Role of single pulse electrical stimulation (SPES) to guide electrode implantation under general anaesthesia in presurgical assessment of epilepsy.

PURPOSE: Single-pulse electrical stimulation (SPES) during intracranial recordings is part of the epilepsy presurgical evaluation protocol at King's College Hospital (London). Epileptiform responses correlated to the stimulus (delayed responses - DRs) tend to occur in areas of seizure onset, thereby allowing interictal identification of epileptogenic cortex in patients suffering refractory epilepsy. This preliminary study investigated the validity of SPES in the operating theatre under general anaesthesia (GA) during the implantation procedure, aiming to improve the positioning of intracranial electrodes. METHODS: Twelve drug-resistant epilepsy patients implanted with depth and/or subdural electrodes were studied. SPES (1 ms pulses, 4-8 mA, 0.2 Hz) was performed during both intra-operative electrode implantation under GA and chronic intracranial ECoG recordings, and the two recordings were compared in terms of cortical responses produced by stimulation and their electrode location. RESULTS: In 8/12 patients, SPES during chronic recordings produced DRs positively correlated to seizure onset and/or early seizure propagation areas. Of those eight patients, four showed DRs during electrode implantation under GA over the same electrode contacts. Among the four patients without DR during GA, three had continuous localized spontaneous epileptiform discharges, which made interpretation of SPES responses unreliable. CONCLUSION: This study showed that, under GA, DRs can be reliably replicated, without false positive epileptiform responses to SPES, although the method's sensitivity is greatly reduced by spontaneous discharges. Results support SPES as a complementary technique that can be used to improve electrode placement during epilepsy surgery when no profound interictal activity is present.

Value of scalp delayed rhythmic ictal transformation (DRIT) in presurgical assessment of temporal lobe epilepsy.

OBJECTIVES: To estimate the localising and lateralising value of delayed rhythmic ictal transformation (DRIT) on the scalp EEG during presurgical assessment for temporal lobe epilepsy. METHODS: Two hundred and eighty eight seizures recorded simultaneously with scalp and foramen ovale (FO) electrodes were studied retrospectively in 110 patients. DRIT was defined as sustained regular rhythmic waveforms seen on scalp recordings after scalp seizure onset. The incidence and laterality of scalp DRIT was evaluated by comparison to FO electrodes. RESULTS: Scalp DRIT was seen in 192 seizures, 65% of which showed focal mesial temporal onset (FMTO) recorded by FO electrodes. FMTO onset was seen in 73% of the 122 seizures showing unilateral scalp DRIT, in 32% of the 62 seizures showing bilateral asymmetrical scalp DRIT, and in 14% of the seven seizures showing bilateral symmetrical DRIT on the scalp. Among the 89 seizures showing unilateral scalp DRIT and FMTO, both were ipsilateral in 70 seizures (79%). Among the 38 seizures showing bilateral asymmetrical DRIT and FMTO, the largest amplitude of scalp DRIT was ipsilateral to the FO onset in 27 seizures (71%). CONCLUSION: Two thirds of seizures showed unilateral scalp DRIT, which had a lateralising value of up to 79%. SIGNIFICANCE: Scalp DRIT is a reliable marker to lateralise seizure focus in patients without a focal seizure onset on the scalp EEG.

Presurgical assessment of memory-related brain structures: the Wada test and functional neuroimaging.

Medial temporal lobe structures are known to play a major role in memory processing. Recent work has revealed that extratemporal structures (e.g. the frontal lobe and thalamus) may also be important in memory function. In candidates for epilepsy surgery, particularly in those with temporal lobe seizures, presurgical evaluation of memory function is essential, since seizures may originate in the neural substrate that is critical for memory. In this article, we review the tools used for presurgical evaluation and their contribution to the understanding of memory function, focusing on the Wada test, [18F]fluorodeoxy-glucose positron emission tomography ([18F]FDG-PET) and functional magnetic resonance imaging (fMRI). We also explore perspectives on future studies that may elucidate the role of the temporal and extratemporal structures in memory function and the mechanisms of cerebral plasticity.

Lateralising value of neuropsychological protocols for presurgical assessment of temporal lobe epilepsy.

PURPOSE: To estimate the value of neuropsychological measurements in determining the side of seizure onset for presurgical assessment in patients with temporal lobe epilepsy. The lateralising value of neuropsychological protocols was evaluated for all patients and in subpopulations depending on surgical outcome with regard to seizure control, speech dominance, neuropathology, and need for intracranial EEG recordings. METHODS: A battery of neuropsychological procedures was carried out preoperatively in 125 patients who underwent left (n = 66) or right (n = 59) temporal lobectomies. Binary logistic regression analysis was performed to find sets of variables that allowed the best prediction of the side of seizure onset (assumed to be the operated-on side). RESULTS: Combinations of noninvasive neuropsychological tests and Wada subscores showed the highest lateralising values: 80.8% for all patients, 79.4% in seizure-free patients, 86.0% in patients not rendered seizure free, 85.7% in left speech patients, 77.8% in non-left speech patients, 89.3% in patients with mesial temporal sclerosis (MTS), 78.1% in non-MTS patients, 80.3% in patients who underwent intracranial EEG recordings, and 77.3% in those who did not. CONCLUSIONS: The lateralising value (80-90%) of neuropsychological protocols appears similar to that of other tests widely accepted for lateralisation (ictal and interictal scalp EEG and neuroimaging). Attention should be paid to neuropsychological results, particularly from the Wada test, during presurgical assessment of temporal lobe epilepsy, as they can provide strong support for findings from other lateralising tests, particularly in patients with presumed MTS or in left-speech patients.