Beyond the nerve: Understanding cortical contributions to post-paralytic facial synkinesis

Patients with post-paralytic facial synkinesis (PPFS) experience a diverse array of neuronal changes from aberrant peripheral neurogenesis to cortical changes. The aim of this review is to utilize a comprehensive search strategy to (1) identify studies using functional magnetic resonance imaging (fMRI) to evaluate PPFS, and (2) synthesize the available evidence for any unifying structural and functional cortical changes which hold promise for future investigation. A search of Pubmed and Embase databases was conducted with a research librarian. Articles were then screened and reviewed independently by two authors. Inclusion criteria specified studies reporting quantitative imaging evaluation of post-paralytic facial synkinesis. Cerebral cortical changes were evaluated using diffuse tensor imaging, task-based, and resting-state fMRI. Specific assessments of sameness (Dice coefficients), transmission patterns (functional anisotropy), synchronization (Voxel-Mirrored Homotopic Connectivity: VMHC), intrinsic activity (Amplitude of Low Frequency Fluctuations: ALFF), and network efficiency (rich-club nodes) were all examined. Of the 359 studies identified, 13 studies met inclusion criteria. The cortical changes observed included diffuse/overlapping activation patterns, impaired interhemispheric communication, structural brain changes, and structural/functional connectivity mismatch. Evidence of cortical reorganization is apparent in post-paralytic facial synkinesis. Maladaptive neuroplasticity is a hallmark of multiple neuropathologic disorders and should be investigated as a future therapeutic target.