Parkinson's disease is a common progressive neurological disorder that results from the degeneration of nerve cells, or neurons, in a region of the brain that controls the body's movement.

This degeneration is known to create a shortage of the brain-signalling neurotransmitter dopamine, causing impaired movement. Dopamine is a chemical that transmits electrical signals to and from the brain. With old age the dopamine secretions get depleted, causing involuntary body movements and impair coordinated movements of the limbs.

The symptoms and potential therapies find their mention in Ayurveda, a system of medicine practiced in India as early as 5000 BC, and in the first Chinese medical text, Nei Jing, which appeared 2,500 years ago.

The most obvious symptom of Parkinson's disease is tremor of a limb, especially when the body is at rest. Bradykinesia (slow movement), akinesia (inability to move), stooped posture, and rigid limbs are some of the common symptoms associated with Parkinson's disease.

The symptoms first appear between ages 50 and 65 years and tend to worsen over time. Occasionally, the disease also causes depression, personality changes, dementia, sleep disturbances, speech impairments, or sexual difficulties.

According to estimates, the disease affects one million people and is found all over the world. It is more common in men than in women.

Causes
Theoretically, there are many causes of Parkinson's disease. For more than a century, researchers have reported families with apparently inherited Parkinson's disease.

Until recently, environmental factors were linked to the onset of disease. Severe Parkinson's-like symptoms have been reported in people who have consumed an illegal drug contaminated with the chemical MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and in people who suffered a particularly severe form of influenza during an epidemic in the early 1900s.

In 1996, a collaborative study headed by scientists at the National Human Genome Research Institute, or NHGRI, and the National Institute of Neurological Disorder and Stroke, or NINDS, at the National Institute of Health reported strong evidence that a gene on chromosome 4 can lead to Parkinson's disease in some families.

The NHGRI researchers suspected that the abnormal gene may account for only a small portion of the total number of Parkinson's disease cases. However, this gene and other similar genes that are known to exist elsewhere in the human genome are now expected to help scientists decipher additional causes of Parkinson's and perhaps shed light on other devastating and common brain disorders such as Alzheimer's disease.

Diagnosis
Parkinson's disease is usually diagnosed by a neurologist who can evaluate symptoms and their severity. There is no test to clearly identify the disease. Tests such as brain scans can help doctors decide if a patient has true Parkinson's disease or some other disorder that resembles it. Microscopic brain structures called Lewy bodies, which can be seen only during an autopsy, are regarded as a hallmark of classical Parkinson's disease.

Treatments
There is no cure for Parkinson's disease. Many patients are only mildly affected and need no treatment for several years after the initial diagnosis. In case of severe symptoms, levodopa is prescribed to replace the brain's dopamine. Various kinds of brain surgery have reportedly been effective in reducing symptoms in severely affected people.