The course of the facial nerve (CN VII) is depicted in Fig. 29. The numbers in the figure refer to the following locations of the lesions:
1.Peripheral to chorda tympani in facial canal or outside stylomastoid foramen. Peripheral upper and lower facial weakness (motor aspects of CN VII) only. Usually related to trauma.
2.Facial canal (mastoid), involving chorda tympani. In addition to upper and lower facial weakness, patients have loss of taste over the anterior two-thirds of tongue and decreased salivation.
3.Facial canal, involving the stapedius nerve. Upper and lower facial weakness as in 1 and 2, plus hyperacusis.
4.Geniculate ganglion. Usually associated with pain in the ear. May have decreased lacrimation.
5.Internal auditory meatus. Complete CN VII (facial weakness; decreased taste, salivation, and lacrimation) plus CN VIII dysfunction (deafness or vestibular symptoms).
6.Extrapontine, subarachnoid. May have other cranial nerve involvement. Hemifacial spasm is more commonly associated with more proximal lesions of CN VII.
7.Pontine (nuclear or infranuclear). Millard-Gubler, Foville, and Brissaud syndromes (see Ischemia).
8.Supranuclear. Lesions may occur anywhere from mid-pons to motor cortex and are usually associated with other findings such as hemiparesis, hemisensory deficit, language disturbance, or homonymous hemianopia, depending on location. Taste, salivation, and lacrimation are not involved. Lower facial weakness is much more prominent than upper because of bilateral input to the portions of the facial nucleus controlling the upper face; input for the lower face is from contralateral cortex. Mild weakness may appear only as slight drooping of the angle of the mouth, slight widening of the palpebral fissure, or flattening of the nasolabial fold. (Table 65 compares clinical findings in the upper and lower motor neuron facial nerve weakness.)
Idiopathic Facial Paralysis (Bell palsy): the most common form (60% of cases) and is often a result of HSV-1 infection. Typified by a sudden, unilateral onset often preceded by a viral prodrome, the paralysis often regresses spontaneously in 8 to 10 weeks, with complete recovery in 85% to 90% of patients. Acyclovir and prednisone are often prescribed to treat the condition, but their use is controversial. Exposure keratitis may be ameliorated by lubricating ointment but rarely requires tarsorrhaphy. Melkersson syndrome represents episodes of recurrent facial palsy with a deeply furrowed tongue (lingua plicata) and recurrent facial edema. Overall prognosis for this condition is good, but permanent paralysis may result after many recurrences. Aberrant regeneration of the facial nerve with resolution of the paralysis has several variations. The most common of these is involuntary tearing of the eye on the involved side when eating (“crocodile tears”) or synkinesis of the facial musculature when chewing. This often takes the form of a jaw-winking phenomenon, wherein the lid closes on the involved side when the jaw opens (Marin-Amat syndrome).
Posttraumatic Facial Paralysis: Trauma is the second most common cause of peripheral facial paralysis in adults. Most often injury occurs in the intrapetrous portion. Nevertheless, traumatic lesions along the brainstem, the cerebellopontine angle (CPA), or in the parotid gland also occur.
Tumors may cause sudden or relapsing facial paralysis, but progressive facial paralysis with a slow onset of more than 3 weeks strongly indicates the presence of a tumor.
I.Brainstem primary or secondary tumors produce impairment in multiple cranial nerve pairs.
II.CPA and the internal acoustic meatus (IAM) can be sites of tumors. Most frequently, lesions are neurinomas of CN VIII and meningiomas (consider neurofibromatosis-2). Epidermoid tumors have cerebrospinal fluid (CSF) signal on T1 and T2 sequences of MRI (Table 66). Arachnoid cysts are isointense to the CSF, have sharp contours, and remain confined to the subtentorial space. Fluid-attenuated inversion recovery (FLAIR) can help detect epidermoid tumors; diffusion sequences show reduced diffusion within an epidermoid cyst compared with an arachnoid cyst.
III.Internal auditory meatus and the region of the ganglion geniculi may be sites for hemangiomas, which are small lesions associated with notable clinical manifestations: perception deafness, rigidity, or facial paresis.
IV.Tympanic cavity and mastoid: Otoscopic examination can suggest tumor in the tympanic cavity and the mastoid by showing a whitish (primary cholesteatoma), grayish, or bluish (glomus) retrotympanic mass. Primary or secondary bone tumors can be seen in the petrosal bone. Metastatic tumors are the most common type.
V.Parotid gland: Every parotid mass associated with facial paralysis should suggest a malignancy.
This movement disorder is characterized by sudden, painless, involuntary clonic contractions of the muscles innervated by CN VII, usually starting with the orbicularis oculi. It is most often due to compression by vascular structures of the facial nerve at the root exit zone, which is an approximately 3-mm area beginning at the emergence of CN VII from the brainstem, at the junction of the central-type and peripheral-type myelin fibers. The vascular structures involved are, by decreasing order of frequency, the posteroinferior cerebellar artery, the vertebral artery, the anteroinferior cerebellar artery, and a vein (respectively 70%, 40%, 30%, and 20% in). Treatment usually consists of botulinum toxin injection or surgical decompression.
