The antimuscarinics benzatropine, orphenadrine, procyclidine and trihexyphenidyl are used to reduce symptoms of tremor and rigidity in parkinsonism induced by antipsychotic drug treatments. The antimuscarinics are effective because they reduce cholinergic excess caused by lack of dopamine. However, they are not generally used in idiopathic Parkinson’s disease because they are less effective than dopaminergic drugs and are associated with cognitive impairment.

Dosage regimen

* Antimuscarinic drugs are taken in divided doses throughout the day.

Patient knowledge of medicine use

* Check the patient knows how to take their medication, as they are likely to be taking several treatments.

* Explain how the treatment helps, and that it is necessary to take it in divided doses.

Is the medicine working?

* Has the patient been taking their medication correctly?

* Is the medication controlling the symptoms? If not, refer them to their GP.

Side effects

* Antimuscarinic drugs commonly cause constipation, dry mouth, nausea and vomiting.

* There may also be tachycardia, dizziness, confusion, euphoria, hallucinations, impaired memory, anxiety, urinary retention and blurred vision.

Monitoring

* Antimuscarinics should not be withdrawn abruptly in patients receiving long-term treatment. However, they should be monitored for potential drug abuse.

* Antimuscarinics should be avoided in gastrointestinal obstruction and myasthenia gravis, and should be used with caution in patients with cardiovascular disease, psychosis, prostatic hypertrophy, pyrexia and glaucoma, and in the elderly in general.

Lifestyle

* Patients should be warned that treatment with antimuscarinic drugs may affect driving and other skilled tasks.

Victoria Winterburn, community pharmacist and supplementary prescriber, in Bradford, West Yorkshire / GMA

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