This state of abnormal rippling muscle activity may be generalized or limited to one part of the body such as the muscles of the shoulders or of the lower extremities. It is observed most often with demyelination of peripheral nerve following injury, and thus it is neuropathic in nature. The common underlying conditions are multiple sclerosis or Guillain-Barré syndrome affecting the facial nerve and radiation damage to the brachial or lumbar plexus. In the EMG, myokymic discharges consist of repetitive firing of 1 motor unit, firing at 5 to 60 Hz and recurring regularly at 0.2to 10-s intervals. The driving impulses arise in the most peripheral parts of the axon of chronically damaged nerves. In some patients cramping is associated, and those muscles about to cramp may twitch or show premonitory spontaneous rippling contractions; the cramping may be associated with sweating. Thus, myokymia, fasciculation, and cramping seem to be related but not clinically identical conditions.
The relation of myokymia to the state called continuous muscle fiber activity is ambiguous. Sporadically in the neurologic literature there have been descriptions of patients whose muscles at some point begin to “work” continuously (see Isaacs). Terms such as neuromyotonia and widespread myokymia with delayed muscle relaxation are additional names that have been applied to what is essentially the same condition. At the moment, there is little reason to distinguish one from another except in gradations of severity. In each case the excessive and spontaneous activity can be attributed to hyperexcitability of terminal parts of motor nerve fiber, possibly as a result of a partial loss of motor innervation and compensatory collateral sprouting of surviving axons (Cöers et al; Valli et al). The association of this type of disorder to anti-voltage gated potassium channels has been mentioned in relation to thymoma and to Morvan’s syndrome.
Twitching, spasms, and rippling of muscles (myokymia) are evident, the latter being the main clinical sign. In advanced cases there is generalized muscle stiffness and a sense of weakness. Complaints of muscle aching are usual, but severe myalgia is uncommon. The tendon reflexes may be reduced or abolished. Any muscle group may be affected. The stiffness and slowness of movement make walking laborious (“armadillo” syndrome); in extreme cases, all voluntary movement is blocked. The muscle activity persists throughout sleep. The continuous visible and painful cramps of the above-described Satoyoshi disease may be difficult to distinguish from myokymia clinically, but they represent a different phenomenon. General and spinal anesthesia do not always suppress the muscular activity but curare does; nerve block usually has no effect or may reduce the activity, as in the case described by Lütschg and colleagues. The EMG findings are much the same as those described earlier.
Special types of myokymia arise in childhood or adult life, sometimes in association either with a polyneuropathy or rarely with an inherited type of episodic ataxia that is variably responsive to acetazolamide or remits spontaneously (see Chap. 5). An inherited form of continuous muscle fiber activity has been traced to a mutation of the peripheral nerve K channel (Gutmann and Gutmann). In addition to the association with polyneuropathy, a state of continuous muscular activity has also been described in association with lung cancer and thymoma in which cases an immune mechanism has been inferred as noted above in Morvan syndrome (see reviews by Thompson and by Newsom-Davis and Mills and the discussion of paraneoplastic syndromes in Chap. 30).
Treatment Phenytoin or carbamazepine often abolish the continuous muscular activity and cause a return of reflexes. Acetazolamide has been helpful in other cases (Celebisoy et al). Many of the idiopathic cases will improve spontaneously after several years, however, plasma exchange may be tried if the symptoms are intractable.