Stratifying chronic stroke patients based on the influence of contralesional motor cortices: An inter-hemispheric inhibition study

•The relationship between interhemispheric inhibition and motor impairment is nonlinear (bimodal).•We have defined a criterion of Upper Extremity Fugl-Meyer score of 43, which can be used for designing tailored treatments.•Our results are in line with the bimodal-balance recovery model. A recent “bimodal-balance recovery” model suggests that contralesional influence varies based on the amount of ipsilesional reserve: inhibitory when there is a large reserve, but supportive when there is a low reserve. Here, we investigated the relationships between contralesional influence (inter-hemispheric inhibition, IHI) and ipsilesional reserve (corticospinal damage/impairment), and also defined a criterion separating subgroups based on the relationships. Twenty-four patients underwent assessment of IHI using Transcranial Magnetic Stimulation (ipsilateral silent period method), motor impairment using Upper Extremity Fugl-Meyer (UEFM), and corticospinal damage using Diffusion Tensor Imaging and active motor threshold. Assessments of UEFM and IHI were repeated after 5-week rehabilitation (n = 21). Relationship between IHI and baseline UEFM was quadratic with criterion at UEFM 43 (95%conference interval: 40–46). Patients less impaired than UEFM = 43 showed stronger IHI with more impairment, whereas patients more impaired than UEFM = 43 showed lower IHI with more impairment. Of those made clinically-meaningful functional gains in rehabilitation (n = 14), more-impaired patients showed further IHI reduction. A criterion impairment-level can be derived to stratify patient-subgroups based on the bimodal influence of contralesional cortex. Contralesional influence also evolves differently across subgroups following rehabilitation. The criterion may be used to stratify patients to design targeted, precision treatments.