Characterization of Snarl Types in Healthy Individuals: Establishing a Comparative Baseline for Nonflaccid Facial...

Nonflaccid facial paralysis (NFFP) is characterized by hypertonicity of facial muscles, often resulting in a snarl-like expression. This abnormal movement arises from facial synkinesis, where involuntary co-activation of multiple muscles during intentional movement leads to movement dysfunction. There is a lack of literature classifying the underlying mechanism and muscular involvement behind the snarl expression. This study analyzes the snarl in healthy individuals to create a clinically useful comparative baseline for NFFP patients, serving as a clinical guide for NFFP treatment approaches. Fifty participants were included and demographic data and pictures of their faces at rest, right and left unilateral snarl, bilateral snarl, and smiling were obtained. Adobe Photoshop for Clinician Facial Assessment, Emotrics AI software, and Noldus FaceReader AI software were utilized for analysis. Significant results included increased nasolabial fold severity rating, ipsilateral brow depression and decreased glabella distance, increased ipsilateral nasal deviation, decreased nasal tip mid-glabella distance, increased ipsilateral alar base elevation, decreased ipsilateral MRD1 and MRD2, increased ipsilateral oral commissure elevation, and increased upper and lower lip movement (all P<0.0001). Bilateral snarl followed the same pattern except for having even brown depression, even oral commissure elevation, and no nasal tip deviation. Snarl patients had significantly increased conveyance of disgust than resting and smiling expressions. Snarl expressions retained an increased percentage of happiness and fear compared with resting. The findings concluded by this study provide a neuromuscular framework of how NFFP can present, thus highlighting what specific facial structures should be treated to restore normal facial expressions.