A 68-year-old man with a history of benign prostatic hyperplasia and chronic urinary retention presents to the emergency department with acute abdominal distension, nausea, and inability to void. His post-void residual volume is 450 mL. He is started on bethanechol 25 mg orally three times daily. After 2 hours, he experiences severe cramping abdominal pain, profuse salivation, lacrimation, and diarrhea. His heart rate is 52 bpm and blood pressure is 88/54 mmHg. Which of the following is the most appropriate immediate management?

Explanation

## Cholinergic Crisis from Bethanechol Overdose **Key Point:** Bethanechol is a direct-acting muscarinic agonist used to treat urinary retention and atonic bladder. Overdose or excessive dosing causes a cholinergic crisis characterized by excessive muscarinic and nicotinic stimulation. ### Clinical Presentation of Cholinergic Toxicity The patient exhibits classic **SLUDGE** syndrome: - **S**alivation (profuse) - **L**acrimation (tears) - **U**rination (retention paradoxically worsens with overdose due to smooth muscle spasm) - **D**efecation (diarrhea) - **G**astric cramping (severe abdominal pain) - **E**mesis (nausea) Additionally: **bradycardia** (52 bpm) and **hypotension** (88/54 mmHg) indicate muscarinic effects on the heart and vasculature. ### Management Algorithm ```mermaid flowchart TD A["Cholinergic Crisis<br/>SLUDGE + Bradycardia + Hypotension"]:::outcome A --> B{"Severity?"}:::decision B -->|"Mild (salivation, mild GI)"| C["Discontinue agent<br/>Observation"]:::action B -->|"Moderate-Severe<br/>(Hemodynamic instability)"| D["Atropine 0.5-1 mg IV<br/>Repeat q5-10 min PRN"]:::action D --> E["Target HR >60 bpm<br/>SBP >90 mmHg"]:::outcome E --> F{"Response?"}:::decision F -->|"Yes"| G["Continue supportive care"]:::action F -->|"No"| H["Repeat atropine<br/>Consider ICU admission"]:::urgent ``` **High-Yield:** Atropine is the **antidote** for cholinergic excess. It is a competitive antagonist at **muscarinic receptors** and rapidly reverses salivation, lacrimation, bronchospasm, bradycardia, and hypotension. Doses are titrated to effect (target: HR >60 bpm, SBP >90 mmHg, dry mouth). ### Why Atropine, Not Other Agents? | Agent | Mechanism | Use in Cholinergic Crisis | |-------|-----------|-------------------------| | **Atropine** | Muscarinic antagonist | **First-line** — rapid reversal of SLUDGE and cardiac effects | | Pralidoxime | Acetylcholinesterase reactivator | Used in **organophosphate/carbamate poisoning** (irreversible enzyme inhibition), NOT direct agonist overdose | | Propranolol | β-blocker | Treats tachycardia; **contraindicated** here (patient is bradycardic and hypotensive) | | Observation alone | None | Inadequate — patient is hemodynamically unstable | **Clinical Pearl:** Bethanechol is a **quaternary ammonium compound** (charged, polar) and does NOT cross the blood–brain barrier, so it causes only peripheral cholinergic effects. Atropine (tertiary amine, lipophilic) crosses the BBB and is therefore superior to quaternary ammonium antimuscarinics (e.g., methscopolamine) in crisis management. **Warning:** Do NOT use pralidoxime — it is specific for **irreversible acetylcholinesterase inhibitors** (organophosphates, carbamates). Bethanechol is a direct agonist; the enzyme is not inhibited, so pralidoxime is ineffective and wastes critical time.