G. Klem

Cortical somatosensory evoked potentials in response to hand stimulation

Somatosensory evoked potentials were recorded from chronically implanted subdural electrodes in six patients with intractable seizures. The following conclusions were reached: 1) The initial cortical negativity-positivity (N1 with a latency of about 20 msec and P2 with a latency of about 24 msec) recorded in the postcentral area was an expression of the classical primary surface positivity, but N1 was generated by the posterior pole of an early horizontal dipole in area 3b, and P2 was generated by the positive pole of a slightly delayed vertical dipole in area 1 and 2.2) P2 permitted the most accurate localization of the primary somatosensory area. 3) No potentials were elicited in the primary somatosensory area by stimulation of the ipsilateral hand. 4) No cortical potentials were seen at stimulation intensities below the sensory threshold. The cortical distribution of evoked potentials evoked by weak and strong intensities had significantly different distribution. 5) The recovery function of cortical evoked potentials showed a U-curve with an early period of facilitation (10 to 30 msec) followed by a prolonged period of subnormality which peaked at about 50 msec. The recovery curve at different cortical loci differed.

The localizing value of ictal EEG in focal epilepsy

Objective: To investigate the lateralization and localization of ictal EEG in focal epilepsy. Methods: 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. Results: 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. Conclusions: 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.

Epileptogenicity of Focal Malformations Due to Abnormal Cortical Development: Direct Electrocorticographic–Histopathologic Correlations

Summary:  Purpose: 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.

Cortical Afterdischarge and Functional Response Thresholds: Results of Extraoperative Testing

Summary: 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 > 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.”

Effect of stimulus intensity on subcortical and cortical somatosensory evoked potentials by posterior tibial nerve stimulation

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.

Origin of far-field subcortical potentials evoked by stimulation of the posterior tibial nerve

Evoked potentials to stimulation of posterior tibial nerves were recorded from cervical-scalp derivation (cervical electrode: surface electrode on spinal process of the fifth cervical vertebrae; scalp: vertex). Four subcortical potentials labeled as N24, P27, N30 and P32 were identified. Evidence indicating that these components are originated in the following structures is presented: N24, posterior columns at the level of C5; P27, high cervical posterior columns or brain stem; N30, medial lemniscus or thalamus; P32, thalamo-cortical radiations.

The value of closely spaced scalp electrodes in the localization of epileptiform foci: A study of 26 patients with complex partial seizures

Twenty-seven patients with complex partial seizures were studied electrographically utilizing a large number of closely spaced scalp electrodes around the epileptogenic focus. Skull roentgenograms were made with the electrodes in place in order to relate the electrode positions to underlying brain anatomy. Field distribution maps were constructed from reference montages employing the closely spaced electrode set. Electrodes other than 10-20 were maximal most often and the single electrodes most often maximal were D9-D10, anterior temporal in location. This method of localization is of help in the evaluation of patients with complex partial seizures who may be surgical candidates.

Electrical stimulation of Wernicke's area interferes with comprehension

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.

A new technique for intraoperative monitoring of spinal cord function: multichannel recording of spinal cord and subcortical evoked potentials.

A new technique for intraoperative monitoring of spinal cord function is described. Evoked potentials to peripheral nerve stimulation are recorded with interspinous ligament needles from below and from two or more levels above the surgery. Simultaneously subcortical potentials are recorded with scalp electrodes from cervical-scalp derivations. Simultaneous recordings from multiple levels (below and above the surgical procedure) permit reliable differentiation between technical problems and alteration of cord function due to the surgical procedure. The interspinous ligament technique for recording of spinal potentials has significant advantages over previous monitoring techniques. One case in which monitoring during surgery detected a reversible acute spinal cord dysfunction is described.

k tibial nerve stimulation: Normative values

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: 1. (1) 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. 2. (2) All absolute latencies other than N2 were correlated with body height. However, interpeak latency differences were independent of body height. 3. (3) 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. 4. (4) Absolute amplitudes and amplitude ratios (left/right and uni/bilateral) showed marked interindividual variability and have very limited value in defining abnormality. 5. (5) The use of restricted filter windows facilitated the selective recording of postsynaptic potentials (30–250 Hz) and action potentials (150–1500 Hz).