P 28. Comparison of a focal high intensity-coil and common round-coil for standard MEP diagnostics

Abstract

Introduction Diagnostic Motor Evoked Potentials (MEPs) are commonly elicited with a round coil for cortical and spinal stimulation in combination with peripheral electric stimulation. Especially for lower extremity MEPs, the stimulator output often limits the efficacy of the examination. This raises the issue, whether the lack of MEPs is the result of true pathophysiological changes or the result of non-optimal stimulation. Objectives High-intensity coils (HIC) induce a higher electric field thought for deeper tissue penetration. Thus, those might be useful for standard diagnostic setting especially for lower extremity muscles, as well for spinal and peripheral nerve stimulation. Another advantage is that the exam could be performed with one coil only. Thus motor thresholds and latencies obtained with an H-coil and a round coil were compared. Methods In 25 healthy volunteers (29 ± 11.6; 11 m), bilateral Abductor pollicis brevis muscle (APB) and Abd. hallucis muscle (AH) MEPs were elicited with the following protocol: (a) “high-intensity-coil” (Max e-field at 25 mm depth 204 V/m; Nexstim Co., Finland) for cortical, spinal (cervical and lumbar) and peripheral (median nerve at wrist, tibial nerve at medial malleolus) and (b) with a commercial round-coil (9 cm diameter, Magstim Co., UK) for cortical, spinal (cervical and lumbar) and electric anodal stimulation at the median nerve at wrist and tibial nerve at the medial malleolus. Cortical stimulation was performed with slight preinnervation, all other stimulations were performed in relaxed muscle. Motor thresholds and latencies were compared with the paired t-test, the level of significance was set at 0.01. Results MEPs and peripheral responses could be elicited in all healthy volunteers. There were any significant differences between right and left muscles allowing for common comparison into one group. High intensity-coil Round coil Electrical stim. Muscle Cortical Spinal Peripheral Cortical Spinal Peripheral APB MT [mA] 34.0 ± 7.5 36 ± 4.4 31.8 ± 5.3 42.5 ± 8.7 33.9 ± 4.7 10.9 ± 4.2 Paired ttest HIC vs RC 0.0006 n/a Latency [ms] 22.8 ± 1.6 15.3 ± 1.9 3.8 ± 1 23.2 ± 1.8 15.4 ± 2.5 3.7 ± 0.9 Latency (median; ms) 22. Jul 15. Mrz Mrz 70 23. Mrz 15. Jun 03. Jun Paired ttest HIC vs RC 0.0117 0.466 0.176 AH MT [mA] 53.4 ± 9.5 31.0 ± 5.6 32.1 ± 4.0 64.8 ± 10.6 33.0 ± 4.8 14.8 ± 10.0 Paired ttest HIC vs RC 0.0002 n/a Latency [ms] 44.7 ± 4.5 27.4 ± 3.6 5.1 ± 1.2 45.1 ± 4.8 28.1 ± 3.7 5.7 ± 0.8 Latency (median; ms) 44.5 26. Apr 05. Feb 44.9 27. Apr 05. Mai Paired ttest HIC vs RC 0.2284 0.0113 0.0001 Conclusions MT for HIC was significantly shorter compared to the round coil, demonstrating the efficacy this coil type. As both coils were used at motor threshold, average latencies elicited with the H-coil were only 0.1 ms shorter, which reached a weak significance for cortical stimulation with the APB and for spinal stimulation for the AH, as well as for AH elicited by peripheral stimulation. This is explained by the deeper penetration of the electromagnetic field by the H-coil and in case of the peripheral stimulation by smaller distance to the recording electrode. The HIC may be used for standard diagnostics, but requires new normative values.