Weakness from magnesium-containing cathartics: electrophysiologic studies.

The case history of a patient with severe muscular weakness resulting from magnesium intoxication following oral and rectal administration of magnesium citrate cathartics is reported. The findings of the electrophysiologic studies were characteristic of the neuromuscular blockade seen in this disorder, including marked reduction in evoked muscle action potential amplitude which progressively decline on nerve stimulation at low rates, and a striking degree of facilitation of evoked muscle action potential after exercise or on high-frequency stimulation. When the hypermagnesemia was corrected, strength returned to normal.