Abstracts

Rationale: Children with tumors are at risk for intractable epilepsy. The ictal onset zone may involve the tumor or may encompass a wider field involving the surrounding tissue. When there is concern the epileptogenic zone is not isolated to the tumor, or abuts eloquent cortex, invasive monitoring may therefore be required. We proposed that depth and inter-hemispheric electrodes would better define the ictal onset zone. The depth and inter-hemispheric electrodes may also act as a neuro-anatomical border, hence assisting the neurosurgeon during surgical resection.Methods: Children with tumors and intractable epilepsy, from June 2006-present were retrospectively reviewed. 13 cases were identified, two of whom were repeat surgeries after tumor resection did not terminate the seizures. Of the 11 children without prior surgical resection, 3 children had both depth and inter-hemispheric subdural strip electrodes placed. In two children depth electrode(s) traversed the grid between specific grid electrodes placed over the surface of the lesion and in one case the depth was just adjacent to the grid. The depth electrodes were embedded within the tumor allowing for tumor tissue sampling for epileptogenicity. Two children had tumors approximating the midline and inter-hemispheric electrodes were placed in both casesResults: In 3 cases (100%), seizure onset, involved the depth electrode and when placed inter- hemispheric electrodes, and in two cases the subdural grid became involved over 15 seconds after seizure onset. Grid involvement in all cases encompassed an area larger than the epileptogenic zone; involving eloquent cortex defined by fMRI,and cortical motor and sensory mapping in two children. Inter-hemispheric strips and depths also complimented stereotactic guided (Stealth) MRI in defining the neuro-anatomical borders of the lesions during surgical resection. All lesions were successfully resected (a low grade oligodendroglioma, a high grade oligodendroglioma, and a harmatomatous lesion). All have remained seizure free.Conclusions: If a subdural grid was used in isolation for the cases described, the region of ictal onset would not have been captured. Additional coverage with depths and or inter-hemispheric strips was instrumental in better defining the epileptogenic zone. It also allowed for avoidance of critical cortical functional areas. The authors suggest a surface grid may be insufficient in identifying the region of ictal onset in children with intractable epilepsy who have cortical lesions. Unless contraindicated, depth electrodes and, for parasagittal lesions, inter-hemispheric electrodes, should be used when invasive monitoring is required.