Pros and Cons of Insular Resection

Pro: Insular Resection Should Be the First-Line Intervention for Drug-Resistant Insular Epilepsy
Chengyuan Wu, MD
Surgical resection of the epileptogenic zone remains the first-line option for patients with drug-resistant epilepsy. Long-term seizure freedom rates are 66% in temporal lobe epilepsy, 46% in occipital and parietal lobe epilepsy, and 27% in frontal lobe epilepsy.47 With the increased adoption of invasive EEG, the diagnosis of epilepsy of insular onset has increased. The literature supports surgical resection for insular epilepsy. Across 5 separate case series involving 74 patients with an average follow-up of 3.5 years, 73% of patients were seizure free.48
As with all surgical interventions, we must weigh benefits with surgical risk. Unfortunately, early experience with open insular resections resulted in high morbidity and mortality.49 Resection of insular tumors continues to associate with morbidity rates from 20% to 45.5%.50 The insula certainly challenges surgeons with its deep-seated location, hidden by the frontal and temporal opercula, and its intimate relationship with the “candelabra” of the middle cerebral artery (MCA). Surgical risk, therefore, stems primarily from retraction injury and from damage of lenticulostriate arteries or MCA branches.51 An improved understanding of these concerns along with advances in surgical techniques has significantly reduced the risk of insular lobe surgery. In a more recent series, insular lesionectomy was associated with a permanent morbidity of 8% and no mortalities.50
With invasive EEG serving as the means by which insular epilepsy can be properly diagnosed, we believe that the technique of electrode implantation should be informed by the method of surgical resection. Although the Talairach’s method of insular investigation involves orthogonal transopercular electrode trajectories,41 we favor an oblique approach as described by Afif et al.52 We have taken this approach one step further by implanting 3 or 4 electrodes in a manner that recreates the borders of the tetrahedron-shaped insula (Figure 5).
Figure 5.  The insula is shaped like a tetrahedron or triangular pyramid (left). By taking oblique approaches to the insula and implanting 4 electrodes, we are able to mimic this anatomy and define the borders of the insula (right). In addition to improving our ability to localize the seizure onset zone to a particular region of the insula, this method of SEEG implantation allows us to take a “fence-post” approach to surgical resection of the insula.53 Because their entry points are distant from the craniotomy needed to access the insula, the electrodes remain in place during surgical resection. Consequently, the electrodes can then be used as internal landmarks to facilitate adequate, appropriate, and safe insular resection (Figure 6).
Figure 6.  Intraoperative view of SEEG electrodes serving as internal landmarks during insular resection. A complete anterior insulectomy has been performed, as superior, inferior, and anterior insular electrodes can be seen at the borders of the resection. SEEG indicates stereo electroencephalogram. In scenarios where the epileptogenic zone cannot be safely resected, neuromodulation is a viable option. When considering responsive neurostimulation (RNS), deep brain stimulation (DBS), or vagal nerve stimulation (VNS), however, we must understand that these modalities provide significant seizure reduction, but rarely seizure freedom. Specifically, although RNS, DBS, and VNS, in general, have reported seizure reduction rates of 70%, 40%, and 44%, respectively, they are associated with seizure freedom rates of 15% (for 1 year), 6%, and less than 10%.54-56 In comparison to the potential for seizure freedom with surgical resection, neuromodulation is definitively inferior and as such should only be considered when resection cannot be performed.
Overall, open resection of the insula is a safe and the most effective approach to drug-resistant insular epilepsy and should be considered as the first-line surgical option.

Con: Diagnosis and Treatment of Medically Refractory Insula Epilepsy: Challenges and Pitfalls
Jorge Gonzalez-Martinez, MD, PhD
Insular epilepsy is a particularly challenging topic in medically refractory epilepsies. The clinical and surgical challenges are intrinsically related to the heterogeneous semiological features of seizures arising from the insula and adjacent structures, the difficult access to the insular cortex, and the relative high morbidity associated with insula and perisylvian resections.
Regarding the technique of insular implantation, several series have addressed the technique and safety of insular exploration by the SEEG methodology.40, 41 The SEEG is arguably the most common and appropriate surgical method to explore the insula cortex among other invasive techniques. Since its inception by Talairach and Bancaud,57 the SEEG methodology, and in particular its technique, evolved over the years. Nevertheless, the common denominator among different techniques is accurate vascular imaging, particularly important for insula explorations. Even in the highly vascular insular cortex, SEEG can still be performed safely if planning and technique are performed carefully.
The most common SEEG depth electrode implantation technique is the transopercular approach,27 in which orientation of electrodes is perpendicular to the sagittal plane, as defined by the anterior commissure–posterior commissure line. The advantages of this approach include its common and widespread clinical application, its safety and efficacy for accessing the insula cortex and adjacent areas, and its ability to sample medial and lateral portions of the insula, as well as the adjacent frontal and temporal opercula.
Once localization methods confirm that seizures are in fact arising from the insula cortex, and the extent of the epileptogenic zone is defined, the surgical treatment strategies become the main challenge. Several published manuscripts reported the outcome and morbidity related to insula epilepsy surgeries.11,35, 48, 58 Alomar et al40 reported the results of 17 patients with nonlesional imaging who were surgically treated for medically intractable epilepsy (15 resection and 2 laser ablations). Overall, 11 of 15 patients with insular resection had favorable outcome (Engel I and Engel II), of whom 5 (33.3%) had an Engel I outcome and 6 (40%) an Engel II outcome. The remaining patients had either an Engel III (n = 3, 20%) or an Engel IV outcome (n = 1, 6.7%). In this cohort, 3 patients developed permanent neurological deficits related to hemiparesis (17.6%), with an additional 4 patients developing transient deficits and mild complications. Therefore, the total complication rate in this series was approximately 41%. Interestingly, all permanent motor deficits were related to the resection of the caudal dorsal insula and adjacent parietal operculum, possibly due to damage of the small caliber performant arteries originating from the MCA, which exclusively provide vascular supply to the more caudal aspect of the corona radiata, harboring motor, and sensory fibers (Figure 7).
Figure 7.  Postoperative T2 MRI image (coronal orientation) after right caudal rostral insula resection in a 22-year-old female, resulting in symptomatic infarct in the ipsilateral corona radiata (red arrow). The procedure resulted in seizure freedom, but with a only partially recovered left side hemiparesis. MRI indicates magnetic resonance imaging. In conclusion, the authors highlighted the dangers related to the dorsal caudal insula resections. Because of the relatively high morbidity associated with open resections in the most caudal–rostral aspect of the insular cortex, preoperative discussion should include alternative treatment approaches including RNS and focal laser ablation. In order to overcome the potential high morbidity related to this area, 2 patients at our center underwent MRI-guided stereotactic laser ablation targeting the dorsal–caudal insula cortex, resulting in no complications and good control of focal seizures.