What is spinal muscular atrophy?
Spinal Muscular Atrophy (SMA) is a neuromuscular condition causing weakness of the muscles.
Is SMA Hereditary?
The gene for SMA is passed from parents to their children, but SMA can only affect a child if both parents carry a defective gene (this is called an autosomal recessive pattern). Genes come in pairs, one from each parent. If a person has one normal and one affected gene they do not show the symptoms of SMA but are carriers. If both genes are affected they will have SMA.
How Spinal Muscular Atrophy is Inherited.
Each child of carrier parents has a 1 in 4 chance of being affected by SMA, a 2 in 4 chance of being a carrier themselves, and a 1 in 4 chance of being completely clear of SMA. Boys and girls are affected equally.
Is There More Than One Type SMA?
There are several types of SMA. These differ in the age at which they have an obvious effect, how severe this effect is, and which muscles are most affected. The intermediate form described here starts in infancy. However, it does not have such a severe effect as Werdnig-Hoffmann Syndrome with which it is sometimes confused.
Types of SMA
All ages of onset are approximate
| Type 1 (Severe) |
Also known as Werdnig-Hoffman Syndrome. Onset before or shortly after birth. Unable to sit. Do not usually survive past 2 years old. |
| Type II (Intermediate) |
Onset between 3 months and 2 years. Able to sit, but not stand without aid. Survival into adulthood possible. |
| Type III (Mild) |
Also known as Kugelberg-Welander Disease. Onset usually around 2 years. Able to walk. Normal lifespan. |
| Adult Onset SMA |
Number of forms differing in age of onset. Degree of weakness is variable. |
How do Normal Muscles Work?
Muscles contract, moving parts of the body, when signals from the brain pass down the spinal cord through the anterior horn cells. Each anterior horn cell is responsible for passing a signal down another nerve to a muscle. The group of structures made up by one anterior horn cell, the nerve fibre and the muscle it supplies, is known as a motor unit (see diagram).
What Happens in SMA?
In SMA, it is the anterior horn cell, which is abnormal. Not all nerve impulses can be passed from the brain to the muscles. This results in some of the muscles becoming weak and wasted (atrophied). | 
