Trihexyphenidyl is an antiparkinson agent of the antimuscarinic class of agents and is chemically a tertiary amine. It has been in clinical usage for decades. The drug is available as hydrochloride.

Pharmacology

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The exact mechanism of action in parkinsonian syndromes is not precisely understood, but it is known that Trihexyphenidyl blocks efferent impulses in parasympathetically innervated structeres like smooth muscles (spasmolytic activity), salvia glands and eyes (mydriasis). In higher doses direct central inhibition of cerebral motor centers may contribute. In very high doses central toxicity as seen in atropine overdose is noted.

Pharmacokinetics

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Trihexyphenidyl is rapidly absorbed from the GI-Tract. The onset of action is within 1 hour after oral dosing. The peak activity is noted after 2 to 3 hours. The duration of action of one single dose is 6 to 12 hours in a dose dependent manner. It is excreted in the urine, probably as unchanged drug. More precise data in animals and humans have so far not been determined.

Established uses

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Trihexyphenidyl is used for the symptomatic treatment of Morbus Parkinson in mono- and combination therapy. It is active in postencephalitic, arteriosclerotic, and idiopathic forms. The drug is also commonly used to treat extrapyramidal side effects occurring during antipsychotic treatment. It reduces the frequency and duration of oculogyric crises as well as of dyskinetic movements and spastic contractions. Excessive salivation may also respond. Trihexyphenidyl may improve psychic depression and mental inertia frequently associated with Morbus Parkinson and symptomatic problems caused by antipsychotic treatment.

Therapeutic prospects

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The drug is not able to cure Morbus Parkinson, but may provide substantial alleviation of symptoms. An estimated 50 to 75% of patients with M. Parkinson will react positively and experience a 20 to 30% symptomatic improvement. To increase therapeutic activity Trihexylphenidyl is often given concomitantly with levodopa, other antimuscarinic or antihistaminic (e.g. diphenhydramine) agents. Combination treatment with dopaminergic agonists such as cabergoline is also possible. This is often termed a 'multidimensional approach'.

Investigational uses

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Equivocal preleminary results from small studies exist for:

• Other Dyskinesias
• Chorea Huntington
• Spasmodic Torticollis

Trihexyphenidyl does not improve cerebral palsy and hemiplegia.

Contraindications and cautions

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See Biperiden.

• Patients under 18 yrs. of age should not be treated due to a lack of clinical experience.

Pregnancy and lactation

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The safe use of Trihexyphenidyl during pregnancy and lactation has so far not been assured.

Side-effects

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See Biperiden. Tolerance may develop during therapy which requires dose adjustments.

Interactions

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See Biperiden.

Dosage

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• Morbus Parkinson : One mg is given on the first day. Increments are usually 2mg every 3 days until 6 to 10mg are reached. In postencephalitic cases up to 15 mg might be necessary, but then excessive dryness of mouth or nausea could be a problem. To increase tolerability Trihexyphenidyl may be given in 3 divided doses.

• Extrapyramidal side effects : Usually, 5 to 15 mg daily are needed in 2 or 3 divided doses. Some patients, however, are successfully treated with as little as 1 mg daily.

Overdose

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See Biperiden.

Brand names and dose forms

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• Artane® : elixir 2mg/5ml, tabletts 2mg and 5mg
• Generics (e.g. Cypress, URL, Watson, West-Ward) : elixir and/or tabletts

External links

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• AHFS online database
• http://www.mentalhealth.com/drug/p30-t04.html