A 68-year-old man presents with acute onset of severe mydriasis, cycloplegia, dry mouth, tachycardia (HR 118/min), and agitation. He is a known glaucoma patient who accidentally instilled an anticholinergic agent into both eyes instead of his prescribed timolol. Which investigation is most appropriate to confirm anticholinergic toxicity and differentiate it from other causes of mydriasis?

Explanation

## Diagnosis of Anticholinergic Toxicity via Pupil Response ### The Clinical Problem Anticholinergic overdose causes fixed, dilated pupils (mydriasis) and cycloplegia. However, mydriasis can also result from sympathomimetic toxicity, opioid withdrawal, or neurological emergencies (e.g., third nerve palsy, increased intracranial pressure). A specific pharmacological test is needed to confirm the anticholinergic mechanism. ### Why Pilocarpine Testing is Diagnostic **Key Point:** Pilocarpine is a muscarinic agonist that directly stimulates the pupillary sphincter muscle, bypassing the blocked muscarinic receptors. 1. **In anticholinergic toxicity:** Pilocarpine drops fail to constrict the pupil because the muscarinic receptors are competitively blocked by the anticholinergic agent. The pupil remains dilated (non-reactive to pilocarpine). 2. **In sympathomimetic toxicity or third nerve palsy:** Pilocarpine will cause pupillary constriction because the muscarinic pathway is intact. ### Interpretation Table | Cause of Mydriasis | Pilocarpine Response | Mechanism | |---|---|---| | **Anticholinergic toxicity** | No constriction (fixed dilated) | Muscarinic receptors blocked | | Sympathomimetic overdose | Brisk constriction | Muscarinic pathway intact | | Third nerve palsy | Brisk constriction | Muscarinic pathway intact | | Opioid withdrawal | Brisk constriction | Muscarinic pathway intact | **High-Yield:** Pilocarpine challenge test is the gold standard for confirming anticholinergic-induced mydriasis and cycloplegia because it directly tests muscarinic receptor responsiveness. **Clinical Pearl:** In atropine or anticholinergic poisoning, the pupil is classically described as "blown" (fully dilated) and unresponsive to both light and accommodation. Pilocarpine testing confirms the anticholinergic etiology by demonstrating receptor blockade rather than structural or sympathetic dysfunction. ### Why Other Investigations Are Insufficient - **Slit-lamp & gonioscopy:** Structural assessment only; does not confirm the pharmacological mechanism of mydriasis. - **ECG & electrolytes:** Nonspecific; useful for systemic complications (arrhythmias, electrolyte disturbances) but not diagnostic of anticholinergic toxicity. - **Fundus fluorescein angiography:** Assesses retinal perfusion; irrelevant to pupillary pharmacology.