Abstract
Following a stroke, the resulting lesion creates contralateral motor impairment and an interhemispheric imbalance involving hyperexcitability of the contralesional hemisphere. Neuronal reorganization may occur on both the ipsilesional and contralesional hemispheres during recovery to regain motor functionality and therefore bilateral activation for the hemiparetic side is often observed. Although ipsilesional hemispheric reorganization is traditionally thought to be most important for successful recovery, definitive conclusions into the role and importance of the contralesional motor cortex remain under debate. Through examining recent research in functional neuroimaging investigating motor cortex changes post-stroke, as well as brain-computer interface (BCI) and transcranial magnetic stimulation (TMS) therapies, this review attempts to clarify the contributions of each hemisphere toward recovery. Several functional magnetic resonance imaging studies suggest that continuation of contralesional hemisphere hyperexcitability correlates with lesser recovery, however a subset of well-recovered patients demonstrate contralesional motor activity and show decreased functional capability when the contralesional hemisphere is inhibited. BCI therapy may beneficially activate either the contralesional or ipsilesional hemisphere, depending on the study design, for chronic stroke patients who are otherwise at a functional plateau. Repetitive TMS used to excite the ipsilesional motor cortex or inhibit the contralesional hemisphere has shown promise in enhancing stroke patients' recovery.
Highlights
Stroke remains a leading cause of long-term disability (Hoyer and Celnik, 2011), affecting nearly 800,000 people annually within the United States (Mozaffarian et al, 2016), and causing almost 80% of patients to have persistent motor impairment following traditional therapy (Mayo et al, 1999)
This study suggests that increased chronicity of stroke causes impairment of beneficial effects of rTMS in motor recovery (Rose et al, 2014)
Increased contralesional M1 recruitment has been demonstrated for well-recovered patients with larger lesions (Touvykine et al, 2016)
Summary
Stroke remains a leading cause of long-term disability (Hoyer and Celnik, 2011), affecting nearly 800,000 people annually within the United States (Mozaffarian et al, 2016), and causing almost 80% of patients to have persistent motor impairment following traditional therapy (Mayo et al, 1999). Initial stroke recovery correlates to resolution of necrotic tissue, edema, and inflammation (Furlan et al, 1996; Stinear and Byblow, 2014), while later recovery relates mainly to disinhibition of redundant neural circuits, recruitment of functionally homologous pathways, and the creation of neural connections to overtake the previous functions of the damaged neurons (Rossini et al, 2007; Murphy and Corbett, 2009; Ackerley et al, 2011) Such neuronal processes may occur on the ipsilesional and contralesional hemispheres and are not completely understood (Hoyer and Celnik, 2011; Buetefisch, 2015); the relationship between the two motor cortex (M1) hemispheres post-stroke remains a topic of great interest (Baron et al, 2004; Hummel et al, 2008). This mini review attempts to broaden the scientific community’s understanding of the roles of the M1 hemispheres in stroke rehabilitation by: (1) Analyzing current research on the role of the M1 hemispheres for spontaneous stroke recovery, and (2) Examining results from a therapy that relies upon specific M1 neuronal signals for feedback (BCI) as well as therapy that targets specific M1 hemispheres (TMS)
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