8.1. Neurogenic Disorders
Let us take a situation in which axonal injury occurs to a nerve such that some, but not all of it, will regrow and eventually reinnervate a muscle it controls. For simplicity, this muscle is normally only innervated by three neurons. Figure 12A shows the initial state with a healthy muscle and nerve, with individual muscle fibers having a checkerboard pattern of innervation from three different motor neurons. Figure 12B shows the injured nerve and the associated denervated muscle shortly after the injury. The motor neuron has its basement membrane intact and can regrow, whereas the other two do not, and cannot regrow. The nerve is now not connected, and if a needle were placed in the muscle, marked spontaneous activity would be observed (as shown), but no MUPs would be identified. Figure 12C shows early reinnervation. Only one of the three axons in the nerve has managed to grow back, and it has attached to only three muscle fibers. If an EMG were performed at this stage, one motor unit would appear, but it would be made up of only three muscles and would be recruited very rapidly because there are no other motor neurons to assist. This unusual situation of a low-amplitude, short-duration motor unit with reduced recruitment is called a "nascent" MUP (a MUP in the process of being born). Gradually more muscle fibers will be added to the motor unit, including muscle fibers that are more distant that were not normally part of the initial unit. These developing nerve twigs are poorly myelinated (dotted lines) and are long and, hence, the resulting potential is demonstrated in Fig. 12D—polyphasic with low amplitude but long duration. In this example, there is a single satellite potential attached to the end of MUP; this potential represents the firing of a distant recently reinnervated muscle fiber. The recruitment remains reduced, because there are no other associated motor units able to contribute to the muscle contraction. Finally, in Fig. 12E, reinnervation is complete. The terminal nerve twigs are well-myelinated and a new, single, large motor unit has formed, now doing the work of nearly three motor units (not all of the fibers were successfully reinnervated in this example). This MUP has a larger amplitude, longer-duration, and, perhaps, some mild polyphasia; recruitment remains reduced. Should all of the fibers be successfully reinnervated, spontaneous activity will also normalize and no fibrillation potentials or PSWs will be identified.
In summary, not all neurogenic disorders are the same, and the appearance of the MUP will depend on the severity of the lesion, when in the course of recovery the neurophysiological study is being completed, and the relative success of reinnervation. Of course, if no reinnervation occurs (as does happen in some severe nerve injuries), than no MUPs will be identified and fibrillation potentials will persist in the muscle indefinitely.
Myopathic disorders also produce changing characteristics on needle EMG over time. Acute and subacute myopathies generally are identified by short-duration, low-amplitude MUPs, usually accompanied by fibrillation potentials and PSWs. The spontaneous activity itself is often of low amplitude (small fibrillation potentials). The reason for the MUP
changes is demonstrated in Fig. 13A and B, which shows a MUP before and after a myopathic process has begun. Several processes likely contribute to the development of small motor units, including destruction of muscle fibers associated with a given motor unit, truncation of muscle fibers leading to smaller individual muscle fiber action potentials, and destruction of intramuscular nerve twigs by inflammation. Ischemia and other pathological processes may also play a role. The result, however, is that the number of individual motor units is preserved, but their amplitude and duration are markedly reduced. In addition to these changes, the force-generating capacity of a single motor unit will also decrease, making it necessary to recruit additional motor units earlier than normal to produce even small movements of the extremity. Hence, short-duration, low-amplitude motor units with "early" recruitment—many motor units participating to produce small degrees of force—are seen in subacute or ongoing myopathy, as detailed above in Section 7.4.
As a myopathic condition persists and there is ongoing destruction of muscle fibers and some motor nerve terminals are lost, a gradual degree of reinnervation occurs, such that motor neurons sprout twigs to still-functioning individual muscle fibers. In time, the previous clear myopathic picture may become more and more clouded by the presence of some occasional large motor units. In some situations, the myopathic motor units become almost impossible to find as they are replaced by a few, usually very polyphasic, high-amplitude, long-duration units. If there are muscle fibers or segments of muscle fiber that remain without reinnervation, then ongoing denervation will persist.
Muscle diseases are either hereditary or acquired. Hereditary muscle diseases, including the muscular dystrophies, can show a more chronic pattern of large MUPs mixed with short-duration, low-amplitude, polyphasic MUPs. This reflects the chronicity of the disease process, which, in adult cases, is longstanding. Acquired muscles diseases, especially inflammatory myopathies such as dermatomyositis or polymyositis, typically have short-duration, low-amplitude MUPs with prominent denervating features. In contrast to there inflammatory myopathies, inclusion body myositis, an acquired long-standing disease, characteristically has a mixture of large and small MUPs and prominent fibrillation potentials and positive sharp waves.
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