G Klem

Cleveland Clinic, Cleveland, Ohio, United States

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Publications (30)135.32 Total impact

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    ABSTRACT: Malformations due to abnormal cortical development (MCDs) are common pathologic substrates of medically intractable epilepsy. The in situ epileptogenicity of these lesions as well as its relation to histopathologic changes remains unknown. The purpose of this study was to correlate the cellular patterns of MCDs with the expression of focal cortical epileptogenicity as assessed by direct extraoperative electrocorticographic (ECoG) recordings by using subdural grids. Fifteen patients with drug-resistant focal epilepsy due to pathologically confirmed MCD who underwent subdural electrode placement for extraoperative seizure localization and cortical mapping between 1997 and 2000 were included in the study. Areas of interictal spiking and ictal-onset patterns were identified and separated during surgery for further pathologic characterization (cellular and architectural). Three pathologic groups were identified: type I; architectural disorganization with/without giant neurons, type IIA; architectural disorganization with dysmorphic neurons, and type IIB; architectural disorganization, dysmorphic neurons, and balloon cells (BCs). The focal histopathologic subtypes of MCDs in cortical tissue resected were then retrospectively correlated with in situ extraoperative ECoG patterns. Cortical areas with histopathologic subtype IIA showed significantly higher numbers of slow repetitive spike pattern in comparison with histopathologic type I (p = 0.007) and normal pathology (p = 0.002). The ictal onset came mainly from cortical areas with histopathologic type IIA (nine of 15 patients). None of the seizures originated from neocortical areas that showed BC-containing MCD (type IIB). This study shows that areas containing BCs are less epileptogenic than are closely located dysplastic regions. These results suggest a possible protective effect of BCs or a severe disruption in the neuronal networks in BCs containing dysplastic lesions. Further studies are needed to elucidate the nature and the potential role(s) of balloon cells in MCD-induced epileptogenicity.
    Epilepsia 02/2003; 44(1):69-76. · 3.91 Impact Factor
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    ABSTRACT: To investigate the lateralization and localization of ictal EEG in focal epilepsy. A total of 486 ictal EEG of 72 patients with focal epilepsy arising from the mesial temporal, neocortical temporal, mesial frontal, dorsolateral frontal, parietal, and occipital regions were analyzed. Surface ictal EEG was adequately localized in 72% of cases, more often in temporal than extratemporal epilepsy. Localized ictal onsets were seen in 57% of seizures and were most common in mesial temporal lobe epilepsy (MTLE), lateral frontal lobe epilepsy (LFLE), and parietal lobe epilepsy, whereas lateralized onsets predominated in neocortical temporal lobe epilepsy and generalized onsets in mesial frontal lobe epilepsy (MFLE) and occipital lobe epilepsy. Approximately two-thirds of seizures were localized, 22% generalized, 4% lateralized, and 6% mislocalized/lateralized. False localization/lateralization occurred in 28% of occipital and 16% of parietal seizures. Rhythmic temporal theta at ictal onset was seen exclusively in temporal lobe seizures, whereas localized repetitive epileptiform activity was highly predictive of LFLE. Seizures arising from the lateral convexity and mesial regions were differentiated by a high incidence of repetitive epileptiform activity at ictal onset in the former and rhythmic theta activity in the latter. With the exception of mesial frontal lobe epilepsy, ictal recordings are very useful in the localization/lateralization of focal seizures. Some patterns are highly accurate in localizing the epileptogenic lobe. One limitation of ictal EEG is the potential for false localization/lateralization in occipital and parietal lobe epilepsies.
    Neurology 01/2002; 57(11):2022-8. · 8.25 Impact Factor
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    ABSTRACT: A 44-year-old man with a right frontal lobe tumor and intractable seizures underwent subdural grid evaluation before resection. The electrode locations were identified on a three-dimensional surface-reconstructed image of the brain after subdural grid placement. Electrical stimulation of electrodes placed over the right cingulate gyrus revealed evidence of tonic posturing of the left forearm and wrist and tonic extension of the left leg. This finding provides further evidence of a motor area in the cingulate gyrus in humans.
