In most myopathic conditions, spontaneous activity is increased, although it is most prominently increased in inflammatory or necrotizing myopathic processes. The presence of myotonic discharges can be very helpful in limiting the differential diagnosis. Complex repetitive discharges, which are present in myopathic conditions, are nonspecific. Motor unit potentials in myopathy are generally of low-amplitude and short duration, except in very chronic conditions, in which the motor unit potentials can actually become of long duration and high amplitude.
Finally, it is critical to remember that an entirely normal EMG does not exclude the presence of myopathy. Unable to display preview. Download preview PDF. Skip to main content. Advertisement Hide. This is a preview of subscription content, log in to check access. Aminoff MJ. Electromyography in Clinical Practice, 3rd ed. Google Scholar. Ball RD. Basics of needle electromyography. Electromyography in idiopathic myositis. Mt Sinai J Med ;55 6 : — PubMed Google Scholar. Daube JR. AAEM minimonograph needle examination in electromyography. Muscle Nerve ; — Electrodiagnosis of muscle disorders.
Pathology of skeletal muscle.
Am J Pathol ; — Dumitru D. In: Electrodiagnostic Medicine Dumitru D, ed. Hanley and Belfus, Philadelphia, PA Essentials of Neural Science and Behavior.
Depending on the pattern of affected muscles, it is possible to distinguish between radiculopathies, plexopathies, and neuropathies and also to determine whether a neuropathy involves one or several nerves. A specific etiologic diagnosis cannot be made by the electrophysiologic findings, however. The configuration of motor unit potentials is helpful in determining the duration of nerve injury and in indicating whether reinnervation is occurring.
Immediately after acute denervation, the number of motor units is reduced to a level that depends on the extent of the lesion no motor units are activated if the muscle is denervated completely , but surviving motor unit potentials are unchanged in configuration. If axon loss has occurred as opposed to conduction block with moderate nerve injury, subsequent reinnervation occurs first by noninjured axons sending out collateral sprouts. Thus, over 1 to several months after injury, motor units become larger.
With severe nerve injuries or transection, subsequent reinnervation requires axonal regeneration; reinnervated motor units initially appear as small, short-duration, polyphasic motor unit potentials, which then evolve to longer, larger potentials as more muscle fibers come to be reinnervated.
Electrodiagnosis in Diseases of Nerve and Muscle. Principles and Practice
Long-duration, high-amplitude, polyphasic motor unit potentials are characteristic of reinnervation after a denervating lesion table 1. The electromyographic findings may thus provide a guide to the time of onset of the lesion and to its chronicity, and this may have medicolegal implications. If a patient reports that a wrist drop has developed immediately after an operative procedure and needle electromyography performed shortly thereafter reveals abnormal spontaneous activity fibrillation potentials and positive sharp waves in the extensor muscle of the wrist, it is likely that the lesion is at least 1—3 weeks old and therefore that it preceded the surgery.
Similarly, the presence of long-duration, large-amplitude, polyphasic motor unit potentials indicates that the denervation occurred several months or more before surgery because some reinnervation has occurred. Nerve conduction studies permit assessment of function in motor and sensory nerves. For motor studies, the nerve is stimulated supramaximally at two points or more along its course, and a recording is made of the electrical response of one of the muscles that it innervates.
This permits conduction velocity to be determined in the fastest-conducting fibers to that muscle. The size of the muscle response i. An abnormal reduction in size of the response with stimulation of the nerve at one point along its course, compared with stimulation at a more distal site, may be indicative of conduction block, acutely evolving axon loss, or anomalous innervation in which some nerve fibers follow an aberrant course to reach their target. Sensory conduction studies typically involve stimulating supramaximally the nerve fibers at one point and recording the nerve action potentials from them at another.
The latency of the response can be measured and, if desired, converted to a conduction velocity, and the size of the sensory nerve action potential can also be recorded as a reflection of the number of functioning sensory axons. Nerve conduction studies are an important means of evaluating the functional integrity of peripheral nerves.
