Intracranial EEG for seizure focus localization: evolving techniques, outcomes, complications, and utility of combining surface and depth electrodes

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Intracranial electroencephalography (iEEG) provides valuable information that guides clinical decision-making in patients undergoing epilepsy surgery, but it carries technical challenges and risks. The technical approaches used and reported rates of complications vary across institutions and evolve over time with increasing experience. In this report, the authors describe the strategy at the University of Iowa using both surface and depth electrodes and analyze outcomes and complications.


The authors performed a retrospective review and analysis of all patients who underwent craniotomy and electrode implantation from January 2006 through December 2015 at the University of Iowa Hospitals and Clinics. The basic demographic and clinical information was collected, including electrode coverage, monitoring results, outcomes, and complications. The correlations between clinically significant complications with various clinical variables were analyzed using multivariate analysis. The Fisher exact test was used to evaluate a change in the rate of complications over the study period.


Ninety-one patients (mean age 29 ± 14 years, range 3–62 years), including 22 pediatric patients, underwent iEEG. Subdural surface (grid and/or strip) electrodes were utilized in all patients, and depth electrodes were also placed in 89 (97.8%) patients. The total number of electrode contacts placed per patient averaged 151 ± 58. The duration of invasive monitoring averaged 12.0 ± 5.1 days. In 84 (92.3%) patients, a seizure focus was localized by ictal onset (82 cases) or inferred based on interictal discharges (2 patients). Localization was achieved based on data obtained from surface electrodes alone (29 patients), depth electrodes alone (13 patients), or a combination of both surface and depth electrodes (42 patients). Seventy-two (79.1%) patients ultimately underwent resective surgery. Forty-seven (65.3%) and 18 (25.0%) patients achieved modified Engel class I and II outcomes, respectively. The mean follow-up duration was 3.9 ± 2.9 (range 0.1–10.5) years. Clinically significant complications occurred in 8 patients, including hematoma in 3 (3.3%) patients, infection/osteomyelitis in 3 (3.3%) patients, and edema/compression in 2 (2.2%) patients. One patient developed a permanent neurological deficit (1.1%), and there were no deaths. The hemorrhagic and edema/compression complications correlated significantly with the total number of electrode contacts (p = 0.01), but not with age, a history of prior cranial surgery, laterality, monitoring duration, and the number of each electrode type. The small number of infectious complications precluded multivariate analysis. The number of complications decreased from 5 of 36 cases (13.9%) to 3 of 55 cases (5.5%) during the first and last 5 years, respectively, but this change was not statistically significant (p = 0.26).


An iEEG implantation strategy that makes use of both surface and depth electrodes is safe and effective at identifying seizure foci in patients with medically refractory epilepsy. With experience and iterative refinement of technical surgical details, the risk of complications has decreased over time.

ABBREVIATIONS ATL = anterior temporal lobectomy; iEEG = intracranial electroencephalography; MCD = malformation of cortical development; MTS = mesial temporal sclerosis; VNS = vagal nerve stimulator.

Article Information

Correspondence Yasunori Nagahama: University of Iowa Hospitals and Clinics, Iowa City, IA.

INCLUDE WHEN CITING Published online May 25, 2018; DOI: 10.3171/2018.1.JNS171808.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.



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    Placement of intracranial electrodes. A: Intraoperative photograph showing a left-sided craniotomy and exposure for a pediatric patient who underwent iEEG with bilateral but predominantly left-sided coverage. Note that the peripheral dural cuff is tented up with black dural sutures to the pericranium or temporalis muscle. B: Intraoperative photograph obtained in the same patient, showing placement of surface and depth electrodes secured to the dural cuff. C: Postoperative axial CT scan showing the elevated placement of the bone flap on the left side. D: The postoperative reconstruction image showing the locations of surface electrode contacts and entry points for depth electrodes. Figure is available in color online only.

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    Axial CT images obtained in 3 patients with postimplantation hemorrhagic complications. A: Left-sided acute epidural hematoma. B: Right-sided acute subdural hematoma, as well as subgaleal hematoma. C: Left frontal intraparenchymal hemorrhage (arrow) associated with a depth electrode (the high density dot is due to the artifact).

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    Axial CT images obtained in 2 patients with postimplantation edema/compression complications. Right-sided (A) and left-sided (B) electrode placement with associated brain edema and compression, causing significant mass effect and midline shift.





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