(163) Difficult-to-localize epilepsy after stereoelectroencephalography: safety, efficacy, and technique of placing additional electrodes during the same admission

Rationale: Stereoelectroencephalography (SEEG) is an invasive procedure used to identify the epileptogenic zone (EZ) in patients with epilepsy for potential surgical intervention. While SEEG has excellent rates of identifying the presumptive EZ, a minority of patients will have an EZ that is difficult-to-localize even after extensive invasive monitoring. We attempted to develop a technique for placing additional electrodes in these situations, and then performed an analysis on complications and surgical outcomes.

Methods: An operative technique which involves maintaining previously placed electrodes and sterilely placing new electrodes was developed and implemented. Figure 1 and Figure 2 demonstrate patients who underwent additional electrode placements with this technique. All patients who underwent placement of additional SEEG electrodes during the same admission between December 2011 and November 2019 were retrospectively reviewed regarding demographic and surgical variables, peri-operative complications, post-operative complications, and Engel classification at last follow up.

Results: Fourteen patients (five male, nine female) met criteria, and had undergone SEEG evaluation with 198 electrodes. The vast majority of patients (13/14) had non-lesional epilepsy.  After unsuccessful localization of the epileptogenic zone after a mean of 9.6 days (std. dev. 6.0 days) of monitoring, they underwent placement of 77 additional electrodes (5.5 average electrodes per patient) to augment the original implantation. 42.9% of patients (6/14) had additional electrodes placed to expand coverage to an alternative hypothesis, 42.9% of patients (6/14) had additional electrodes placed to better define the surgical resection, and 14.2% (2/14) of patients had additional electrodes placed to expand coverage on the opposite side. No patients had new hemorrhage after implantation of additional electrodes. At no point in the peri-operative or post-operative period did any patients develop infection, wound breakdown, or require any kind of additional antimicrobial treatment or wound revision. 64.3% (9/14) of patients underwent surgery guided by the additional SEEG electrodes. Overall, 44.4% (4/9) of surgical patients had Engel class I outcomes. Figure 1 demonstrates post-technique placement of additional electrodes (*) in a patient who had undergone multiple prior invasive evaluations. Additional electrodes demonstrated a focal epilepsy amenable to surgery. The patient is now seizure-free at last follow-up after surgical intervention [A: anterior, S: superior, P: posterior]. Figure 2 demonstrates post-technique placement of additional electrodes (*) in another patient. Additional electrodes helped better define the surgical resection area. The patient is seizure-free at last follow-up after surgical intervention.

Conclusions: Occasionally patients undergoing SEEG evaluation continue to have difficult-to-localize epilepsy even after invasive monitoring. This technique for placing additional SEEG electrodes, while maintaining the previously placed electrodes, appears to be safe, effective, and had no infectious complications. A majority of patients who received additional electrodes went on to have surgery guided by the new data collected, of whom 44.4% were Engel Class I at last follow-up. When confronted with difficult-to-localize epilepsy even after invasive monitoring, it appears to be safe and potentially clinically effective to place additional electrodes during the same admission.

Funding: Please list any funding that was received in support of this abstract.: No funding was received in support of this abstract.