The distribution of transcallosal inhibition to upper extremity muscles is altered in chronic stroke

Objective: To determine if the distribution of transcallosal inhibition (TI) acting on proximal and distal upper extremity muscles is altered in chronic stroke. Methods: We examined thirteen healthy controls and sixteen mildly to moderately impaired chronic stroke patients. We used transcranial magnetic stimulation (TMS) to probe TI from the contralesional onto ipsilesional hemisphere (assigned in controls). We recorded the ipsilateral silent period in the paretic biceps (BIC) and first dorsal interosseous (FDI). We measured TI strength, distribution gradient (TI difference between muscles), and motor impairment (Fugl-Meyer Assessment). Results: Both groups had stronger TI acting on their FDIs than BICs (p < 0.001). However, stroke patients also had stronger TI acting on their BICs than controls (p = 0.034), resulting in a flatter distribution of inhibition (p = 0.028). In patients, stronger FDI inhibition correlated with less hand impairment (p = 0.031); BIC inhibition was not correlated to impairment. Conclusion: TI is more evenly distributed to the paretic FDI and BIC in chronic stroke. The relative increase in proximal inhibition does not relate to better function, as it does distally. Significance: The results expand our knowledge about segment-specific neurophysiology and its relevance to impairment after stroke.