    Neurology 10/2000; 55(5):725-8. · 8.25 Impact Factor
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    ABSTRACT: Recognizing epileptic seizures during video polysomnography (VPSG) can be challenging, particularly when using standard, limited EEG montages and paper speed. Few sleep laboratories have PSG equipment that allows for the recording of 18 channels of EEG without compromising the ability to detect sleep apnea, periodic limb movements, and parasomnias. We studied the ability of sleep medicine- and EEG-trained polysomnographers to correctly identify epileptic seizures during PSG using 4, 7, and 18 channels of simultaneous EEG, recording at conventional PSG and EEG paper speeds. The purpose of this study was to determine the value of limited EEG montages viewed with EEG reformatting capability in the identification of seizures during PSG. Blinded EEG analysis of seizures and arousals during VPSG. Tertiary care hospital with sleep laboratory and epilepsy monitoring unit. Subjects with focal (partial) epilepsy that underwent video-EEG monitoring. We designed two 7-channel EEG montages that might facilitate the identification of seizures arising from the frontal and temporal lobes. Sleep medicine- and EEG-trained polysomnographers were asked to review tracings containing frontal or temporal lobe epileptic seizures and arousals from sleep. Utilizing the capability of our digital recording equipment to reformat EEG channels and change paper speeds, we asked the readers to classify events recorded with 4, 7, and 18 channels of simultaneous EEG, at paper speeds of 10 and 30 mm/sec. 6 readers viewed 32 sleep-related events (13 frontal lobe seizures, 11 temporal lobe seizures, and 8 arousals). The following factors were analyzed for their influence on accuracy of event detection: 1) the type of training of the reader (EEG vs. sleep medicine); 2) the number of EEG channels (4, 7, or 18); and 3) paper speed (10 vs. 30 mm/sec). Pair-wise comparisons and generalized estimating equations were used to identify factors leading to more accurate detection of seizures and arousals. 77% of events were correctly identified: 74% of seizures and 88% of arousals. Seizure detection was better using 7 and 18 channels (sensitivity of 82% and 86%, respectively) than 4 EEG channels (sensitivity of 67%) for temporal lobe seizures only. The number of EEG channels did not affect the accuracy of frontal lobe seizure detection. For EEG-trained readers, accuracy was greater using 30 mm/sec than 10 mm/sec paper speed (85% vs. 78% correct, respectively). Adding EEG channels and EEG reformatting capabilities to PSG interpretation improves the detection of some types of epileptic seizures. Accuracy of temporal lobe seizure detection using an abbreviated 7-channel montage approximates that of an 18-channel EEG recording. However, the same is not true of frontal lobe seizures in which accuracy was similar regardless of the number of EEG channel available. Further studies are needed to identify specific EEG montages that would best detect epileptiform activity during VPSG.
    Sleep 04/2000; 23(2):221-9. · 5.10 Impact Factor
  • Electroencephalography and clinical neurophysiology. Supplement 02/1998; 48:58-69.
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    ABSTRACT: Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2-17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias, occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.
    Electroencephalography and Clinical Neurophysiology 06/1997; 104(3):189-98.
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    ABSTRACT: Electrical stimulation studies have demonstrated that a "supplementary motor area" (SMA) exists in humans. However, its precise functional organization has not been well defined. We reviewed the extraoperative electrical stimulation studies of 15 patients with intractable epilepsy who were evaluated with chronically implanted interhemispheric subdural electrodes. SMA-type positive motor responses were elicited not only from the mesial portion of the superior frontal gyrus but also from its dorsal convexity, and from the paracentral lobule, cingulate gyrus, and precuneus. Sensory symptoms, that could not be attributed to stimulation of the primary sensory area, were elicited from the superior frontal and cingulate gyri in addition to the precuneus. Therefore, human SMA, as defined by electrical stimulation, is not always confined to the mesial portion of the superior frontal gyrus as described previously. It is also not strictly "motor" but "sensorimotor" in representation. We propose referring to this region as the "supplementary sensorimotor area" (SSMA). We observed a somatotopic organization within the SSMA with an order of lower extremity, upper extremity, and head from posterior to anterior. Sensory representation in an individual was either anterior or posterior to the positive motor representation but never both. There was a supplementary eye field within the head representation. A supplementary negative motor area was noted at the anterior aspect of the SSMA. No language area was demonstrated within the SSMA. The physiologic significance of the SSMA and functional consequences of its resection must be addressed in further studies.
    Electroencephalography and Clinical Neurophysiology 10/1994; 91(3):179-93.
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    ABSTRACT: We report five patients who experienced delayed patchy hair loss after prolonged EEG monitoring. The location of the patches corresponded to the electrode sites in four. The hair loss was temporary and most probably was due to traumatic noncicatricial alopecia. The frequency of hair loss was approximately 2% of monitored patients.
