Facial Nerve Lesions
Facial nerve lesions usually occur at the facial canal or after the facial nerve exits through the stylomastoid foramen but before it divides into its terminal branches (Figure 182.1 [D]). A distribution of weakness in neonates with facial nerve lesions includes the corner of the mouth, nasolabial fold, lower eyelid, upper eyelid, and forehead equally and ipsilaterally to the injury.

Figure 182.1.— Anatomical localizations of injuries in the facial motor system. T: thalamus; AC: 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.

The causes of facial nerve lesions are trauma, malformation, or parotid gland tumor. There is a familial congenital form.

Facial nerve trauma
Facial nerve trauma (Figure 182.2) usually occurs in the facial canal or immediately after the styloidmastoid orifice. The side of the facial nerve injury relates to the obstetrical position. Those with left occipital obstetrical position have left facial nerve injury. Those with right occipital obstetrical position have right facial nerve injury.

Figure 182.2— Traumatic facial nerve injury. [A] At rest, no asymmetry is noted. [B] During crying, the facial asymmetry becomes obvious.

Facial nerve trauma may be associated with neurological or non-neurological findings. Neonates with facial nerve trauma may also have brachial plexus lesions or evidence of central nervous system trauma. They may also have evidence of facial trauma, hemotympanum, or skull fracture.