Abnormalities of Facial Movement
The ophthalmologist may be asked to evaluate CN VII weakness, which is frequently seen in clinical settings. Assessment of CN VII includes testing not only motor function, but also sensory and autonomic functions. Motor function can be readily assessed by observation. With the patient at rest, any asymmetry of facial expression or eyelid blink is noted. The palpebral fissure on the side of CN VII paresis will be wider as a result of the relaxed tone of the orbicularis oculi muscles. The clinician can test the various muscle groups by asking the patient to smile, to close the eyes forcibly, and to wrinkle the forehead. The degree to which the eyelashes become buried on each side can reveal subtle orbicularis oculi weakness. The corneal blink reflex provides a functional assessment of CN VII and CN V.
Testing autonomic functions such as salivation and lacrimation as well as testing sensation can help localize CN VII lesions. Cutaneous sensation can be tested along the posterior aspect of the external auditory canal. Lesions of CN VII from the cerebellopontine angle to the geniculate ganglion typically impair all functions of the nerve, whereas lesions distal to the geniculate ganglion affect only certain functions, depending on their location (Fig 11-8).
Any aberrant facial movements at rest or during volitional movement should be noted (see Fig 11-4). After any facial neuropathy, but most commonly as a result of Bell palsy, regenerating axons may reinnervate different muscles from those originally served; such aberrant regeneration can cause synkinetic movements. In this situation, the involved facial muscles may remain weak. When axons originally destined for the orbicularis oculi reinnervate the lower facial muscles, each blink may cause a twitch of the corner of the mouth or a dimpling of the chin. Conversely, movements of the lower face—such as pursing the lips, smiling, or chewing with the mouth closed—may produce involuntary eyelid closure.
Other disorders of aberrant facial innervation include lacrimation caused by chewing (crocodile tears), in which fibers originally supplying mandibular and sublingual glands reinnervate the lacrimal gland by way of the greater superficial petrosal nerve. This syndrome usually develops after severe injury to the proximal CN VII and may be accompanied by decreased reflex tearing and decreased ability to taste from the anterior two-thirds of the tongue.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.