How can a clinician differentiate a myasthenic crisis from a cholinergic crisis?

• A. Improvement after edrophonium administration indicates myasthenic crisis; deterioration indicates cholinergic crisis.
• B. Both conditions improve after edrophonium administration.
• C. Edrophonium worsens symptoms in myasthenic crisis but improves cholinergic crisis.
• D. Edrophonium has no diagnostic utility in distinguishing these crises.

Answer: (A)

The main concept is using an acetylcholinesterase inhibitor to distinguish types of crisis in myasthenia gravis. Edrophonium temporarily increases acetylcholine at the neuromuscular junction. In a myasthenic crisis, the weakness comes from too little acetylcholine effect; boosting acetylcholine with edrophonium improves muscle strength, so the patient gets better briefly. In a cholinergic crisis, there is already excess acetylcholine from too much acetylcholinesterase inhibition, so giving more acetylcholine worsens both nicotinic weakness and muscarinic symptoms. Therefore, improvement after edrophonium points to a myasthenic crisis, while deterioration points to a cholinergic crisis.