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# Pacitane (trihexyphenidyl)
## Overview
Trihexyphenidyl is an anticholinergic agent used primarily for Parkinson’s disease and extrapyramidal symptoms (EPS) caused by antipsychotics. It acts by blocking central muscarinic acetylcholine receptors.
## Primary Indications
- Idiopathic Parkinson’s disease (adjunct to levodopa/carbidopa in younger patients or those with intolerable tremor)
- Drug-induced extrapyramidal symptoms (acute dystonia, parkinsonism, akathisia)
- Not recommended for tardive dyskinesia (may worsen)
## Adult Dosing
**Parkinson’s disease (adjunct):**
- Initial: 1 mg PO daily; increase by 2 mg every 3–5 days
- Usual maintenance: 6–10 mg/day in three divided doses
- Maximum: 15 mg/day (higher doses rarely used due to side effects)
**Drug-induced EPS:**
- Initial: 1 mg PO once or twice daily
- Titrate up to 5–15 mg/day in divided doses based on response
- Maximum: 15 mg/day; use lowest effective dose for shortest duration
## Pediatric Dosing
**Drug-induced EPS (age ≥4 years):**
- Initial: 1 mg PO daily; increase by 1 mg every 3–5 days as needed
- Usual range: 1–5 mg/day in two to three divided doses
- Maximum: 6 mg/day (or 0.2 mg/kg/day, whichever is lower)
*Dosing per local protocol may vary; consult specialist.*
## Dose Adjustments
**Hepatic impairment:** No specific adjustment; use caution in severe disease.
**Renal impairment:** Not well studied; monitor for anticholinergic toxicity in advanced disease.
**Elderly:** Start at 1 mg/daily; titrate slowly with close monitoring (falls, confusion).
## Contraindications
- Narrow-angle glaucoma (absolute)
- Prostatic hypertrophy with urinary retention (relative; avoid if symptomatic)
- Myasthenia gravis
- Severe GI obstruction (paralytic ileus, pyloric stenosis)
- Hypersensitivity to trihexyphenidyl or any component
## Adverse Effects
**Common:** Dry mouth, blurred vision, constipation, urinary retention, nausea, dizziness, drowsiness
**Serious:** Confusion, hallucinations, delirium (especially in elderly), tachycardia, hyperthermia (risk with heat stress), impaired cognitive function
**Overdose:** Anticholinergic crisis (flushing, fever, agitation, seizures, coma); treat with physostigmine (only if severe)
## Key Drug Interactions
- **Antipsychotics (e.g., haloperidol, chlorpromazine):** Additive anticholinergic effects; may reduce antipsychotic levels (GI motility)
- **Anticholinergics (e.g., benztropine, diphenhydramine, TCAs):** Synergistic toxicity; avoid combination unless necessary
- **Levodopa:** May enhance therapeutic effect but increase anticholinergic side effects
- **CNS depressants (e.g., alcohol, benzodiazepines):** Increased sedation; use caution
## Monitoring
- Efficacy: tremor severity, EPS improvement (Abnormal Involuntary Movement Scale if applicable)
- Toxicity: cognitive function (MMSE in elderly), heart rate, urine output, intraocular pressure if glaucoma suspected
- Drug-induced dyskinesia: monitor for tardive or choreiform movements with chronic use
## Clinical Pearls
- **Not first-line** for Parkinson’s in elderly or cognitively impaired (risk of delirium).
- **Abrupt withdrawal** can cause cholinergic rebound (nausea, vomiting, sialorrhea, dyskinesia); taper over 1–2 weeks.
- Consider **less anticholinergic options** (e.g., amantadine for EPS) in older adults or those with dementia.
- Use lowest effective dose; tolerance to dry mouth often develops.
- For **acute dystonia** (e.g., oculogyric crisis), IV/IM benztropine is often preferred over oral trihexyphenidyl.
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**Disclaimer:** This information is for educational use only and does not replace clinical judgment. Always verify current dosing, safety data, and local protocols (e.g., hospital formulary, prescribing guidelines) before initiating therapy.
