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Normal values are depending on used stimulation hardware flash stimulus vs. Auditory evoked potentials AEP can be used to trace the signal generated by a sound through the ascending auditory pathway.

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The evoked potential is generated in the cochlea, goes through the cochlear nerve , through the cochlear nucleus , superior olivary complex , lateral lemniscus , to the inferior colliculus in the midbrain, on to the medial geniculate body , and finally to the cortex. ERPs are brain responses that are time-locked to some "event", such as a sensory stimulus, a mental event such as recognition of a target stimulus , or the omission of a stimulus.

For AEPs, the "event" is a sound. AEPs and ERPs are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc. Brainstem auditory evoked potentials are small AEPs that are recorded in response to an auditory stimulus from electrodes placed on the scalp. AEPs serve for assessment of the functioning of the auditory system and neuroplasticity. They are recorded by stimulating peripheral nerves, most commonly the tibial nerve , median nerve or ulnar nerve , typically with an electrical stimulus.

The response is then recorded from the patient's scalp. Although stimuli such as touch, vibration, and pain can be used for SSEP, electrical stimuli are most common because of ease and reliability. Because of the low amplitude of the signal once it reaches the patient's scalp and the relatively high amount of electrical noise caused by background EEG , scalp muscle EMG or electrical devices in the room, the signal must be averaged. The use of averaging improves the signal-to-noise ratio. Typically, in the operating room, over and up to 1, averages must be used to adequately resolve the evoked potential.

The two most looked at aspects of an SSEP are the amplitude and latency of the peaks. The most predominant peaks have been studied and named in labs. Each peak is given a letter and a number in its name. For example, N20 refers to a negative peak N at 20ms. This peak is recorded from the cortex when the median nerve is stimulated. It most likely corresponds to the signal reaching the somatosensory cortex. When used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient's post-intubation baseline is a crucial piece of information.

Dramatic increases in latency or decreases in amplitude are indicators of neurological dysfunction. During surgery, the large amounts of anesthetic gases used can affect the amplitude and latencies of SSEPs. Any of the halogenated agents or nitrous oxide will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected.

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For this reason, an anesthetic utilizing less halogenated agent and more intravenous hypnotic and narcotic is typically used. Conventional SSEPs monitor the functioning of the part of the somatosensory system involved in sensations such as touch and vibration. The part of the somatosensory system that transmits pain and temperature signals is monitored using laser evoked potentials LEP.

LEPs are evoked by applying finely focused, rapidly rising heat to bare skin using a laser. In the central nervous system they can detect damage to the spinothalamic tract , lateral brain stem , and fibers carrying pain and temperature signals from the thalamus to the cortex. In the peripheral nervous system pain and heat signals are carried along thin C and A delta fibers to the spinal cord, and LEPs can be used to determine whether a neuropathy is located in these small fibers as opposed to larger touch, vibration fibers. Somatosensory evoked potentials provide monitoring for the dorsal columns of the spinal cord.

Sensory evoked potentials may also be used during surgeries which place brain structures at risk. They are effectively used to determine cortical ischemia during carotid endarterectomy surgeries and for mapping the sensory areas of the brain during brain surgery. Electrical stimulation of the scalp can produce an electric current within the brain that activates the motor pathways of the pyramidal tracts.

This technique is known as transcranial electrical motor potential TcMEP monitoring. This technique effectively evaluates the motor pathways in the central nervous system during surgeries which place these structures at risk. These motor pathways, including the lateral corticospinal tract, are located in the lateral and ventral funiculi of the spinal cord.

Since the ventral and dorsal spinal cord have separate blood supply with very limited collateral flow, an anterior cord syndrome paralysis or paresis with some preserved sensory function is a possible surgical sequela, so it is important to have monitoring specific to the motor tracts as well as dorsal column monitoring.

Transcranial magnetic stimulation versus electrical stimulation is generally regarded as unsuitable for intraoperative monitoring because it is more sensitive to anesthesia. Electrical stimulation is too painful for clinical use in awake patients. The two modalities are thus complementary, electrical stimulation being the choice for intraoperative monitoring, and magnetic for clinical applications. Motor evoked potentials MEP are recorded from muscles following direct stimulation of exposed motor cortex, or transcranial stimulation of motor cortex, either magnetic or electrical. During the s, there were attempts to monitor "motor evoked potentials", including "neurogenic motor evoked potentials" recorded from peripheral nerves, following direct electrical stimulation of the spinal cord.

It has become clear that these "motor" potentials were almost entirely elicited by antidromic stimulation of sensory tracts—even when the recording was from muscles antidromic sensory tract stimulation triggers myogenic responses through synapses at the root entry level.

Because MEP amplitude is correlated with motor excitability, they offer a quantitative way to test the role of various types of intervention on the motor system pharmacological, behavioral, lesion, etc. From Wikipedia, the free encyclopedia. For other uses, see VEP. See also: Event-related potential. Electrical potential in the nervous system. APA dictionary of psychology 2nd ed.

Evoked Potentials. 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.

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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.

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.

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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. 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.

Intraoperative Electrophysiological Monitoring (peripheral nerve disorders)

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.


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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. 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. To test the function of an individual sensory nerve root, dermatomal somatosensory evoked potentials DSSEPs can be recorded. The sensory dermatomes resulting from dorsal sensory nerve root innervation of the skin can be recorded by direct dermatomal electrical stimulation of the skin surface with surface electrodes and recorded as DSSEPs.

