Clinically silent magnetic resonance imaging findings after subdural strip electrode implantation

Subdural strip electrodes (SSEs) are often used as part of the workup in patients being considered for epilepsy surgery. To assess for complications or to confirm electrode placement, postoperative imaging is often performed. Imaging performed with the electrodes in situ is limited by streak artifact on CT and susceptibility artifact on MR imaging. Therefore, the first opportunity for high-quality postoperative imaging is following explantation of electrodes. There is no data available to determine what would be the expected MR imaging appearance following insertion of SSE. The purpose of this study is to describe the MR imaging findings in asymptomatic patients who underwent insertion of SSEs.

Twenty consecutive patients who underwent SSE insertion were studied. Within 24 hours after removal of the electrodes, each patient underwent MR imaging that included axial T2-weighted, gradient echo, diffusion weighted, and coronal FLAIR sequences. No significant symptoms were reported by any of the patients. The studies were reviewed by an experienced, blinded neuroradiologist and categorized.

Of the 20 patients studied, 11 were female (mean age 36 years). Clinically silent postexplantation MR imaging abnormalities were found in all patients: subdural hematomas in 7 (35%), cortical contusions in 5 (25%), local edema in 5 (25%), trans-bur hole cortical herniation in 5 (25%), subdural hygromas in 2 (10%), and pneumocranium in 4 (20%). The MR imaging abnormalities were subdivided into 2 types: Type A, abnormalities related to the site of electrode insertion; and Type B, abnormalities related to the location of the electrodes. The most common location for a Type A abnormality was occipitotemporal, with cortical contusions occurring in this location in 18% of cases, local edema in 24%, and trans-bur hole herniation in 24%. The next most common location was frontal, with cortical contusions found in this location in 10% of cases, local edema in 5% and trans-bur hole herniation in 5%. The most common Type B abnormality was a subdural hematoma, followed by pneumocranium and subdural hygroma.

Clinically silent MR imaging abnormalities are common following SSE placement. Knowledge of these findings would be of assistance in interpreting MR imaging results in patients being assessed for complications.