Myotonia and Myotonic Disorders

Abstract

Myotonia is the delayed relaxation of muscles following voluntary contraction (action myotonia) or mechanical stimulation (percussion myotonia). Action myotonia can be demonstrated by asking the patient to release a tightly clinched fist quickly or by having a patient look down after sustained upgaze. (The upper lids will remain hung for up to 10 seconds.) Typically, action myotonia improves with repeated activity although an occasional patient will report the reverse situation (paradoxical myotonia). Exposure to cold frequently worsens the problem while heat has the opposite effect. Percussion myotonia is useful diagnostically when elicited by the examiner but does not cause symptoms. It can be evoked in the thenar eminences, tongue, and wrist extensors by briskly tapping these areas with a reflex hammer. A positive response is seen as sustained dimpling in the region struck with the hammer. Action and percussion myotonia are not equally expressed in myotonic patients and a specific search for each type is necessary on routine physical examination.