Information pages published by

The Jennifer Trust for Spinal Muscular Atrophy

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Background to
Spinal Muscular Atrophy


CONTENTS

 
 
WHAT IS SPINAL MUSCULAR ATROPHY?
Spinal Muscular Atrophy is a neuromuscular condition causing weakness of the muscles.

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).

 

Nerves
 

WHAT HAPPENS IN SMA?
In Spinal Muscular Atrophy it is the anterior horn cell which is abnormal. Not all nerve impulses can be passed from the brain to the muscles. The result of this is that some muscles become weak and wasted (atrophied).

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 severe form described here starts shortly before or shortly after birth.

 

TYPES of SMA
All ages of onset are approximate
Severe (Type I)Also known as Werdnig-Hoffmann Syndrome.
Onset before or shortly after birth.
Unable to sit. Do not usually survive past 2 years old.
Intermediate (Type II)Onset between 3 months and 2 years.
Able to sit, but not stand without aid.
Survival into adulthood possible.
Mild (Type III)Also known as Kugelberg-Welander Disease.
Onset usually around 2 years.
Able to walk. Normal lifespan.
Adult Onset SMANumber of forms differing in age of onset.
Degree of weakness is variable
 
 
IS SMA HEREDITARY?
The gene for SMA is passed from parents to their children. A child can only be affected by SMA if both parents carry a defective gene (this is called an autosomal recessive pattern). Genes come in pairs, one from each parent. If a child 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. 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 equally affected.

Genes
Approximately one million of the United kingdom's population are SMA carriers. It is estimated that about 1 in 50 people are SMA carriers (of all types).
The chances of both parents being carriers is 1 in 2,500 which explains the approximate incidence of SMA of around 1 in 10,000 births (all types).

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From an original booklet ©The Jennifer Trust for Spinal Muscular Atrophy, 1993

These pages ©The Jennifer Trust for Spinal Muscular Atrophy, 1997 & 2000

Illustrations and HTML by DanE, 1993, 1997 & 2000