ELECTROMYOGRAPHIC1 investigations of the myotonias reveal a dramatic and pathognomonic picture. Thus, in patients with myotonia congenita (Thomsen's disease), dystrophia myotonica (myotonia atrophica), and myotonia acquisita (Talma's disease)2 we have found identical abnormal action potentials generated in the skeletal musculature.
These abnormal action potentials are elicited from a myotonic muscle immediately when the needle electrode is inserted into such a muscle. Large bursts of spontaneous, involuntary motor-unit voltages are seen in the cathode-ray oscilloscope, accompanied by a high-pitched, pulsating sound. Both the abnormal wave forms and the sounds reach a peak immediately and then gradually recede to an isoelectric level and silence. The slightest motion of the needle electrode in the hyperirritable muscle reproduces this characteristic response, which is seen and heard only in a myotonic muscle. Voluntary motion of the muscle produces normal, simple motor-unit contraction voltages, which persist, however, for a considerable period after motor relaxation
SHEA PA, WOODS WW, TRATAR AA. THE MYOTONIASAn Electromyographic Study, with Special Reference to Myotonia Acquisita. AMA Arch NeurPsych. 1952;68(6):794-796. doi:10.1001/archneurpsyc.1952.02320240069007