“Mail-slot” Technique for Minimally Invasive Placement of Subdural Grid Electrodes: A Single-institution Experience

In the management of multi-drug-resistant focal epilepsies, intracranial electrode implantation is employed for precise localization of the ictal onset zone. In select patients, subdural grid electrode implantation is utilized. Subdural grid placement traditionally requires large craniotomies to visualize the cortex prior to mapping. However, smaller craniotomies may enable shorter operations and reduced risks. We aimed to compare surgical outcomes between patients undergoing traditional large craniotomies with those undergoing tailored 'mini' craniotomies (the "Mail-Slot" technique) for subdural grid placement. This retrospective cohort study included 23 patients who underwent subdural electrode implantation for epilepsy monitoring between 2014 and 2020. Patients were categorized into mini craniotomies (N=9) and traditional large craniotomies (N=14) groups. Demographics, operative details, and outcomes were reviewed. Craniotomy size and number of electrodes were determined via post-hoc radiographs. Of the 23 patients studied, the mini group had smaller craniotomy sizes (mean=22.71 cm2 vs. 65.17 cm2, p<0.001) and higher electrode-to-size ratios (mean=4.25 vs. 1.71, p<0.0001). The mini group had slightly fewer total electrodes (mean=88.67 vs. 107.43, p=0.047). No significant differences were found in operative duration, blood loss, invasive EEG duration, complications, or Engel scores between the groups. One patient per group required further invasive epilepsy monitoring for localization; all patients underwent therapeutic surgery. Our findings suggest that mini craniotomies for subdural grid placement in epilepsy monitoring offer significant advantages, including smaller craniotomy sizes and shorter operation durations, without compromising safety or efficacy. These results support the trend towards minimally invasive, patient-tailored surgical approaches in epilepsy treatment.