Role of contralesional cortico-reticulospinal tract compensation in walking recovery after stroke.

Purpose: Evidence suggests the contralesional cortico-reticulospinal tract (cCRST) upregulates after stroke, and that this upregulation correlates with worse motor function, suggesting it may be harmful for walking recovery. However, this relationship may be confounded by the extent of ipsilesional corticospinal tract (CST) and CRST damage, which could cause both greater cCRST upregulation and worse walking function. No previous studies have tested whether this confounding relationship exists, nor whether the amount of damage to the ipsilesional motor tracts is related to the amount of cCRST upregulation. We hypothesized that lower ipsilesional motor tract strength would: (1) be associated with greater cCRST compensation; and (2) explain the negative association between cCRST compensation and worse walking function. Methods: Ten individuals with chronic stroke and ten age- and sex-matched controls completed diffusion MRI, from which quantitative anisotropy was derived to evaluate the strength of the ipsilesional and contralesional CRST and CST. Walking capacity was assessed using 6-minute walk distance (6MWD). Linear regressions were applied to examine relationships among ipsilesional corticomotor tract strength (iCRST and iCST combined), cCRST strength, and 6MWD. Results: Compared with controls, participants with stroke had lower ipsilesional and higher contralesional strength for both motor tracts. Lower ipsilesional tract strength was associated with greater cCRST strength z-score (-0.12 SDs [-0.23, -0.02]). The unadjusted association between greater cCRST strength z-score and lower walking capacity (-72 meters [-136, -9]) was no longer present after adjusting for ipsilesional tract strength (-3 meters [-28, 23]). Conclusions: Greater damage to ipsilesional motor tracts (lower strength) was associated with increased cCRST strength. The extent of ipsilesional tract injury fully explained the negative association between cCRST strength and worse walking capacity. These findings suggest that cCRST upregulation is an adaptive compensation mediated by the extent of ipsilesional tract damage, and unlikely to impede walking recovery.