Descriptive Study of Facial Motor Cocontractions During Voluntary Facial Movement in a Healthy Population: A New Hypothesis Contributing to Synkinesis.

Background: Motor overflow refers to involuntary movements that accompany voluntary movements in healthy individuals. This may have a role in synkinesis. Objective: To describe the frequency and magnitude of facial motor overflow in a healthy population. Methodology: Healthy participants performed unilateral facial movements: brow elevation, wink, snarl, and closed smile. Two reviewers analyzed the magnitude of each movement and cocontraction. Patterns of movements are described. Univariate analysis was used to assess the relationship between efficacy of unilateral facial control and the frequency and magnitude of cocontractions. Results: Eighty-nine participants completed the videos. Consensual mirror movements occurred in 96% of participants during unilateral eye closure and 86% during brow elevation. The most common associated movement was ipsilateral eye constriction occurring during snarl (90.1%). Improved unilateral facial control was associated with a decrease in frequency and magnitude of associated movements during brow elevation, wink, and snarl. Conclusion: This study showed stereotyped patterns of motor overflow in facial muscles that resemble those in synkinesis and become more evident as unilateral control of the face decreases.