Anticholinergics and Amantadine

For a more detailed discussion of this topic, see Anticholinergics and Amantadine, Sec. 31.6, p. 3014 in Comprehensive Textbook of Psychiatry, 9th Edition.

Anticholinergic drugs block the actions of atropine. In the clinical practice of psychiatry, the anticholinergic drugs are primarily used to treat medication-induced movement disorders, particularly neuroleptic-induced parkinsonism, neuroleptic-induced acute dystonia, and medication-induced postural tremor. Amantadine (Symmetrel) is used primarily for the treatment of medication-induced movement disorders, such as neuroleptic-induced parkinsonism. It is also used as an antiviral agent for the prophylaxis and treatment of influenza A infection.

Anticholinergics

Pharmacologic Actions

All anticholinergic drugs are well absorbed from the gastrointestinal (GI) tract after oral administration, and all are sufficiently lipophilic to enter the central nervous system (CNS). Trihexyphenidyl (Artane) and benztropine (Cogentin) reach peak plasma concentrations in 2 to 3 hours after oral administration, and their duration of action is 1 to 12 hours. Benztropine is absorbed equally rapidly by intramuscular (IM) and intravenous (IV) administration; IM administration is preferred because of its low risk for adverse effects.

All six anticholinergic drugs listed (Table 4-1) in this section block muscarinic acetylcholine receptors, and benztropine also has some antihistaminergic effects. None of the available anticholinergic drugs has any effects on the nicotinic acetylcholine receptors. Of these drugs, trihexyphenidyl is the most stimulating agent, perhaps acting through dopaminergic neurons, and benztropine is the least stimulating and thus is least associated with abuse potential.

Therapeutic Indications

The primary indication for the use of anticholinergics in psychiatric practice is for the treatment of neuroleptic-induced parkinsonism, characterized by tremor, rigidity, cogwheeling, bradykinesia, sialorrhea, stooped posture, and festination. All of the available anticholinergics are equally effective in the treatment of parkinsonian symptoms. Neuroleptic-induced parkinsonism is most common in elderly persons and is most frequently seen with high-potency dopamine receptor antagonists (DRAs), for example, haloperidol (Haldol). The onset of symptoms usually occurs after 2 or 3 weeks of treatment. The incidence of neuroleptic-induced parkinsonism is lower
with the newer antipsychotic drugs of the serotonin-dopamine antagonist (SDA) class.

Table 4-1 Anticholinergic Drugs

Generic Name	Brand Name	Tablet Size	Injectable	Usual Daily Oral Dosage	Short-Term Intramuscular or Intravenous Dosage
Benztropine	Cogentin	0.5, 1, 2 mg	1 mg/mL	1–4 mg one to three times	1–2 mg
Biperiden	Akineton	2 mg	5 mg/mL	2 mg one to three times	2 mg
Ethopropazine	Parsidol	10, 50 mg	—	50–100 mg one to three times	—
Orphenadrine	Norflex, Dispal	100 mg	30 mg/mL	50–100 mg three times	60 mg IV given over 5 min
Procyclidine	Kemadrin	5 mg	—	2, 5–5 mg three times	—
Trihexyphenidyl	Artane, Trihexane, Trihexy-5	2, 5 mg elixir 2 mg/5 mL	—	2–5 mg two to four times	—
IV, intravenous.

Another indication is for the treatment of neuroleptic-induced acute dystonia, which is most common in young men. The syndrome often occurs early in the course of treatment; is commonly associated with high-potency DRAs (e.g., haloperidol); and most commonly affects the muscles of the neck, tongue, face, and back. Anticholinergic drugs are effective both in the short-term treatment of dystonias and in prophylaxis against neuroleptic-induced acute dystonias.

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