Huntington’s disease is diagnosed through a combination of genetic testing, neurological examination, and brain imaging. The most definitive step is a blood test that counts the number of CAG repeats in the HTT gene. A result of 40 or more repeats confirms the diagnosis, while 26 or fewer rules it out. But the path to that confirmation often involves several stages, especially when symptoms are just beginning or family history is unclear.
The Genetic Test
The core diagnostic tool is a DNA blood test that measures a specific repeating segment in the gene responsible for Huntington’s disease. Everyone has this gene, and in most people, the CAG segment repeats 10 to 35 times. Problems arise when that number grows too large.
The results fall into four categories:
- 26 or fewer repeats: Normal. No risk of developing or passing on the disease.
- 27 to 35 repeats: The person won’t develop symptoms, but there’s a chance (particularly for fathers) of passing on an expanded version to children that could reach the disease-causing range.
- 36 to 39 repeats: Reduced penetrance. Some people in this range develop symptoms, others never do. Children are at risk of inheriting the expansion.
- 40 or more repeats: Full penetrance. This causes Huntington’s disease, assuming a normal lifespan. Each child has a 50% chance of inheriting the expanded gene.
The higher the repeat count, the earlier symptoms tend to appear. Juvenile-onset Huntington’s, which strikes before age 20, is typically associated with repeat counts above 60.
Testing Before Symptoms Appear
If you have a parent with Huntington’s disease, you can choose to be tested before any symptoms develop. This is called predictive or presymptomatic testing, and it follows a structured protocol designed to protect your emotional wellbeing.
At specialized centers, the process typically spans multiple visits. You first meet with a genetic counselor to discuss what the results could mean for you, your family planning, your insurance, and your mental health. A second visit includes a neurological exam and continued discussion, with the option to have blood drawn. Results are then disclosed in person at a third visit, followed by a prearranged check-in call or appointment. Some people change their minds partway through the process, choosing to delay or skip the test entirely. That’s built into the design. There is no obligation to proceed at any point.
If you’ve experienced depression, anxiety, or suicidal thoughts in the past, discussing your decision to test with a mental health professional beforehand is especially important. The right not to know is a recognized and respected choice.
Neurological Examination
When a person shows up with possible symptoms, the neurologist conducts a detailed physical and cognitive assessment before or alongside genetic testing. The exam focuses on involuntary movements (chorea), slow or rigid movement, coordination problems, and eye movement abnormalities.
Early motor signs can be subtle: slight involuntary twitching, difficulty walking heel-to-toe in a straight line, slowed rapid alternating hand movements, or sluggish eye tracking. Clinicians rate their confidence that these signs represent Huntington’s rather than some other neurological condition. Even a relatively low motor score can be enough to confirm clinical onset if the pattern fits, particularly in someone who previously had no motor abnormalities at all.
Family history plays a major role. A neurologist will ask detailed questions about relatives, since Huntington’s follows an autosomal dominant inheritance pattern, meaning each child of an affected parent has a 50% chance. When there is no known family history, the condition is classified as sporadic Huntington’s disease. In those cases, genetic testing and imaging become even more critical for ruling out other conditions that can look similar.
Early Cognitive and Psychiatric Signs
Huntington’s disease isn’t purely a movement disorder. It produces a characteristic triad of motor, cognitive, and psychiatric symptoms, and the non-motor signs often appear first. Personality changes like irritability, apathy, mood swings, depression, or uncharacteristic aggression can precede any visible movement problems by years.
Cognitive decline in the early stages shows up as difficulty with multitasking, poor judgment, trouble making decisions, problems with driving, and struggling to organize daily activities. Memory and learning difficulties emerge, and concentration on intellectual tasks gets progressively harder. These changes are different from Alzheimer’s-type dementia. People with Huntington’s tend to have more trouble retrieving information and planning sequences of actions rather than forming new memories.
Clinicians assess these changes through standardized cognitive tests and psychiatric interviews. While cognitive or psychiatric symptoms alone aren’t enough for a formal diagnosis of “manifest” Huntington’s disease (which traditionally requires unequivocal motor signs), they are important diagnostic clues that often prompt genetic testing.
What Brain Imaging Shows
MRI scans aren’t required for diagnosis when genetic testing and clinical signs are clear, but they provide powerful supporting evidence, especially in ambiguous cases or when family history is unknown.
The hallmark finding on MRI is shrinkage of the caudate and putamen, two deep brain structures involved in movement control and cognitive processing. This shrinkage is remarkably early and dramatic. Reduced volume in these structures can be detected more than 20 years before motor symptoms appear. By the time a person receives a clinical motor diagnosis, the caudate has typically lost 52 to 70% of its volume compared to healthy individuals of the same age, and the putamen has lost 43 to 67%.
In one analysis from the PREDICT-HD study, baseline caudate volume predicted clinical motor diagnosis within two years with 100% accuracy. Subcortical brain volumes alone classified individuals within five years of diagnosis with 88% accuracy. Cortical thinning and white matter loss also occur, though these become more widespread later in the disease course. While brain imaging is used more in research than routine clinical practice, it can be invaluable when genetic results fall in the intermediate or reduced penetrance range, or when confirming progression in someone already known to carry the gene.
Diagnosis Without Family History
About 5 to 10% of Huntington’s cases appear without any known family history. This can happen for several reasons: a parent may have died young before symptoms appeared, the family history may have been hidden or misdiagnosed, or the gene expansion may have grown into the disease-causing range during transmission from a parent who carried an intermediate allele (27 to 35 repeats) without being affected themselves.
For these patients, diagnosis typically takes longer. The neurologist works through a broader list of possible conditions, relying more heavily on brain imaging to look for the characteristic pattern of caudate and putamen atrophy. Genetic testing ultimately confirms the diagnosis, but reaching the point of ordering that test may involve ruling out other movement disorders, autoimmune conditions, or medication side effects first.