Which drug used for differentiating myasthenia gravis from cholinergic crisis?

Ans. (A) Edrophonium(Ref: KDT 8th/e p120)Patients with myasthenia gravis may present in the emergency with muscle weakness. This could be because of the aggravation of their primary problem (myasthenic crisis) or due to the overdose of the drugs like neostigmine (cholinergic crisis). The etiology of the two conditions is important to understand because in myasthenic crisis, there is deficiency of acetylcholine whereas in cholinergic crisis, there is excess of acetylcholine.Edrophonium is an inhibitor of the enzyme acetylcholinestrase and thus, increases the concentration of acetylcholine at the neuromuscular junction. The increased availability of ACh causes dramatic improvement in the symptoms of patients with myasthenic crisis.In contrast, the symptoms of patients with cholinergic crisis worsen with edrophonium (because it further increases ACh in cholinergic crisis). The drug, therefore, is useful to clinically differentiate the actual cause of muscle weakness in an emergency situation.The reason for preferring edrophonium over other inhibitors of acetylcholinesterase is its short duration of action. This may be desirable for patients presenting with cholinergic crisis as the worsening in their condition would be only for a short time.