This is an interrelated group of clinical states, all characterized by some degree of regional continuous muscular activity that in some cases cannot be fully differentiated from one another. From a clinical perspective we have found it is useful to categorize them in groups that are caused by (1) hyperexcitability of the peripheral motor nerves (fasciculations and myokymia), (2) centrally mediated hyperexcitability of motor output (Isaacs syndrome, stiff man syndrome), and (3) nonmyotonic hyperexcitability of muscle (rippling muscle disease, Schwartz-Jampel syndrome).
Hyperexcitability of Peripheral Nerve
This comprises a set of disorders in which peripheral motor nerve activity is augmented such that there are excessive, sometimes sustained contractions of the motor unit. Its mildest manifestation is benign fasciculation. In more severe form the manifestations include neuromyotonia, Isaacs disease, and a disease of potassium-gated ion channels (Morvan disease, or Morvan fibrillary chorea discussed below) that may also involve the brain. These processes are not generally familial and several lines of investigation suggest an acquired autoimmune nature (Newsom-Davis). For example, all but benign fasciculations are associated more often than might be expected with other autoimmune diseases such as myasthenia gravis and some respond to autoimmune therapies such as plasmapheresis, the patient’s serum possesses antibodies to either voltage-gated potassium channels as mentioned or, less frequently, to nicotinic acetylcholine (ACh) receptors (Vernino and Lennon).
A few random fasciculations in the muscles of the calf, small muscles of the hand or of the face, or elsewhere are seen in most normal individuals. They are of little significance but can be a source of worry to physicians and patients who have read that fasciculations are an early sign of amyotrophic lateral sclerosis. A simple clinical rule is that fasciculations in relaxed muscle are not indicative of motor system disease unless there is associated weakness, atrophy, or reflex change.
Healthy individuals experience intermittent twitching of a muscle (or even part of a muscle), such as one of the muscles of the thenar eminence, eyelids, calves, or orbicularis oculi. They may continue for days. Electromyographically, benign fasciculations tend to be more constant in location and more frequent and rhythmic than the ominous fasciculations of amyotrophic lateral sclerosis but such distinctions are not entirely reliable. The physiology of fasciculations is discussed in Chap. 2 and illustrated in Fig. 2-20. Quantitative study of the motor unit size may be helpful in these circumstances by demonstrating normally modeled units in the benign form and abnormally large units because of reinnervation in the case of motor neuron disease.