This baby boy was born at term to an 18-year-old primigravida via spontaneous vaginal delivery. The membranes ruptured about 6 hours before delivery. The amniotic fluid was heavily stained with meconium. Forceps were not used during the delivery. The newborn initially had poor tone and no spontaneous respirations, but his heart rate exceeded 100 beats per minute. Bulb and deep suctioning as well as supplemental oxygen were provided. Apgar scores were 3 and 8 at 1 and 5 minutes.
In the nursery, his vital signs remained stable, and he was able to feed without difficulty. He was noted to have facial asymmetry. While at rest, his facial features (including the forehead and nasolabial folds) appeared symmetrical (A); while crying, only the right side of the lower lip depressed (B). The baby's eyes were tightly closed during episodes of crying. No palatal abnormalities were observed. Other physical findings were normal.
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The mother had fewer than 3 prenatal care visits; she had tested positive for group B streptococci and was given 1 dose of vancomycin because of a penicillin allergy.
The asymmetric facies was determined to be the result of agenesis of the depressor anguli oris muscle. This can be an isolated anomaly or it may be associated with cardiovascular, musculoskeletal, genitourinary, and respiratory defects.1,2 The cause is unknown. Rarely, agenesis or hypoplasia of the anguli oris muscle is familial.3
In newborns with agenesis of the depressor anguli oris, the lower lip on the affected side looks thinner because of the lack of eversion. It also feels thinner because of the muscle agenesis. When the newborn cries, 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.4 However, forehead wrinkling, eye closure, and nasolabial fold depth are symmetrical. Affected infants suck well without drooling from either corner of the mouth.
It may be difficult to distinguish this disorder from a mandibular branch deficit. However, signs of trauma are usually present in persons with a mandibular branch lesion, and the facial asymmetry often resolves within a few days.5-7
The diagnosis of agenesis of the depressor anguli oris may be confirmed by electrophysiological studies. The facial nerve conduction time to the mentalis and orbicularis oris muscle is normal. There is no fibrillation in the area normally occupied by the depressor anguli oris. Motor units are decreased or absent in the same area.8,9 Further testing for associated conditions is usually unnecessary if no other abnormalities are apparent on the physical examination.
No treatment is required. The asymmetry, which is easily recognizable when the newborn cries, becomes less obvious with age, because smiling and facial expressions that use muscles other than the depressor anguli oris become more dominant. *