Repetitive stimulation in myasthenia gravis: decrementing response revealed by anticholinesterase drugs.

ABSTRACT

In myasthenia gravis (MG) a typical decrementing response is frequently revealed with repetitive stimulation (RS) in clinically unaffected muscles. However, RS is unlikely to give normal results in weak muscles. In two of our patients we found a normal response in clinically affected muscles. However, a decrementing response surprisingly appeared after the administration of anticholinesterase drugs. A possible explanation for this apparently paradoxical effect is as follows some junctions are already blocked at rest, whereas others function normally both at rest and during RS. As a consequence, the initial compound muscle action potential (CMAP) is reduced in amplitude and shows no further decrement on RS. Anticholinesterase drugs reverse some blocks, therefore causing the CMAP increase in amplitude. However, these labile junctions are more prone than normal to synaptic fatigue, so a decrementing response is produced on RS. This behavior, though not frequently encountered, can account for some negative results of RS in MG.