A 46-year-old woman has variable ptosis, worse on the right, present for 6 weeks without limb weakness. To differentiate myasthenic from non-myasthenic ptosis at the bedside, an ice pack is applied to the closed right eye for 2 minutes. Which finding supports myasthenia gravis?
- A Ptosis worsens after cooling
- B Ptosis is unchanged but pupil becomes smaller
- C Ptosis improves measurably after cooling ✓
- D Ptosis worsens only after 10 minutes of cooling
Correct answer: C. Ptosis improves measurably after cooling
Explanation
The ice pack test exploits the temperature dependence of the neuromuscular junction: cooling reduces acetylcholinesterase activity and slows transmitter breakdown, so myasthenic ptosis measurably improves after about 2 minutes of local cooling. Improvement supports myasthenia gravis and warrants antibody testing and single-fiber EMG. Worsening with cooling would argue against myasthenia, since warm conditions aggravate myasthenic weakness.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.