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Localizing mild peroneal neuropathies to the fibular head by sensory nerve conduction studies
S162 For presurgical assessment of potential candidates for selective amygdalohippocampectomy we have developed a less invasive extra-cerebral recording technique. It consists of bilateral insertion of solid 4-contact electrodes via the foramen ovale with positioning of the tips of the electrodes at the end of the ambient cistern. This technique permits stable and excellent recording from the mediobasal limbic structures of both temporal lobes.
LOCALIZING MILD PERONEAL NEUROPATHIES TO THE FIBULAR HEAD BY SENSORY NERVE CONDUCTION STUDIES. A.J. Wilbourn and R.W. Shields. Jr. (Cleveland
Clinic Foundation,
Cleveland,
OH, USA)
Most common peroneal mononeuropathies at the fibular head (CPN-FH) present as footdrop, due to motor fiber compromise. Occasionally, however, sensory symptoms predominate. In such a patient, we demonstrated conduction block (CB) along the superficial peroneal sensory (SPS) fibers at the FH, using a nerve conduction study (NCS) technique originally described by Gilhatt and Sears in 1958. A 15year-old girl lean, without recent weight loss, but a chronic leg-crosser developed footdrop and sensory symptoms in a left common peroneal nerve distribution (CPND) while sitting in class (with left leg crossed over right). The former rapidly resolved; the latter persisted. EMG examination two months later revealed motor conduction velocity, recording slowed ‘across-FH’ tibiahs anterior (50 m/set vs. 67, right) and fibrillations in a CPND distal to knee; the peroneal motor, recording EDB, and the standard SPS NCS were normal. Proximal left SPS conduction was abnormal, however: recording with surface electrodes proximal and distal to FH, while stimulating over the foot dorsum, responses were detected distal to FH bilaterally but proximal to FH only on right, indicating CB along SPS fibers at left FH. Although this study has significant limitations, in selected cases it is quite helpful for localization.
EEG MAPPING OF EPILEPTIC SPIRES. P. K. H. Wong (Children’s
Hospital,
Vancouver,
Canada)
Spatial domain analysis of epileptiform discharges may go further than traditional time-domain analysis and extract additional information from the same data. Basically, the spike (or other activity) is not treated as a series of voltage tracings, but as chronologic sets of instantaneous scalp voltage readings, each reading being derived from one electrode, with 1 set per time point. Salient topographic features may be inferred and ongoing changes monitored once every few milliseconds. Interpretation of such data which may be in the form of maps can
be on 3 levels: 1) visual identification of prima;y features. voltage peaks and valleys; 2) their spatial symmetry. gradient and spread; 3) additional data processing for the estimation and localization of underlying generators (‘equivalent dipoles’). The first step is merely descriptive. With the second step. some informatton of the spatial organization of the data can be appreciated, including possibly how many distinct generators there are and where they may he. The third step requires some form of mathematical model to describe the head, from which estimation of the generator configuration (‘dipole localization’) can be made using numerical techniques. Alternate secondary processing of the raw data may generate new descriptive parameters which may be used to characterize or classify data from various clinical populations. Examples will be provided based on EEG spike data, showing how topographic or spatial descriptors can be effectively constructed and applied to a clinical problem with better results than can be obtained by using time-domain parameters alone.
