Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic, as well as by pathologic mechanism and electrophysiologic studies (i.e., acetylcholine receptor (AChR) deficiency, slow-channel or fast-channel kinetic defects at the AChR).  Approximately 10% of CMS cases are presynaptic, 15% are synaptic, and 75% are postsynaptic, the majority of which are caused by AChR deficiency. Slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the postsynaptic NMJ characterized by early-onset progressive muscle weakness. The disorder results from kinetic abnormalities of the AChR channel, specifically prolonged opening and activity of the channel, which causes prolonged synaptic currents resulting in a depolarization block. This is associated with calcium overload, which may contribute to subsequent degeneration of the endplate and postsynaptic membrane. [From OMIM]