Huntington Disease 101
A Brief History of Huntington’s Disease
Huntington’s disease (HD) is named after George Huntington, who described it among residents of East Hampton, Long Island in 1872. It is a hereditary neurodegenerative disease. In 1993, a collaborative group of investigators discovered the gene that causes HD. As a result of this discovery it is now possible to diagnose HD with blood or tissue samples.
HD is caused by mutation in a gene located on chromosome 4. This gene is found in every human being, and contains a CAG repeat sequence. We have not yet discovered the gene’s normal function. In a case of HD, the gene contains an abnormally large number of CAG repeats. The larger the number of triplet repeats, generally speaking, the earlier in life one will develop HD. Furthermore, when the gene is passed from father to child (but not when passed from mother to child) the gene may lengthen even more, resulting in an earlier age of onset for the disease. This phenomenon is known as anticipation.
Genes for diseases can be either dominant or recessive. The gene for HD is dominant. Each child of an affected parent has a 50/50 chance of getting the mutant gene, and therefore has a 50% chance of inheriting the disease. On the other hand, if people with a parent suffering from HD do not inherit the mutant gene, they cannot pass it on to anyone else.
The Scope of 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 will develop symptoms 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.
Important Information About HD Testing
It is important to understand that while people are born with the mutated gene for HD, in most cases they will not develop the symptoms until later in life. Therefore someone can be without symptoms or for a number of years. In the past, there was no way to test for the abnormal gene, but now a blood test can determine whether or not an individual carries the gene for HD. This test can be used in cases of suspected HD, to confirm the diagnosis, or it can be used as a predictive test in individuals who are at risk for HD. People who are at risk may want to undergo predictive testing in order to put their minds at ease, to plan for their medical needs, or prior to having children. The decision to have such a test is a serious one and should not be taken lightly. Most centers that do predictive testing require a period of counseling before and after the test.
What Are the Characteristics of HD?
Onset is usually in mid-life, but can occur any time from childhood to old age. The initial signs of this disorder may be subtle. HD is characterized by a movement disorder, and psychiatric disturbances. Additional characteristics of HD include, weight loss (probably from a combination of difficulty eating, and calories burned by the involuntary movements), difficulty swallowing, and hard-to-understand speech.
Signs & Symptoms of HD
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.
More About the Movement Disorder in HD
A person with HD typically develops involuntary movements, which initially may just seem like fidgetiness or a nervous restlessness, but which usually evolve into 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.
More About the Cognitive Disorder in HD
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.
More About the Psychiatric Disorder in HD
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).
HD Gene Size & Symptoms
Research has shown that the age at which an individual develops signs and symptoms is related to the size of the expansion of the HD gene in individuals having large expansions. 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 and Observational Studies in Progress.
The Course of HD
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. The observational studies being conducted are important in helping us to understand the different manifestations of HD.
Once an individual develops signs of HD the course of the disease can last anywhere from ten to thirty years. Typically, the course of HD can be roughly divided into three stages.
In this stage patients can still perform most of their usual activities. They may still be working and may still be able to drive. Involuntary movements are mild and infrequent, speech is still clear, and dementia, if present at all, is mild.
At this stage patients are more disabled and may need assistance with some of their activities of daily living. Falls, weight loss, and swallowing difficulties may become a problem. Dementia is more obvious to the casual observer. Involuntary movements are more pronounced.
Patients entering this stage of the disease require almost total care and may reside in hospitals or nursing homes, although some remain at home. They may no longer be able to walk or speak. They may be more rigid now and show fewer involuntary movements. Individuals in this stage may or may not be able to swallow food. At this stage most patients do not seem to suffer much as they are apparently unaware of their surroundings. When the patient passes away, the cause of death is usually related to malnutrition, pneumonia, or heart failure.
What Treatments Are Available?
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 and Observational Studies in Progress which aim to identify such treatments.
Medications currently indicated for other conditions may be prescribed by a physician to treat symptoms of HD. These medications should be used with caution in a person with HD since the medication was tested and approved for a condition other than HD.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.
Proper nutrition, exercise and precautions in the home can help minimize many of the potential consequences of HD such as weight loss, falls, and choking on food. Finally contact with other HD sufferers, family members, and care-providers can be a vital source of support for HD patients and their families.
People with HD should discuss their concerns and wishes about treatments/interventions (e.g. feeding tubes, resuscitation requests) and autopsies with their families and doctors while they are still able to speak for themselves.