How to Spot Congenital Myasthenic Syndromes Resembling the Lambert-Eaton Myasthenic Syndrome? A Brief Review of Clinical, Electrophysiological, and Genetics Features.

Congenital myasthenic syndromes (CMS) are heterogeneous genetic diseases in which neuromuscular transmission is compromised. CMS resembling the Lambert-Eaton myasthenic syndrome (CMS-LEMS) are emerging as a rare group of distinct presynaptic CMS that share the same electrophysiological features. They have low compound muscular action potential amplitude that increment after brief exercise (facilitation) or high-frequency repetitive nerve stimulation. Although clinical signs similar to LEMS can be present, the main hallmark is the electrophysiological findings, which are identical to autoimmune LEMS. CMS-LEMS occurs due to deficits in acetylcholine vesicle release caused by dysfunction of different components in its pathway. To date, the genes that have been associated with CMS-LEMS are AGRN, SYT2, MUNC13-1, VAMP1, and LAMA5. Clinicians should keep in mind these newest subtypes of CMS-LEMS to achieve the correct diagnosis and therapy. We believe that CMS-LEMS must be included as an important diagnostic clue to genetic investigation in the diagnostic algorithms to CMS. We briefly review the main features of CMS-LEMS.