You are in: eMedicine Specialties > Neurology > Electroencephalography and Evoked Potentials Somatosensory Evoked Potentials: Clinical ApplicationsArticle Last Updated: Sep 28, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 6
  Author: Jorge G Burneo, MD, MSPH, Assistant Professor of Neurology, Epidemiology, and Biostatistics, Department of Clinical Neurological Sciences, Epilepsy Program, London Health Sciences Centre, University of Western Ontario, Canada Jorge G Burneo is a member of the following medical societies: American Academy of Neurology and American Epilepsy Society Coauthor(s): Gregory L Barkley, MD, Director of the Comprehensive Epilepsy Program, Department of Neurology, Henry Ford Health System; Associate Professor, Case Western Reserve University Editors: Donald B Sanders, MD, EMG Laboratory Director, Professor of Medicine (Neurology), Division of Neurology, Duke University Medical Center; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Norberto Alvarez, MD, Assistant Professor, Department of Neurology, Harvard Medical School; Consulting Staff, Department of Neurology, Boston Children's Hospital; Selim R Benbadis, MD, Professor, Director of Comprehensive Epilepsy Program, Departments of Neurology and Neurosurgery, University of South Florida School of Medicine, Tampa General Hospital; Nicholas Y Lorenzo, MD, Chief Editor, eMedicine Neurology; Consulting Staff, Neurology Specialists and Consultants Author and Editor Disclosure Synonyms and related keywords: SEP, SEPs, SEP waveform INTRODUCTIONSection 2 of 6
  Somatosensory evoked potentials (SEPs) are generated by stimulation of afferent peripheral nerve fibers by either physiological or electrical means. SEPs were first recorded more than 40 years ago. For an explanation of their physiologic and anatomic basis and description of techniques, see Somatosensory Evoked Potentials: General Principles. In this article, the focus is on the clinical applications of SEPs. SEPs can be recorded after physiological stimuli (eg, muscle stretch). However, electrical stimulation usually is administered to elicit the potential. Typically, a square wave of 0.2- to 2-millisecond duration is delivered to a peripheral nerve by electrodes, usually surface electrodes. For intraoperative monitoring, needle electrodes are used for stimulation since they require smaller currents, which reduce the stimulus artifact. The usual sites for SEP stimulation are the median nerve at the wrist, the common peroneal nerve at the knee, and the posterior tibial nerve. A SEP also may be recorded by stimulating the skin in various dermatomal areas, but the response is much weaker. In mixed peripheral nerves, the threshold for sensory perception is lower than the threshold to elicit movement. For stimulation of mixed peripheral nerves, the stimulating current is adjusted to produce a minimal movement of the joint involved. This stimulation intensity typically is well tolerated by patients. Recording electrodes are placed on the scalp and over the cervical spine. For recording upper extremity SEPs, electrodes are placed over the Erb point. For recording lower extremity SEPs, electrodes are placed over the lumbosacral spine. Waveforms are described in terms of morphology, amplitude, and dispersion. Each laboratory should establish reference values for latencies and interpeak latencies that are based on a patient's age and height. Because limb cooling affects peripheral nerve conduction velocity, minimum skin temperature norms should be established for each laboratory. Responses recorded are classified according to specific latencies. "Short-latency" SEPs refer to the portion of the SEP waveform that occurs within 25 milliseconds after stimulation of the upper extremity nerves, 40 milliseconds after stimulation of the peroneal nerve, or 50 milliseconds after stimulation of the tibial nerve. "Long-latency" refers to the waveforms recorded more than 100 milliseconds following stimulation of these nerves. "Middle-latency" SEP refers to waveforms that occur between these 2 periods. Mixed nerve stimulation has become the standard for clinical use. Other methods include cutaneous nerve stimulation, dermatomal stimulation (which is more specific than cutaneous nerve stimulation), motor point stimulation, and paraspinal stimulation. CLINICAL INTERPRETATIONSection 3 of 6
The SEP stimulus preferentially excites only the largest myelinated fibers in the peripheral nerve. These include cutaneous and subcutaneous fibers, somesthetic and proprioceptive fibers, and motor axons of equivalent diameter. The stimulus activates predominantly the large-diameter, fast-conducting group in muscles and group II cutaneous fibers. Stimulation produces an action potential that travels up the axon toward the spinal cord and past the cell bodies of the sensory axons of the large-fiber sensory system in the dorsal root ganglia to the ipsilateral posterior columns of the spinal cord. These signals then traverse a synapse in the dorsal column nuclei at the cervicomedullary junction. The signals travel in second-order neurons to the ventroposterolateral nucleus of the thalamus (VPL) via the medial lemniscus. In the VPL, a synapse is made with a third-order neuron that travels to area 3b of the parietal sensory cortex. SEPs provide information concerning the integrity of the pathway through the brain, brain stem, spinal cord, dorsal roots, and peripheral nerves. CLINICAL USESSection 4 of 6
Clinical uses of SEPs include the following:
Although SEP findings are not disease specific, they can confirm afferent conduction impairment associated with certain disorders. Peripheral nervous system SEPs may be used in evaluation of the peripheral nervous system when traditional nerve conduction studies (NCSs) are not possible (for any reason) or are not reliable (eg, technical problems, or artifacts). Peripheral neuropathy: SEPs rarely are used to assess peripheral neuropathy since standard NCSs are the test of choice. The stimulation is applied at 2 or more sites and the responses are recorded over the scalp. In the presence of polyneuropathies and mononeuropathies, SEP waveforms recorded over the scalp may be absent or show delayed latencies with normal central conduction velocities. In this way, SEPs can be used to measure the afferent fiber conduction velocities of proximal segments. Higher stimulation currents typically are required in patients with peripheral neuropathies. Use of SEPs has been reported for the following peripheral nerve disorders:
