Comparison of bipolar versus monopolar extraoperative electrical cortical stimulation mapping in patients with focal epilepsy.

OBJECTIVE: Extraoperative cortical stimulation (CS) for mapping of eloquent cortex in patients prior to epilepsy surgery is not standardized across centres. Two different techniques are in use, referred to as bipolar and monopolar CS. We compared the ability of bipolar versus monopolar CS to identify eloquent cortex and their safety profile in patients undergoing subdural EEG recordings. METHODS: Five patients undergoing intracranial EEG recordings and extraoperative CS. Systematic comparison of stimulus parameters, clinical signs and afterdischarges of bipolar versus monopolar CS. RESULTS: Bipolar CS requires less stimulation current but is more time consuming and more likely to produce afterdischarges when compared to monopolar CS. None of the stimulations elicited seizures. The area defined as eloquent by either bipolar or monopolar CS reveals only minor discordances, involving mainly the outer row and edge of the electrode array producing clinical signs with monopolar CS only. Qualitatively, bi- and monopolar CS reproduced similar movements and types of muscle contractions. CONCLUSIONS: Bipolar and monopolar CS are safe procedures identifying similar cortical areas as eloquent, although monopolar cortical stimulation is less time consuming. SIGNIFICANCE: Findings advocate the use of monopolar CS in a clinical setting.

Severe ictal hypoxemia following focal, subclinical temporal electrographic scalp seizure activity.

Ictal hypoxemia has been reported in focal seizures and can be particularly severe during sustained seizure activity involving both hemispheres. Oxygen desaturations have been linked to sudden unexplained death in epilepsy (SUDEP). We report a 71-year-old patient with subclinical electrographic seizure discharges involving the left temporal lobe. Electrographic seizures were followed by apneas and severe oxygen desaturations below 70% SpO(2) even after cessation of electrographic scalp seizure activity, suggesting inhibition of respiratory brainstem centers outlasting neocortical seizure activity. Seizures led to disrupted night's sleep due to arousals. Our case illustrates that severe hypoxemia can occur in association with subclinical seizures involving the temporal lobe and after scalp EEG seizure activity has terminated. Electrographic seizures followed by hypoxemia, such as observed in our patient, could contribute to SUDEP without overt clinical seizure activity.

Extraoperative electrical cortical stimulation: characteristics of motor responses and correlation with precentral gyrus.

PURPOSE: To characterize properties and stimulation thresholds of extraoperative cortical stimulation with respect to the anatomic area stimulated. METHODS: Analysis of 1,496 bipolar extraoperative cortical stimulations with respect to stimulation threshold, anatomic area stimulated, afterdischarges (ADs), and latencies to observe a clinical sign on video analysis. RESULTS: Cortical stimulation mapping stimulus thresholds to induce clinical motor responses are lower when compared with stimulus thresholds to induce nonmotor responses (P < 0.01). Motor thresholds vary depending on the anatomic area stimulated with the precentral gyrus, showing lower stimulation thresholds when compared with cortex outside the precentral gyrus (P < 0.001). Afterdischarges are more likely to occur with motor stimulations outside the precentral gyrus (χ = 1; n = 123; P < 0.05), and those stimulations show longer latencies to observe an arm motor response on video analysis when compared with precentral gyrus stimulations (P < 0.05). Within the precentral gyrus, stimulation of the hand knob was achieved with lower stimulation intensities compared with stimulation outside the hand knob (P < 0.001). CONCLUSION: Clinical response thresholds, afterdischarge occurrence, and latencies to observe clinical signs during extraoperative cortical stimulation vary depending on the cortex stimulated. Findings provide information about motor cortex organization and might aid interpreting results from extraoperative cortical stimulation in a clinical setting.