Abstracts

RATIONALE: Central sulcus (CS) localization with cortical median nerve SSEPs is essential in localizing eloquent sensoriomotor area in functional neurosurgery as direct cortical electrical stimulation occasionally fails in CS localization. The cortical potentials of median SSEP N20-P30, P20-N30, and P25-N35 have been hypothetically attributed to activation of area 3b and area 1 in the postcentral sulcus, but the clinical relevance of the phase reversal of these potentials has not been systemically investigated. The criteria of locating the phase reversal have not been established.
METHODS: Cortical median nerve SSEPs were recorded from subdural electrodes placed in the perirolandic area (electric pulse 2-3Hz of 0.1ms duration, bandpass 30-2000Hz) in 20 patients as a part of the presurgical evaluation of medically intractable partial epilepsy. First and second negative and positive peaks (N1, P1, N2, P2) were identified in responses recorded from each electrode. The accuracy of the following 3 criteria for CS localization was studied.
1) N1 and P1 potentials with the earliest peak latency, regardless of the amplitude of potentials.
2) Largest N1 and P1 potentials. These first peaks should be located between the N1-P1 and P2-N2 peaks as defined in criterion 1.
3) N1 and P1 potentials that are followed by the largest later components (N2, P2, N3, P3).
CS as correlated with electrode location was identified in 3D-MRI co-registered with subdural electrodes. After locating N1 and P1 potentials, P25 potential was also evaluated.
RESULTS: The identification of N1 and P1 potentials that are followed by the largest later components was the most accurate criteria for CS localization (95% versus 80% for criterion 1 or 2). P25 potential was identified in 15 out of 20 patients (75%). The location of the P25 in CS or the postcentral gyrus was confirmed with MRI in 11 out of 15 patients. Cortical median nerve SSEPs failed to identify CS location in only one patient.
CONCLUSIONS: Cortical median nerve SSEPs are very accurate in CS localization for the presurgical evaluation of patients with medically intractable epilepsy. The identification of a phase reversal of N1 and P1 with following large amplitude potentials is highly predictive of CS location. Further studies on larger number of patients are needed to confirm those results and to elucidate the generator mechanism of the following large potentials.
Support: This abstract summaries work done without the financial support of commercial interests.