    Neurology 08/1992; 42(7):1401-2. · 8.25 Impact Factor
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    ABSTRACT: Functional localization prior to cortical resections for intractable seizures has usually been performed in the operating room in awake patients. Chronically placed subdural electrodes offer the possibility of performing such testing outside of the operating room and without the unavoidable stresses and time limitations of the surgical setting. The use of the technique is reviewed.
    Journal of Clinical Neurophysiology 02/1987; 4(1):27-53. · 1.45 Impact Factor
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    ABSTRACT: The relationship of intraoperative monitoring of spinal cord somatosensory evoked potentials and postoperative deficit in 220 cases (121 with scoliosis, 41 with neoplasms, and 58 others) is reported. Bilateral posterior tibial nerve stimulation was used in 181 cases and unilateral median nerve stimulation in 39. Spinal cord (interspinous ligament needles), subcortical (neck surface), and cortical (scalp surface) SEP's were monitored. Seven patients had worsening of neurological function after surgery, three of whom demonstrated significant changes in SEP's monitored. In an additional four cases, there was more than a 50% decrease in amplitude of subcortical/cortical SEP's during monitoring, but no change in neurological status postoperatively. Combined monitoring of spinal cord, subcortical, and cortical SEP's enhanced the certainty of detecting spinal cord dysfunction even though there was a significant number of false-negative and false-positive results. A marked change in the SEP's indicated a high chance of developing a neurological deficit (three or 43% of seven cases), and if there was no change the chance of any neurological postoperative deficit was extremely low (four or 1.87% of 213 cases). These data justify the use of intraoperative SEP monitoring.
    Journal of Neurosurgery 01/1987; 65(6):807-14. · 3.15 Impact Factor
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    ABSTRACT: Arrays of subdural electrodes were placed over the lateral convexity of the dominant hemisphere for propositional language in four patients with epilepsy as part of an evaluation prior to cortical resections. Stimulation was performed over several days. When we stimulated the posterior temporal language area, reading and comprehension of complex verbal information were impaired, but comprehension of nonverbal and simple verbal data was not affected. Impairment produced by stimulation seemed to be due to language comprehension difficulties, rather than impaired praxis or initial word storage.
    Neurology 06/1986; 36(5):658-63. · 8.25 Impact Factor
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    ABSTRACT: We describe 6 patients who demonstrated postoperative neurological deficits despite unchanged somatosensory evoked potentials during intraoperative monitoring. Although there is both experimental and clinical evidence that somatosensory evoked potentials are sensitive to some types of intraoperative mishap, the technique should be employed with an awareness of its possible limitations.
    Annals of Neurology 02/1986; 19(1):22-5. · 11.19 Impact Factor
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    ABSTRACT: Twelve patients with medically intractable epilepsy had plates of chronic subdural electrodes placed over the lateral and basal cortical hemispheres during evaluations for surgical therapy. During cortical stimulation, ipsilateral sensations involving any of the branches of the trigeminal nerve were noted in the eye, face, and mouth. Some responses could have been due to dural or direct trigeminal nerve trunk stimulation, but others were probably due to electrical stimulation of trigeminal fibers accompanying the pial-arachnoidal vessels. These fibers had been demonstrated in animals, but not in humans.
    Neurology 01/1986; 35(12):1760-3. · 8.25 Impact Factor
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    ABSTRACT: Controversy exists in the literature concerning whether Parkinson's disease (PD) results in prolongation of pattern evoked potential (PEP) responses. PEPs were obtained in 20 patients with PD. The latencies of the first major positive potential (P2) in response to independent left and right eye stimulation were evaluated. There was no statistically significant difference (group or individual) when patients with PD were compared with age-corrected controls. There was no difference in results in comparing patients with mild, moderate, and severe PD. These results contradict previous reports of markedly abnormal PEPs.
    Neurology 05/1985; 35(4):610-3. · 8.25 Impact Factor
  • Cleveland Clinic quarterly 02/1985; 52(2):223-8.
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    ABSTRACT: We have evaluated the afterdischarge thresholds and functional response thresholds in 21 patients with chronically implanted arrays of subdural electrodes. Afterdischarge thresholds varied from 2 to greater than 15 mA over the tested cortex, by as much as 12 mA in individual patients, and by as much as 12 mA between adjacent electrodes. Thresholds for functional alteration varied from 2 to 15 mA in tested cortex, by as much as 9.5 mA in individually tested patients, and by as much as 6.5 mA between adjacent electrodes. We conclude that the optimal localization of functional cortical areas requires different stimulation intensities at different points. The use of too high an intensity would produce afterdischarges at some positions. The use of too low an intensity would falsely make others appear functionally "silent."
