gms | German Medical Science

Objective: Epidural stimulation (EDS) for tailored dura opening was introduced to minimize unnecessary exposure of primary cortical motor areas. To overcome dura impedance, higher stimulation intensities to elicit motor evoked potentials (MEP) are needed, but subdural cerebrospinal fluid (CSF) facilitates electrical current spreading. Both might result in activation of more distant cortical areas and spatial inaccuracy. EDS and direct cortical stimulation (DCS) were compared with regard to spatial distribution of motor responses and their respective motor thresholds.

Method: 19 patients (60 ± 14 years; 13 male) with subject to craniotomy exposing the motor cortex were studied. A bone cement template of the size of the craniotomy with grid-like burr holes was made for point-to-point stimulation. EDS and DCS were performed with an anodal monopolar probe (2mm diameter) referenced to Fpz and a train of 5 pulses (0.5ms individual pulse width, 4ms interstimulus interval, maximum current 30mA). Motor threshold was established for each stimulation point. MEPs were recorded from contralateral biceps brachii, extensor digitorum communis, first interdigitorum, abductor hallucis and additional muscles according to the cortical area of interest.

Results: DCS elicited MEPs in 142/259 stimulation points (54%). EDS elicited MEPs in 149/259 stimulation points (58%). In 13/142 DCS-positive stimulation points (9%) EDS was not successful (false negatives). This was observed in 6/19 patients (32%). In 12/13 of those false negative stimulation points (92%), EDS elicited MEPs in the close neighborhood (i.e. maximum distance of 2cm or two point distances). EDS elicited MEPs in 20/117 DCS-negative stimulation points (17%; false positives). The results yield in a sensitivity of 0.91 and specificity of 0.83. The average EDS intensity to elicit MEPs was 16mA. For DCS, the average stimulation intensity to elicit MEPs was 13mA. Eliciting an MEP with DCS that caused a false negative during EDS required an average stimulation intensity of 21.5mA.

Conclusions: In the majority of the stimulation points, EDS allowed for point-to-point epidural location of motor cortex and thus is useful for tailored dura opening. False negative results were related to higher DCS intensities compared to congruent EDS-DCS-points. This can be explained by the limited maximum stimulation intensity and unpredictable current spreading towards areas of lower resistance. This has to be taken into consideration when using EDS.