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The clinical role of evoked potentials

2005; BMJ; Volume: 76; Issue: suppl_2 Linguagem: Inglês

10.1136/jnnp.2005.068130

1468-330X

P. J. Walsh,

Neural dynamics and brain function

euro-electric responses to sensory stimuli can be readily and non-invasively recorded using averaging techniques first employed by Dawson in 1947. 1 The evoked responses can be quantified by measuring peak amplitudes and latencies, in the millisecond (ms) domain, and they provide numerical data that are quantitative extensions of the neurological examination.The clinical utility of evoked potentials (EPs) is based on their ability to: c demonstrate abnormal sensory system conduction, when the history and/or neurological examination is equivocal c reveal subclinical involvement of a sensory system (''silent'' lesions), particularly when demyelination is suggested by symptoms and/or signs in another area of the central nervous system c help define the anatomic distribution and give some insight into pathophysiology of a disease process c monitor changes in a patient's neurological status.In theory almost any sensory modality may be tested, although in routine clinical practice pattern reversal visual evoked potentials (VEPs), short latency somatosensory evoked potentials (SSEPs), and brainstem auditory evoked potentials (BSAEPs) are tested most frequently.Longer latency responses that are related to higher ''cognitive'' functions such as event related potentials (ERPs), contingent negative variant (CNV), and sensory potentials after stimulation with CO 2 lasers are not routinely used in clinical practice and are beyond the scope of this article.EPs have the advantages of being objective, often more sensitive than detailed neurological examination, and they can be recorded in patients who are anaesthetised or comatose.The latter fact, along with improvements in recording equipment, has led to newer applications in the operating theatre and intensive care unit (ICU)-at a time when the role of EPs in the assessment of multiple sclerosis has largely been replaced by magnetic resonance imaging (MRI).A recent evidence based review recommended that VEPs are probably useful and SSEPs are possibly useful in identifying patients who are at increased risk for developing clinically definite MS, but that there was insufficient evidence to recommend BSAEPs. 2 Disadvantages of EPs in clinical practice are that they are rarely disease specific and can be confounded by end organ disease (for example, VEPs may be abnormal in ocular disease, SSEPs in patients with peripheral neuropathy, and BSAEPs in conductive and sensorineural deafness), are affected by age, and require a degree of patient cooperation to obtain artefact-free recordings. VISUAL EVOKED POTENTIALS: WHAT ARE THEY? cVEPs provide a sensitive indication of abnormal conduction in the visual pathway.Increases in retino-striate conduction time caused by processes such as demyelination can be detected by measuring the latency of this cortical response.Abnormalities in the amplitude and waveform of the VEPs may also be caused by the loss of axons in the pathway.VEPs are therefore widely used in the investigation of demyelinating disease, optic neuritis, and other optic neuropathies.The standard clinical test involves the recording of the pattern reversal VEPs.The visual stimulus is a high contrast black-and-white checkerboard spanning the central 20 ˚-30 ˚of the visual field whose black and white squares periodically exchange places.The VEP is the averaged response to this reversal.Normal responses to binocular and monocular ''full field'' stimulation are illustrated in the upper part of fig 1.The responses are recorded from three electrodes spanning the occipital region with a mid frontal electrode as the voltage reference.The signal at the midline occipital electrode normally contains a prominent positive component which occurs approximately 100 ms after the pattern reversal (called P100).It is usually preceded by a smaller negative component with a latency of about 75 ms (N75).The waveforms at the lateral electrodes are rather variable and so the latency of P100 at the midline electrode is taken as the measure of retino-striate conduction time.The cursors in fig 1 show the mean latency for this group of normal subjects ¡2.5 SDs, the range used in our laboratory to define the range of normal variation.