The diagnosis of Parkinson's disease is based on the demonstration of a parkinsonian syndrome associated with at least 2 "support" criteria.
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Parkinson’s syndrome and its symptoms

Parkinson’s syndrome is evoked when at least 2 of the following 3 symptoms of Parkinson’s disease are observed: akinesia, muscular hypertonia and tremor.

Akinesia is defined as slowness, delay or difficulty in initiating movement. Patients are no longer able to perform rapid, fine, repetitive movements such as writing.

Patients express the sensation that their limbs respond a long time after they intend to make a movement, as if the command takes longer to reach the muscles. This sensation is actually true, because when there is 50% or more loss of dopaminergic neurons, there is no longer enough dopamine for the neurons to communicate effectively with each other and with the muscles.

Hypertonia is defined as a permanent increase in muscle tone. The patient feels stiff and has a peculiar posture due to permanent contraction of the muscles that allow the limbs to flex. Over time, abnormal flexion of the neck, trunk, arms and legs may be observed. Facial expression is affected, with a much reduced frequency of blinking. The voice may also be altered, becoming quieter and with fewer intonations (monotone voice).

Tremor, the best-known symptom of the disease, is only present in 30 to 40% of cases of Parkinson’s disease. It is a resting tremor that affects the extremities of the arms, legs or jaw. This tremor disappears during sleep and during voluntary movements.