gms | German Medical Science

Objective: Involvement of the corticospinal tract (CST) may cause primary motor deficits in patients with a malignant brain tumor. Diffusion tensor imaging (DTI) has been developed to visualize the subcortical fiber tracts and can contribute valuable diagnostic information by integrating navigated transcranial magnetic stimulation (nTMS) data for CST mapping. In particular, the fractional anisotropy (FA) is a metric measure of the directionality of molecular motion in DTI and may possibly reflect the integrity of the fibers. The clinical relation remains yet unclear.

Method: 30 patients with a malignant brain tumor of various entities adjacent to the M1 region or the CST underwent an anatomical standard MRI protocol including diffusion tensor imaging (DTI). Cortical M1 representations for the hand, foot and tongue were assessed by navigated transcranial magnetic stimulation (nTMS). Function associated DTI based tractography of the CST was carried out by a region of interest analysis of the coordinate revealing the highest motor evoked potential and the brainstem level. The spatial relationship of the CST and the tumor as well as the surrounding T2 lesion was assessed. The minimum fractional anisotropy (FA) was determined in the function associated voxels of the CST.

Results: 22 out of the 30 patients had a primary motor deficit recorded while 8 patients had not developed any deficit prior to or at admission. The minimum FA value of the CST was significantly lower in the group of patients with a primary motor deficit (0.22 ± 0.06 units) than the ones without any motor impairment (0.28 ± 0.06 units, p=0.04). However, there was no statistical correlation observed in relation to the degree of weakness or the entity (glial vs. metastatic). Independent of the motor function, there was no difference detected in the FA values of motor fibers infiltrated by tumor bulk (0.22 ± 0.06 units, n=7) compared to the preserved fibers (0.24 ± 0.06, n=23). However, the FA values of the CST affected by the perifocal T2 lesion (0.22 ± 0.06, n=19) were significantly lower than the unaffected ones (0.27 ± 0.05 units, n=11, p = 0.02).

Conclusions: Preoperative involvement of the CST as determined by the FA in function associated DTI based tractography was correlated to the presence or absence of motor deficits. Further study is needed to define its role in predicting postoperative motor function as well as possibly differentiate the pattern of the CST’s involvement by tumor growth.