Abstracts

Rationale: Complications from subdural electrode insertion are rare and include hemorrhage, cortical contusions and infections. To assess for complications, post-operative imaging is often required. Imaging performed with the electrodes in situ is limited by streak artifact on computed tomography (CT) and susceptibility artifact on magnetic resonance imaging (MRI). In addition, most commercially available subdural electrodes are not MRI compatible due to safety concerns. Therefore, the first opportunity for high quality post-operative imaging is following the explantation of the electrodes. There are no data available to determine what would be considered a normal MRI appearance following explantation of subdural electrodes. Knowledge of these normal findings would be of great assistance in interpreting MRI results in the rare symptomatic patients being assessed for complications. The purpose of this study is to describe the post-explantation MRI findings in asymptomatic patients who underwent insertion of subdural electrodes. Methods: Twenty consecutive patients who underwent subdural electrode insertion were studied. Within 24 hours after the removal of the electrodes, each patient underwent MR imaging that included axial T2-weighted, gradient echo, diffusion weighted and coronal FLAIR sequences. No significant adverse symptoms were reported by any of the patients. The studies were reviewed by an experienced, blinded neuroradiologist and categorized. Results: Of the 20 patients studied, 9 were male and the mean age was 36 years. Clinically silent post-explantation MRI abnormalities were found in all patients: Small subdural hematomas in 7 patients (25%), cortical contusions in 5 (18%), local edema in 5 (18%), transburrhole cortical herniation in 5 (18%), subdural hygromas in 2 (7%) and pneumocranium in 4 patients (14%). MRI abnormalities were subdivided into two groups: A) Abnormalities related to the site of electrode insertion and B) abnormalities related to the location of the electrodes. In group A, the entry site most commonly associated with MRI abnormalities was posterior temporal where cortical contusions were identified in 3 (18%) of 17 cases, local edema in 4 (24%), and transburrhole herniation in 4(24%). With frontal entry, cortical contusion was found in 2 (10%) of 20 cases, local edema in 1 (5%) and transburrhole herniation in 1 (5%). There were no abnormalities identified in the 6 parietal accesses. In group B, the most common location for subdural hematoma was interhemispheric (47%), followed by inferior (29%), lateral (14%) and inferior (14%) temporal. Subdural hygromas were identified over the frontal convexities and in the interhemispheric fissure. Pneumocranium was identified exclusively over the frontal convexities. Conclusions: Clinically silent MR abnormalities are common following insertion of subdural electrodes. Knowledge of these findings will be of great assistance in interpreting MRI results in the rare symptomatic patients being assessed for complications.