19. Tailored dura opening in eloquent areas – Is epidural monopolar stimulation a useful tool?

Introduction Direct cortical stimulation (DCS) is considered the gold standard for intraoperative mapping of motor cortex. As wide dura opening for exposure of the motor cortex is not always possible, transdural monopolar stimulation might help for tailored dura opening and 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. Patients and material 25 patients (57  ±  12 years; 11 male) with eloquent area-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, co-localization was given within a 1 cm range. 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 train-of-5 pulses (0.5 ms individual pulse width, 250 Hz, maximum 30 mA). Motor threshold was established for each stimulation point. Results In 4/25 (40%) patients EDS did not elicit MEPs, but DCS was successful in 22 (88%) patients. In the 20 patients, where EDS and DCS elicited MEPs, the minimum threshold of EDS was 17 ± 6.4 mA compared to 14.1  ±  5.3 mA (median 15 mA) in DCS ( p  = 0.006; paired t-test); thus being true positive in 0.97, true negative in 0.5 cases and resulting in a sensitivity of 0.91 and specificity of 0.67. EDS elicited MEPs in 1.8  ±  1.1 muscles compared to 1.6  ±  1 muscles with DCS. Discussion Stimulation intensities to elicit MEPs were expectedly higher for EDS, which explains 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 tumour surgery where dural scarring hampers dura opening.