Presurgical and intraoperative mapping of the motor system in congenital truncation of the precentral gyrus.

A 43-year-old man presented with a grade II astrocytoma in the left postcentral gyrus and superior parietal lobule. Preoperative functional MR imaging and diffusion tensor imaging mapped distal upper-extremity primary motor cortex and white matter, respectively, adjacent to the tumor, within a congenitally truncated precentral gyrus. Because of the congenital anomaly, this region of primary motor cortex was inaccessible to direct visualization or intraoperative electrocortical stimulation. The integration of preoperative and intraoperative mapping data facilitated resection of the tumor while avoiding a postoperative motor deficit.