The phenomenon of myotonia, or neuromyotonia denotes a failure of voluntary relaxation of muscle because of sustained firing of the muscle membrane (see Chaps. 45 and 46), is characterized by high-frequency repetitive discharges generally having a positive sharp waveform. These myotonic discharges wax and wane in amplitude and frequency, producing a “dive-bomber” sound on the audio monitor. The discharges can be elicited mechanically by percussion of the muscle or movement of the needle electrode and are also seen following voluntary contraction or electrical stimulation of the muscle via its motor nerve. The MUPs may appear normal during voluntary contraction, but they are not followed by the silence that normally occurs on relaxation; instead, there is a “prolonged afterdischarge” consisting of long trains of fibrillation-like potentials that may take as long as several minutes to subside (Fig. 2-19A). These EMG findings can be seen with any myotonic disorder. If the muscle is activated repeatedly at short intervals, the late discharge becomes briefer and briefer and eventually disappears (see Fig. 2-19B), as the patient becomes able to relax the exercised muscle (“warmup” effect).
In paradoxical myotonia the myotonia worsens after each of a succession of voluntary contractions. This is the converse of what happens in myotonia congenita (Thomsen disease). As shown by single-fiber EMG studies, myotonia is generated by single muscle fibers and the mechanism of the membrane instability, at least in some forms, seems to involve changes in the chloride conductance. These disorders are discussed in subsequent chapters.
The cramp-like contracture of McArdle disease and phosphofructokinase deficiency is associated with electrical silence of contracting muscle. This feature is an important part of the definition of true physiologic muscle contracture (as distinguished from chronic shortening of a muscle and its tendon which, strictly speaking, is a pseudocontracture).
In the syndrome of continuous muscle fiber activity or Isaacs syndrome (see Chap. 46), which is a generalized form of myokymia, the EMG discloses high-frequency (up to 300-Hz) repetitive discharges of varying waveforms. In the stiff man syndrome, painful muscle spasms and stiffness are generated by a spinal mechanism; the EMG potentials resemble normal motor units but are abnormal by virtue of continuous firing at rest.
Abnormalities in Amplitude, Duration, and Shape of Motor Unit Potentials
Figure 2-20 depicts the ways in which disease processes affect the motor unit and the appearance of the MUP in the EMG.
Motor unit potentials in denervation Early in the course of denervation of muscle by disease of the nerve, many motor units with functional connections to the spinal cord are unaffected, and although the number of MUPs appearing during contraction is reduced, the configurations of the remaining ones are quite normal. In time, the remaining MUPs often increase in size and in electrical amplitude, perhaps two to three times normal, and become longer in duration and sometimes polyphasic (more than four phases).