The crossed frontal aslant tract: A possible pathway involved in the recovery of supplementary motor area syndrome

Abstract

Introduction: Supplementary motor area (SMA) syndrome is a constellation of temporary symptoms that may occur following tumors of the frontal lobe. Affected patients develop akinesia and mutism but often recover within weeks to months. With our own case examples and with correlations to fiber tracking validated by gross anatomical dissection as ground truth, we describe a white matter pathway through which recovery may occur.

Methods: Diffusion spectrum imaging from the Human Connectome Project was used for tractography analysis. SMA outflow tracts were mapped in both hemispheres using a predefined seeding region. Postmortem dissections of 10 cadaveric brains were performed using a modified Klingler technique to verify the tractography results.

Results: Two cases were identified in our clinical records in which patients sustained permanent SMA syndrome after complete disconnection of the SMA and corpus callosum (CC). After investigating the postoperative anatomy of these resections, we identified a pattern of nonhomologous connections through the CC connecting the premotor area to the contralateral premotor and SMAs. The transcallosal fibers have projections from the previously described frontal aslant tract (FAT) and thus, we have termed this path the "crossed FAT."

Conclusions: We hypothesize that this newly described tract may facilitate recovery from SMA syndrome by maintaining interhemispheric connectivity through the supplementary motor and premotor areas.

Keywords: supplementary motor area syndrome; tractography; white matter connectivity.

Figure 1

Interhemispheric white matter connections of the supplementary motor area (SMA) and premotor area (PMA) through the corpus callosum (CC). (a) Tractography illustrating white matter path from the frontal aslant tract (FAT) to contralateral SMA and PMAs. Fibers in blue are originating from the left hemisphere; yellow fibers are originating from the right. (b) Gross anatomical dissection of fiber bundles, tracts are similar to the ones illustrated in (a). Fiber tracts (blue and yellow arrows) originate from the FAT and course through the CC to end at the contralateral PMA. (c) Same image as (b) without markings. (d) Closer view of white matter bundles exiting the CC toward left and right PMAs

Figure 2

Permanent supplementary motor area (SMA) syndrome, case 1. Patient with a low‐grade glioma of the left frontoparietal region, hyperintensities seen on coronal T2 (a) and on sagittal T2 FLAIR (b) imaging. Postresection of tumor revealing dissection through the SMA into the corpus callosum (CC) on T1 with (d) and without (c) contrast (blue arrows). Asterisks in (a) designate location of CC

Figure 3

Permanent supplementary motor area syndrome, case 2. Hyperintensities seen on T2 sagittal (a) and coronal (b) imaging demonstrating a patient with glioblastoma. As in Figure 2, postoperative images of T1 with contrast enhancement showing dissection into the corpus callosum (CC) from coronal (c) and sagittal (d) views (blue arrows). Asterisks in preoperative images of (a) and (b) designate CC location

Figure 4

Simplified illustration of the frontal aslant tract (FAT) and newly described transcollasal projections of the FAT. (a) FAT connecting the supplementary motor area (SMA) and inferior frontal gyrus (IFG). Right FAT shown in green and left FAT in blue. (b) Transcollasal FAT fibers originate from the previously described FAT. The fiber bundles transverse the corpus callosum (CC) connecting controlatereal premotor areas. (c) Proposed mechanism of permanent SMA syndrome. Dissection into the CC with complete separation of the SMA from the contralateral hemisphere may result in permanent SMA syndrome