Spectrum of clinical and histopathologic responses to intracranial electrodes: From multifocal aseptic meningitis to multifocal hypersensitivity-typemeningovasculitis

Purpose: We sought to characterize and compare the histopathologic and clin ical changes elicited by subdural and depth electrodes in subjects undergoi ng epilepsy surgery evaluation. Methods: A retrospective review of clinical records, imaging and histopatho logic studies of epilepsy surgery cases requiring subdural strips and depth electrodes for localization of epileptogenic tissue was performed between 1993 and 1999. Forty-nine subjects had a combination of subdural and depth, whereas 10 had depth electrodes only. Histopathologic changes were classif ied as mild, moderate, or severe based on the density, extent, and composit ion of the inflammatory infiltrate. Results: Subdural electrodes induced a clinical picture of transient asepti c meningitis; histopathologically, the infiltrates were moderate in degree in the majority (73%) and severe in the remainder (27%), with T cells and e osinophils infiltrating the cortex and arteriolar walls (hypersensitivity-t ype response). Depth electrodes alone caused minimal or no symptoms of meni ngeal irritation; the cellular response elicited by these electrodes was mi ld in five and moderate in the remaining five cases; severe inflammation wa s not observed in this group. Although the proportion of small clinically s ilent hematomas was larger in cases with depth (five of 59) compared with s ubdural electrodes(one of 49), microhemorrhages were considerably more nume rous with subdural than with depth electrodes. Conclusions: These results suggest that the spectrum of brain responses to foreign bodies is wide, ranging from self-limited physiologic to hypersensi tivity-type reactions of varying severity. Subdural strips elicited more in tense inflammation than did depth electrodes. The histopathologic extent of the reaction to either type of electrodes could not be precisely defined b ecause of the retrospective nature of this study. History of allergy to lat ex or previous craniotomies are probable risk factors for the hypersensitiv ity-type reaction. Surgical outcome, excellent in the majority, was indepen dent of the severity or type of inflammation, and there have not been neuro logic or systemic sequelae.