congenital myasthenic syndrome synaptic congenital myasthenic syndrome The congenital myasthenic syndromes (CMS) are a diverse group of disorders that have an underlying defect in the transmission of signals from nerve cells to muscles. These disorders are characterized by muscle weakness, which is worsened upon exertion. Term added in response to Mondo restructuring. To be considered for replacement or obsoletion in EFO also. Orphanet:98915