Lesions at the lower third of the precentral gyrus or at the corticopontine tract prior to its decusation may be associated with limb weakness, ocular abnormalities, and signs of head trauma.

Figure 179.1.— 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 lesion; E: mandibular branch lesion; F: depressor angularis oris muscle.

Upper limb monoplegia or hemiparesis ipsilateral to the affected facial musculature may occur. The limb findings are due to involvement of the corticospinal fibers that travel very near to the corticopontine fibers. They manifest initially as a flaccid paralysis (weakness, hypotonia, and decreased Moro reflex and muscle stretch reflexes) followed after a few days by a spastic paralysis (weakness, hypotonia, and exaggerated Moro and muscle stretch reflexes). Gaze abnormalities are due to involvement of the neurons in the cortical frontal area that control rapid eye movements or their descending fibers. Damage to this system produces conjugate eye deviation away from the side of the facial weakness during the first weeks after the damage. Cephalohematomas and other signs of skull trauma may be present.
The diagnosis of an upper motor neuron facial asymmetry warrants an MRI of the brain. The lesions frequently found are subdural hemorrhages, infarcts, porencephalic cysts, and tumors. Upper motor neuron facial asymmetry also occurs after hemispherectomy (Figure 179.2).

Figure 179.2— Computed tomography of the brain several days after hemispherectomy in the patient shown in Figure 178.1.