Focal neuropathy: The test of choice in focal neuropathy is standard NCSs. Entrapment neuropathies, such as carpal tunnel syndrome, may be found incidentally when SEPs are recorded. The use of SEP for detection of saphenous neuropathy, intercostal neuropathy, and trigeminal neuropathy has been reported. However, standard NCSs are the preferred diagnostic test for these conditions. Plexopathy: SEPs are useful for evaluation of brachial plexopathy and traumatic plexopathies. In thoracic outlet syndrome, SEPs are of limited value with regard to the neurogenic variety of plexopathy and have no established value in diagnosis of the nonneurogenic (ie, vascular) variety. The value of SEPs in preventing or minimizing intraoperative damage of the peripheral nervous system is unproven. Ventral rootlets and roots: Recent studies suggest that SEPs may have some utility in the evaluation of rootlet and root dysfunction. However, needle electromyography (EMG) provides superior information in these disorders and remains the test of choice. Lumbosacral root disease: SEPs may have some utility in the evaluation of acute lumbosacral root disease or in lumbosacral spinal stenosis. Thoracic root disease: No data are available. Cervical root disease: EMG is the best neurophysiological tool for evaluation of this condition. SEPs may or may not have a limited role in these conditions. Spinal cord Trauma: Scalp-recorded potentials are absent in complete lesions of the cervical spinal cord. However, incomplete lesions yield varying abnormalities on SEPs. SEPs can help localize the sensory level in trauma and also may help in determining prognosis for functional outcome. Surgical monitoring: SEP monitoring is used widely to assess integrity of the spinal cord during surgery in which the spinal cord is manipulated. SEPs are used most often during scoliosis correction. Ischemia of ascending somatosensory pathways produces a drop in amplitude or loss of waveforms, thus warning the surgeon in time to take corrective action. Ischemic changes are usually widespread; rarely, motor function is lost when somatosensory pathways have not been affected. Other spinal procedures in which SEP monitoring is used include reduction of vertebral fractures and other vertebral injuries and spinal cord tumor surgery. Conditions for which SEPs might be useful
Brain stem and brain Multiple sclerosis: SEPs of the upper extremities are abnormal in about 60% of all patients with MS (definite, probable, and possible) and in about 40% of patients with asymptomatic MS. SEPs of the lower extremities have a higher frequency of abnormality, presumably since longer axons are more likely to have an area of demyelination. In clinical practice, SEPs of both the upper and lower extremities usually are performed. The change most frequently seen in MS is prolongation of central latencies. MRI has largely supplanted SEP testing in MS. Other demyelinating diseases: Abnormalities in SEPs are seen in adrenoleukodystrophy, adrenomyeloneuropathy, metachromatic leukodystrophy, heterozygotes for adrenoleukodystrophy and adrenomyeloneuropathy. Coma: SEPs are useful in the evaluation of a comatose patient, not only for diagnostic purposes but also for prognosis. Central conduction times are not affected significantly by level of consciousness, hypothermia, or sedative-hypnotic medications such as barbiturate anesthesia. Thus, SEPs are often useful in conditions in which clinical examination is of limited value. A favorable outcome is expected in about 53% of comatose patients with hypoxic-ischemic encephalopathy and in 49% of patients with traumatic brain injury coma who have preservation of bilateral cortical SEP responses. The absence of cortical responses bilaterally is associated with an unfavorable prognosis (ie, an outcome of persistent vegetative state in the vast majority of patients). Unilateral preservation of cortical SEP responses offers hope for a favorable outcome. Brain death: In brain death, peripheral SEP components are preserved. However, potentials generated by structures above the lower medulla are absent. Surgical monitoring techniques Intraoperative SEPs are recorded routinely using electrode strips or grids or electrodes applied directly to the exposed cortex during neurosurgical procedures in and around the somatosensory cortex. The N20 response is used to identify the primary somatosensory cortex in the postcentral gyrus. By inference, the motor cortex in the precentral gyrus can be localized as well, and this information is used to guide the surgical procedure. Scalp-recorded SEPs also have been used to monitor cerebral ischemia during vascular surgery, such as carotid endarterectomy; however, EEG monitoring is used more widely for this application. Somatosensory response can be measured by magnetoencephalography. The generators of the N20m and later waveforms can be modeled as equivalent current dipoles. Usually, individual digits and the lip are stimulated, and the Degenerative disorders In Friedreich ataxia, SEPs recorded from the scalp usually show normal peripheral nerve conduction latencies but prolonged central conduction times. Degenerative disorders of the brain stem, such as spinal cerebellar degeneration, usually show a similar SEP pattern. In Huntington disease, latencies are normal but the amplitudes may be low (similar to the low-voltage pattern found in the EEG). SEPs are normal in many degenerative disorders, including Parkinson disease, Alzheimer disease, and amyotrophic lateral sclerosis (particularly early in the course of the disease). An unusually large amplitude response is seen in the SEPs of patients with progressive myoclonic epilepsy. In cerebrovascular disease or head trauma, abnormal SEPs are recorded when infarction, hemorrhage, or direct injury disrupts the integrity of ascending somatosensory pathways. SEPs are unaffected and normal Other disorders SEPs have been used to confirm the presence of normal conduction pathways in patients with conversion disorder, malingering, or other psychological disturbances. Findings must be interpreted carefully, because normal SEPs can be seen in patients with "organic" sensory loss. For example, some patients with pure sensory strokes due to lacunar infarcts may have normal SEPs. MULTIMEDIASection 5 of 6
REFERENCESSection 6 of 6
Somatosensory Evoked Potentials: Clinical Applications excerpt Article Last Updated: Sep 28, 2006 | |||||||||||||||||||||||||||||||||
