Unilateral or asymmetrical weakness of the mimetic facial muscles, levator palpebrae muscle, and the muscles of Müller can produce facial asymmetry. Mimetic facial muscle weakness is most noticeable when the patient is trying to close his eyes, grimace, or smile. Levator palpebrae muscle weakness and muscle of Müller weakness are most noticeable when the neonate is trying to open his eyes. The mimetic facial muscles are innervated by the facial motor system; the levator palpebrae muscles are innervated by the common oculomotor system; and the muscles of Müller are innervated by the sympathetic ocular system.

The facial motor system can produce facial asymmetry that may involve the upper, lower, or both quadrants. The asymmetry is only present or is most noticeable when the neonate grimaces. The muscles used by a neonate when grimacing are those that close the eyes and those that spread the angles of the lips apart and turn the corners of the lower lip down. The muscles used to close the eyelids are the orbicularis oculi muscles. The primary muscles used to spread the lips apart are the buccinator muscles. The muscles used to turn the corners of the lower lip down are the depressor angularis oris muscles. These muscles are innervated by the facial motor system.