Intraoperative subcortical mapping of a language-associated deep frontal tract connecting the superior frontal gyrus to Broca's area in the dominant hemisphere of patients with glioma

OBJECT The deep frontal pathway connecting the superior frontal gyrus to Broca's area, recently named the frontal aslant tract (FAT), is assumed to be associated with language functions, especially speech initiation and spontaneity. Injury to the deep frontal lobe is known to cause aphasia that mimics the aphasia caused by damage to the supplementary motor area. Although fiber dissection and tractography have revealed the existence of the tract, little is known about its function. The aim of this study was to determine the function of the FAT via electrical stimulation in patients with glioma who underwent awake surgery. METHODS The authors analyzed the data from subcortical mapping with electrical stimulation in 5 consecutive cases (3 males and 2 females, age range 40–54 years) with gliomas in the left frontal lobe. Diffusion tensor imaging (DTI) and tractography of the FAT were performed in all cases. A navigation system and intraoperative MRI were used in all cases. During the awake phase of the surgery, cortical mapping was performed to find the precentral gyrus and Broca's area, followed by tumor resection. After the cortical layer was removed, subcortical mapping was performed to assess language-associated fibers in the white matter. RESULTS In all 5 cases, positive responses were obtained at the stimulation sites in the subcortical area adjacent to the FAT, which was visualized by the navigation system. Speech arrest was observed in 4 cases, and remarkably slow speech and conversation was observed in 1 case. The location of these sites was also determined on intraoperative MR images and estimated on preoperative MR images with DTI tractography, confirming the spatial relationships among the stimulation sites and white matter tracts. Tumor removal was successfully performed without damage to this tract, and language function did not deteriorate in any of the cases postoperatively. CONCLUSIONS The authors identified the left FAT and confirmed that it was associated with language functions. This tract should be recognized by clinicians to preserve language function during brain tumor surgery, especially for tumors located in the deep frontal lobe on the language-dominant side.