Association of Interhemispheric Structural Integrity With Transcallosal Interhemispheric Inhibition in People With Chronic Stroke.

After stroke, interhemispheric communication is often disrupted. Interhemispheric inhibition after transcranial magnetic stimulation (TMS) is thought to be mediated through the corpus callosum, but no studies have directly tested this association in people with chronic stroke. This study determined the association between TMS interhemispheric inhibition and structural integrity of the pathway connecting the bilateral motor cortices through the corpus callosum in people with chronic stroke. 33 individuals with chronic stroke [60.7 (8.3) years old; 24 male; 8.6 (6.6) years post-stroke] were enrolled. From a diffusion-weighted scan, fractional anisotropy (FA) values were extracted from the pathway between the lower limb representations of the motor cortices via the corpus callosum. TMS interhemispheric inhibition was assessed as the ipsilateral silent period (iSP) in both legs. We tested associations between FA of the corpus callosum, TMS iSP, motor impairment [Fugl Meyer Lower Extremity Assessment (FMLE)], and interlimb coordination. Fractional anisotropy values (mean [SD]: 0.35 [0.07]) were indicative of white matter damage in the corpus callosum. Ipsilateral silent periods were identified in the non-paretic (n = 31, mean [SD]: 59.9 [41.3]) and paretic TA (n = 24, mean [SD]: 60.8 [22.2]). Larger FA values were associated with shorter iSP duration in the paretic TA (R2 = 0.24, p = 0.02), better interlimb coordination (R2 = 0.19, p = 0.02), and lesser motor impairment (R2 = 0.14, p < 0.05). Results suggest that TMS interhemispheric inhibition is at least partially mediated via the corpus callosum. Preservation of corpus callosum after stroke may allow interhemispheric inhibition and motor function and coordination that more closely resembles that of individuals without stroke.