Antiparkinsonism Drugs Nursing Pharmacology Study Guide

Antiparkinsonism Agents: Generic and Brand Names

• Dopaminergic Agents
  • amantadine (Symmetrel)
  • apomorphine (Apokyn)
  • bromocriptine (Parlodel)
  • carbidopa-levodopa (Sinemet)
  • levodopa (Dopar)
  • ropinirole (Requip)
• Anticholinergic Agents
  • benztropine (Cogentin)
  • diphenhydramine (Benadryl)
  • trihexyphenidyl (Artane)
• Others
  • entacapone (Comtan)
  • tolcapone (Tasmar)
  • selegiline (Carbex)

Disease Spotlight: Parkinson's Disease and Parkinsonism

Parkinson's disease is a chronic, progressive neurological disorder whose first sign is rhythmic tremor. The tremors progress to rigidity and weakness that interfere with posture. Other findings: bradykinesia (extremely slowed movement), shuffling gait, drooling, and slow, slurred speech. Cranial nerve involvement produces a mask-like expression. Parkinson's spares the higher cerebral cortex, so intelligence and other higher brain functions stay intact. The cause is unknown, but the signs trace directly to damaged neurons in the basal ganglia. Suspected triggers of that damage include viral infections, head trauma, brain infections, atherosclerosis, and drug and environmental exposures.

Parkinsonism refers to Parkinson's-like extrapyramidal symptoms caused by certain drugs or brain injuries.

Age-Group Considerations (Both Drug Classes)

• Children. Safety and effectiveness are not established. Parkinson's is rare in children, but they can develop parkinsonian symptoms from other drugs; diphenhydramine is the drug of choice in that case.
• Adults. Teach them how to cope with the disease and the drug effects. Herbal and alternative therapies must be reported to the care team because of interactions that can blunt therapy. Vitamin B6 causes serious problems with these drugs. Women of childbearing age should use contraception, and lactating women should feed by another method, because of fetal and infant risk.
• Older adults. More susceptible to drug effects, so doses often need frequent adjustment. These drugs can worsen glaucoma and benign prostatic hypertrophy. An extensive written drug-teaching protocol is essential.

Dopaminergic Agents

Dopaminergics increase the effects of dopamine at receptor sites.

Therapeutic Action

Dopamine itself does not cross the blood-brain barrier, so therapy relies on drugs that mimic dopamine or raise its concentration in the substantia nigra (the area governing muscle tone), restoring the balance between stimulatory and inhibitory neurotransmitters. Levodopa, the precursor of dopamine and the mainstay of treatment, crosses the barrier and converts to dopamine. Combining it with carbidopa inhibits dopa decarboxylase from breaking levodopa down, so more crosses the barrier.

Indications

• Relief of the signs and symptoms of idiopathic Parkinson's disease.
• Levodopa is the drug of choice and acts as replacement therapy.
• Amantadine is an antiviral that increases dopamine release.
• Apomorphine binds directly to postsynaptic dopamine receptors.

Pharmacokinetics

Here are the characteristic interactions of dopaminergics and the body in terms of absorption, distribution, metabolism, and excretion:

Contraindications and Cautions

• Allergy to dopaminergics. Prevents hypersensitivity reactions.
• Angle-closure glaucoma. Worsened by drug effects.
• Lactation. Enters breast milk and harms the baby.
• Pregnancy. Potential risk to the fetus; safety not established.
• Suspicious skin lesions. Levodopa is linked to melanoma.
• CV disease, bronchial asthma, psychiatric disorders. Worsened by dopamine receptor stimulation.
• Hepatorenal disease. Interferes with metabolism and excretion.
• Apomorphine raises the risk of hypotension and prolonged QT interval.

Adverse Effects

• CNS: anxiety, nervousness, headache, malaise, fatigue, confusion, mental changes, blurred vision, muscle twitching, ataxia
• CV: arrhythmias, hypotension, palpitations
• Respiratory: bizarre breathing patterns
• GI: anorexia, nausea, vomiting, dysphagia, constipation or diarrhea
• GU: urinary retention
• Other: flushing, increased sweating, hot flashes

Interactions

• MAOIs: increased therapeutic effects and risk of hypertensive crisis. Stop the MAOI 14 days before starting dopaminergic therapy.
• Vitamin B6, phenytoin: decreased levodopa efficacy.
• Over-the-counter vitamins: decreased dopaminergic effectiveness.

Nursing Considerations

Assessment. Screen for contraindications and cautions (drug allergy, GI depression or obstruction, BPH, glaucoma). Do a full physical (CNS, respiratory status, vital signs, CV status, bowel sounds, urine output) for baseline data. Inspect the skin for suspicious lesions before giving levodopa, since it can cause or worsen melanoma. Check for a history of prolonged QT and get an ECG before apomorphine. Monitor labs (liver and renal function, CBC) for dose adjustment and bone marrow suppression.

Nursing diagnoses. Disturbed thought processes related to CNS effects; constipation related to dopaminergic effects; risk for urinary retention related to dopaminergic effects; risk for injury related to CNS effects and orthostatic hypotension.

