Parkinson's disease has been classified into five stages by Hoehn and Yahr. Their classification is based on the severity of symptoms and the degree of disability the patient experiences.

When prescribing treatment, the neurologist will take into account the stage at which the patient is perceived to be, among other things.

Stage 1 is called unilateral disease. The term means that only one side of the body shows symptoms of Parkinson's disease.

This is considered an early stage of the illness and may last for several years. Neuroprotective treatment is common at this stage. This treatment aims to prevent further damage to the nervous system.

Vitamin E was used at this stage, but many researchers are of the opinion that this is not very effective. A selective MAO-B inhibitor called Rasigiline has shown promise.

Stage 2 of Parkinson's disease is labeled bilateral disease. Meaning, symptoms of Parkinson's now show up on both sides of the body. The illness is considered to be at Stage 2 even if there is an insignificant symptom (an occasional tremor, for example) on the side of the body that was symptom-free earlier.

Recent therapies at this stage include use of dopamine agonists. Till some years ago, dopamine agonists were used only at later stages of the disease. Currently, researchers are of the view that if such treatment is started earlier, complications from using drugs like Levodopa may not arise at later stages.

The disease is said to be at stage 3 when symptoms show progression and especially when the patient has difficulty in maintaining their posture. Symptoms at this stage include postural instability and falling down.

Traditionally, this is the stage at which dopamine agonists or Levodopa was prescribed, but as mentioned above, some of these drugs are now used at earlier stages itself.

Stage 4 occurs when there is an increase in the severity of symptoms including postural instability and falling. Surgery is one option for treatment at this stage. Surgery can alleviate some of the symptoms. However, this is generally recommended only for relatively young patients who are in good health otherwise.

Stage 5 is the most advanced stage of Parkinson's disease. The patient is usually wheelchair bound. Walking is possible only with assistance.

Levodopa continues to be a prescribed drug at this stage, along with a COMT inhibitor, which ensures that the effects last longer. Surgery may also be an option, including pallidotomy, which destroys a small group of brain cells. This helps avoid the rigidity often experienced with Parkinson's disease and may stop tremors as well.

Another possible treatment is deep brain stimulation, where electrodes are implanted into the brain tissue and stimulated with low level electric currents.

The symptoms of Parkinson's disease progress as the patient moves from one stage to the next. Ongoing research is producing new treatments including stem cell implants and new drugs.