Such large and sometimes giant polyphasic potentials (Fig. 2-20C) arise from motor units containing more than the usual number of muscle fibers that are spread out over a greatly enlarged territory within the muscle. Presumably, new nerve twigs have sprouted from nodal points and terminals of undamaged axons and have reinnervated previously denervated muscle fibers, thus adding them to their own motor units. Soon after reinnervation, the MUPs generated will be low in amplitude, extremely prolonged, and polyphasic, findings that constitute a transitional configuration of early reinnervation. These amplitudes disappear as the motor unit is reestablished. Increased amplitude is usually associated with very chronic, proximal axon loss, for example, with remote poliomyelitis and chronic radiculopathy. These MUPs are to be differentiated from (1) polyphasic potentials of normal duration, which, as has been mentioned, make up as much as 10 percent of the total number of MUPs in normal muscle, and (2) polyphasic MUPs of short duration and low amplitude, which are characteristic of most myopathies and of myasthenia gravis and other disorders of neuromuscular transmission.
The motor unit potential in myopathy As Fig. 2-20B shows, diseases such as polymyositis, the muscular dystrophies, and other myopathies that randomly destroy muscle fibers or render them nonfunctional, and obviously reduce the population of muscle fibers per motor unit. Therefore, when such a unit is activated, its potential is of lower voltage and shorter duration than normal, and it may also appear polyphasic as the compound MUP becomes fragmented into its constituent single-fiber potentials. Slowing of the propagated muscle fiber action potential in affected muscle fibers also contributes to the changes in the “myopathic” MUP. When most of the muscle fibers are affected, the MUPs are very small and of short duration and are recruited out of proportion to the tension generated, the so-called early recruitment. Both types of alterations produce a characteristic high-pitched crackling sound from the audio monitor that has been likened to rain falling on a tin roof. They occur in all forms of chronic myopathies. Identical MUP changes are seen occasionally with other processes that cause disintegration of the motor unit, for example, early Guillain-Barré syndrome (because of conduction block along some of the terminal nerve fibers), and rarely with disorders of neuromuscular transmission (myasthenia gravis, other myasthenic syndromes), but they are most characteristic of primary muscle disease.
Abnormalities of the interference pattern Diseases that reduce the population of functional motor neurons or axons within the peripheral nerve decrease the number of motor units that can be recruited in the affected muscles. The decreased number of motor units available for activation produces a low-amplitude interference pattern with only a few remaining units firing at a moderate to rapid rate. A severe reduction in the interference pattern may result in the recruitment of only a single unit (see Fig. 2-17B). Structural damage to nerve, as well as demyelinating block, can produce this pattern of reduced recruitment; indeed, a reduced recruitment pattern coupled with the absence of denervation usually indicates a conduction block.
If muscle power is reduced in diseases such as polymyositis or muscular dystrophy, in which individual muscle fibers are affected, there may be little or no reduction in the number of motor units available for recruitment until the process is far advanced and entire MUPs have been lost as a result of random loss of all their constituent muscle fibers. Nonetheless, each motor unit will consist of fewer muscle fibers than normal, so more motor units must be activated to reach a certain degree of force. A modest effort can thus produce a full interference pattern despite marked weakness (increased recruitment). Because fewer muscle fibers are firing, the amplitude of the pattern will be reduced from normal. This type of full, highly complex interference pattern of less-than-usual amplitude in the face of dramatic weakness is the hallmark of myopathy (see Fig. 2-17C).