Huntington Disease (cont.)
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How is Huntington's disease diagnosed?
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The great American folk singer and composer Woody Guthrie died on October 3, 1967, after suffering from HD for 13 years. He had been misdiagnosed, considered an alcoholic, and shuttled in and out of mental institutions and hospitals for years before being properly diagnosed. His case, sadly, is not extraordinary, although the diagnosis can be made easily by experienced neurologists.
A neurologist will interview the individual intensively to obtain the medical history and rule out other conditions. A tool used by physicians to diagnose HD is to take the family history, sometimes called a pedigree or genealogy. It is extremely important for family members to be candid and truthful with a doctor who is taking a family history.
The doctor will also ask about recent intellectual or emotional problems, which may be indications of HD, and will test the person's hearing, eye movements, strength, coordination, involuntary movements (chorea), sensation, reflexes, balance, movement, and mental status, and will probably order a number of laboratory tests as well.
People with HD commonly have impairments in the way the eye follows or fixes on a moving target. Abnormalities of eye movements vary from person to person and differ, depending on the stage and duration of the illness.
The discovery of the HD gene in 1993 resulted in a direct genetic test to make or confirm a diagnosis of HD in an individual who is exhibiting HD-like symptoms. Using a blood sample, the genetic test analyzes DNA for the HD mutation by counting the number of repeats in the HD gene region. Individuals who do not have HD usually have 28 or fewer CAG repeats. Individuals with HD usually have 40 or more repeats. A small percentage of individuals, however, have a number of repeats that fall within a borderline region (see table 1).
The physician may ask the individual to undergo a brain imaging test. Computed tomography (CT) and magnetic resonance imaging (MRI) provide excellent images of brain structures with little if any discomfort. Those with HD may show shrinkage of some parts of the brain—particularly two areas known as the caudate nuclei and putamen—and enlargement of fluid-filled cavities within the brain called ventricles. These changes do not definitely indicate HD, however, because they can also occur in other disorders. In addition, a person can have early symptoms of HD and still have a normal CT scan. When used in conjunction with a family history and record of clinical symptoms, however, CT can be an important diagnostic tool.
Another technology for brain imaging includes positron emission tomography (PET,) which is important in HD research efforts but is not often needed for diagnosis.
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