The human brain undergoes significant plastic change after stroke, yet the details of how this occurs remain obscure. A better understanding of this process may allow us to amplify functional recovery using brain stimulation, pharmacologic manipulation, or even stem cell therapy. In lieu of peering directly into the brain of a living human, multiple MRI techniques including resting state fMRI have recently emerged. Zhang and colleagues used a multimodal MRI approach to better characterize functional reorganization in chronic stroke patients.
The authors studied 26 chronic subcortical stroke patients who had achieved either partial or complete motor recovery. They compared cortical thickness, task-related cortical activation in M1, amplitude of low-frequency fluctuations (ALFF), and resting state functional connectivity (rsFC) between the stroke patients and 25 age-matched healthy controls. They discovered that stroke patients had an area of decreased cortical thickness in lesional M1 that also showed increased task-related activation, ALFF and rsFC in comparison to control subjects. In addition, stroke patients had a separate area of lesional M1 with no cortical atrophy that demonstrated increased cortical activation, ALFF, and rsFC. Those with complete motor recovery had dramatically increased ALFF in these areas while those with partial recovery had an intermediate increase in ALFF.
Resting state fMRI is an emerging field that holds promise to help elucidate the subtleties of brain reorganization after stroke. Unlike task-related fMRI, which requires some form of motor movement, even severely disabled stroke patients can participate in resting state studies. In general, functional connectivity decreases immediately after stroke then gradually returns to normal or slightly increased levels in those with good recovery. The ALFF also comes from resting state fMRI data. Though poorly understood, ALFF is thought to represent spontaneous regional neuronal activity in the resting state as opposed to connectivity. The findings in the current study, showing progressively increasing levels of ALFF depending on the state of recovery, suggest ALFF may emerge as an important imaging biomarker for stroke recovery. The next step would entail proving this in a follow up longitudinal study. Such an imaging biomarker would provide targets for focal brain stimulation and other rehabilitation strategies. The authors have shed light on how various MRI approaches characterize neural reorganization after stroke. Now we need to translate these findings into better therapies for our patients.
