Parkinson’s disease is chronic, progressive disease, which means it is a long-term condition in which the symptoms worsen over time. It causes several characteristic symptoms, including tremors, slowness of movement (bradykinesia) and limb and trunk rigidity. It is a motor system disorder. As symptoms progress, patients with Parkinson’s disease may have difficulty performing basic tasks, including talking and walking.

The condition was identified in 1817 by James Parkinson, a British physician who first described the principal symptoms of the disease.

Parkinson’s disease is the second most common neurodegenerative disease after Alzheimer’s disease, according to the National Parkinson Foundation (NPF). Some estimates suggest that roughly 1 million people in the United States have Parkinson’s disease. However, the disease is often not diagnosed or misdiagnosed, so this figure may not be accurate. The NPF estimates that Parkinson’s disease affects one in every 100 people over the age of 60, and the average age of onset is 60. The National Institute of Neurological Disorders and Stroke (NINDS) estimates that “early-onset” (in which symptoms develop before the age of 50) Parkinson’s disease occurs in about 5 to 10 percent of people with Parkinson’s disease. These cases are often inherited, and scientists have linked several cases to identified gene mutations. In rare cases, symptoms may appear in people under 20, a condition known as juvenile parkinsonism.

For unknown reasons, more men than women are affected. The NINDS estimates that 50 percent more men than women have Parkinson’s disease.

In healthy people, cells within a small region of the brain stem known as the substantia nigra produce and release a neurotransmitter called dopamine, which controls movement and balance in the body. Dopamine is vital to proper central nervous system functioning and helps electrochemical signals move from one neuron to another.

However, patients with Parkinson’s disease experience a destruction of the cells in the substantia nigra. By the time 60 to 80 percent of these cells are destroyed, there is a sharp decline in dopamine production triggering the symptoms of Parkinson’s disease.

Recent studies also have found that destruction of the nerve endings that produce the neurotransmitter norepinephrine may contribute to many of the nonmotor-related symptoms of Parkinson’s disease. These include fatigue and blood pressure problems. Norepinephrine is the chief chemical messenger within the sympathetic nervous system (the part of the autonomic nervous system that controls automatic functions in the body).  

Some patients who appear to have Parkinson’s disease may actually have a separate disorder that mimics Parkinson’s disease. These disorders may be grouped with Parkinson’s disease and are collectively known as parkinsonism. Examples of non-Parkinson’s disease parkinsonism include:

• Drug-induced parkinsonism. Caused by taking certain medications, including antipsychotics, calcium-channel blockers, the dopamine blocker metoclopramide and the blood pressure medication reserpine.
  
  
• Vascular parkinsonism. Results from blockage of small blood vessels to the brain.
  
  
• Essential tremor. A progressive condition that tends to run in families and usually affects both hands, particularly when the hands are moving. It may affect the head as well. Patients with this condition usually do not have other parkinsonian features.
  
  
• Lewy body dementia. Condition marked by early dementia, hallucinations and fluctuations in cognitive status.
  
  
• Multiple system atrophy. An illness that causes symptoms similar to Parkinson’s disease, but it is much less common and tends to develop more rapidly. The cause of this neurodegenerative disease is unknown.
  
  
• Other conditions. Parkinsonian symptoms may appear in patients with other neurological disorders, including Wilson’s disease, Huntington’s disease, Alzheimer’s disease, spinocerebellar ataxias and Creutzfeldt-Jakob disease.  

Without treatment, patients with Parkinson’s disease usually experience a steady worsening of symptoms and a corresponding decline in quality of life. However, both the nature and severity of symptoms vary significantly from patient to patient, and overall prognosis usually is difficult to predict. Generally, most people with Parkinson’s disease have a similar life expectancy to healthy people. However, some of the symptoms of Parkinson’s disease (e.g., difficulties chewing and swallowing) may cause choking and aspiration pneumonia, which can be fatal.