A 42-year-old woman with known myasthenia gravis (MuSK antibody-positive) on pyridostigmine develops acute respiratory failure. She has been taking ciprofloxacin for a urinary infection. Her Tensilon test is equivocal. The most appropriate immediate investigation to differentiate myasthenic crisis from cholinergic crisis is:
- A Repetitive nerve stimulation — decrement confirms myasthenic crisis
- B Serum MuSK antibody titre — rising titre confirms cholinergic crisis
- C Bedside clinical assessment: pupil size, secretions and fasciculations — miosis/hypersalivation/fasciculations suggest cholinergic; mydriasis/dry skin suggests myasthenic ✓
- D Chest CT scan for thymoma to guide dosing decisions
Explanation
Differentiating myasthenic crisis (insufficient acetylcholine at NMJ) from cholinergic crisis (excess acetylcholine from pyridostigmine overdose or organophosphate) is critical because treatment is opposite. Cholinergic crisis features: SLUDGE (salivation, lacrimation, urination, defaecation, GI cramps, emesis), miosis, bradycardia, fasciculations. Myasthenic crisis features: ptosis, diplopia, dry skin, absent fasciculations, mydriasis. Clinically assessing these signs avoids the risk of a Tensilon (edrophonium) test which can worsen a cholinergic crisis. Ciprofloxacin can worsen MG by neuromuscular junction blockade, suggesting myasthenic deterioration here.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.