Although there is no cure for Parkinson's disease, medicines can often provide dramatic relief from the symptoms. However , there are limits to their effectiveness, and scientists are working to find better ways to treat the disease .
Three classes of medications
Medications for Parkinson's fall into three groups.
- drugs that increase the level of dopamine in the brain
- drugs that affect other neurotransmitters in the body in order to ease some of the symptoms of the disease
- medications that help control non-motor symptoms (those that do not affect movement) of Parkinson's
Levodopa Is the Main Therapy
The main therapy for Parkinson's is levodopa, also called L-dopa. It is a simple chemical found naturally in plants and animals. Nerve cells use levodopa to make dopamine to replenish the brain's dwindling supply. People cannot simply take dopamine pills because dopamine does not easily pass through the blood-brain barrier. The blood-brain barrier is a lining of cells inside blood vessels that controls the transport of drugs (as well as oxygen, glucose, and other substances) into the brain.
Usually, people take levodopa along with another medication called carbidopa. Carbidopa delays the body's conversion of levodopa into dopamine until the levodopa reaches the brain. This prevents or reduces some of the side effects that often accompany levodopa therapy. Carbidopa also reduces the amount of levodopa needed to improve symptoms.
Levodopa's Effect on Symptoms
Levodopa successfully reduces the tremors and other symptoms of Parkinson's during the early stages of the disease. It allows most people with Parkinson's to extend the period of time in which they can lead relatively normal, productive lives.
However, not all symptoms respond equally to levodopa. It usually helps most with bradykinesia (slowness of movement) and rigidity. Problems with balance and other non-motor symptoms may not be helped at all.
People with Parkinson's disease often see dramatic improvement in their symptoms after starting levodopa therapy. However, they may need to increase the dose gradually over time to maintain maximum benefit.
Levodopa is often so effective that some people may temporarily forget they have Parkinson's during the early stages of the disease. But levodopa is not a cure. Although it can reduce the symptoms, it does not replace lost nerve cells or stop the gradual loss of brain cells that causes the disease.
Side Effects of Levodopa
Levodopa can have a variety of side effects including nausea, vomiting, low blood pressure, and restlessness. It can also cause drowsiness or sudden sleep onset. Levodopa in excess sometimes causes hallucinations and psychosis.
Dyskinesias, or involuntary movements such as twitching, twisting, and writhing, commonly develop in people who take large doses of levodopa for a long time. The dose of levodopa is often reduced in order to lessen the movements brought on by the drug. However, symptoms of Parkinson's disease often reappear even with lower doses of medication. People with Parkinson's must work closely with their doctor to find a tolerable balance between the drug's benefits and side effects.
Other troubling and distressing problems may occur with levodopa use in people with advanced Parkinson's disease. People may begin to notice more pronounced symptoms before their first dose of medication in the morning, and they may develop muscle spasms or other problems when each dose begins to wear off. The period of effectiveness after each dose may begin to shorten, called the "wearing-off" effect.
One approach to reducing these side effects is to take levodopa more often and in smaller amounts. People with Parkinson's disease should never stop taking levodopa without telling their doctor because suddenly stopping the drug may have serious side effects, such as being unable to move or having difficulty breathing.
Other Drug Treatments
Fortunately, doctors have other treatment choices for some symptoms and stages of Parkinson's disease. These include
- dopamine agonists
- MAO-B inhibitors
- COMT inhibitors
- Amantadine, an old antiviral drug which can reduce dyskinesia
- anticholinergic drugs.
Dopamine agonists are drugs that mimic the role of dopamine in the brain as opposed to levodopa which requires conversion in the brain to dopamine. They may be used in the early stages of the disease, or later on to give a more prolonged, steady dopaminergic effect in people who experience wearing off or on-off effects. Dopamine agonists are generally less effective than levodopa in controlling rigidity and bradykinesia. They can cause confusion in older adults.
MAO-B inhibitors are drugs that slow down the enzyme monoamine oxidase B, or MAO-B, which breaks down dopamine in the brain. MAO-B inhibitors cause dopamine to build up in surviving nerve cells, thereby reducing symptoms of Parkinson's.
One MAO-B inhibitor, rasagiline, may be used alone or in combination with other medication to treat Parkinson’s disease symptoms.
COMT, which stands for catechol-O-methyltransferase, is another enzyme that helps to break down dopamine. COMT inhibitors prolong the effects of levodopa by preventing the breakdown of dopamine. They can decrease the duration of "off" periods, and they usually make it possible to reduce the person's dose of levodopa.
- Amantadine is an old antiviral drug that can help reduce symptoms of Parkinson's and dyskinesias caused by levodopa. It is often used alone in the early stages of the disease, and again in later stages to treat dyskinesias.
- Anticholinergics decrease the activity of the neurotransmitter acetylcholine and help to reduce tremors and muscle rigidity.
When recommending a course of treatment, a doctor will tailor therapy to the person's particular condition. Since no two people react the same way to a given drug, it may take time and patience to get the dose just right. Even then, symptoms may not go away completely.