Damage to the corticospinal tract is widely studied following unilateral subcortical stroke, whereas less is known about changes to other sensorimotor pathways. This may be due to the fact that many studies investigated morphological changes in the brain, where the majority of descending and ascending brain pathways are overlapping, and did not investigate the brainstem where they separate. Moreover, these pathways continue passing through separate regions in the spinal cord. Here, using a high-resolution structural MRI of both the brainstem and the cervical spinal cord, we were able to identify a number of microstructurally altered pathways, in addition to the corticospinal tract, post stroke. Moreover, decreases in ipsi-lesional corticospinal tract integrity and increases in contra-lesional medial reticulospinal tract integrity were correlated with motor impairment severity in individuals with stroke.

Following hemiparetic stroke, precise, individuated control of single joints is often replaced by highly stereotyped patterns of multi-joint movement, or abnormal limb synergies, which can negatively impact functional use of the paretic arm. One hypothesis for the expression of these synergies is an increased dependence on bulbospinal pathways such as the rubrospinal and especially the reticulospinal tracts, which coactivate multiple muscles of the shoulder, elbow, wrist and fingers. Despite indirect evidence supporting this hypothesis in humans post-stroke, it still remains unclear whether it is correct. Therefore, we used high-resolution diffusion tensor imaging to quantify white matter microstructure in relation to severity of arm synergy and hand related motor impairments. Diffusion tensor imaging was performed on 19 moderately to severely impaired chronic stroke individuals and 15 healthy, age-matched controls. In stroke individuals compared to controls, there was significantly decreased fractional anisotropy and significantly increased axial and radial diffusivity in bilateral corona radiata and body of the corpus callosum. Furthermore, post-stroke the contralesional reticulospinal tract fractional anisotropy correlated significantly with both upper extremity synergy severity (r=-0.606, p=0.003) and hand impairment (r=-0.609, p=0.003). Fractional anisotropy in the ipsilesional rubrospinal tract significantly correlated with hand impairment severity (r=-0.590, p=0.004). For the first time, we separately evaluate reticulospinal and rubrospinal tract microstructure in chronic stroke individuals with upper extremity motor impairment. We demonstrate that individuals with the greatest upper extremity synergy severity and hand impairments post-stroke have the highest fractional anisotropy in the contralesional reticulospinal tract, a pattern consistent with increased myelination and suggestive of neuroplastic reorganization. Since the reticulospinal pathway microstructure, in particular, is sensitive to abnormal joint coupling and hand related motor impairment in chronic stroke, it could help test the effects of specific, and novel, anti-synergy neurorehabilitation interventions for recovery from hemiparesis.