- A rather uncommon pathologic disease in adolescents is masseter muscle unilateral hypertrophy.. Its origin is unknown and because it needs to be distinguished from other masseter-related disorders, it calls for a high level of suspicion. We describe a rare instance of unilateral masseter muscle hypertrophy in a 13-year-old female patient with a painless swelling in the posterior left cheek that led to facial asymmetry and negatively impacted the girl's self-image because there are few pathology studies that explain this condition. Clinical assessment, ultrasonic scanning, sialography, histological testing, and surgical correction of the unilateral masseter muscle hypertrophy were all used to support the diagnosis