Occasionally, benign fasciculations are widespread and may last for months or even years. In several of our patients they have recurred in bouts separated by months and lasting several weeks. No reflex changes, sensory loss, nerve conduction, EMG abnormality (other than fasciculations), or increase in serum muscle enzymes is found. Low energy and fatigability in some of these patients may suggest an endogenous depressive illness yet the fasciculations are not explained by this mechanism. Patients commonly report a sense that the muscles affected by the twitching are weak but this cannot be confirmed by testing and several of our patients, curiously most of whom have been physicians, complained of equally troubling migratory zones of paresthesias. Pain of aching or burning type may increase after activity and cease during rest. Fatigue and a sense of weakness are frequent complaints. We suspect that this fasciculatory state reflects a disease of the terminal motor nerves, for a few of our patients have shown slowing of distal latencies, and Cöers and associates have found degeneration and regeneration of motor nerve terminals in similar cases. However, most such cases are of benign nature and settle down in a matter of weeks or months. In the cases reported by Hudson and colleagues, the condition, even after years, did not progress to spinal muscular atrophy, polyneuropathy, or amyotrophic lateral sclerosis. This conforms to our experience and to that reported from the Mayo Clinic where 121 patients with benign fasciculations, followed in some cases for more than 30 years, showed no progression of symptoms and did not acquire motor neuron disease or neuropathy (Blexrud et al). It should be acknowledged, however, that there are infrequent patients with seemingly benign fasciculations in whom the EMG shows some abnormal features (e.g., rare fibrillations) in numerous muscles and who later develop the other features of motor neuron disease. Carbamazepine, and to a lesser extent phenytoin and other antiepileptic drugs, has been helpful in reducing the fasciculations and sensations of weakness in a proportion of cases and numerous other medications have been reportedly helpful.
This is probably a variant of the above-described benign entity in which fasciculations are conjoined with cramps, stiffness, and systemic features such as exercise intolerance, fatigability, and muscle aches. Although affected individuals may be to some degree disabled by these symptoms, the prognosis is good. The salient finding on physiologic studies is that stimulation of peripheral nerves results in sustained muscle firing due to prolonged trains of action potentials in the distal motor nerve. This phenomenon may be brought out in special electrophysiologic testing, as described by Tahmoush and colleagues. In effect, this is a mild form of neuromyotonia, which is described further on. In a small number of patients with cramp-fasciculation syndrome it is possible to demonstrate the presence of autoantibodies directed against voltage-gated axonal potassium channels. Carbamazepine or gabapentin may be beneficial.
There are, in addition to these benign states, several syndromes of abnormal muscle activity. The main ones are myokymia, a state of successive contractions of motor units imparting an almost continuous undulation or rippling of the overlying body surface, and several syndromes of continuous muscle fiber activity described below.