A 75-year-old man had a 3-month history of shooting and burning pain in all four limbs, with painless muscle twitches, fatigue and moderate weight loss. During this time, he had suffered four episodes of syncope. He had longstanding stable epilepsy, controlled with carbamazepine and lamotrigine.
On examination, there was frequent fasciculation and myokymia in the limb and trunk muscles. Muscle tone was increased, tendon reflexes were brisk, and he had a glove and stocking reduction in pain appreciation. His systolic blood pressure dropped considerably with standing.
Nerve conduction studies showed compound muscle action potential (CMAP) with repetitive after discharges, both on single nerve stimulation and during F-wave acquisition respectively (figures 1A and 2A). Motor conduction velocities and sensory studies were normal. There was no neurophysiological evidence of neuromuscular transmission defect. Electromyography identified widespread and continuous spontaneous activity of motor units, in the form of fasciculation, doublets,…