Ataxia is caused by damage to a part of the brain known as the cerebellum and sometimes part of the spinal cord.
The spinal cord is a cable of nerves that runs from the brain to the rest of the body.
The cerebellum sits at the base of the brain and is responsible for controlling:
- walking and sitting balance
- limb co-ordination
- eye movements
Damage can occur as a result of injury or illness (as is the case with acquired ataxia) or because the cerebellum or spinal cord degenerates (as is the case with hereditary ataxia).
In a minority of cases there is no clear cause why the cerebellum and spinal cord become damaged as is the case with idiopathic late onset cerebellar ataxia (ILOA).
Acquired ataxia can have a wide range of potential causes, such as:
- severe head injury, such as the type of injury that can occur during a car crash or a fall
- bacterial infection that affects the brain, such as meningitis or encephalitis (an infection of the brain itself)
- viral infection – some types of viral infection such as chickenpox or measles can spread to the brain, although this is very uncommon
- conditions that disrupt the supply of blood to the brain, such as a stroke, haemorrhage (bleeding in or around the brain) or a transient ischaemic attack (a so-called mini-stroke)
- cerebral palsy – a series of conditions that can disrupt a child’s normal growth and development
- multiple sclerosis – a long-term condition that causes damage to the nerve fibres of the central nervous system
- prolonged long-term alcohol misuse
- underactive thyroid gland
- certain toxic chemicals, such as mercury, lead, solvents and some types of pesticides, can trigger ataxia if a person is exposed to enough of them
- certain medications, such as benzodiazepines (a medication used to relax people who are anxious or have problems sleeping), can occasionally trigger ataxia as a side effect
- health conditions where the immune system attacks healthy tissue (autoimmune conditions), such as lupus
- in children, epilepsy – a condition that can cause a person to experience repeated seizures (fits)
Hereditary ataxia is caused by genetic mutations. A genetic mutation is when the instructions carried in all living cells become scrambled in some way meaning that one or more of the functions of the body does not work as it should do. Read more about genetics.
The genetics of ataxia
To have a better understanding of the genetics of hereditary ataxia, it is useful to learn about chromosomes.
Chromosomes are blocks of deoxyribonucleic acid (DNA). They contain a detailed set of instructions that control a wide range of factors – from how the body’s cells develop, to what colour eyes a baby will have and what sex a baby will be.
You receive two sets of chromosomes, one from your mother and one from your father. As all the genes in your body are created from these chromosomes, you will receive two sets of every gene, one gene from your mother and one gene from your father. In each pair of genes you receive, one gene is recessive and one is dominant.
There are two ways that a genetic mutation can be passed down through families:
- autosomal recessive: such as the mutations responsible for Friedreich’s ataxia and ataxia-telangiectasia
- autosomal dominant: such as the mutation responsible for spinocerebellar ataxia
These are described in more detail below.
If the mutated gene is autosomal recessive, it means that you will only develop ataxia if you receive a pair of mutated genes (one from your mother and one from your father).
If you only receive one of the mutated genes, the other normal gene will cancel out the effects of the mutation. However, you will be a carrier of one of the mutated genes.
It is estimated that around 1 person in every 75 is a carrier of the mutated gene that causes Friedreich’s ataxia and around 1 person in 100 carries the mutated gene that causes ataxia-telangiectasia.
The chances of two carriers meeting, having a relationship and then having a baby is low, which explains why these types of ataxia are so rare.
If two carriers have a baby:
- There is a 1 in 4 chance that the baby will receive a pair of normal genes.
- There is a 1 in 2 chance that the baby will receive one normal gene and one mutated gene. In this case, they will not develop ataxia but they will be a carrier.
- There is a 1 in 4 chance that the baby will receive a pair of mutated genes and will develop ataxia.
If you have autosomal recessive ataxia and your partner is a carrier:
- There is a 1 in 2 chance that your baby will receive one normal gene and one mutated gene and become a carrier.
- There is a 1 in 2 chance that your baby will receive a pair of mutated genes and develop ataxia.
If you have autosomal recessive ataxia and your partner does not and is not a carrier, there is no risk that any children that you have will develop ataxia because your mutated gene will be cancelled out by your partner’s normal gene.
If the mutated gene is autosomal dominant, as is the case with the various types of spinocerebellar ataxias, you can develop ataxia if you receive a single mutated gene, either from your mother or your father. This is because the mutation is strong enough to override the other, normal gene.
If you have autosomal dominant ataxia, any children that you have will have a 1 in 2 chance of developing ataxia.