Bilateral muscle activation in postparalytic facial synkinesis: a cross-sectional high-resolution surface electromyography study

Facial synkinesis (FS) is a facial muscle coordination disorder following facial paralysis. We hypothesized that FS involves the entire network of facial muscles on both sides of the face. Bilateral facial high-resolution surface electromyography (HR-sEMG), using a total of 58 electrodes, was performed during 11 standardized facial movements in 36 healthy adult participants (53% female, 18-67 years) and 36 patients with FS (81% female, 24-70 years). Differences between the synkinetic and contralateral sides of patients and healthy controls were evaluated using linear mixed-effects models. Each movement task elicited a complex activation pattern involving nearly all measured facial muscles. In healthy controls, HR-sEMG activity was highest in the muscles primarily responsible for generating the intended voluntary movement. Notably, in patients with FS, muscles not normally expected to be strongly involved in a given movement showed markedly higher activity on the synkinetic side, and often also on the contralateral side, compared with healthy controls. Thus, facial movements in FS evoke a complex pattern of activity that only partially serves the intended movement and is further characterized by involuntary activation in muscles outside the primary movement region and on the contralateral side.