SCA6

Symptoms:
Spinocerebellar ataxia (SCA6) is a rare, slowly progressive, disease that can result in dizziness, difficulty with balance, vision disturbances, and difficulty with speaking clearly. These symptoms are often not experienced until later in life, but can occur earlier with a usual range of presentation between 43 and 52 years of age. One difference between SCA6 and other ataxia types is that the progression of symptoms tends to be slow. Life expectancy and mentation (ability to think and remember clearly) does not seem to be significantly altered by SCA6.

Diagnosis:
SCA6 can be diagnosed by collecting and testing saliva. An individual who wants to be tested can request a saliva kit through a qualified physician and often with an offer of genetic counseling. Either in the office or at home, the individual spits into a tube to collect the saliva. The saliva specimen is then mailed to the testing lab. The result is provided to the physician who then shares it with the requesting individual. Genetic counseling may be offered for those who want it. There are laws against genetic discrimination by health insurance companies and employers; however, life insurance companies are less restricted. As such, an individual may choose to pay out-of-pocket for genetic testing to assure the confidentiality of the results. The damage to the cerebellum can be detected by MRI imaging of the brain with a decrease in size of the cerebellum. With a family history of SCA6 and thinning of the cerebellum identified by MRI, the individual likely does have SCA6, but still may want confirmation by genetic testing. Of note, MRI results may not correlate with symptoms. In other words, some people with severe cerebellar degradation may have mild symptoms; while others with very little degradation may have more severe symptoms.

Cause:
SCA6 is caused by an increase in CAG repeats in the CACNA1A gene (SCA6 Molecular Basis). Affected individuals have 20 to 33 repeats; unaffected individuals have 18 or fewer CAG repeats; the clinical presentation with 19 repeats is variable. The increased number of repeats in the CACNA1A gene results in creation of an abnormal protein that can lead to loss of Purkinje cells in the cerebellum. Purkinje cells in the cerebellum are important for balance and speech.

Inheritance:
The risk of SCA6 in children of affected individuals is 50%. SCA6 is inherited in an autosomal dominant manner meaning that abnormal CAG expansion on just one chromosome causes the disease. At times, the symptoms of the disease are so late onset and/or so mild that SCA6 may not be diagnosed. Children of such individuals still likely have a 50% risk for SCA6.

Additional Information:
It is possible to perform preimplantation diagnosis and in vitro fertilization if a couple with an affected or even at risk individual were planning a pregnancy and desired this. An embryo could be selected that lacks the CAG expansion for implantation. Due to the general risks, costs, inconvenience, and potential complications with such a procedure, and because SCA6 is usually such a mild late onset disease, this may not be desired. Genetic testing of children is strongly discouraged because of the late onset and mild nature of SCA6 with no way to prevent the onset of SCA6 symptoms currently available.