They enable a focal nerve lesion to be localized in patients with a mononeuropathy. Localized peripheral nerve damage leads to evoked motor or sensory responses that are reduced or change abnormally in amplitude depending on the site of stimulation and recording; conduction velocity may also be slowed. Nerve conduction studies combined with needle electromyography can determine whether a nerve injury is complete or incomplete and thus guide prognosis and the likely course of recovery. With a complete lesion, motor units cannot be activated volitionally in a distal muscle, and, if axonal loss has occurred, fibrillation potentials and positive waves are found on needle examination after an appropriate interval that varies with the site of injury and recording ; electrical stimulation of the nerve above the lesion does not elicit a response in muscles supplied by branches arising distal to a complete lesion, or it elicits a smaller response with a partial injury.
In patients presenting with a mononeuropathy, nerve conduction studies may reveal the presence of a subclinical polyneuropathy that has made the individual nerves more susceptible to injury. In patients with multiple affected nerves, such studies can distinguish between a polyneuropathy in which there is symmetrical involvement of multiple nerves at the same time, usually in a length-dependent manner and mononeuropathy multiplex in which involvement of several nerves occurs, usually noncontiguously and at different times , which is important because different causes are likely to be responsible.
Finally, nerve conduction studies may suggest whether the underlying pathologic process is axon loss or demyelination, which has important implications regarding clinical course and prognosis. Axon loss is characterized electromyographically by signs of denervation, and nerve conduction studies reveal small or absent compound muscle or sensory nerve action potentials, with little or no change in conduction velocity while this can be measured.
Demyelination, by contrast, is manifest by markedly slowed nerve conduction velocities. Conduction block may also occur: Some or all of the axons in the nerve become unable to transmit impulses through a segment of nerve but can function more distally. Stimulation proximal to the block then leads to a smaller muscle response or no response at all than when the nerve is stimulated distally. Other techniques for evaluating neuromuscular function have been developed over the years. These include repetitive nerve stimulation or single-fiber electromyography to evaluate neuromuscular transmission, quantitative electromyographic techniques, late-response studies F-wave or H-reflex studies and recording of somatosensory evoked potentials to detect proximal pathology, and various techniques to evaluate reflex function.
These are beyond the scope of the current article, but monitoring of somatosensory evoked potentials is sometimes helpful for preventing intraoperative damage to neural structures, especially the spinal cord. Electromyography and nerve conduction studies provide helpful information for anesthesiologists in several settings. They are helpful in determining the basis of any clinical deficit, in localizing the responsible lesion, and in defining its severity and prognosis. They do not indicate directly the cause of the injury, although the location and age of the lesion and underlying pathologic process axon loss or demyelinative changes may help to distinguish between various possibilities.
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As mentioned previously, the mechanism of perioperative nerve injury is sometimes obscure. Injury may certainly result from compression of nerves occurring while the patient is anesthetized and receiving muscle relaxants, and proper positioning of patients is therefore imperative. Ulnar or radial neuropathies in the arm are particularly common in this context, and the peroneal nerve may be compressed against the fibular head.
Other nerves are involved less commonly.
Individual peripheral nerves may also be injured by direct injury, as from intraneural injection of local anesthetics or other substances, or by the placement of a tourniquet to limit blood flow to the limb. In these situations, electrodiagnostic studies are important in localizing the lesion and defining the prognosis. Mechanical damage is probably the major cause of injury in tourniquet paralysis, but ischemia may be contributory.
Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice - Oxford Medicine
In the upper limb, several nerves are usually affected by tourniquet injuries, with the radial occasionally affected in isolation; in the legs, the sciatic nerve is affected most often. Electrodiagnostic studies typically reveal a focal conduction block in affected nerves and have sometimes localized the lesion to the upper or lower edge of the tourniquet. It may be difficult, particularly for nonneurologists, to distinguish clinically between, for example, a peroneal or sciatic neuropathy and lumbar radiculopathy, all of which may lead to foot drop in the perioperative period, or between a radial neuropathy and a cervical radiculopathy that is causing wrist drop.