    Epilepsia 11/1984; 25(5):615-21. · 3.91 Impact Factor
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    ABSTRACT: The recovery function of evoked potentials to posterior tibial nerve stimulation was studied. Intrasurgical recordings were made from interspinous ligaments at lumbar levels and from high thoracic-low cervical level. In addition, surface recordings were obtained from neck-scalp derivations. The recovery function of the potentials recorded from lumbar and from high thoracic-low cervical spinal cord were very similar, showing an early period of supernormality (5-20 ms) followed by a period of subnormality which reached its lowest point at 40-60 ms. Assuming that the potentials recorded at the lumbar level reflect activity in the cauda equina, we conclude that the results support the hypothesis that the potentials recorded from the thoraco-cervical level reflect activity in the dorsal columns. The recovery curve of the amplitude between the far field potentials P27 (which most probably reflects activity of the afferent volley at the level of foramen magnum) and N30 (which, by latency criteria, would reflect lemniscal or thalamic activity) showed a similar shape but with a shorter duration of the periods of super- and subnormality. It is likely that this modification was due to the synapse at the gracilis nucleus. The first cortical component (P32) recorded in the neck-scalp derivation was totally abolished within the recovery period studied (50 ms interval).
    Brain Research 09/1984; 309(1):27-34. · 2.88 Impact Factor
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    ABSTRACT: Cortical somatosensory evoked potentials to posterior tibial nerve stimulation were obtained in 29 normal controls varying in age and body height. In obtaining these potentials we varied recording derivations and frequency settings. Our recordings demonstrated the following points: N20 (dorsal cord potential) and the early cortical components (P2, N2) were the only potentials that were consistently recorded. All other subcortical components (N18, N24, P27, N30) were of relatively low amplitude and not infrequently absent even in normals. All absolute latencies other than N2 were correlated with body height. However, interpeak latency differences were independent of body height. Below the age of 20, subcortical but not cortical peak latencies correlated with age, but this appeared to be due to changes in body height in this age group. Absolute amplitudes and amplitude ratios (left/right and uni/bilateral) showed marked interindividual variability and have very limited value in defining abnormality. The use of restricted filter windows facilitated the selective recording of postsynaptic potentials (30-250 Hz) and action potentials (150-1500 Hz).
    Electroencephalography and Clinical Neurophysiology 07/1984; 59(3):214-28.
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    ABSTRACT: The effects of stimulus intensity on subcortical and cortical somatosensory evoked potentials (SEPs) to posterior tibial nerve (PTN) stimulation were studied in 16 normal controls. Stimulus intensity was evaluated as a function of sensory threshold (S). Motor threshold (M) varied between 1 S and 2 S. The amplitude of N18 (afferent volley immediately before it enters the spinal canal) increased approximately linearly up to at least 4.5 S. N20 (dorsal cord potential) also demonstrated a linear increase up to at least 4 S but the rate of increase was significantly smaller. All central components (subcortical brain-stem components P27 and N30, and cortical components N1 and P2) showed an even smaller rate of increase which was non-linear and reached a plateau at 3 S. The relatively higher rate of increase of N18 as compared with N20 was most probably due to the recording of sensory impulses plus antidromic impulses in motor fibers. The smaller rate of increase and early saturation of all the central components compared with N20 suggests that of all the afferent fibers generating N20 only the low threshold fibers participate in the generation of more central components. Stimulus intensities of 3 S are recommended for clinical studies of the central SEPs to PTN stimulation.
    Electroencephalography and Clinical Neurophysiology 07/1984; 59(3):229-37.
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    ABSTRACT: With the aid of chronic subdural electrodes we have been able to record from the posterior banks of the sylvian fissure, auditory evoked potentials (AEPs) that had morphologies and peak latencies compatible with the primary AEPs described by Celesia and Puletti (1969). These AEPs had amplitudes that were not only affected by the side of stimulus presentation but were maximal in an area close to the primary auditory cortex. The AEPs also displayed an extremely steep spatial gradient and were not altered by pentobarbitone sodium and nitrous oxide anaesthesia. Together, these properties suggest that these subdurally recorded potentials are near-field evoked potentials from the primary auditory cortex. The focal nature of these potentials also allows them to be used as effective electrophysiological tools for localization of the primary auditory cortex in patients.
    Brain 04/1984; 107 ( Pt 1):115-31. · 10.23 Impact Factor