In the early 1960s, researchers identified the fundamental brain defect behind Parkinson’s disease: loss of the brain cells producing the neurotransmitter dopamine, which helps control muscle activity. This led to the first successful treatment for PD – levodopa, the metabolic precursor of dopamine. Levodopa remains the gold standard of treatment, and is now joined by newer variations such as Sinemet (carbidopa/levodopa) and by agents such as Symmetrel (amantadine hydrochloride), an antiviral that appears to affect the use of dopamine in the brain.
However, their association with so many motor and psychiatric effects – ranging from uncontrolled movements and shakiness to difficulty speaking, memory loss and decreased attention span – has brought about an arsenal of other drug treatments, all with different side-effects.
These include antimuscarinics like benztropin (which causes drowsiness, dry mouth, fast or irregular heartbeat, confusion, delusions and hallucinations); COMT inhibitors, such as tolcapone, which inhibit levodopa breakdown (and cause sleep problems, dizziness, vomiting, hallucinations, irregular heartbeat); dopamine agonists, such as bromocriptine mesylate, which activate dopamine-sensitive brain cells (associated with dizziness, nausea, hallucinations, uncontrolled movements of the body); and monoamine-oxidase-B inhibitors like selegiline (producing mood/mental changes, unusual body movements, nausea, sleep problems).
These are powerful and complex drugs that can interact with other medications, whether also for PD or for any concomitant condition (common in this age group), as well as with food. Because levodopa’s beneficial effects decrease after around five years, doctors vary in how soon they prescribe it – reserving it as the court of last resort, or starting it low and gradually increasing the dose.
Taking drugs for PD is, thus, a balancing act of weighing the loss of motor control due to the illness against loss of motor control caused by the drugs. As a result of this, doctors regularly adjust medication in an attempt to keep one step ahead of what medicine believes is an inevitable decline in motor function.