Which indirect cholinergic agent is used to treat urine retention and GI atony and acts as an antidote to neuromuscular blockers?

• A. Physostigmine
• B. Neostigmine
• C. Pyridostigmine
• D. Organophosphate

Answer: (B)

Indirect cholinergic agents inhibit acetylcholinesterase, raising acetylcholine levels at both muscarinic and nicotinic receptors. This extra acetylcholine boosts smooth muscle activity in the gut and bladder, helping with urinary retention and GI atony. At the neuromuscular junction, more acetylcholine competes with a nondepolarizing neuromuscular blocker, reversing the blockade and restoring muscle strength.

Neostigmine is ideal here because it’s a quaternary ammonium compound that largely stays in the periphery, giving strong reversal of nondepolarizing nerve blockers with manageable muscarinic effects. Its peripheral action also makes it effective for stimulating GI motility and bladder contraction.

Physostigmine, being a tertiary amine, crosses the blood–brain barrier and is mainly used for anticholinergic toxicity rather than NMJ blockade reversal. Pyridostigmine is useful for myasthenia gravis and can reverse blockade but has a longer duration and is not the typical agent for immediate perioperative reversal. Organophosphates are irreversible inhibitors and cause cholinergic toxicity, not a therapeutic antidote in this context.