Spinal muscular atrophy (SMA) is a serious, debilitating genetic condition that causes muscle weakness and a progressive loss of movement.
SMA causes the motor neurons in an area of the spinal cord, known as the anterior horn, to deteriorate. This results in:
- progressive muscle wasting,
- loss of motor function (the ability to move parts of the body), and
- loss of mobility.
Motor neurons
Motor neurons are nerve cells which ‘connect’ the brain and spinal cord to the body’s muscles. The motor neurons send out electrical signals to the muscles, which ‘tell’ the muscles when to expand and contract.
In people with spinal muscular atrophy, the motor neurons deteriorate, causing the link between the brain and muscles to gradually break down.
As the link between the brain, spinal cord, and muscles breaks down, the muscles which are used for activities such as crawling, walking, sitting up, and moving the head, are used less and less and become weaker, or shrink (atrophy).
How common is SMA?
In the UK, it is estimated that there are somewhere between 5,500-6,000 people with SMA at any one time. Listed below are some statistics about SMA.
- Approximately, 1 in every 6,000 births is affected by SMA.
- About 1 in 40 people are carriers of the defective gene that causes SMA.
- Children of parents who are both carriers of the defective gene have a 25% chance of developing SMA.
- SMA can affect both men and women, but it is more common in men.
Types of SMA
SMA is classified into a number of different types, based on the age at which the condition develops, and the severity of symptoms. Types 0, I, II, and III develop in childhood, and type IV occurs during adulthood.
The different types of SMA are discussed in more detail below.
Type 0 (prenatal)
Type 0 SMA is a very severe form of spinal muscular atrophy. It develops before birth, causing reduced movement of the foetus, which is usually first noticed between weeks 30 and 36 of pregnancy.
Type I
Type I SMA, known as Werdnig-Hoffmann disease, is also a severe form of spinal muscular atrophy.
Symptoms may be apparent at birth, or during the first few months of life. They usually occur before six months of age.
In type I SMA, the proximal muscles (those closest to the centre of the body, such as the shoulders, hips, and back) become increasingly weak, causing difficulties with moving, eating, swallowing, and breathing.
Type II
Type II SMA, known as Dubowitz disease, is not as severe as types 0 and I, but it still causes weakness in the proximal muscles. Type II SMA usually develops when an infant is between 6-18 months of age.
Type III
Type III SMA, known as Kugelberg-Welander disease, is the mildest form of childhood spinal muscular atrophy. The condition often develops later than types I and II, with a diagnosis usually being made after a child reaches two years of age.
Type IV
Type IV SMA begins in adulthood, with the onset of symptoms usually occurring after the age of 35.
Compared with the four types of childhood SMA, the symptoms of type IV spinal muscular atrophy are usually milder.
Kennedy's syndrome
Kennedy's syndrome, or spinal-bulbar muscular atrophy (SBMA), is another type of adult SMA.
SBMA only affects men and is caused by a defect on the X chromosome. It usually develops between 20-40 years of age, although it can sometimes also affect young, teenage men and those who are over 40 years of age.
