Obviously this important group of diseases, appearing as they do in the same periods of life as the congenital myopathies and of certain of the congenital muscular dystrophies, must figure in the differential diagnosis of early-onset muscle weakness, hypotonia, and of arthrogryposis. Indeed, they represent the main problems faced by the clinician studying neuromuscular diseases of the infant. Their hereditary nature, their progression to fatal outcome or delayed motor attainments, and their tendency in certain instances to produce disabling contractures are shared with the primary muscle diseases. The proper application of current laboratory techniques sets them apart in most instances. In deference to their neuronal origin, we have placed them with the other degenerative diseases in Chap. 38.
DISORDERS OF MUSCLE CHARACTERIZED BY CRAMP, SPASM, PAIN, AND LOCALIZED MASSES (SEE ALSO CHAP. 46)
Quite apart from spasticity and rigidity, which are caused by a disinhibition of spinal motor mechanisms, there are forms of muscular stiffness and spasm that can be traced to abnormalities of the lower motor neuron and its spinal inhibitory mechanisms or to the sarcolemma of the muscle fiber. Muscles may go into spasm because of an unstable depolarization of motor axons, sending volleys of impulses across neuromuscular junctions—as occurs in myokymia, hypocalcemic tetany, and pseudohypoparathyroidism. In other states, the innervation of muscle is normal, but contraction persists despite attempts at relaxation (myotonia). Or, after one or a series of contractions, the muscle may be slow in decontracting, as occurs in paradoxical myotonia and hypothyroidism. In the contracture of McArdle phosphorylase deficiency and phosphofructokinase deficiency, muscle, once contracted, lacks the energy to relax. In yet another type of muscle stiffness, the muscle may ripple or respond to percussion by mounding and rapidly contrasting and relaxing. Cramping should also be differentiated from restless leg syndrome (see Chap. 18), which is primarily a nocturnal disorder but may carry over into the daytime hours.
Each of these conditions evokes the complaint of cramp or spasm, which is variably painful and interferes with free and effective voluntary activity. Each condition has its own identifying clinical and EMG characteristics and most of them respond favorably to therapy.
Muscle Cramp (See Also Chap. 46)