Huntington's disease (HD), also known as Huntington's chorea, is a rare degenerative inherited genetic disorder of the brain.

HD is named after George Huntington, an American medical doctor who described the disease accurately in 1872. His description was based on observations of HD affected families from the village of East Hampton, Long Island, New York (USA), where Huntington lived and worked as a physician. He was the first person to identify the pattern of inheritance of HD.

HD is caused by a mutation in the gene coding for a certain protein, called the huntingtin (Htt) protein. This mutation produces an altered form of the Htt protein, which results in death of nerve cells (neurones) in some areas of the brain.

The exact mechanism of the disease is still unclear. Two pathways have been proposed: Firstly, the protein can no longer exert its normal function (loss of function). Secondly, the mutant protein may be toxic to the nerve cells (gain of function).

Certain functions of the brain, such as the ability to move, think and talk, gradually deteriorate as crucial nerve cells become damaged and die. The part of the brain most affected by HD is the striatum, which is a structure of the basal ganglia located in the central region of the brain and consisting of two main subdivisions called caudate nucleus and putamen. The striatum is primarily responsible for planning and controlling movements, but is also involved in many other cognitive (thinking) processes. Loss of cortex (grey matter in the outermost layers of the brain) occurs during disease progression which contributes to worsening of the cognitive function. In general, HD causes atrophy of the whole brain.

Most individuals develop the disease during mid-adult life, i.e. between 35 and 55 years of age. Approximately 10% of people develop symptoms prior to the age of 20 (juvenile HD) and another 10% after the age of 55. More rarely, symptoms appear before the age of 10 years (infantile HD).

HD is a fatal illness, developing at a gradual and relentless rate. The average duration of the disease is 15-20 years, but this varies between individuals.

Most people with HD do not die as a direct result of the disease, but rather from medical problems that arise from the body’s weakened condition, particularly choking, infections (such as pneumonia) and heart failure.

If you suspect that you have HD, you should consult an HD specialist (usually a neurologist) for diagnostic testing.

HD is a rare disease which affects up to approximately 1 in 10,000 people in most European countries. In Germany for instance, about 10,000 people have HD and a further 50,000 are considered at risk for inheriting the HD gene because they have (or had) a parent with HD. Men and women are equally likely to inherit the gene and develop the disease.

HD can affect people of all ethnic groups, but is more common among European descendants. The prevalence of HD in countries predominantly of European descent (e.g. USA, Canada and Australia) is similar to Europe. In the USA for example, approximately 30,000 people have HD and a further 150,000 are at risk. HD is less common in Asian and African countries, where the frequency has been estimated as 1 per 1,000,000 people. However, detailed studies have not been conducted in these countries, with the exception of Japan, where it has been well-documented that there are fewer HD cases than in Europe.