A 28-year-old woman has variable ptosis worse with fatigue. The ice pack test is performed: an ice pack is placed over the closed eye for 2 minutes, after which the ptosis measurably improves. This improvement occurs because cold:
- A Increases release of acetylcholine from the presynaptic terminal
- B Reduces antibody binding at the postsynaptic receptor
- C Inhibits acetylcholinesterase activity at the neuromuscular junction ✓
- D Enhances calcium influx into the motor nerve ending
Correct answer: C. Inhibits acetylcholinesterase activity at the neuromuscular junction
Explanation
In myasthenia gravis, cooling inhibits acetylcholinesterase, prolonging the residence time of acetylcholine in the synaptic cleft and improving neuromuscular transmission, hence the positive ice pack test. It is a simple bedside test with high specificity for myasthenic ptosis. The mechanism is not related to acetylcholine release, antibody binding, or calcium influx.
Reference: Yanoff and Duker Ophthalmology, 5th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.