The complex facial muscle activation pattern in patients with postparalytic facial synkinesis: A prospective observational study using synchronous high-resolution surface electromyography on the synkinetic and the contralateral side

Facial synkinesis (FS) is a facial muscle coordination disorder following facial paralysis. We hypothesize that FS involves the entire network of all facial muscles on both sides of the face. Bilateral facial high-resolution surface electromyography (HR-sEMG) was performed during 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 tasks, synkinetic and contralateral side in patients, and compared to healthy probands were evaluated by linear mixed-effects models. Each movement task led to a complex activation pattern of nearly all measured facial muscles. In healthy controls HR-sEMG activity was highest in the facial muscles considered as target muscles for the voluntary movement. Vice versa, the facial muscle activity was highest on the synkinetic side in facial areas not related to the region of voluntary activity. The activity was often also higher on the contralateral side in patients in these no target regions compared to healthy probands. In patients with FS, facial movement tasks evoke a complex but partly functionally decoupled pattern of facial muscle activity related to the voluntary movement but also to involuntary muscle activation aside the target region and on the contralateral side.