Structural white matter changes in descending motor tracts correlate with improvements in motor impairment after undergoing a treatment course of tDCS and physical therapy

Xin Zheng, Gottfried Schlaug, Xin Zheng, Gottfried Schlaug

Abstract

Motor impairment after stroke has been related to the structural and functional integrity of corticospinal tracts including multisynaptic motor fibers and tracts such as the cortico-rubral-spinal and the cortico-tegmental-spinal tract. Furthermore, studies have shown that the concurrent use of transcranial direct current stimulation (tDCS) with peripheral sensorimotor activities can improve motor impairment. We examined microstructural effects of concurrent non-invasive bihemispheric stimulation and physical/occupational therapy for 10 days on the structural components of the CST as well as other descending motor tracts which will be referred to here as alternate motor fibers (aMF). In this pilot study, ten chronic patients with a uni-hemispheric stroke underwent Upper-Extremity Fugl-Meyer assessments (UE-FM) and diffusion tensor imaging (DTI) for determining diffusivity measures such as fractional anisotropy (FA) before and after treatment in a section of the CST and aMF that spanned between the lower end of the internal capsule (below each patient's lesion) and the upper pons region on the affected and unaffected hemisphere. The treated group (tDCS + PT/OT) showed significant increases in the proportional UE-FM scores (+21%; SD 10%), while no significant changes were observed in an untreated comparison group. Significant increases in FA (+0.007; SD 0.0065) were found in the ipsilesional aMF in the treated group while no significant changes were found in the contralesional aMF, in either CST, or in any tracts in the untreated group. The FA changes in the ipsilesional aMF significantly correlated with the proportional change in the UE-FM (r = 0.65; p < 0.05). The increase in FA might indicate an increase in motor fiber alignment, myelination, and overall fiber integrity. Crossed and uncrossed fibers from multiple cortical regions might be one reason why the aMF fiber system showed more plastic structural changes that correlate with motor improvements than the CST.

Keywords: brain stimulation; diffusion tensor imaging; motor recovery; plasticity; rehabilitation; tDCS.

Figures

Figure 1
Figure 1
Lesion density map of treated and untreated groups. Lighter blue indicates voxels with greater lesion overlap across patients.
Figure 2
Figure 2
Axial slices showing the location of the CST (Green) and alternate motor fibers (aMF) (Red) overlaid onto a T1 template. The z-coordinates are in Tailarach space.
Figure 3
Figure 3
Graphs showing fractional anisotropy (FA) changes (± standard deviation) in the CST and aMF ROIs, in the treated and untreated group, on both hemispheres. Only the FA extracted from the aMF ROI on the lesion side of the treated group showed a significant (p ≤ 0.01, Bonferroni corrected) increase in FA (see panel A*). Remaining regional values in the treated group (B,E,F) or regional values in the untreated group (C,D,G,H) did not show significant changes.
Figure 4
Figure 4
Slice-by-slice cross-sectional plot of the FA change in the aMF ROI (red color) on the lesional side of the treated group (mean and SD). ROIs (red = aMF; green = CST) are superimposed onto a standardized T1-weighted image. Most pronounced and most consistent increases in FA across the entire group can be seen at the pontine level (z = −21, −20) and in a location in the vicinity of the red nucleus which typically can be found between z = 0 to −8, Talairach coordinates.
Figure 5
Figure 5
Correlation between changes in FA (obtained from the aMF on the lesional hemisphere) and proportional changes in UE-FM in the treated group (r = 0.65; p < 0.05). Red dots are those from the diffusion tensor imaging (DTI) scan that used 30 directions. No differences were seen between the DTI sequences that used 25 directions and the one that used 30 directions.

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