Interhemispheric subdural electrodes: technique, utility, and safety.

BACKGROUND: Invasive monitoring using subdural electrodes is often valuable for characterizing the anatomic source of seizures in intractable epilepsy. Covering the interhemispheric surface with subdural electrodes represents a particular challenge, with a potentially higher risk of complications than covering the dorsolateral cortex. OBJECTIVE: To better understand the safety and utility of interhemispheric subdural electrodes (IHSE). METHODS: We retrospectively reviewed the charts of 24 patients who underwent implantation of IHSE by a single neurosurgeon from 2003 to 2010. Generous midline exposure, meticulous preservation of veins, and sharp microdissection were used to facilitate safe interhemispheric grid placement under direct visualization. RESULTS: The number of IHSE contacts implanted ranged from 10 to 106 (mean = 39.8) per patient. Monitoring lasted for 5.5 days on average (range, 2-24 days), with an adequate sample of seizures captured in all patients before explantation, and with a low complication rate similar to that reported for grid implantation of the dorsolateral cortex. One patient (of 24) experienced symptomatic mass effect. No other complications clearly related to grid implantation and monitoring, such as clinically evident neurological deficits, infection, hematoma, or infarction, were noted. Among patients implanted with IHSE, monitoring led to a paramedian cortical resection in 67%, a resection in a region not covered by IHSE in 17%, and explantation without resection in 17%. CONCLUSION: When clinical factors suggest the possibility of an epileptic focus at or near the midline, invasive monitoring of the paramedian cortex with interhemispheric grids can be safely used to define the epileptogenic zone and map local cortical function.