Clinical definition and localization of a peripheral nerve lesion may be especially difficult when selective nerve fascicles are injured, leading to an atypical or incomplete presentation. In particular, they indicate which muscles have been affected, clarify the site of the lesion, and may localize any dysfunction with precision to a short segment of peripheral nerve. The electrophysiologic findings are also helpful in determining the underlying pathologic process and thus the prognosis.
In patients with mild lesions, segmental demyelination is typically responsible, and recovery is then likely to occur quickly and completely. By contrast, if axonal loss has also occurred, evidence of denervation can be found if the examination is conducted at a suitable time after onset of the lesion, as indicated above , and recovery may be delayed and incomplete. With mixed lesions, the neurapraxic component typically recovers quickly, but the axonal-loss component requires longer for recovery to occur.
Recordings from the abductor digiti minimi muscle to show the likely changes with an ulnar nerve lesion at the elbow. The location of a lesion may be important in determining the likely underlying cause. For example, the development of acute foot drop may be attributed clinically to a nerve injury as a result of sciatic nerve block, but electrophysiologic evidence of a focal lesion at the head of the fibula would make this unlikely.
The optimal timing of the electrodiagnostic examination depends on the reason that it is undertaken. In a patient with postoperative reports of weakness or sensory changes, electrophysiologic evaluation even in the first 2 or 3 days may provide useful information. At this early time, the examination can help to determine whether a nerve lesion is indeed present as evidenced by a reduced recruitment of motor units in involved muscles.
The presence of at least some motor units under voluntary control shows that any such lesion is incomplete; this implies a more favorable prognosis than otherwise in patients with an apparently complete lesion clinically. The presence of abnormal spontaneous activity fibrillation potentials and positive waves at this time indicates that a long-standing lesion is present, as does a small muscle response to distal nerve stimulation table 1.
This is of medicolegal importance, suggesting either that perioperative nerve injury is not responsible for the findings and perhaps also for the clinical deficit or that any perioperative injury was superimposed on a longstanding lesion that may have made the nerve more susceptible to injury. More information is provided if the examination is repeated approximately 4 weeks after injury, when adequate time has elapsed for the electrophysiologic changes to have evolved more fully.
At this time, more definitive information can be obtained about the site, nature, and severity of the lesion, which can guide prognostication. Serial studies are generally not required because progress can be followed clinically, unless patients have a clinically complete axon-loss lesion that is seemingly not improving and surgical repair is a consideration.
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In this latter circumstance, serial electrophysiologic studies every 3 months may then be worthwhile: Needle electromyography may indicate whether recovery is occurring, because voluntary motor unit activity reappears before any clinical signs of recovery. In recent years, intraoperative recordings from peripheral nerves by similar techniques to those used in nerve conduction studies have proved useful in the surgical management of nerve injuries. Recording intraoperatively has facilitated the identification of individual nerves, the determination of whether they are in continuity, and the localization of damage to a specific site.
When a nerve has been identified but its continuity is uncertain, the failure of stimulation to elicit a muscle response may reflect conduction block or nerve transection or a lack of proximity to the nerve. Mechanical stimulation e.
Loss of such responses may indicate that the nerve has been injured, and in this circumstance, the response to electrical stimulation should be assessed. By stimulating or recording at different sites along the course of a nerve, the site of damage can be localized precisely. For example, the ulnar nerve can be monitored by stimulating it directly and recording action potentials from the nerve itself or from a muscle supplied by the nerve.
Intraoperative monitoring has also helped in the early recognition of nerve damage caused by surgery close to limb or cranial nerves so that the ongoing surgical procedure can be modified before damage is irreversible. Therefore, it is common to monitor the facial nerve during surgery in the cerebellopontine angle e. Depending on the operative field, other cranial or spinal nerves may also be monitored: the cranial nerves to the extraocular muscles during surgery on the cavernous sinus, the lower cranial nerves during surgery on the skull base, and the spinal nerve roots during spinal surgery.