A patient on long-term treatment with a selective M3 muscarinic antagonist for overactive bladder develops blurred near vision. The most likely mechanism responsible for this ocular side effect is:
- A Mydriasis due to unopposed sympathetic stimulation of the iris dilator muscle
- B Increased aqueous humour production due to ciliary body stimulation
- C Cycloplegia due to inhibition of the ciliary muscle, preventing accommodation ✓
- D Miosis due to parasympathetic blockade of the iris sphincter
Correct answer: C. Cycloplegia due to inhibition of the ciliary muscle, preventing accommodation
Explanation
The ciliary muscle is innervated by parasympathetic fibres acting on M3 receptors; contraction of the ciliary muscle allows lens accommodation for near vision. Selective M3 blockade causes cycloplegia (paralysis of accommodation), resulting in blurred near vision. Mydriasis (not miosis) is indeed a separate consequence, but the near vision blurring specifically reflects cycloplegia, not mydriasis.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.