gms | German Medical Science

Objective: We recently showed that the critical subcortical mapping motor threshold (MT) for irreversible MEP changes and motor deficits is lower (<2 mA) than previously thought. A lower safe mapping MT could potentially increase the rate of gross total resections in tumors adjacent to the corticospinal tract (CST). However, mapping at MTs of 1-5 mA is potentially dangerous due to insufficient temporal and spatial coverage of the surgical site with the conventional monopolar fingerstick mapping probe during resection. We therefore developed a new continuous and dynamic mapping technique.

Method: We prospectively studied 41 patients, who underwent tumor surgery adjacent to the CST (<1 cm) with simultaneous subcortical monopolar motor mapping (train-of-five stimuli, interstimulus interval 4.0ms, pulse duration 500 µs) and DCS-MEP monitoring (same parameters, 4-contact strip electrode). Continuous (temporal coverage) and dynamic (spatial coverage) mapping were achieved by integrating the stimulation probe at the tip of a new suction device to enable close contact with tissue at the site of tumor resection. Motor function was assessed 1 day after surgery, at discharge, and at 3 months.

Results: In the first 24 patients, both stimulation probes were used and a 1:1 correlation of motor MTs for stimulation sites simultaneously mapped with the suction device and the fingerstick probe was found (74 stimulation points, r=0.996, p<0.001). Thereafter, mapping was performed using only the continuous dynamic technique. All procedures were technically successful. Lowest individual motor thresholds (MTs) were (MT, number of patients): >20 mA, n=3; 11–20 mA, n=5; 6–10 mA, n=4; 4–5 mA, n=13; 1–3 mA, n=16. MEP monitoring showed stable signals in 30 patients, unspecific changes in 8, irreversible alterations in 3, and irreversible loss in none. At 3 months, one patient had a persisting postoperative motor deficit (2.4%). Surgeons using the continuous dynamic mapping unanimously reported a higher confidence of knowing the exact location and safe distance from the CST compared to the interruptive workflow with the classical fingerstick probe.

Conclusions: Continuous dynamic mapping is a feasible and ergonomic technique for localizing the exact site and distance to the CST. The acoustic feedback and the ability to continuously stimulate exactly at the site of tissue removal improves the accuracy of mapping especially at low (<5 mA) MTs.