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Q: My daughter has been diagnosed as having Huntington's disease. Will you please explain this disease and its progression?

- P.T., Rancho Cucamonga, Calif.

A: Huntington's disease is caused by a genetic error. A specific site on chromosome 4 has been identified that causes an important protein in cells of the brain to become deformed. This deformation then causes some brain cells to malfunction and die. A similar problem occurs in Alzheimer's disease, but different proteins are involved.

The resulting symptoms include abnormal movements and changes in the person's intellect. Often a person becomes more restless and fidgets. Later on, the classic problem of this disease - Huntington's chorea - develops. Chorea is defined as involuntary, purposeless, rapid muscle movements. These include flexing and extending the fingers; raising and lowering the shoulders; and grimacing.

Earlier mental changes also include irritability, moodiness and antisocial behavior. Eventually, almost everyone with this disease will develop dementia.

The symptoms usually start in someone between the ages of 30 and 50. But rarely it starts during childhood. The earlier the symptoms appear, the faster the disease progresses. The average length of life after diagnosis is 10 to 15 years.

There is no cure for Huntington's disease, and the progression and worsening of symptoms cannot be slowed. Drug treatments help some people control some of the symptoms, but often they are only marginally effective.

Anyone with this disease should be treated by a neurologist who works very closely with the person and their family. Potential benefits and side effects of drugs need to be understood before deciding to take them. Equally important is to have a plan of what to do if the benefits don't happen or the side effects do.

It's also very important to talk about the impact of this being an inherited disease. It is an autosomal dominant genetic disease. This means that if someone with the abnormal gene has children with someone who does not, then each child of this couple has a fifty-fifty chance of having a child with the disease. Because the disease is uncommon, it is very rare that two people with the disease will have children together.

There is now a very reliable test (about 99 percent effective) to detect the abnormal chromosome. So children of someone with it are able to determine if they have the abnormality long before symptoms occur. Although having this information may help some people better plan their lives, it can also cause a great deal of emotional anguish decades before symptoms even begin.

A major benefit of knowing early in life is the ability to consider the risk of having a child with the disease before conception. And, for some, testing the chromosomes very early in a pregnancy may also provide information that can be used in determining whether to carry the pregnancy to term.

It is very important to deal with the potential emotional problems that often occur in the parents of someone diagnosed with this disease. Even though the vast majority of these parents did not know they carried the abnormal gene, that is often poor comfort for many.