Loss of Control of Muscle Movement in Children and Adults:  Spinal Muscular Atrophy

Spinal muscular atrophy is defined as a disease that negatively affects the control of muscle movement. The cause is a loss of motor neuron found in the spinal cord and the brain stem (it is the part that is connected to the spinal cord). This loss causes weakness and shrinkage (atrophy) of the voluntary muscles used for everyday activities like crawling, sitting up, walking, and head movement.

Types and Symptoms of Spinal Muscular Atrophy

There are many subtypes of the disease. This is based on the severity and age of onset of symptoms.

Type I Spinal Muscular Atrophy (Werdnig-Hoffman disease) happens to be the most severe form of this neuromuscular disease. Symptoms of this stage are present at birth or within the first few months of life. Affected infants usually have difficulty breathing and swallowing, and they are not able to sit or stand.

Children with Type II Spinal Muscular Atrophy typically develop muscle weakness between 6 to 12 months of age. Children with Type II can sit, but they cannot stand or walk without assistance.

Type III Spinal Muscular Atrophy ( Kugelberg-Welander disease or juvenile type) is the mildest form, and patients are able to stand and walk unaided. Symptoms develop between early childhood (greater than 1 year of age) and early adulthood.

Usually after age 30, two types of spinal muscular atrophy, Type IV and Finkel type, occur in adulthood. Adult-onset symptoms of this disease are usually mild to moderate and do often include tremor, muscle weakness and twitching.

How much of the population is affected?

Approximately 1 in 10,000 people are affected by neuromuscular disease, according to the latest statistics. It is not that common, but does affect many.

Are genes related to spinal muscular atrophy?

Genetics do play a role in spinal muscular atrophy.  SMN1 and VAPB gene Mutations are responsible for the disease. 

Mutations in the SMN1 gene are responsible for the formation of the SMA, types I-IV. SMN1 mutations cause a shortage (deficiency) of the protein needed for the survival of motor neurons. In the absence of this protein, motor neurons die, and nerve impulses are blocked between the brain and muscles. The end result is that some muscles cannot perform their normal functions, leading to impaired movement and weakness.

Extra copies of the SMN2 gene can also change the course of spinal muscular atrophy.

In some people with spinal muscular atrophy, the number of copies of the SMN2 gene goes from two to three or four. Changes that increase copies of the SMN2 gene can change the severity of symptoms. On a more limited basis, extra SMN2 genes may help replace the protein  that is needed for the survival of motor neurons. Generally, symptoms are less severe and begin at a later point in life.  Finkel type is caused by a mutation in the VAPB gene. It remains unclear how a VAPB mutation leads to the loss of motor neurons.

Inheritance of Spinal Muscular Atrophy Disease

Most cases, this disease is inherited via an autosomal recessive pattern, meaning that two copies of the gene in each cell are altered. Usually, the parents of a person with an autosomal recessive disorder are the carriers of one copy of the altered gene but lack the signs and symptoms of the disorder.

Finkel type is inherited as an autosomal dominant pattern, which means one copy of the altered gene in each cell is more than sufficient to cause the disorder.