Continuing Education Activity
Trihexyphenidyl works as an anticholinergic and is used for the treatment of tremors, spasms, stiffness, and weak muscle control seen in patients with Parkinson disease. It can also be used for the prevention or treatment of similar muscular conditions, which are caused by certain central nervous system (CNS) drugs such as fluphenazine, haloperidol, chlorpromazine. It was approved for the management of all types of parkinsonism (idiopathic, postencephalitic, and arteriosclerotic) in June 2003 by the FDA. This activity outlines the indications, mechanism of action, methods of administration, important adverse effects, contraindications, toxicity, and monitoring, of trihexyphenidyl so providers can direct patient therapy as indicated as part of the interprofessional team.
- Describe the mechanism of action of trihexyphenidyl.
- Summarize the adverse event profile of trihexyphenidyl.
- Review the toxicity and misuse potential for trihexyphenidyl.
- Explain the importance of collaboration and coordination among the interprofessional team and how it can enhance patient care with trihexyphenidyl therapy to improve patient outcomes for patients with Parkinsonism.
Trihexyphenidyl works as an anticholinergic used for the treatment of tremors, spasms, stiffness, and weak muscle control seen in patients with Parkinson disease. It can also be useful for the prevention or treatment of similar muscular conditions, which are caused by certain central nervous system (CNS) drugs such as fluphenazine, haloperidol, chlorpromazine. Although it has been pertinent in clinical trials investigating the treatment of Parkinson disease since 1949, it was approved for the management of all types of parkinsonism (idiopathic, postencephalitic, and arteriosclerotic) in June 2003 by the FDA. Trihexyphenidyl is often used as an adjuvant therapy when treating the forms mentioned above of parkinsonism with levodopa.
In 2008, there were reports that Iraqi police and soldiers were using trihexyphenidyl for recreational purposes along with other prescription drugs. Reportedly the drugs were taken as they seemed to relieve combat stress. Although this could have been the primary cause for use in some, some also seen as a substitute or a stronger version of LSD by some users.
Mechanism of Action
Although the precise mechanism of action of trihexyphenidyl remains poorly understood, it appears to act on the parasympathetic nervous system by inhibiting the efferent impulses directly. Structures innervated by the parasympathetic system, such as the salivary glands, eyes, and smooth muscles (directly and indirectly), are affected, even on smaller doses. The direct central inhibition of cerebral motor centers may occur with higher doses. Researchers believe the receptors affected are the dopamine and M1 muscarinic receptors. The drug is absorbed from the gastrointestinal tract, and the onset of action occurs 60 minutes after an oral dose with peak activity occurring after 2 to 3 hours. One dose has a duration of action of around 6 to 12 hours and is then excreted in the urine, most likely as an unchanged drug.
The dosage of trihexyphenidyl HCl varies with the individual and is determined empirically. Patients start with a low initial dose, which is increased gradually, especially in adults who are older than 60 years of age. The patient can take the oral drug before or after meals; this depends on the individual patient as in those with excessive xerostomia (due to trihexyphenidyl’s anticholinergic effects) could take the drug before meals and those who feel nauseous or are prone to excessive salivation could take the drug after meals. Trihexyphenidyl is better tolerated if taken in 3 separate doses daily, with food. Higher doses, such as more than 10 mg per day, could be divided into four doses daily (one taken with each meal and one at bedtime).
Clinicians should avoid abrupt withdrawal of trihexyphenidyl in patients undergoing treatment for parkinsonism symptoms, as this could cause an acute exacerbation of existing symptoms. There are reports of neuroleptic malignant syndrome in patients who had an abrupt withdrawal of treatment.
For initial therapy for symptoms of idiopathic parkinsonism, 1 mg tablet of trihexyphenidyl is given on the first day. This dose could then increase by 2 mg accretions at intervals of around four days until a total dose of 6 to 10 mg is being administered daily to the patient. The final daily dose is determined by the physician, to what is deemed the optimal dosage for symptom control. Most patients display significant improvement in symptoms on total daily doses of 6 to 10 mg, but some, especially those of the postencephalitic sub-group, could require daily doses of around 12 to 15 mg for symptom management.
The optimal dose and dosing frequency of trihexyphenidyl required to control the extrapyramidal symptoms of commonly used CNS drugs, for example, thioxanthenes, and phenothiazines, are determined empirically by the physician. For most patients, the total daily dosage ranges from 5 to 15 mg, although there have been reported cases of symptoms being sufficiently controlled on as minimal as 1 mg daily. Therefore, recommendations are to start treatment with a single dose of 1 mg. Failure of the extrapyramidal manifestations to resolve in a few hours could prompt an increase in dosage until achieving satisfactory control of symptoms. Adequate control of symptoms is sometimes possible in a shorter duration by briefly decreasing the CNS drug dosage when initiating trihexyphenidyl and gradually adjusting the dose of both drugs for the desired effects without the onset of extrapyramidal symptoms.
