Chapter 16: Antiparkinsonian Drugs – Treatments & Mechanisms

The chapter offers an extensive overview of medications used to manage Parkinson’s disease (PD), a chronic and progressive neurodegenerative condition resulting from a critical imbalance in the basal ganglia caused by a deficiency of inhibitory dopamine relative to excitatory acetylcholine. The cardinal manifestations of PD are grouped under the acronym TRAP: tremor, rigidity, akinesia (or bradykinesia), and postural instability. Since the underlying pathology involves significant dopamine depletion, drug therapy focuses on either supplementing dopamine, enhancing its effects, or reducing the impact of acetylcholine, although current treatments aim only to relieve symptoms rather than halt disease progression. Pharmacological approaches are categorized into several classes: direct-acting dopaminergic drugs, which include dopamine replacement agents like levodopa–carbidopa—the traditional cornerstone of therapy—and nondopamine dopamine receptor agonists such as ropinirole and bromocriptine. Indirect-acting agents include selective monoamine oxidase B (MAO-B) inhibitors (selegiline, rasagiline) that prevent dopamine breakdown, dopamine modulators (amantadine) that promote dopamine release, and catechol ortho-methyltransferase (COMT) inhibitors (entacapone). Entacapone is particularly vital as an adjunct drug because it helps to prolong levodopa's action and minimize the wearing-off phenomenon that occurs as the disease advances. Finally, anticholinergic drugs, such as benztropine, are utilized to target and reduce the muscle rigidity and tremors resulting from unopposed cholinergic activity. Nursing care involves detailed assessment for symptoms like dyskinesia and orthostatic hypotension, and extensive patient education regarding avoiding abrupt cessation of medication, managing GI distress, timing levodopa doses away from high-protein meals, and avoiding tyramine-rich foods when taking certain MAOIs. Effective, long-term PD management relies on continuous dosage adjustment to counter complications like the on–off phenomenon and ensuring adherence to complex, individualized dosing schedules.