A specialist can help identify the characteristic symptoms of Huntington’s disease as part of a diagnosis. This includes exams to measure any changes in motor function, neuropsychological assessments that look for cognitive changes, and psychiatric evaluations to assess any behavioral changes.
A brain imaging scan, such as magnetic resonance imaging (MRI), may also be carried out. This may be used to identify changes that Huntington’s disease causes in the brain, but also to rule out other conditions that may affect the brain.
For a definitive diagnosis, a genetic test is required. This will normally involve a blood sample being taken and sent off to a specialized center for examination. The test can determine the size of the HTT gene, or how many CAG repeats are present.
Huntington’s disease will almost always be present in people with 40 or more CAG repeats. The CAG segment can be repeated more than 120 times and, generally, the more expanded the gene is, the earlier disease symptoms will start.
It is possible to be tested for Huntington’s disease before any symptoms appear if someone already has the disease in the same family. However, a diagnosis can be difficult if there are between 36 to 40 CAG repeats, as the disease may or may not develop in the person. For this reason, a decision to undergo such testing is recommended only after careful discussion with a genetic counselor, in order to better understand the impact results can have on the person and his or her family.