What is Spinal Muscular Atrophy?
Spinal Muscular Atrophy (SMA) is a neuromuscular condition causing a
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 a1in 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. Both boys and girls are affected equally.
Is there more than one type of 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 knows 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.
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). |
