Treatment of pyridostigmine-induced AV block with hyoscyamine in a patient with myasthenia gravis.

Myasthenia gravis is an autoimmune disorder of the nervous system typically mediated by antibodies against the nicotinic acetylcholine receptor at the neuromuscular junction. Treatment of myasthenia gravis frequently involves the use of cholinesterase inhibitors such as pyridostigmine. Treatment with these agents has been associated with bradycardia and syncope requiring pacemaker implantation. We report a case of a 60-year-old man with a 1-year history of myasthenia gravis treated with pyridostigmine who presented with syncope due to high degree AV block. Before committing the patient to a permanent pacemaker, a trial of medical therapy with hyoscyamine was attempted. Hyoscyamine is a muscarinic antagonist commonly used to block cholinergic side effects associated with pyridostigmine without reducing its efficacy at the neuromuscular junction. Treatment with hyoscyamine resulted in complete resolution of AV block, thereby avoiding pacemaker implantation.