CHILDREN WITH FOCAL SEIZURES MAY NOT HAVE THE PRIMARY MOTOR CORTEX FOR THE HAND IN THE PRECENTRAL GYRUS

Objectives: Using functional cortical mapping in children with focal seizures evaluated for epilepsy surgery, we determined whether the primary motor cortex for the hand is most frequently located in the precentral gyrus.

Methods: Fifty-five children with focal seizures aged between 1 year and 19 years were studied. Patient selection criteria were as follows: Subjects underwent an initial two-stage epilepsy surgery using chronic subdural EEG monitoring; both intraoperative pictures of brain surface and three-dimensional reconstructed MR images demonstrated a well-defined central sulcus (Talairach and Tournoux, 1988; Yousry et al, 1997); and subdural electrodes were placed on the pre- and post-central gyri at least 4cm above the Sylvian fissure. As a part of presurgical evaluation, a pair of subdural electrodes was electrically stimulated with a stimulus frequency of 10Hz. The stimulus intensity was increased from 6.5 to 16.5mA in a stepwise manner by 5mA. A brain region with the contralateral hand movement induced by the lowest-intense electrical stimulation was defined as the primary motor cortex.

Results: Contralateral hand movement was induced without afterdischarges in 37 children but not in the remaining 18 children. The Mann-Whitney test revealed that failure to induce contralateral hand motor movement was associated with younger age of subjects (p<0.001). The primary motor cortex was confined to the precentral gyrus in five children (13%), confined to the postcentral gyrus in 11 children (30%), and located to both the pre- and post-central gyri in the remaining 21 children (57%).

Conclusion: Age is an important factor to determine the success rate of functional motor mapping. Our study failed to support the generally accepted notion that the primary motor cortex is most frequently confined to the precentral gyrus. Whether the common involvement of the postcentral gyrus is due to motor reorganization associated with epileptic foci, or it represents a normal variation, remains to be determined. Positive motor response associated with electrical stimulation of the postcentral gyrus is consistent with the observations that TMS-induced electrical current running posterior-anterior direction most frequently induces movement of the contralateral hand.