A 68-year-old man with Parkinson's disease on levodopa presents to the emergency department with acute urinary retention, constipation, and blurred vision. His wife reports he started taking an over-the-counter cold medicine 3 days ago for rhinitis. On examination, he has a dry mouth, dilated pupils, and a distended bladder on palpation. His heart rate is 102/min and blood pressure is 148/92 mmHg. Which of the following drugs in the cold medicine is most likely responsible for his acute presentation?

Explanation

## Clinical Diagnosis: Anticholinergic Toxicity from Diphenhydramine ### Why Diphenhydramine? **Key Point:** First-generation antihistamines like diphenhydramine possess significant anticholinergic properties due to their lipophilic structure and ability to cross the blood-brain barrier. They block muscarinic receptors in the central and peripheral nervous systems. **High-Yield:** Anticholinergic effects of diphenhydramine include: - Urinary retention (detrusor muscle paralysis) - Constipation (reduced GI motility) - Blurred vision (ciliary muscle paralysis → mydriasis) - Dry mouth (reduced salivary secretion) - Tachycardia (loss of vagal tone) - Hypertension (sympathetic unopposed effects) ### Anticholinergic Syndrome Triad **Mnemonic: "Hot as a hare, Dry as a bone, Red as a beet, Mad as a hatter"** - **Hot:** Hyperthermia (impaired sweating) - **Dry:** Dry mucous membranes, urinary retention - **Red:** Flushed skin, mydriasis - **Mad:** Confusion, agitation, delirium This patient exhibits the classic triad: mydriasis (dilated pupils), urinary retention, and dry mouth. ### Risk Factors in This Patient 1. **Age >65:** Elderly patients have reduced hepatic metabolism and increased CNS sensitivity to anticholinergics 2. **Parkinson's disease:** Already has reduced cholinergic tone; additive anticholinergic effects are more pronounced 3. **Drug interaction:** Levodopa metabolism may be affected; anticholinergics can delay gastric emptying and reduce levodopa absorption ### Mechanism of Anticholinergic Toxicity ```mermaid flowchart TD A[Diphenhydramine ingestion]:::action --> B[Blocks muscarinic receptors<br/>M1, M2, M3, M4, M5]:::outcome B --> C{Organ System Effects}:::decision C -->|Bladder| D[Detrusor paralysis<br/>Urinary retention]:::outcome C -->|Eye| E[Ciliary muscle paralysis<br/>Mydriasis + Blurred vision]:::outcome C -->|GI| F[Reduced motility<br/>Constipation]:::outcome C -->|Salivary glands| G[Reduced secretion<br/>Dry mouth]:::outcome C -->|Heart| H[Tachycardia<br/>Loss of vagal tone]:::outcome C -->|CNS| I[Confusion, agitation<br/>if severe]:::urgent ``` ### Management **Clinical Pearl:** Anticholinergic toxicity is treated symptomatically. In severe cases (altered mental status, seizures), **physostigmine** (a tertiary amine anticholinesterase that crosses the BBB) may be used as a specific antidote. For mild-to-moderate cases, supportive care and discontinuation of the offending agent suffice. **Tip:** Always counsel elderly patients with Parkinson's disease to avoid OTC cold medicines containing first-generation antihistamines due to additive anticholinergic effects. [cite:KD Tripathi 8e Ch 12]