Huntington's disease (HD) is a neurodegenerative genetic disorder that affects muscle coordination and leads to cognitive decline and dementia. It typically becomes noticeable in middle age. HD is the most common genetic cause of abnormal involuntary writhing movements called chorea, and indeed the condition used to be called Huntington's chorea. It is caused by an autosomal (there is an equal number of copies of the chromosome in males and females) dominant mutation on either of an individual's two copies of a gene called Huntingtin, which means any child of an affected parent has a 50% risk of inheriting the disease. Physical symptoms of Huntington's disease can begin at any age from infancy to old age, but usually begin between 35 and 44 years of age. About 6% of cases start before the age of 21 years; they progress faster and vary slightly.
Genetic testing can be performed at any stage of development, even before the onset of symptoms. This fact raises several ethical debates: at what age is an individual considered mature enough to choose testing, do parents have the right to have their children tested, as well as confidentiality issues and disclosure of test results. Genetic counselling has developed to inform and aid individuals considering genetic testing and has become a model for other genetically dominant diseases.
Symptoms of the disease can vary between individuals and among affected members of the same family, but the symptoms progress predictably for most individuals. The earliest symptoms are a general lack of coordination and unsteady movements. As the disease advances uncoordinated, jerky body movements become more apparent, along with a decline in mental abilities and behavioural and psychiatric problems. Physical abilities are gradually impeded until coordinated movement becomes very difficult. Mental abilities generally decline into dementia. Complications such as pneumonia, heart disease, and physical injury from falls reduce life expectancy to around twenty years after symptoms begin. Full-time care is required in the later stages of the disease.