Calcium and myokymia of brainstem origin.

In five patients with Guillain-Barré syndrome, clinical myokymia increased and myokymic burst amplification occurred when ionized Ca++ was lowered by hyperventilation. Myokymia decreased when ionized Ca++ was increased after IV infusion of CaCl2. These responses were absent or diminished in the four patients with myokymia due to brainstem lesions, suggesting that the blood-brain barrier impedes the effects of altered serum ionized Ca++ on axonal excitability. Altering serum Ca++ can distinguish peripheral and central myokymia.