HUNTINGTON'S DISEASE OVERVIEW
WHAT IS HUNTINGTON'S DISEASE? Huntington's disease (HD) is an inherited, degenerative brain disorder named after physician George Huntington, who first described the illness in 1872. It is also sometimes referred to as Huntington's chorea, from a Greek word meaning "dance", referring to the characteristic movement disorder that is a part of the illness.
WHAT CAUSES HD? HD is a genetic (inherited) disease, meaning that it can be passed down from one generation to the next. In 1993, scientists identified the gene that causes HD. Genes are comprised of a long string of chemical building blocks, and in individuals with HD, there are too many of these building blocks (designated as CAG) in a particular place on chromosome 4. The protein encoded by this gene does not work properly and eventually leads to the symptoms of HD. Although every cell of the body has two copies of every gene, only one abnormal or expanded gene is needed to cause HD.
Every child of a parent who carries the HD gene has a 50:50 chance of inheriting the abnormal gene. A child who inherits the HD gene will eventually develop the illness, although usually not until adult life. If a child does not inherit the HD gene, he/she will not get HD, nor will their children: HD does not "skip" generations.
WHO GETS HD? About 30,000 people in North America have HD, and another 150,000 are considered "at risk" for inheriting the illness because they have (or had) a parent with HD. Most of those affected by the illness develop it in middle adult life, but about 10% will have onset prior to age 20 (juvenile HD) and another 10% after age 55. Men and women are equally likely to inherit the gene and develop the illness. HD can affect people of all ethnic groups, but is more common among those of European descent.
WHAT ARE THE SIGNS AND SYMPTOMS OF HD, AND WHEN DO THEY BEGIN? A person inheriting the HD gene usually develops signs of the illness very gradually, so that they may go unnoticed for years. The age at which an individual develops signs and symptoms is related to the size of the expansion of the HD gene, with larger expansions leading to an earlier age of onset. However, this relationship is very variable: it is not currently possible to predict accurately the age of onset from genetic test results or any other information. The Huntington Study Group is currently investigating these issues in important clinical trials involving at-risk individuals; see Clinical Trials in Progress.
HD causes a movement disorder, psychiatric difficulties, and cognitive changes, usually beginning in middle adult life. Signs and symptoms vary from person to person. For instance, one person affected with HD may have a very obvious movement disorder, but only mild psychiatric symptoms and intellectual deterioration, while another might suffer with depression and anxiety for years before showing any abnormal movements.
- The movement disorder
A person with HD typically develops involuntary movements, which initially may just seem like fidgetiness or a nervous restlessness, but which usually evolve into obviously abnormal movements that are difficult to control. Children and teenagers with HD, as well as those in the later stages of the illness, may have slow, stiff movements rather than the quick, random, dance-like movements called chorea.
- The cognitive disorder
HD causes difficulties in mental flexibility, so that it may become hard for the person with HD to switch quickly from one mental task to another. It may become difficult to learn new information or to remember things. However, a person with HD can usually recognize previously-learned information when it is presented to them. These cognitive changes can cause difficulties in the workplace and home before other symptoms of HD become evident.
- The psychiatric disorder
Many of the psychiatric aspects of HD can be treated successfully. Depression is very common in HD, very treatable, and may precede the onset of other signs and symptoms of the illness. Severe depression may even lead to suicide attempts in a few individuals. Since depression is also common in the general population, it can be difficult to tell whether depression really represents the onset of HD in an at-risk person.
Other psychiatric difficulties seen in some people with HD include anxiety, obsessiveness, irritability, impulsiveness, social withdrawal, and trouble initiating activity. A few individuals with HD may have aggressive outbursts or even psychosis (hallucinations or delusions).
HOW DOES HD PROGRESS OVER TIME? Although HD is a progressive illness, the rate of progression varies from person to person, and many people with HD are able to live independently or with only minor assistance for many years. However, HD eventually causes physical and mental disability requiring significant assistance at home or in a care facility.
WHAT TREATMENTS ARE AVAILABLE FOR HD? Treatments for HD can be divided into two categories: those that are aimed at treating the symptoms of the illness (like depression or chorea) and those that slow down the underlying progression of the illness. Although there are currently no treatments proven to slow down progression or to delay its onset, there are clinical trials in progress which aim to identify such treatments.
Medications should be used with caution in a person with HD, who may be especially sensitive to side effects. A person with HD may respond differently to medications in different stages of the illness, so the medication list should be reviewed frequently. In general, an approach which starts medications at a low dose and increases slowly as needed works best. Ideally, medications should be given under the supervision of a physician experienced in the care of patients with HD. A person with HD may have difficulty taking medications on a schedule, so it is a good idea to keep the schedule as simple as possible and to use pill boxes or direct supervision by a care provider as necessary.
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