Congenital and Acquired Facial Palsies

Anatomy

The facial nerve ( Figure 13.1 ) emerges from multiple functionally specialized ventrolateral brainstem nuclei. The motor nucleus of cranial nerve VII originates in the caudal pons. Five groups of cells are defined, corresponding to a topographic organization of motor neurons innervating different muscles. The ventral groups of cells supply the periorbital muscles; the dorsal groups supply the perioral muscles. Taste fibers for the anterior two-thirds of the tongue, and sensory fibers from the external acoustic canal, project to the upper part of the nucleus of the tractus solitarius in the vicinity of the motor nucleus of cranial nerve VII. The lacrimal, sublingual, and submandibular preganglionic parasympathetic fibers of the facial nerve originate in the nearby superior salivary nucleus. The twin sensory and motor roots of the facial nerve emerge from the brainstem at the level of the bulbopontine sulcus, between the sixth and eighth cranial nerves, at the cerebellopontine angle, then enter the pars petrosa ossis temporalis via the internal acoustic meatus and follow a common path in the first part of the facial canal. The sensory components enter the geniculate ganglion at this level. Motor fibers of the facial nerve traverse peripherally through the internal auditory canal in the temporal bone, in company with the nervus intermedius, consisting of the sensory fibers of cranial nerve VII as well as with the VIIIth cranial nerve. The greater petrosal nerve arises from the labyrinthine segment of the facial nerve and carries parasympathetic fibers to the pterygopalatine ganglion, where it synapses and sends fibers to the lacrimal gland. More distally, the facial nerve’s mastoid segment travels inferiorly and laterally to the jugular fossa. Here, the stapedius nerve arises to innervate the stapedius muscle. The facial nerve gives off the chorda tympani in the distal mastoid segment. The chorda tympani then passes through the middle ear to supply afferent taste fibers to the anterior two-thirds of the tongue, as well as parasympathetic fibers to the submandibular and sublingual glands. The primary motor bundle of the facial nerve traverses the second and third parts of the facial canal alone. The facial motor nerve emerges from the skull at the stylomastoid foramen, gives off the posterior auricular branch, and then divides into two main terminal branches in the parotid gland. The inferior cervicofacial branch passes down along the mandible to supply the muscles in the lower part of the face, giving off a buccal branch to the risorius, the buccinator, and the orbicularis oris muscles; a mental branch to the depressor anguli oris, the depressor labii inferioris, and the mentalis muscles; and a cervical branch to platysma. The superior temporofacial branch runs horizontally forward, giving off frontal branches to the frontalis and orbicularis oculi muscles; suborbital branches to the levator labii superioris, zygomaticus, levator anguli oris, and dilatator naris muscles; and buccal branches to the buccinator and orbicularis oris muscles. Although there is considerable diversity in its trajectory and divisions, cranial nerve VII innervates all muscles of facial expression except the levator palpebrae superioris. The close anatomic proximity of cranial nerves V, VI, VII, and VIII in the brainstem and cerebellopontine angle explains the involvement of multiple nerves in malformations, and in ischemic or compressive disorders. Interneuronal and synaptic V–VII connections in the brainstem provide the basis for the neurophysiologic studies exploring the blink reflex. The geniculate ganglion, located within the internal auditory canal, contains the soma of sensory facial fibers. Reactivation of viral particles latent in the geniculate ganglion is believed to be involved in the pathogenesis of idiopathic and Ramsay Hunt facial palsies. Lesions of the facial nerve trunk localized distal to the ganglion may involve different facial nerve branches (i. e. greater superficial petrosal nerve, nerve to stapedius muscle, and chorda tympani), causing variable dysfunction of lacrimation, salivation, taste, and/or hearing. Nerve entrapment is most likely to occur at the narrowest intraosseus segment, the meatal foramen, where it is not protected by epineurium and perineurium.

Clinical Examination

Clinical-Anatomic Relationships

When partial or complete facial paralysis has developed, associated features may help in distinguishing central from peripheral palsies, and determining the specific site of the lesion in the cranial nerve VII pathways or nucleus. Lesions in the pons may induce hyperacusis due to dysfunction of motor fibers to the stapedius muscle, and may progress to other features of brainstem involvement. A lesion between pons and geniculate ganglion may produce hyperacusis and impairment of lacrimation, salivation, and taste of the anterior two-thirds of the tongue. Between the geniculate ganglion and stapedius nerve, hyperacusis and impairment of salivation and taste are expected, with preservation of lacrimation. Between the stapedius nerve and chorda tympani, salivation and taste will be impaired but hearing is normal. Beyond the exit of chorda tympani, the only symptom will be facial weakness.

Central facial palsies (suprabulbar) show more widespread clinical features and a progressive course. Upward and outward eye movement when blinking (Bell’s sign) is usually absent. Typically, spontaneous facial expression, such as smiling as an emotional reaction, is preserved, while no voluntary movement is obtained in response to a command, such as asking the patient to smile.

Complementary Investigations

Congenital facial palsies (CFPs) have a more standardized diagnostic algorithm than acquired facial palsies (AFPs). Most CFPs are either related to a congenital malformation or are of traumatic origin. A combination of neurophysiologic tests and neuroimaging provides an opportunity for early diagnosis, management, and determination of prognosis, but their use in AFPs is still controversial. Globally, electrodiagnostic studies are used to search for prognostic indicators. Magnetic resonance imaging (MRI) is mandatory in traumatic cases or whenever an intracranial lesion is suspected. Cerebrospinal fluid (CSF) examination is not a routine test but may be useful to rule out neoplastic, infectious, or inflammatory meningeal involvement. Polymerase chain reaction (PCR) of serum, CSF, or saliva may allow early detection of Lyme disease, varicella or other herpes viruses, taking into account geographical and seasonal differences.