Subdural Electrodes Beneficial for Epilepsy Monitoring

By: SHARON WORCESTER, Clinical Neurology News Digital Network

The implantation of subdural electrodes for the treatment of intractable epilepsy is beneficial, but requires careful surveillance during the monitoring period, according to findings from a study of 91 consecutive patients.

This is especially true for those who undergo large subdural grid placement, as these patients have an increased risk for complications, Dr. Fernando L. Vale of the University of South Florida, Tampa, and his colleagues reported online in Clinical Neurology and Neurosurgery.

Of 508 patients who underwent surgical intervention for the evaluation and treatment of medically resistant epilepsy at a single center from 1999 to 2010, 91 (18%) required invasive subdural electrode placement and were included in this study.

Twenty-eight of those patients (31%) had a radiographic lesion on preoperative high-resolution magnetic resonance imaging (MRI), including 13 with evidence of neuronal migrational disorder, 9 with radiographic evidence of gliosis or encephalomalacia of unknown origin, 3 with benign neoplastic lesions, 2 with documented arachnoid cysts, and 1 with radiographic evidence of mesial temporal lobe sclerosis. Resective epilepsy surgery was performed in 70 (77%) of these patients, and 24 of those (34%) were seizure free at last follow-up, the investigators said (Clin. Neurol. Neurosurg. 2012 Nov. 5 [doi: 10.1016/j.clineuro.2012.10.007]).

A very strong trend was seen for improved outcomes in those with positive lesions on preoperative MRI, compared with those with a normal brain MRI, the investigators said.

No significant associations were found between outcomes and preoperative positron emission tomography (PET) results, ictal single photon emission computed tomography (SPECT) results, type of implant, or lateralization or localization of subdural implants, they noted.

Ten surgical complications occurred, including radiographically evident subdural or epidural hemorrhage in eight patients, a transient cerebrospinal fluid leak in one patient, and a subdural empyema following removal of the electrodes in one patient. The latter required prolonged intravenous antibiotics and removal of the bone flap.

Of the eight patients with hemorrhage, four were symptomatic and required evacuation of the hematoma, and two underwent removal of the electrodes during emergency craniotomy. All required observation with a prolonged intensive care unit stay.

“All but one of these patients had undergone placement of an electrode array that included a grid[,] and more significant[ly], all symptomatic subdural hemorrhage patients had undergone placement of a grid with or without additional subdural strip electrodes,” the investigators noted.

Indeed, placement of a subdural grid in any combination was significantly associated with complications, they said.

However, none of the patients died or experienced permanent morbidity.

The patients included in the study were 55 men and 36 women with an average age of 32.2 years who underwent subdural electrode placement between 1999 and 2010. The electrodes were placed when ictal recordings were inadequate due to extensive muscle artifact or poorly localized ictal onset, when preoperative scalp electrode encephalogram and neuroimaging findings were discordant, and/or when the epileptogenic zone was localized near eloquent cortex. More than half (57%) of the patients underwent strip placement only, 5 (5.5%) underwent grid placement only, and 34 (37%) underwent both. The mean duration of monitoring was 7 days, and patients were followed for at least 18 months (mean of 42 months).

The findings indicate that although the use of subdural electrode placement has diminished over the last two decades due to improvements in brain imaging, better definition of syndromes amenable to surgery, and better patient selection, such invasive monitoring can improve seizure control and the possibility of cure. Subdural electrode placement thus remains “a useful and necessary technique for the surgical treatment of intractable epilepsy,” despite the possibility of complications, they said.

Careful surveillance during the monitoring period, along with a good working hypothesis, assessment of the risk-benefit ratio, patient selection, and meticulous surgical technique, is a must for minimizing complications and achieving better outcomes, they concluded.

Given the limitations of a single-center, retrospective study, however, the authors recommend additional study to corroborate the clinical findings.

Dr. Vale said there are no disclosures for any of the authors of this article.