One of the newest group of medications, the COMT inhibitors, in fact don’t treat the symptoms of Parkinson’s directly. They are always used with levodopa, preventing levodopa from breaking down and allowing more of the dopamine replacement medication to reach the brain.. This helps to reduce the wearing “off” periods and motor fluctuations that can occur with levodopa therapy by providing a more stable level of the drug. It’s also helpful for patients who want to have more effective symptom control without increasing their dose of levodopa.
Examples of COMT inhibitors include entacapone (Comtan) and tolcapone (Tasmar). Entacapone is also one of the ingredients along with levodopa, in the combination medication, Stalevo.
Because COMT inhibitors increase the amounts of available levodopa, they also can cause an increase in the number of side effects that occur with dopamine replacement such as dyskinesias (involuntary movements), hallucinations and nausea. Other side effects that they have themselves include abdominal pain, diarrhea and abnormally colored urine (can be bright yellow – orange). Tolcapone requires regular liver function tests as part of the monitoring process.
Amantadine (Symmetrel) is an older drug that was used to treat and prevent influenza. It is also effective at controlling mild symptoms of Parkinson’s early in the course of the disease such as fatigue, tremor and bradykinesia. It is also effective in reducing dyskinesias that occur as a result of dopamine replacement in more advanced stages of this disease.
Amantadine may take a couple of weeks to start working effectively. Side effects may include dizziness, hallucinations, dry mouth, constipation, insomnia, swelling and agitation.
Anticholinergics (e.g. benztropine, biperident and trihexyphenydil) are not commonly used but have been shown to be effective in younger patients for tremor control. Potential significant side effects including hallucinations, memory and cognition problems, dry mouth, constipation, blurred vision and difficulty with urination limit their use.
These medications do not affect dopamine levels but instead impact another neurotransmitter or chemical messenger in the brain called acetylcholine. The effects of acetylcholine and dopamine must be balanced in order for there to be normal movement. In Parkinson’s disease the levels of dopamine are low and therefore the acetylcholine levels are relatively too high. Anticholinergics block acetylcholine bringing the levels of these two neurotransmitters into better balance.
Belonging to a group of medications known as cholinesterase inhibitors, rivastigmine tartrate (Exelon) is approved for the treatment of dementia in Parkinson’s. It helps improve memory, awareness and the ability to perform activities of daily living. Side effects include nausea, vomiting and diarrhea which are worse when the drug is first started. There is a potential, minimal risk of stomach ulcers as well.
These medications round out the tools available to physicians to manage the symptoms of Parkinson’s disease. Not an easy task. Effective management depends on ongoing adjustments and care titration to create of fine balance of medications that hopefully leads to optimal symptom control and quality of life.
"Anticholinergics." Parkinson's UK. Parkinson's UK, n.d. Web.
Sethi, Kapil D., MD. "Timing and Choice of Treatment Options in PD: The Role of Dopamine Agonists, MAO-B Inhibitors, and COMT Inhibitors." Medscape Family Medicine. N.p., 2009. Web.