Concomitant Use with Levodopa
The usual dose of both levodopa and trihexyphenidyl may require reduction when administering both of these drugs concomitantly. Any adjustment in dosage needs to be made carefully, depending on the level of symptom control and subsequent side effects. An adequate dosage for symptom control with minimal side effects is generally around 3 to 6 mg daily, given in divided doses.
Adverse effects of trihexyphenidyl are frequently seen in a dose-dependent manner but usually decrease over time as tolerance develops, and the body adapts to the drug. Even with all of the adverse effects considered, trihexyphenidyl demonstrated dramatic and consistent improvement of neurologic defects in people of ages between 16 to 86 years, over a 5-year course. Confusion and delirium frequently occur in patients who were older than 86 years or had a psychiatric condition.
Adverse effects of trihexyphenidyl include but are not limited to:
Ocular effects: Mydriasis may present in patients with and without photophobia. This condition can lead to blurred vision or precipitate narrow-angle glaucoma by angle closure, which increases intraocular pressure.
CNS effects: Frequently reported side effects include a headache, dizziness, drowsiness, and vertigo. Anxiety, nervousness, confusion, and agitation occurred in patients who were on higher doses. Trihexyphenidyl also produces a short-acting euphoric and mood-elevating effect, and that is why it is a drug of abuse. There have been cases of the disruption of normal sleep architecture (REM sleep depression). It could also potentially lower the seizure threshold, and use requires caution in people with epilepsy or other seizure disorders.
Peripheral side effects: As with other anticholinergics, impaired sweating, dry mouth, abdominal discomfort, nausea, urinary retention, and constipation are frequently seen (patients require monitoring for long-term use). Some patients also develop tachycardia. Although allergic reactions are infrequent, they could occur with the use of trihexyphenidyl. Fatal hyperthermia and severe anhidrosis are also possible, and that is why caution is advised when using the drug during exercise or in extremely hot weather.
Patients taking trihexyphenidyl have also reported weight gain.
Tolerance could develop with the prolonged use of the drug, and dosing adjustments may be necessary.
Trihexyphenidyl classifies as FDA pregnancy risk factor class C. 
Trihexyphenidyl is contraindicated in those with hypersensitivity to the drug (trihexyphenidyl HCl) or any of its drug formulation ingredients.
It is also contraindicated in patients with narrow-angle glaucoma because it has anticholinergic activity that could cause mydriasis, further narrowing the angle of the lens, increasing the IOP, and worsening the condition.
Trihexyphenidyl is not strictly contraindicated for patients who have liver, kidney, or cardiac disorders, but recommendations are to monitor these patients closely when using the drug. Patients with hypertension should also have their blood pressure monitored for the duration of therapy.
Some patients may require the indefinite use of trihexyphenidyl, and since it has properties similar to atropine, constant, and long-term supervision should be implemented to prevent allergic and other unwanted reactions. Because of the parasympathetic activity of trihexyphenidyl, it should be used precautiously and monitored closely in patients with obstructive genitourinary or gastrointestinal diseases, glaucoma, and in older males with prostatic hypertrophy. Geriatric patients, especially those who are older than 60, commonly develop an increased sensitivity to these types of drugs and require strict regulation of their dosage.
As with some other antiparkinsonian medications, trihexyphenidyl is known to be a drug of abuse. Abuse has been reported primarily in both chronic schizophrenics and those who abuse other substances, the former being less frequent abusers of other substances. Trihexyphenidyl toxicity resembles atropine intoxication (antimuscarinic effects) with xerostomia, anhidrosis, mydriasis, nausea/vomiting, tachycardia, hyperpyrexia, decreased bowel and bladder movements, rash, and hyperthermia, which usually accompany excessive doses. CNS symptoms observed with overdose include confusion, restlessness, agitation, incoordination, paranoid and psychotic reactions, delirium, and hallucinations. Reports exist of CNS depression leading to coma, respiratory, and circulatory failure, and death in cases of severe overdose. If the overdose does not receive prompt treatment, it could be fatal, especially in smaller children.
The management of overdoses is always supportive; establish adequate airway patency immediately. Physostigmine is a specific antagonist that acts centrally and peripherally to counter the antimuscarinic effects. Convulsions and hyperactivity require management with diazepam, but with caution as the risk of CNS depression could be exacerbated. Acidosis and hypoxia should have appropriate therapy. Dysrhythmias should not have treatment with antiarrhythmic drugs. Atonic bladder and bowel are treatable with carbachol.
Enhancing Healthcare Team Outcomes
Due to the significant potential toxicity of trihexyphenidyl, its dosing and management require an interprofessional healthcare team consisting of the clinician, nurse, and pharmacist monitoring the patient for complications and communicating any concerns to the healthcare team. [Level V]