The first subtle signs may be slight personality or mood changes. Forgetfulness, clumsiness and random, brief, "fidgeting" movements of the fingers or toes might also be a hint. Often, medical advice is not sought during these very early stages of the disease, and some years may pass by before the disorder is medically diagnosed. Hence, the onset of HD is described as “insidious”, as the disease emerges very slowly.

HD is characterised by a combination of motor (movement), behavioural (mood) and cognitive (thinking) disturbances. The symptoms of HD may vary in range, severity, age at onset and rate of progression from individual to individual, including among members of the same family. For instance, one person affected by HD may have a very obvious movement disorder, but only mild psychiatric symptoms and intellectual deterioration, while another may suffer from depression and anxiety for years before showing any abnormal movements.

One of the earliest motor symptoms of HD is chorea (involuntary “dance-like” movements). Chorea comes from the Greek word choreia, which means dance. At the beginning of the disease, these extra movements occur infrequently and are not especially large. HD patients also have slowed initiation of voluntary movements, which is often very subtle and is best described as bradykinesia (a slowing of movements).

As the disease progresses, these symptoms become more evident. In the middle stages, the chorea may cause relatively large movements of the muscles of the limbs, face and trunk. The slowing of movements will have worsened, but might be masked by the chorea. Furthermore, dystonia may become apparent. This term describes a condition in which abnormally slow and prolonged muscle contractions cause twisting and repetitive movements. The motor signs of HD are therefore a mixture of chorea, bradykinesia and dystonia, which notably affect posture, balance and gait. In some cases, the person may appear stiff and rigid. Oculomotor (eye movement) abnormalities are also frequent. The patient’s speech gradually becomes more slurred and difficulties with swallowing may also appear which might lead to weight loss. Referrals to a speech therapist and a dietician can be helpful.

In addition to the movement disorders listed above, HD causes personality and behavioural (mood) changes. The most typical psychiatric symptoms seen in HD are depression, apathy, anxiety, irritability, outbursts of anger, impulsiveness, obsessive-compulsive behaviours, sleep disturbances and social withdrawal. Occasionally, delusions (false beliefs) and hallucinations (seeing, hearing or feeling things that do not really exist) are also observed.

HD is also characterised by increasing cognitive (thinking) impairments which involve comprehension, reasoning, judgment and memory. Cognitive symptoms include slower thinking, difficulty concentrating, organising, planning, making decisions and answering questions, as well as problems with short-term memory, impaired ability to absorb and understand new information and solve problems.

There are also a number of other symptoms that often occur during the course of the disease including weight loss, sleep disturbance and urinary incontinence.

According to a classification developed by Dr. Ira Shoulson, the progression of HD can be divided into five stages:

• Early Stage: The person is diagnosed as having HD and can function fully both at home and work.
• Early Intermediate Stage: The person remains employable but at a lower capacity. He/she is still able to manage daily affairs despite some difficulties.
• Late Intermediate Stage: The person can no longer work and manage household responsibilities. He/she needs considerable help or supervision to handle daily financial affairs. Other daily activities may be slightly difficult but usually only require minor help.
• Early Advanced Stage: The person is no longer independent in daily activities but is still able to live at home supported by the family or professional carers.
• Advanced Stage: The person requires complete support in daily activities and professional nursing care is usually needed.

When HD starts early in life (i.e. under the age of 20), chorea is less prominent whereas slowness of movement (bradykinesia) and stiffness become more prevalent. In most cases, the rate of progression of juvenile HD tends to be faster than in the adult form. Early features of juvenile HD include strong behavioural changes, learning problems, decline at school and speech problems. Epileptic seizures occasionally occur in HD, being more common among young patients.

When HD starts late in life, chorea tends to be stronger, whereas slowness and stiffness are less prominent. If HD occurs late in life it is likely to be more difficult to establish a family history because the individual’s parents may have already died, perhaps before they themselves showed signs of the disease.