The stimulation and recording parameters used are the same as those for SSEPs. Practically, though, the reliability of DSSEPs in the diagnostic evaluation of patients with suspected radiculopathies is questionable. As a result, they should be regarded as investigational only. MEPs can be used to evaluate the functional integrity of the corticospinal tract system from the brain to the spinal cord or muscle.

This can be accomplished by either magnetic or electric stimulation of the brain. However, in the awake patient only transcranial magnetic stimulation TMS is done because the transcranial electric motor evoked potentials TcEMEPs induce a current in the brain through a large stimulus that would activate pain fibers within the scalp and thus would not be tolerated by an awake patient.

This stimulation produces multiple D direct waves that are the result of multiple action potentials within the descending pyramidal tract neurons. After this initial D wave are I indirect waves, which are the result of synaptic activation of further pyramidal neurons via interneurons. Because anesthetics predominantly take effect at synapses, the I waves would be lost and, hence, there would be no recordable MEPs under general anesthesia. These I waves must then synaptically transmit their impulses to the alpha motor neurons in the spinal cord, then to the peripheral motor nerve, and finally across the neuromuscular junction to muscle where compound muscle action potentials CMAPs can be recorded.

Firing of the alpha motor neuron requires the temporal summation of multiple excitatory postsynaptic potentials. This can be accomplished by the multiple I waves elicited by a single transcranial magnetic stimulus. Because these I waves depend on cortical synaptic function, they can be lost with cortical lesions. These, however, limit testing to the peripheral nervous system, including muscle, neuromuscular junction, peripheral nerve, plexus, and roots.

NCS involve the electrical activation of nerves to provide functional assessment about the peripheral nervous system in the form of conduction and axonal integrity. They may provide diagnostic, descriptive, or prognostic information and are accomplished through the application of a depolarizing square wave electrical pulse over a peripheral nerve, which results in activation of both the sensory and motor axons of the mixed nerve. A nerve action potential is propagated, which can be recorded at a particular distance over the nerve, as well as a CMAP, which can be recorded over the target muscle.

Components such as conduction velocity, amplitude, and shape are assessed. This is typically done with surface electrodes.


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The H wave is the electrical representation of a monosynaptic spinal reflex. It assesses the S1 nerve root, as it is only consistently obtained from recording the gastrocnemius-soleus muscles after stimulation of the tibial nerve in the popliteal fossa. It is the EMG equivalent of the ankle deep tendon reflex. The amplitude can be compared with the contralateral side.

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It is beneficial in that it assesses the function of the sensory root fibers, including the segment proximal to the dorsal root ganglion DRG. However, it can sometimes be normal in S1 radiculopathies, and its abnormality cannot always be attributed to the radiculopathy. It also can be abnormal in polyneuropathy cases and in patients older than 60 years.

F waves are a type of late motor response that may be able to be used to evaluate the motor nerve root. F waves are recorded from muscle after maximal stimulation of its nerve. When a motor nerve axon is stimulated, the action potential propagates in both directions so that an orthodromic potential can be directly recorded in muscle as a CMAP and an antidromic potential conducts proximally to the anterior horn cell. F waves initially were thought to be able to assess proximal nerve motor root segments inaccessible by conventional NCS.

However, they have been found to be insensitive to this. The nerves stimulated contain innervation from usually more than one root, and they are mediated along a pathway that extends muscle, nerve, root, plexus, and spinal cord; thus, an abnormality anywhere along this pathway could lead to an abnormal F wave. There is an exception in which both H waves and F waves may be clinically useful. In the case of an acute myelopathy, both F waves and H waves may disappear during the acute phase of spinal shock only to reappear thereafter.

EMG records the electrical activity in muscles.

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This is done by the insertion of a monopolar or concentric needle into a muscle and recording the observed and corresponding auditory response of spontaneous and recruited activity. It is used to differentiate myopathic from neurogenic disease and, by determining the distribution of neurogenic abnormalities, can localize the site of a lesion, such as differentiating nerve from plexus from radicular lesions. Spinal cord tumors can present with different clinical syndromes depending on the anatomical structures involved.

SSEPs can be used in the clinical diagnosis of neurological disease, in particular for multiple sclerosis. TMS can be used to measure central motor conduction time. Pyramidal tract dysfunction will result in slowing of central motor conduction time. MEPs are most often used in the assessment of multiple sclerosis or amyotrophic lateral sclerosis, conditions in which evidence of corticospinal tract involvement that may not be clinically evident is sought.

For spinal cord lesions for which the predominant injury is demyelination leading to slowing of central motor conduction time, TMS can be useful in assessing the level of functional impairment. Through appropriate muscle sampling, the level of the lesion can be localized to a particular root. Acute myelopathies may show absent F and H waves during only the spinal shock phase of a myelopathy and reappear thereafter. Radiculopathies are usually caused by root compression. They are the most common cause of referral to the EMG laboratory.

There are 31 pairs of spinal nerves attached to the spinal cord by dorsal sensory and ventral motor roots. The ventral roots originate from cells in the anterior and lateral gray columns of the cord; the dorsal roots originate from the DRG, which lie distal to the cord. The dorsal and ventral roots join to form spinal nerves.