The effect of cold on muscle contraction may also prove informative; either paresis or myotonia, lasting for a few minutes, may be evoked or enhanced by cold. This is most prominent in paramyotonia, but it may occur to some degree in all the other myotonic disorders. Also, a cold pack applied to a ptotic eyelid of myasthenia will often reduce the weakness.
Myotonia and myoedema must also be distinguished from the recruitment and spread of involuntary spasm induced by strong and repeated contractions of limb muscles in patients with mild or localized tetanus, with the “stiff man” syndrome and with dystonias of various types. These are not primary muscle phenomena but are neural in origin, a result of an abolition of inhibitory mechanisms and also taken up in Chap. 46.
In practice, the term contracture is applied (somewhat indiscriminately as discussed previously) to all states of fixed muscle shortening. Several distinct types can be recognized. In true physiologic contracture a group of muscles, after a series of strong contractions, remain shortened for many minutes because of failure of the metabolic mechanism necessary for relaxation. In this shortened state, the electromyogram (EMG) remains relatively silent, in contrast to the high-voltage, rapid discharges observed with cramp, tetanus, and tetany. True physiologic contracture occurs in McArdle disease (phosphorylase deficiency), phosphofructokinase deficiency, and possibly in other conditions, where phosphorylase seems to be present but nonfunctional. Yet another type of exercise-induced contracture, described originally by Brody, has been attributed by Karpati and coworkers to an autosomal recessive deficiency of calcium adenosine triphosphatase in the sarcoplasmic reticulum in type 2 muscle fibers. True contracture needs to be distinguished from paradoxical myotonia (see earlier) and from cramp, which in certain conditions (dehydration, tetany, pathologic cramp syndrome, amyotrophic lateral sclerosis [ALS]) can also be initiated by one or a series of strong voluntary muscle contractions.
It is appropriate here to comment on pseudocontracture (myostatic or fibrous contracture), for which the term contracture is used in general medicine. This is the common form of muscle and tendon shortening that follows prolonged fixation and inactivity of the normally innervated muscle (as occurs in a broken limb immobilized by a cast or weakness of a limb that is allowed to remain immobile). Here the shortened state of the muscle and tendons has no clearly established anatomic, physiologic, or chemical basis. Fibrosis of muscle, a state following chronic fiber loss and immobility of muscle, is another cause of muscle shortening. Depending on the predominant position, certain muscles are both weakened and shortened. Flexor fibrous contracture of the arms is a prominent feature of the Emery-Dreifuss form of muscular dystrophy. It also accounts for the rigidity and kyphoscoliosis of the spine, which are so frequently a part of myopathic diseases. The latter state is distinguished from ankylosis by the springy nature of the resistance, coincident with increased tautness of muscle and tendon during passive motion, and from Volkmann contracture, in which there is fibrosis of muscle and surrounding tissues as a result of ischemic injury, usually after a fracture of the elbow.