Agenesis of Depressor angularis oris

Agenesis of the depressor angularis oris {DAOM}. It is a relatively common minor anomaly sometimes confused with facial palsy and it is not associated with birth trauma unlike facial palsy. The depressor angularis oris muscle 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. Its innervation derives from the buccal and mandibular branches of the facial nerve. It draws the lower corner of the lip downward and everts it. The absence or hypoplasia of the DAOM is not uncommon. The condition is usually asymmetrical and benign. The cause for agenesis of the muscle is unknown. 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. But these patients have symmetrical forehead wrinkling, eye closure, and nasolabial fold depth. It can present as an isolated anomaly or in association with cardiovascular, musculoskeletal, genitourinary or respiratory defects {1}. This anomaly without major malformations was present in 0.348, 1,000 newborns. {2} The diagnosis may be confirmed by electrophysiological studies. There is no fibrillation in the area normally occupied by the DAOM. Motor units are decreased or absent in the area of the depressor angularis oris. With increasing age, asymmetry becomes less obvious as other facial muscles dominate. Hence no treatment is required.

References
1. Martínez Granero MA, Argüelles F, Roche Herrero MC, Omeñaca F, López Terradas JM, Ortigado A. Facial asymmetry with crying: a neurophysiological study and clinical account of this entity. An Esp Pediatr. 1998, 48: 44-48
2. Nelson KB, Eng GD. Congenital hypoplasia of the depressor anguli oris muscle: differentiation from congenital facial palsy. J Pediatr. 1972, 81: 16-20

E-published: June 2009, Vol 6 Issue 6 Art # 35