Myasthenia, thymoma, presynaptic antibodies, and a continuum of neuromuscular hyperexcitability

Background: Autoantibodies specific for the nicotinic acetylcholine receptor (AChR) of skeletal muscle impair neuromuscular transmission in myasthenia gravis (MG). Autoantibodies specific for α3 neuronal AChRs or voltage-gated potassium channels have been reported in patients with Isaacs syndrome, an acquired disorder of continuous muscle fiber activity characterized by neuromyotonia. Objective: To report the neuromuscular autoantibody profiles of three patients with a syndrome of MG and neuromuscular hyperexcitability. Results: All three patients reported here had clinical and electrophysiologic evidence of MG and neuromuscular hyperexcitability. None had neuromyotonia. Thymoma was proven in two patients and suspected in the third. One had MG and thymoma and subsequently developed cramp-fasciculation syndrome; MG and rippling muscle syndrome appeared simultaneously in the other two. All patients had muscle and neuronal AChR binding antibodies and striational antibodies. Only one had antibodies reactive with α-dendrotoxin-complexed potassium channels. Conclusions: The coexistence of cramp-fasciculation syndrome and acquired rippling muscle syndrome with MG, thymoma, and neuronal AChR autoantibodies suggests that there is a continuum of autoimmune neuromuscular hyperexcitability disorders related pathogenically to Isaacs syndrome. Manifestations of neuromuscular hyperexcitability may be altered and less apparent in the context of MG because of the coexisting defect of neuromuscular transmission.