A diagnosis of Huntington’s disease is suspected based on the appearance of specific symptoms. In most cases, these symptoms appear around middle age, when a person is between 35 and 45 years of age.
An outline of the steps taken to diagnose this condition is given below:
- A detailed physical examination is performed to assess whether disease onset has started. The physician will test for signs of involuntary body movements. Unintentional, random and abrupt movements will often lead to a diagnosis of Huntington’s being suspected.
- A detailed psychological examination is also performed to check for signs of decline in motor, behavioral and cognitive function.
- Medical imaging techniques, such as computerized tomography (CT) and magnetic resonance imaging (MRI) can reveal atrophy of the caudate nuclei, which is observed in the early stages of Huntington’s disease. However, this feature in itself is not diagnostic of the condition.
- Functional neuroimaging techniques such as fMRI and PET (Positron emission tomography) can reveal changes in brain activity before physical symptoms start to develop. However, these tools have only been used experimentally rather than clinically so far.
- Genetic testing can be performed to check whether someone is at risk of Huntington’s disease. A blood sample is taken and checked for the mutation in the patient’s two copies of the Huntingtin (HTT) gene which codes for the huntingtin protein. The mutation that is checked for is an expansion mutation of the cytosine- adenine-guanine (CAG) triplet found in the HTT gene on chromosome 4. The mutated gene codes for a huntingtin protein that is abnormal and gradually damages brain cells. People born to parents with Huntington’s disease can undergo genetic testing for the condition after the age of 18. If the faulty gene is detected, they will develop the disease but the age at which this will happen cannot be determined.
- There are several other diseases that may have a similar symptom profile to Huntington’s disease such as neuroferritinopathy, chorea acanthocytosis and X-linked McLeod syndrome.
- Prenatal testing can be performed to check whether a fetus or embryo is carrying the mutation that leads to Huntington’s disease. A sample of the amniotic fluid that surrounds the baby may be tested in a process called amniocentesis or a sample can be taken from the placenta using a technique referred to as chorionic villus sampling. Genetic testing is performed when a parent has been diagnosed with the disease, when genetic testing has shown an expansion mutation in the HTT gene or when someone is at a 50% risk of having inherited the disease.