three types of this disorder affect children before the age of 1 year. Type 0 is a very severe form of spinal muscular atrophy that begins before birth. Usually, the initial sign of type 0 is reduced movement of the fetus that is first noticed between 30 and 36 weeks of gestation. After birth, newborns show little movement and have difficulties swallowing and breathing. Type I spinal muscular atrophy (also called Werdnig-Hoffman disease) is a severe form of the disorder that is evident at birth or within the first few months of life.
Typically, affected infants have difficulty breathing and swallowing, and they are unable to sit without support. Children with type II spinal muscular atrophy usually develop muscle weakness between ages 6 and 12 months. Children with type II can sit without support, although they cannot stand or walk unaided.
Type III spinal muscular atrophy (also called Kugelberg-Welander disease or juvenile type) is a milder form of the disorder than types 0, I or II. Symptoms appear between early childhood (older than first year of age) and early adulthood. Individuals with type III spinal muscular atrophy can stand and walk unaided, but usually lose this ability later in life. Two types of spinal muscular atrophy, type IV and Finkel type occur in adulthood. Usually it occurs after age 30. Symptoms of adult-onset spinal muscular atrophy are typically mild to moderate and include muscle weakness, tremor, and twitching.
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. An intellectual impairment is typically not present in SMA. In fact many physicians find children with SMA are unusually alert, interactive, and socially gifted.