EVOKED POTENTIAL TOPOGRAPHIES CORTEX. D.L. Woods, R.J. Andrews (University
of California
OVER HUMAN
and L. Tamas Davis VAMC,
Martinez,
CA, USA)
Although evoked potential recording provided important historical insights into the somatotopic and tonotopic organization of neocortex in various mammalian species, EP techniques have not been systematically applied to organizational studies of sensory areas in humans. We now report the results of studies of human somatosensoty and auditory cortex using multi-channel EP recording from the cortical surface. All patients were undergoing craniotomies for resection of brain tumors. Cortical evoked potentials were obtained using a matrix of 21 electrodes with 10 mm inter-electrode spacings, The electrodes were arranged as a miniature lo-20 pattern so that standard interpolation algorithms could be used to display local EP topographies. Somatosensory evoked potentials were recorded to electrical stimulation of the digits of the contralateral hand. Prominent P25 and N25 components were found on anterior and posterior margins of the central sulcus. These had restricted spatial distributions, with peak amplitudes falling by 4t-50% at distances of 1.0-1.5 cm. Component distributions changed with stimulation of the different digits. Amplitude maxima were more superior following little finger stimulation than with stimulation of the thumb or index finger. Auditory EPs were elicited by monaural and binaural tonebursts (2 msec rise/fall and 8 msec plateau, frequencies 0.25-4.0 kHz) over broadband masking noise. Broadly distributed P40 components were recorded in most patients. These had maximal amplitudes over inferior parietal cortex, and inverted in phase over the Sylvian fissure. In some patients spatially restricted phasic components were also evident over
LOCALIZING MILD PERONEAL NEUROPATHIES TO THE FIBULAR HEAD BY SENSORY NERVE CONDUCTION STUDIES. A.J. Wilbourn and R.W. Shields. Jr. (Cleveland
Clinic Foundation,
Cleveland,
OH, USA)
Most common peroneal mononeuropathies at the fibular head (CPN-FH) present as footdrop, due to motor fiber compromise. Occasionally, however, sensory symptoms predominate. In such a patient, we demonstrated conduction block (CB) along the superficial peroneal sensory (SPS) fibers at the FH, using a nerve conduction study (NCS) technique originally described by Gilhatt and Sears in 1958. A 15year-old girl lean, without recent weight loss, but a chronic leg-crosser developed footdrop and sensory symptoms in a left common peroneal nerve distribution (CPND) while sitting in class (with left leg crossed over right). The former rapidly resolved; the latter persisted. EMG examination two months later revealed motor conduction velocity, recording slowed ‘across-FH’ tibiahs anterior (50 m/set vs. 67, right) and fibrillations in a CPND distal to knee; the peroneal motor, recording EDB, and the standard SPS NCS were normal. Proximal left SPS conduction was abnormal, however: recording with surface electrodes proximal and distal to FH, while stimulating over the foot dorsum, responses were detected distal to FH bilaterally but proximal to FH only on right, indicating CB along SPS fibers at left FH. Although this study has significant limitations, in selected cases it is quite helpful for localization.
EEG MAPPING OF EPILEPTIC SPIRES. P. K. H. Wong (Children’s
Hospital,
Vancouver,
Canada)
Spatial domain analysis of epileptiform discharges may go further than traditional time-domain analysis and extract additional information from the same data. Basically, the spike (or other activity) is not treated as a series of voltage tracings, but as chronologic sets of instantaneous scalp voltage readings, each reading being derived from one electrode, with 1 set per time point. Salient topographic features may be inferred and ongoing changes monitored once every few milliseconds. Interpretation of such data which may be in the form of maps can
be on 3 levels: 1) visual identification of prima;y features. voltage peaks and valleys; 2) their spatial symmetry. gradient and spread; 3) additional data processing for the estimation and localization of underlying generators (‘equivalent dipoles’). The first step is merely descriptive. With the second step. some informatton of the spatial organization of the data can be appreciated, including possibly how many distinct generators there are and where they may he. The third step requires some form of mathematical model to describe the head, from which estimation of the generator configuration (‘dipole localization’) can be made using numerical techniques. Alternate secondary processing of the raw data may generate new descriptive parameters which may be used to characterize or classify data from various clinical populations. Examples will be provided based on EEG spike data, showing how topographic or spatial descriptors can be effectively constructed and applied to a clinical problem with better results than can be obtained by using time-domain parameters alone.
EVOKED POTENTIAL TOPOGRAPHIES CORTEX. D.L. Woods, R.J. Andrews (University
of California
OVER HUMAN
and L. Tamas Davis VAMC,
Martinez,
CA, USA)
Although evoked potential recording provided important historical insights into the somatotopic and tonotopic organization of neocortex in various mammalian species, EP techniques have not been systematically applied to organizational studies of sensory areas in humans. We now report the results of studies of human somatosensoty and auditory cortex using multi-channel EP recording from the cortical surface. All patients were undergoing craniotomies for resection of brain tumors. Cortical evoked potentials were obtained using a matrix of 21 electrodes with 10 mm inter-electrode spacings, The electrodes were arranged as a miniature lo-20 pattern so that standard interpolation algorithms could be used to display local EP topographies. Somatosensory evoked potentials were recorded to electrical stimulation of the digits of the contralateral hand. Prominent P25 and N25 components were found on anterior and posterior margins of the central sulcus. These had restricted spatial distributions, with peak amplitudes falling by 4t-50% at distances of 1.0-1.5 cm. Component distributions changed with stimulation of the different digits. Amplitude maxima were more superior following little finger stimulation than with stimulation of the thumb or index finger. Auditory EPs were elicited by monaural and binaural tonebursts (2 msec rise/fall and 8 msec plateau, frequencies 0.25-4.0 kHz) over broadband masking noise. Broadly distributed P40 components were recorded in most patients. These had maximal amplitudes over inferior parietal cortex, and inverted in phase over the Sylvian fissure. In some patients spatially restricted phasic components were also evident over