Weakness in myotonic syndromes.

Muscle weakness occurs in a number of myotonic syndromes, including those in which muscle bulk is preserved. The possible causes of muscle weakness may be considered in terms of the different stages of the electromechanical activation process and defects in the contractile machinery. In individual myotonic disorders the ability to identify the origin of the muscle weakness (whether neurogenic or myogenic) and the functional level at which the defect occurs would assist the choice of rational treatment.