Brain tumors, especially gliomas, do not respect functional borders and often involve both cortex and subcortical white matter. Surgical planning thus becomes essential, especially when these lesions are located adjacent to functional anatomy.7 Different methods, including diffusion tensor MRI and subcortical electrostimulation, have been developed to further delineate important tracts and avoid them intraoperatively.4 These have been combined with neuronavigation to improve functional outcomes.4,7,11 Other methods, including using diffusion-weighted images combined with T1-weighted postcontrast images, have been used to identify and thus avoid optic radiations.2 Substantial emphasis has been placed on sensorimotor mapping with little attention directed toward the visual system.5 Gras-Combe and colleagues6 present a new target to further define functional boundaries during awake surgeries for glioma resections involving the visual pathways. Previously, efforts have been made to better define functional boundaries involving language and motor function with little emphasis on preservation of visual functions. In their article “Intraoperative subcortical electrical mapping of optic radiations in awake surgery for glioma involving visual pathways,” Gras-Combe et al. report on a series of 14 patients who underwent intraoperative mapping of optic radiations with subcortical electrical stimulation during resection of low-grade gliomas involving the optic radiations. The mean age of the patients was 38 years, and the group included more males (10) than females. All patients presented with seizures. The majority of lesions were located at the temporooccipitoparietal junction. Preoperatively, no visual field deficits were identified. Each patient underwent surgery in the lateral position under local anesthesia. Electrical stimulation was performed utilizing a bipolar electrode with 5-mm spaced tips delivering a biphasic current for 1 msec from 2 to 4 mA. Cortical mapping of sensory, motor, and language pathways (if applicable) was performed first. A second surgical stage was performed with alternating resection and subcortical stimulation. During mapping of the the visual pathways, patients were exposed to 2 pictures placed diagonally on a screen situated in the quadrant to be saved and another in the opposite quadrant. This allowed testing of both language and visual function. While unaware of subcortical stimulation, the patients reported transient visual disturbances (blurred vision, phosphenes, shadow) within the contralateral visual hemifield. The visual disturbances were attributed to subcortical stimulation because they lasted the duration of the stimulation and stopped after cessation of the stimulation. All resections were limited based on functional boundaries especially relating to visual pathways. Ophthalmological examinations were performed between 1 to 6 months postoperatively, using perimetry, and MRI studies were performed immediately and at the 3-month follow-up examination. Six patients had speech disturbances that resolved within 3 months with rehabilitation. One patient suffered a homonymous hemianopia, and 10 others suffered an expected quadrantanopia. Complete resection was obtained in 3 cases, subtotal (residual tumor volume 10 cm3) in 3. One patient had postoperative aphasia, which resolved with rehabilitation. This innovative study does have some limitations. First, the authors acknowledged that evaluation of visual disturbances was subjective, with multiple variations, making it possible that the reported disturbances were not a result of subcortical stimulation of the visual system. The most clearly defined disturbance was that of blurred vision; phosphenes and shadows were less clearly described. The goal of the subcortical mapping of the visual pathways was to prevent homonymous hemianopia. Viegas et al.10 found that patients with a homonymous hemianopia following occipital glioma resection resumed normal social and professional lives with Karnofsky Performance Status scores of 90. This suggests that this visual loss is well tolerated and individuals are able to return to their lives. This brings into question whether, in fact, a homonymous hemianopia prevents someone from having a good quality of life. Is sparing the subcortical visual pathways important enough even to risk leaving gross tumor behind? Clearly, the study was not designed to answer this question. Quality of life was not assessed by means of objective measure before or after the surgery. Furthermore, from an oncological standpoint, follow-up was limited to 3 months, which leaves the question of long-term outcome, in light of there being only 3 patients who had complete resections and 11 who had subtotal resections. The authors did point out that their average extent of resection was 93.6%, but several studies now suggest that the more extensive the resection of low-grade gliomas, the better the long-term survival.3,9 In addition, the study group was a homogeneous population of younger individuals (mean age 38 years) who primarSee the corresponding article in this issue, pp 466–473.
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Aug 1, 2023
Fausto Caruana 
Open Access