Hypoxia-sensitive hyperexcitability of the intramuscular nerve axons in Isaacs' syndrome.

A case of Isaacs' syndrome with Trousseau's phenomenon is reported. Myokymia, pseudomyotonia (difficulty relaxing after forceful contraction), and ischemia-induced carpal spasm (Trousseau's phenomenon) were not abolished by nerve block distal to the cuff or by intravenous infusion of calcium. Inhalation of oxygen suppressed the pseudomyotonia and Trousseau's phenomenon, but myokymia persisted. Phenytoin abolished all types of abnormal discharges. The morphological abnormality in biopsied short peroneal muscle consisted of intraterminal and ultraterminal sprouting. We postulate that the trigger zone for abnormal discharge in our case is in the distal segment of the intramuscular nerve axon, including the nerve terminal. Hypoxia-sensitive hyperexcitability of the axon membrane might be responsible for the generation of pseudomyotonia and Trousseau's phenomenon, although the mechanism underlying myokymia remains unknown.