In PD there is a loss of melanin containing dopaminergic nerve cells within the substantia nigra. Additionally there is a progressive loss of the inhibitory neurotransmitter dopamine in the nigrostriatal tracts and a relative increase in the excitatory neurotransmitter acetylcholine. The imbalance between dopamine and acetylcholine is primarily responsible for the manifestations of PD, but there are other neurotransmitters like norepinephrine and serotonin that are decreased in the parkinsonian brain. There are four classical features of PD which consist of tremor, rigidity, bradykinesia and postural disturbances. Tremor is usually the first symptom and is initially unilateral.
Difficulty initiating and terminating steps result in a hurried or festinating gait.
The posture becomes stooped and postural reflexes are impaired. Symptoms which begin unilaterally often become bilateral and more severe as the disease progresses. In order to assess the degree of disability and evaluate treatment, various scales have been developed. The most commonly used of these is the Hoehn and Yahr scale. In Stage III ADL ' s are affected and treatment may be begun with amantadine or anticholinergics.