edrophonium

Edrophonium is a short-acting acetylcholinesterase inhibitor historically used in the diagnostic assessment of neuromuscular disorders, most notably myasthenia gravis. It is a quaternary ammonium compound, which limits its ability to cross membranes and the blood–brain barrier, concentrating its action at the peripheral neuromuscular junction.

Mechanism of action is to reversibly inhibit acetylcholinesterase, leading to increased levels of acetylcholine at the motor endplate. The resulting enhanced stimulation of nicotinic receptors at the neuromuscular junction temporarily improves skeletal muscle weakness. Its effects are rapid in onset and short in duration, reflecting its reversible and non-covalent mode of binding.

Clinical use centers on the Tensilon test, in which a small dose is administered intravenously to distinguish myasthenia gravis from other causes of weakness and to differentiate MG from a cholinergic crisis. A quick improvement in muscle strength supports MG, while a lack of improvement or deterioration may indicate a cholinergic crisis or another diagnosis. Edrophonium is not used as a long-term therapy for MG due to its transient effect and potential risks.

Pharmacokinetics and administration: edrophonium is given by intravenous injection, with onset within seconds and a duration of several minutes. It is rapidly cleared, and its peripheral action is confined by its hydrophilic, quaternary nature, which also minimizes central nervous system effects.

Safety and adverse effects include bradycardia, hypotension, arrhythmias, bronchospasm, excessive salivation, sweating, and abdominal cramps. It is used with caution in patients with cardiac or pulmonary disease and can be counteracted by antimuscarinic agents if needed. It is generally avoided as a treatment option for MG and is primarily a diagnostic tool. See also other cholinesterase inhibitors such as neostigmine and pyridostigmine.