A new study published in Nature Communications used an innovative MR imaging approach to measure structural changes in motor pathways after a stroke, finding that many patients’ nervous systems appear to shift use to undamaged pathways in the brainstem and spinal cord.
This may actually impair long-term recovery by relying on different motor pathways in a maladaptive way, according to Julius Dewald, PT, PhD, chair of Physical Therapy and Human Movement Sciences, and senior author of the study.
“Patients may not have enough resources left in the hemisphere of the brain where the stroke took place, and start to use the non-affected hemisphere to drive movement in a limb,” said Dewald, who is also a professor of Biomedical Engineering at the McCormick School of Engineering. “On the one hand, that might make a patient stronger and more able to lift their arm, but it may also make them less capable of making dextrous motions required for daily living, like reaching and grasping with their hand.”
The corticospinal tract is a motor pathway beginning at the brain that runs through the spinal cord, controlling the movements of the limbs, face and trunk, and is often affected by stroke.
While damage in these corticospinal tract connections after stroke is well-studied, less has been known about the fate of motor and sensory pathways, Dewald said. In the current study, Northwestern Medicine investigators used structural MRI to identify post-stroke changes in all brain and braistem pathways.
They discovered decreased white matter integrity in a number of sensorimotor pathways travelling through the lesioned hemisphere, as well as increased white matter structural integrity in the medial reticulospinal tract of non-lesioned hemisphere in patients after stroke. Further, the investigators discovered that the two measures were connected: the more severe the damage to the corticospinal tract, the greater the structural integrity of the medial reticulospinal tract became.
“It means the brain is relying more on this ipsi-laterally projecting pathway from the non-lesioned hemisphere,” said Haleh Karbasforoushan, fifth-year PhD student in the Northwestern University Interdepartmental Neuroscience program and lead author of the study.
While this adaption can help patients regain some function, Dewald said he believes it may reduce recovery, resulting in some of the characteristic movement deficits of patients who have had a stroke. When one hemisphere of the brain is impacted by stroke, the brainstem-to-spinal cord pathways at the side of the intact hemisphere become more excitable, he explains. Shifting function from the damaged side to the undamaged, but excitable side, may allow for some function but contribute to long-term distability.
“Whether we can potentially prevent some of these changes from happening, and have patients keep using the remaining resources from the damaged hemisphere, we’re not sure yet,” Dewald said. “That’s what we are in the process of investigating.”
Notably, the scientists used structural MRI to infer function, rather than measure function directly. This offers many advantages both in the study and in possible therapeutic applications, according to Karbasforoushan.
“Functional MRI in the cervical spine would not let us identify altered sensorimotor pathways , so using structural MRI instead could be a good way to measure neural pathway changes as a biomarker,” Karbasforoushan said. “This study opens avenues for further use of spinal cord MRI in brain and spinal cord disorders.”
This research was supported by funding from National Institutes of Health grants R01HD039343, R01NS105759 and F31NS105486.