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Facial asymmetry can be produced by structural abnormalities or by unilateral or asymmetrical weakness of the muscles of the face. Structural abnormalities are due to musculoskeletal deformations, soft tissue masses, and tumors. The weak muscles involved in the production of facial asymmetry are the: (1) facial mimetic muscles; (2) levator palpebrae muscles; or (3) muscles of Müller.
The distinction between functional and structural facial asymmetry can be made by observation. Structural facial asymmetry is diagnosed by observing a physical cause for the facial asymmetry. Functional asymmetry is diagnosed by the absence of a physical deformity. The neurogenic system involved in functional asymmetries can be determined by the area of the face involved, the degree of asymmetry at different behavioral stages (crying and quiet awake), and the presence of associated neurological and general findings. (Figure 169.1) Facial asymmetry due to muscle weakness (functional) will disappear when the weak muscle is not being used. This occurs because the facial mimetic muscles, levator palpebrae muscles (common oculomotor system), and the muscles of Müller (oculosympathetic system) are bilateral and usually have synchronous and symmetrical activity.

Figure 169.1.— Differential diagnosis of neurogenic facial asymmetry. Diagonal blue-line area indicates area of the face involved. CN: cranial nerve; DAOM: depressor angularis oris muscle; M: muscle; SYMP: sympathetic; ABN: abnormality; SYST: system.