Parkinson's disease (PD), or simply Parkinson's, is a long-term neurodegenerative disease of mainly the central nervous system that affects both the motor system and non-motor systems. The symptoms usually emerge slowly, and as the disease progresses, non-motor symptoms become more common. Usual symptoms are tremor, slowness of movement, rigidity, and difficulty with balance, collectively known as parkinsonism. Parkinson's disease dementia, falls and neuropsychiatric problems such as sleep abnormalities, psychosis, mood swings, or behavioral changes may arise in advanced stages. Most of the cases of Parkinson's disease are sporadic and a few contributing factors have been identified. Pathophysiology is characterized by nerve cell death in the locus coeruleus and substantia nigra, latter being a midbrain region that supplies dopamine to the basal ganglia which are involved in voluntary motor control. The cause of this cell death is poorly understood but includes the alpha-synuclein aggregation into Lewy bodies within the neurons. Other possible factors involve genetic and environmental mechanisms as well as medications, lifestyle or previous conditions. Diagnosis is mainly based on signs and symptoms, usually motor-related, found via neurological examination, though medical imaging like neuromelanin MRI can support the diagnosis. Usual onset is in people over 60 years of age of whom about one percent are affected. In those younger than 50, it is termed early-onset PD. No cure is known, and treatment aims to mitigate symptoms. Initial treatment typically includes L-DOPA, MAO-B inhibitors, or dopamine agonists. As the disease progresses, these medications become less effective and produce a side effect marked by involuntary muscle movements. Diet and certain forms of rehabilitation have shown some effectiveness at improving symptoms. Deep brain stimulation has been used to reduce severe motor symptoms where drugs are ineffective. There is little evidence for treatment of the non-movement-related symptoms, such as sleep disturbances and mood instability. The average life expectancy is near-normal.

Reference
Wiki: Parkinson's disease