Interventions. Reduce the dose, as ordered, after any interruption in therapy to prevent systemic dopaminergic effects. Track disease progression and drug response over time. Give with meals for GI irritation. Monitor bowel function and start a bowel program for severe constipation. Have the patient void before dosing to cut urinary retention risk. Monitor renal and liver function and CBC. Provide comfort and safety measures (adequate lighting, raised side rails), and teach the patient.

Evaluation. Monitor response (improvement in Parkinson's signs and symptoms) and adverse effects (CNS changes, urinary retention, GI depression, sweating and flushing). Confirm the patient can name the drug, its indication, and adverse effects, and check compliance.

Anticholinergic Agents

Anticholinergic agents are synthetic drugs built for greater affinity at cholinergic receptor sites in the CNS. They block acetylcholine at the receptor sites of the substantia nigra and corpus striatum.

Therapeutic Action

They return balance to the basal ganglia and reduce the severity of rigidity, akinesia, and tremor. Peripheral anticholinergics also reduce drooling and other secondary effects of parkinsonism.

Indications

• Adjunctive therapy for Parkinson's disease (idiopathic, atherosclerotic, postencephalitic).
• For patients who no longer respond to levodopa.

Pharmacokinetics

Here are the characteristic interactions of anticholinergic agents and the body in terms of absorption, distribution, metabolism, and excretion:

Contraindications and Cautions

• Allergy. Prevents hypersensitivity reactions.
• Angle-closure glaucoma, GI and GU obstruction, prostatic hypertrophy. Worsened by drug effects.
• Myasthenia gravis. Worsened by blockade at acetylcholine receptor sites.
• Tachycardia, dysrhythmia, hyper- or hypotension. Blocking the parasympathetic system can let sympathetic stimulation dominate.
• Hepatic dysfunction. Interferes with metabolism, raising toxicity risk.
• Lactation. Enters breast milk and harms the baby.
• Pregnancy. Potential risk to the fetus; safety not established.

Adverse Effects

• CNS: disorientation, confusion, memory loss, agitation, nervousness, delirium, dizziness, light-headedness, weakness
• CV: tachycardia, palpitations, hypotension
• EENT: blurred vision, photophobia, pupil dilation, blocked lens accommodation
• GI: dry mouth, nausea, vomiting, paralytic ileus, constipation
• GU: urinary retention and hesitancy
• Other: flushing, reduced sweating

Interactions

• TCAs, phenothiazines: increased risk of potentially fatal paralytic ileus and toxic psychoses.
• Antipsychotics: reduced antipsychotic effect from central antagonism between the two agents.

Nursing Considerations

Assessment. Screen for contraindications and cautions (drug allergy, GI depression or obstruction, glaucoma, myasthenia gravis). Do a full physical (CNS, respiratory status, vital signs, CV status, bowel sounds, urine output) for baseline data. Monitor liver and renal function for dose adjustment.

Nursing diagnoses. Disturbed thought processes related to CNS effects; impaired urinary elimination related to GU effects; constipation related to GI effects; risk for impaired thermoregulation related to anticholinergic effects; risk for injury related to CNS effects.

Interventions. Use caution in hot weather or hot environments, since reduced sweating raises the risk of heat prostration. Give with meals for GI irritation. Monitor bowel function and start a bowel program for severe constipation. Have the patient void before dosing. Monitor renal and liver function. Provide comfort and safety measures (adequate lighting, raised side rails), teach the patient, and track disease progression and drug response over time.

Evaluation. Monitor response (improvement in Parkinson's signs and symptoms) and adverse effects (CNS changes, urinary retention, GI depression, decreased sweating). Confirm the patient can name the drug, its indication, and adverse effects, and check compliance.

Frequently Asked Questions

Why is carbidopa given with levodopa? Levodopa crosses the blood-brain barrier and converts to dopamine, but much of it is broken down in the periphery before it gets there. Carbidopa does not cross the barrier; it blocks that peripheral breakdown so more levodopa reaches the brain, which lets a lower dose work and reduces nausea and other systemic effects (StatPearls).

Why not just give dopamine directly? Dopamine itself cannot cross the blood-brain barrier, so it has no effect on the central deficit. Therapy relies on the precursor levodopa, which does cross, or on dopamine agonists that mimic dopamine at the receptor.

Why avoid vitamin B6 supplements with levodopa? Pyridoxine (vitamin B6) accelerates the peripheral conversion of levodopa, leaving less to reach the brain and blunting the therapeutic effect. Teach patients to report any vitamins or herbal products to the care team.

What is the difference between Parkinson's disease and parkinsonism? Parkinson's disease is the idiopathic, progressive disorder tied to loss of dopaminergic neurons in the substantia nigra. Parkinsonism refers to the same Parkinson-like extrapyramidal symptoms (tremor, rigidity, bradykinesia) produced secondarily by certain drugs or brain injuries (StatPearls).

Why should levodopa never be stopped abruptly? Sudden withdrawal can trigger a sharp return of symptoms and, rarely, a dangerous neuroleptic-malignant-like syndrome. Doses are reduced gradually, and any interruption is restarted at a lower dose as ordered.

Why inspect the skin before starting levodopa? Levodopa is linked to melanoma. Check for suspicious skin lesions at baseline and monitor over time, and report new or changing lesions to the prescriber.