An upper motor neuron (UMN) lesion involving cranial nerve VII would most likely produce
Correct Answer: Contralateral weakness of the lower half of the face with sparing of the upper half of the face
Description: The physical finding of facial asymmetry is suggestive of an abnormality involving the facial nerve (CN VII). The facial nucleus, which is located within the pons, is divided in half, the upper neurons innervate the upper muscles of the face, while the lower neurons innervate the lower poion of the face. It is impoant to realize that each half receives input from the contralateral motor coex, while only the upper half receives input from the ipsilateral motor coex. Therefore an upper motor neuron (UMN) lesion will produce a defect involving only the contralateral lower half of the face. Causes of UMN lesions involving the facial nerve includes strokes that involve the coex or the internal capsule. In contrast, lesions that affect the facial nerve from the facial nucleus to the remaining length of the nerve result in LMN lesions. Patients present with facial asymmetry involving the ipsilateral upper and lower quadrants. Lesions to the facial nerve within the facial canal (frequently due to cold weather) cause Bell's palsy. Patients present with paralysis of all muscles of facial expression. Bell's phenomenon refers to the finding of the affected eye looking up and out when patients try to close their eyes. Because the lacrimal punctum in the lower eyelid moves away from the surface of the eye, lacrimal fluid does not drain into the nasolacrimal duct. This produces "crocodile tears." ref - pubmed.com
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