Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android


Complaints referable to muscle such as pain, spasm, stiffness, fatigue and/or abnormal movements within a muscle are commonplace in the practice of medicine. As the cause is often elusive, both patients and physicians may become frustrated as many with these complaints will remain undiagnosed despite thorough investigation. There are many sources for this diagnostic elusiveness. With the exception of cramps and fasciculations, the disorders described in this chapter are uncommon. In addition, most of the disorders that will be described have nonspecific and overlapping clinical features. Successful diagnosis requires a heightened index of clinical suspicion, detailed knowledge concerning each disorder's phenotypic characteristics, and awareness of the serologic and electrodiagnostic (EDX) features of each syndrome.

Motor nerve hyperactivity disorders frequently result in reduced exercise intolerance and impaired mobility. They originate from numerous central and neuromuscular system localizations. This chapter will restrict itself to disorders thought to originate from motor nerves and the upper motor neurons that control them. Cramps, fasciculations, tetany, tetanus, the cramp–fasciculation syndrome (CFS), Isaacs syndrome (IS), Satoyoshi syndrome, stiff person syndrome (SPS) and hyperekplexia will all be discussed. As the differential diagnosis of many of these disorders overlaps, the majority of the differential diagnostic considerations will be primarily emphasized in the first section devoted to muscle cramping.

Historical writings on motor nerve hyperactivity disorders have been potentially confusing. Different names have been used for the same syndrome. Nomenclature to describe clinical observations has overlapped with that used to describe frequently associated electromyographic waveforms. In an attempt to avoid this, and as neurologists deeply appreciative of history and those who created it, we will be preferentially referring to these syndromes by their eponyms whenever appropriate. We will also follow the suggestion of Gutmann et al.1 by using a single term (e.g., fasciculations, myokymia) to refer to clinically observed phenomenon and refer to EDX waveforms as potentials or discharges (e.g., fasciculations potentials, myokymic discharges).



Cramps refer to a sudden, involuntary, and painful shortening of an entire muscle belly accompanied by a squeezing sensation and visible, palpable muscle induration. As cramps tend to incorporate multiple if not all the motor units in one or more muscles, they typically generate sufficient force to induce abnormal posturing of relevant joints. Cramps are characteristically relieved by massage or stretching. They have a tendency to recur if the muscle is prematurely returned to its unstretched position. They will spontaneously remit within minutes in most cases.

Cramps occur commonly. Their prevalence in a "normal" population is estimated at 35% in one study and in 95% of young, healthy people who recently initiated exercise in another.2,3 Their prevalence is increased in the elderly, in pregnant females, and subsequent to exercise in those who have recently begun unaccustomed activity. Cramps are a considerable source of ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.