gms | German Medical Science

Objective: Direct cortical stimulation (DCS) is considered gold standard for intraoperative mapping of motor cortex. Precise intraoperative identification of eloquent areas by transdural monopolar stimulation before dura opening might be advantageous to prevent unnecessary exposure of cortical areas. Thus, the feasibility of monopolar epidural stimulation (EDS) and its relation to DCS with regard to stimulation intensity and amount of muscle responses were studied.

Method: 20 patients (56 ± 12 years, 9 male) with eloquent located lesions and subject to craniotomy exposing the motor region were studied. After craniotomy, EDS and after cortex exposure DCS were performed. Localization was videotaped. MEPs of at least 6 contralateral muscles were stored on a neuromonitoring device (Inomed Co., Germany). EDS and DCS were performed with an anodal monopolar probe (2 mm diameter) and a high frequency stimulation (250 Hz, train-of-5 with 0.5 ms individual pulse width and an interstimulus-interval of 4 ms, maximum 30 mA) Motor threshold was established for each stimulation point.

Results: In 4/20 patients EDS did not elicit MEPs, but DCS was successful in all 20 patients. In the 16 patients, where EDS and DCS elicited MEPs, minimal threshold of EDS was 17 ± 6.1 mA (median 17.5 mA) compared to 15.1 ± 5.6 mA (median 15 mA) in DCS (p = 0.004; paired t-test). EDS elicited MEPs in 1,9 ± 1,7 muscles compared to 1 ± 0,8 muscles with DCS.

Conclusions: Stimulation intensities to elicit MEPs were expectedly higher for EDS and such explain the tendency toward more muscle responses. The close relation to the hot spot of the DCS allows using EDS for tailoring dura opening. This might prove a helpful tool especially in recurrent tumor surgery, where dural scarring hampers dura opening.