After discharges following compound muscle action potential in CASPR2 antibody-related disease

Abstract

© Author(s) (or their employer(s)) 2021. No commercial reuse. See rights and permissions. Published by BMJ. CASE PRESENTATION A 75yearold man had a 3month 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 Fwave 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, triplets and myokymic/neuromyotonic discharges (figure 3). The combination of subacute onset of diffuse muscle twitching and sustained motor unit activity on electromyography studies led us to suspect an acquired peripheral nerve hyperexcitability disorder, Isaacs’ syndrome. His serum contactinassociated proteinlike 2 (CASPR2) antibodies were present at high titre (1/320). A positronemission tomography scan searching for underlying malignancy was negative. Following prednisolone, 50 mg daily he improved promptly and has remained well 6 months later.