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.

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).

Table 1

No. of CAG repeats	Outcome
< 28	Normal range; individual will not develop HD
29-34	Individual will not develop HD but the next generation is at risk
35-39	Some, but not all, individuals in this range will develop HD; next generation is also at risk
> 40	Individual will develop HD

The physician will interview the individual intensively to obtain the medical history and rule out other conditions. He or she will perform a neurological examination including tests of the person's hearing, eye movements, strength, sensation, reflexes, balance, movement, and mental status, and will probably order a number of laboratory tests as well. Together, these tests form the neurological examination. In addition, the physician will ask about recent intellectual or emotional problems, which may be indications of HD.

In addition to direct testing, another 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.

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 physician may ask the individual to undergo a brain imaging test. The computed tomography (CT) scanner provides an excellent image 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 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.

Other technologies for brain visualization, such as magnetic resonance imaging (MRI) and positron emission tomography (PET), are an important part of HD research efforts, but their usefulness to physicians trying to diagnose HD has not yet been established