Parkinson's disease is a progressive neurological disorder that results from the loss of neurons in a region of the brain that controls movement. This creates a shortage of the brain-signaling chemical (neurotransmitter) known as dopamine, causing movement problems that are characteristic of Parkinson's disease. The exact cause of Parkinson's disease
is not currently known.
Although early symptoms of Parkinson's disease
may be subtle, people will eventually develop a characteristic tremor (trembling or shaking) of a limb, especially when the body is at rest. As the disease progresses, symptoms may worsen and new ones may appear.
Although carbidopa-levodopa is an effective Parkinson's medication, its usefulness is often limited to about a few years, when it may begin to lose its effectiveness and cause intolerable side effects. It is not clear at this time why this might occur; some people think it is simply a manifestation of the worsening of the disease that normally happens over time. Adding entacapone to carbidopa-levodopa has been shown to decrease the "off" times (when the medication does not work well) and increase the "on" times (when the medication works well).