Upper Motor Neuron Facial Asymmetry

Upper motor neuron facial asymmetry occurs with lesions at the lower third of the precentral gyrus or at the corticopontine tract prior to its decusation (Figure 178.1 A and B). Upper motor neuron facial lesions produce exclusive or predominantly contralateral lower quadrant weakness. The characteristics of the facial weakness vary with time.
During the first week, facial weakness due to lesions above (Figure 178.1 A) or below (Figure 178.1 B) the thalamus have similar distribution and characteristics.

Figure 178.1.— 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 lesion; E: mandibular branch lesion; F: depressor angularis oris muscle.

During the first week, the facial weakness involves the corner of the mouth, nasolabial fold, and the lower eyelid. The upper eyelid and the forehead are minimally involved or more often not involved. The asymmetry is not noticeable when the neonate sleeps or when the neonate is in a quiet awake state. The asymmetry only becomes apparent when the neonate cries. When crying, the mouth deviates toward the normal side and the eye on the side opposite from the direction the mouth deviates may close properly or may show minimal signs of weakness, such as not burying the eyelashes as deeply as in the other eye.
After the first week, patients with lesions above (Figure 178.1 A) and below (Figure 178.1 B) the thalamus have similar characteristics during quiet awake and during sleep, but not during grimacing. At rest or during quiet awake, no asymmetry is noted. During grimacing, neonates with lesions above the thalamus demonstrate an exaggerated contraction of the affected side (as do adults with simiar lesions with emotional smile but not with voluntary smile) that causes the mouth to deviate toward the affected side (Figure 178.1); during grimacing, neonates with lesions below the thalamus demonstrate mouth deviation towards the normal side.

Figure 178.1.— Facial asymmetry due to a central lesion above the thalamus. The asymmetry is not present in quiet awake [A] but appears during crying [B]. The mouth deviates toward the affected side (right). The patient had a left hemispherectomy.