Electrodiagnostic Evaluation of Congenital Myasthenic Syndromes

Congenital myasthenic syndromes (CMSs) are a group of inborn disorders of neuromuscular (NM) transmission clinically characterized by weakness and muscle fatigability. Recognition of CMS is important because most CMSs are amenable to treatment, and electrodiagnostic (EDX) testing provides an objective method for the detection of failure of NM transmission, which is a feature in common to all forms of CMS. Understanding the principles of NM transmission is fundamental since all components of the EDX evaluation can be affected in CMS and knowledge of the basic physiology of synaptic transmission can assist in the interpretation of the EDX findings and can also provide valuable insights into the underlying pathogenesis of the various types of CMS. Repetitive nerve stimulation (RNS) of distal and proximal muscles at both slow and fast rates is the primary EDX tool used for the detection of failure of NM transmission in CMS. In many cases, RNS can also provide important clues about the underlying mechanism of failure of NM transmission and the type of CMS in question, which can be confirmed by genetic studies. In other cases, failure of NM transmission is subtle and can be demonstrated only by using additional techniques, such as long preconditioning trains of nerve stimulation or single-fiber EMG (SFEMG). Thus, a comprehensive EDX approach is essential not only for diagnosing CMS but also for guiding genetic testing and tailoring targeted treatment strategies, which can ultimately result in improved patient outcomes.