The depressor angularis oris muscle (DAOM) originates from the oblique line of the mandible and extends upward and medially to the orbicularis oris. It attaches to the skin and the mucous membrane of the lower lip. The DAOM draws the lower corner of the mouth downward and everts the lower lip. The cause for agenesis of the muscle is unknown. The absence or hypoplasia of the DAOM produces characteristic findings (Figure 185.1). The lower lip on the affected side looks thinner because of the lack of eversion and feels thinner because of the muscle agenesis. When crying, the corner of the mouth on the affected side is displaced toward the normal side and the lower lip on the normal side moves downward and outward (Figure 185.1). These patients have symmetrical forehead wrinkling, eye closure, and nasolabial fold depth. The diagnosis may be confirmed by electrophysiologic studies. The facial nerve conduction time to the mentalis and orbicularis oris muscle are normal. There is no fibrillation in the area normally occupied by the DAOM. Motor units are decreased or absent in the same area.
Agenesis of the DAOM can occur as an isolated anomaly but it has also been reported in association with cardiovascular, musculoskeletal, genitourinary, and respiratory defects. Our approach to these patients is to assume no associated anomaly is present if the rest of the clinical examination is normal. No treatment is required. The asymmetry will not be noted when the patient grows older.

Figure 185.1.— Absence of the depressor angularis oris muscle. [A] No facial asymmetry is present during quiet awake. [B] The asymmetry becomes apparent when crying.