Causes of Ataxia
The hereditary ataxias are genetic, which means they are caused by a defect in a certain gene that is present from the start of a person's life. All of us have genes that have little mistakes or variations but most of these do not cause disease, they are called mutations. The hereditary ataxias can be divided into those that are dominantly inherited and those that are recessively inherited.
Affects Males and Females Equally
Autosomal dominant inheritance refers to a genetic pattern where the gene that causes the ataxia is located on one of the autosomes and will affect males and females equally. An autosome is any of the chromosomes that are not sex chromosomes. All genes come in pairs and dominantly inherited means that the gene with the disease mutation dominates over the normal copy of the gene. Each child of a parent with an autosomal dominant ataxia gene has a 50/50 chance of whether they will inherit the ataxia gene or not.
Autosomal recessive inherited diseases also affect males and females equally but it takes a "double dose" of the ataxia gene to result in disease symptoms. Both parents must be carriers of the disease gene and each must pass on the ataxia gene to their child for the double dose that is needed to produce symptoms of the recessive disease. Each child of parents who are carriers of a recessive disease has a 25% chance of inheriting two ataxia genes so will develop the disease, a 50% chance of inheriting just one of the ataxia genes and, therefore, be a carrier and a 25% chance of inheriting no ataxia gene and be completely free of ataxia. Because a single recessive ataxia gene does not cause symptoms, it can be passed on in a family for generations without being recognized. Therefore, there is often no "family history" of ataxia if the disease is inherited as a recessive gene.
Ataxia Gene Identified in 1993
The first ataxia gene was identified in 1993 for a dominantly inherited type. It was called "Spinocerebellar Ataxia type1 (SCA1)". Subsequently, as additional dominant genes were found they were called SCA2, SCA3, etc. Generally the number behind the SCA refers to the order in which the gene was found. At this time, 36 different gene mutations have been found. In other words, we have dominant ataxia classifications from SCA1 to SCA 36. Genes have also been located for some of the recessive ataxias, the most common being Friedreich's ataxia (FRDA) but also Ataxia with Oculomotor Apraxia Type 1 and Type 2 (AOA1 and AOA2).
Affects the Nervous System
The various abnormal genes that cause ataxia have in common, is that they make abnormal proteins that affect nerve cells, primarily in the cerebellum (and other parts of the brain) and the spinal cord. The details of which parts of the nervous system are most affected vary with the different types of ataxia. All forms of ataxia cause poor coordination but some forms also cause additional symptoms that make it easier to distinguish one from the other.
Ataxia Results in the Degeneration of Nerve Cells
Eventually the affected nerve cells begin to function poorly and ultimately degenerate. As the disease progresses, muscles become less and less responsive to commands from the brain, causing coordination problems to become more pronounced. Those affected by poor coordination will notice poor balance when walking, inability to run, clumsiness of the hands, a change in speech, or abnormal eye movements.