Facial Nerve Branch Lesions
Facial nerve branch lesions may involve one branch or a combination of the five major branches. The branch most often involved is the mandibular branch (Figure 184.1). The cause is usually trauma.

A	B

Figure 184.1.— Mandibular branch lesion. [A] Asymmetrical facial grimacing involving the lower facial quadrant. [B] No asymmetry during sleep.

Mandibular branch deficits produce complete weakness of the depressor labii inferioris, mentalis, and transversus menti muscles, and incomplete weakness of the depressor anguli oris muscle (Figure 184.2). The depressor anguli oris muscle is innervated by two branches: the mandibular branch and the buccal branch (Figure 184.2). In a mandibular branch lesion the lips will deviate to the opposite side when crying. The lips stay closer together on the side of the lesion than on the normal side. Mandibular branch deficits may be difficult to distinguish from absence of the depressor angularis oris but with mandibular branch injury other signs of trauma are usually present (Figure 184.1) and often the asymmetry improves in a few days.

Figure 184.2.— Anatomical localizations of injuries in the facial motor system. T: thalamus; IAC: internal auditory canal; FC: facial canal; SMO: styloidmastoid orifice; BB: buccal branch; MB: mandibular branch; TB: temporal branch; OOM: orbicularis oculi muscle; RM: risorius muscle; DAOM: depressor angularis oris muscle; BM: buccinator muscle; MM: mentoris muscle. Light blue line indicates components of the facial nerve that have ipsilateral (hence bilateral) cortical innervation; dark blue line indicates components of the facial nerve that have contralateral innervation. A: cerebral lesion above the thalamus; B: cerebral lesion below the thalamus and above the pons; C: pontine lesion; D: facial nerve; E: mandibular branch lesion; F: depressor angularis oris muscle.

Temporal branch deficits produce facial upper quadrant weakness. The weakness is only apparent when the neonate cries.

A	B

Figure 184.3.— Temporal branch lesion. [A] No asymmetry during sleep. [B] Asymmetrical facial grimacing involving the upper facial quadrant.

Facial nerve branch lesions may be associated with brachial plexus injury and signs of facial trauma. The diagnosis is confirmed by finding fibrillations and positive sharp waves in the affected muscles 12 to 14 days after the injury. Facial nerve branch lesions also produce delayed conduction or a conduction block in the affected branch with normal findings in other muscles innervated by other branches. Facial nerve branch lesions are due to trauma and usually do not require treatment.