Neurophysiologie clinique = Clinical neurophysiology
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To be considered specific for nociception, a cortical region should: (a) have plausible connections with ascending nociceptive pathways; (b) be activated by noxious stimuli; (c) trigger nociceptive sensations if directly stimulated; and (d) tone down nociception when injured. In addition, lesions in this area should have a potential to develop neuropathic pain, as is the case of all lesions in nociceptive pathways. The single cortical region approaching these requirements in humans encompasses the suprasylvian posterior insula and its adjoining medial operculum (referred to as "PIMO" in this review). ⋯ Thus, spinothalamic sub-modalities may be partially segregated in the PIMO, in analogy with the separate representation of dorsal column input from joint, muscle spindle and tactile afferents in S1. Specificity, however, may not wholly depend on ascending 'labelled lines' but also on cortical network properties driven by intrinsic and extrinsic circuitry. Given its particular anatomo-functional properties, thalamic connections, and tight relations with limbic and multisensory cortices, the PIMO region deserves to be considered as a third somatosensory region (S3) devoted to the processing of spinothalamic inputs.
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Neurophysiological techniques help in diagnosis, prognosis and treatment of chronic pain, and are particularly useful to determine its neuropathic origin. According to current standards, the diagnosis of definite neuropathic pain (NP) needs objective confirmation of a lesion or disease of somatosensory systems, which can be provided by neurophysiological testing. Lesions causing NP mostly concern the pain-temperature pathways, and therefore neurophysiological procedures allowing the specific testing of these pathways (i.e., A-delta and C-fibres, spino-thalamo-cortical tracts) are essential for objective diagnosis. ⋯ Neurophysiological techniques not exploring specifically nociception, such as standard nerve conduction velocities (NCV) and SEPs to non-noxious stimulation, should be associated to the exploration of nociceptive systems, not only because both may be simultaneously affected to different degrees, but also because some specific painful symptoms, such as paroxysmal discharges, may depend on specific alteration of highly myelinated A-beta fibres. The choice of techniques is determined after anamnesis and clinical exam, and tries to answer a number of questions: (a) is the pain-related to injury of somatosensory pathways?; (b) to what extent are different subsystems affected?; (c) are mechanisms and lesion site in accordance with imaging data?; (d) are results of use for diagnostic or therapeutic follow-up? Neuropathic pain (NP) affects more than 15 million people in Western countries, and its belated diagnosis leads to insufficient or delayed therapy. The use of neurofunctional approaches to obtain a "physiological photograph" of somatosensory function is therefore highly relevant, as it yields significant clues about the type and mechanisms of pain, thus prompting rapid and optimised therapy.
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In the nineties, epidural cortical stimulation (ECS) of precentral region has been performed to treat drug-resistant neuropathic pain and repetitive transcranial magnetic stimulation (rTMS) of prefrontal region has shown antidepressant effects in episodes of major depression. These were among the first attempts to treat neurological or psychiatric disorders with cortical stimulation. Actually, a variety of invasive and noninvasive techniques of cortical stimulation could serve therapeutic purpose, including ECS, rTMS, but also transcranial electrical stimulation using pulsed currents (TCES) or direct currents (tDCS). ⋯ The existence of after-effects relates to processes of synaptic plasticity induced by the stimulation. Cortical stimulation may also have neuroprotective effects against disease-related excitotoxic phenomena. Considering the multiple techniques and the various potential clinical indications, it is a challenge to determine the place of cortical stimulation in the treatment of neurological and psychiatric diseases, in particular by the side of deep brain stimulation.
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Transcranial electrical stimulated motor evoked potential monitoring (TES-MEP) has proven to be a successful and reliable neuromonitoring technique during spinal correction surgery. However, three criteria for TES-MEP monitoring have been described in the literature. This study aims at discussing and comparing the following criteria: (1) the "threshold level criterion" introduced by Calancie et al. (J Neurosurg 88 (1998) 457-70): a more than 100V over more than 1h increase of threshold level to get useful TES-MEP responses indicated neurological impairment; (2) the "amplitude criterion": for TES-MEP monitoring in corrective surgery of the spine, a more than 80% decrease of one or more response amplitudes was considered a valuable criterion for impending neurological deficits by Langeloo et al. (Spine 28 (2003) 1043-50); (3) "the morphology criterion": introduced in 2005 by Quinones et al. (Neurosurgery 56 (2005) 982-93), it is based on the morphology of the MEP-compound muscle action potentials (CMAP). ⋯ Although all methods have been reported to be successful during spinal surgery, the threshold criterion and the morphology change criterion carry several drawbacks. We consider the amplitude reduction method to be most useful during corrective spinal surgery. The sequences of observations and decisions during a TES-MEP monitoring that is based on this criterion are schematized in a flowchart.
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During resection of intramedullary spinal-cord tumors intraoperative neurophysiological monitoring has become a true surgical technology. Motor evoked potentials are the most important modality for this purpose. Its use requires neurophysiological expertise from the surgeon, and a monitoring team in place able to handle the necessary equipment. ⋯ Such adaptation comprises simply waiting for the recordings to spontaneously improve again, irrigating with warm saline solution to wash out blocking potassium. Other measures include the elevation of mean arterial pressure to improve local perfusion. Even staged resection can be considered if intraoperative measures do not sufficiently improve the recordings.