Preoperative multimodal motor mapping: a comparison of magnetoencephalography imaging, navigated transcranial magnetic stimulation, and direct cortical stimulation

Abstract

Object: Direct cortical stimulation (DCS) is the gold-standard technique for motor mapping during craniotomy. However, preoperative noninvasive motor mapping is becoming increasingly accurate. Two such noninvasive modalities are navigated transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG) imaging. While MEG imaging has already been extensively validated as an accurate modality of noninvasive motor mapping, TMS is less well studied. In this study, the authors compared the accuracy of TMS to both DCS and MEG imaging.

Methods: Patients with tumors in proximity to primary motor cortex underwent preoperative TMS and MEG imaging for motor mapping. The patients subsequently underwent motor mapping via intraoperative DCS. The loci of maximal response were recorded from each modality and compared. Motor strength was assessed at 3 months postoperatively.

Results: Transcranial magnetic stimulation and MEG imaging were performed on 24 patients. Intraoperative DCS yielded 8 positive motor sites in 5 patients. The median distance ± SEM between TMS and DCS motor sites was 2.13 ± 0.29 mm, and between TMS and MEG imaging motor sites was 4.71 ± 1.08 mm. In no patients did DCS motor mapping reveal a motor site that was unrecognized by TMS. Three of 24 patients developed new, early neurological deficit in the form of upper-extremity paresis. At the 3-month follow-up evaluation, 2 of these patients were significantly improved, experiencing difficulty only with fine motor tasks; the remaining patient had improvement to 4/5 strength. There were no deaths over the course of the study.

Conclusions: Maps of the motor system generated with TMS correlate well with those generated by both MEG imaging and DCS. Negative TMS mapping also correlates with negative DCS mapping. Navigated TMS is an accurate modality for noninvasively generating preoperative motor maps.

Fig. 1

Axial MR images showing synthetic aperture magnetometry analysis of MEG imaging data in 1 patient, localizing the motor site for the index finger (blue areas).

Fig. 2

Images obtained in a patient with a tumor in the somatosensory cortex. Left: Transcranial magnetic stimulation map; no positive motor sites were identified. Right: Intraoperative photograph; numbered squares indicate sensory sites identified with DCS.

Fig. 3

Example of a multimodal motor map showing MEG imaging (purple sphere), TMS (orange pins), and DCS (orange spheres) sites overlaid. ADQ = abductor digiti quinti; APB = abductor pollicis brevis; O. oris = orbicularis oris.

Fig. 4

Example of a bimodal motor map with MEG imaging (purple sphere) and TMS (orange pins) sites overlaid. This patient had a negative motor map using DCS.

Fig. 5

Scatterplot of distances from MEG imaging of the index finger motor site to TMS of the abductor digiti quinti and abductor pollicis brevis motor sites. The labeled horizontal bar in each series represents the median distance.

Fig. 6

Bar graph showing the median distance from TMS motor sites (aggregated abductor digiti quinti and abductor pollicis brevis data) to corresponding DCS motor sites and to MEG imaging index finger motor sites (error bar = 95% CI).

Fig. 7

Bar graph showing the median distance from interpolated TMS motor sites (midpoint of abductor digiti quinti and abductor pollicis brevis muscles) to corresponding DCS motor sites and to MEG imaging index finger motor sites (error bar = 95% CI).

Fig. 8

Scatterplot of distances from TMS motor sites to corresponding DCS motor sites and to MEG imaging index finger motor sites.