What every neurologist should know about the facial nerve

If a handsome photo of George Clooney didn’t lure you into a blog about the facial nerve, then nothing well. For those of you who fell for it, welcome to BrainWaves. This week’s episode focuses on the anatomy and physiology of the seventh cranial nerve, and the way that diseases can compromise it.

The episode is organized anatomically, beginning with diseases that can affect the cranial nerve most proximally, and continues with diseases that affect the nerve more distally.

Facial nerve and its branches. Image from https://bmc.med.utoronto.ca/cranialnerves/illustrations-by-chapter/facial-vii.

The first major disease discussed is Bell’s Palsy. Bell’s palsy, or any peripheral disturbance of the facial nerve, is really interesting to conceptualize because it tells you so much about the function of the facial nerve in addition to what you already knew about its somatic efferent fibers. Because most patient’s with what looks to be a Bell’s Palsy will have facial nerve inflammation just as it exits the pontomedullary junction, all downstream pathways are affected. For instance, there is a somatic afferent fiber, called the nervus intermedius of Wrisberg—which I just call the nervus intermedius—that conveys tactile stimuli from parts of the external auditory canal. The nervus intermedius also provides visceral afferent information from the anterior 2/3 of the tongue via projections from the geniculate ganglion. So next time when you are seeing a patient and you’re thinking “hey, you might have a Bell’s palsy,” you might wanna check some of these features, like taste on the tongue, sensation to the external auditory canal, hyperacusis, and so on.

Facial nerve nuclei. Image from https://bmc.med.utoronto.ca/cranialnerves/illustrations-by-chapter/facial-vii

As far as etiologies of Bell’s Palsy go, by definition, it is idiopathic. So unless there are specific risk factors, like a recent tick bite, intravenous drug use, or unusual findings on review of systems, experts recommend against diagnostic testing. But if you’re concerned for a more systemic process, things like diabetes, sjogren’s, Lyme disease, HIV, sarcoidosis and other causes of peripheral nerve dysfunction should be considered.

Other disorders of the facial nerve may also have unique localizing features. Starting most proximally in the cranium, and you probably won’t see this as much, but a fracture of the temporal bone can compromise facial and vestibulocochlear nerve function since they both course through the internal acoustic meatus. The first major branch to the VIIth nerve—the greater superficial petrosal branch—takes off in the earliest segment of the facial canal, in a particular part called the labyrinthine segment where the geniculate ganglion rests. This is also the ganglion implicated in the Ramsay Hunt Syndrome. Because the geniculate ganglion is involved, any of the axons that project from it may be affected. I typically think of herpetic eruptions in the inner ear in Ramsay Hunt syndrome, because the sensory nerve to the inner ear branches off the facial nerve distal to the geniculate ganglion, but you can also have herpetic eruptions on the anterior tongue and palate, in addition to hemifacial weakness.

Moving on to the second segment of the facial canal, this part actually lies within the inner ear, and is called the tympanic segment. Here, the nerve is vulnerable to irritation from otitis media in children, and inner ear infections may cause a unilateral facial weakness with hyperacusis and autonomic impairment in 1 out of every 20,000 patients with otitis. Also within the tympanic segment of the facial canal, a cholesteatoma can compress the facial nerve, producing similar symptoms.

More distally in the facial canal, just before the nerve exits at the stylomastoid foramen, the nerve to the stapedius and then the chorda tympani take off (at the pyramidal eminence of the facial canal). And finally, just as the facial nerve exits the cranium at the stylomastoid foramen, it gives off a somatic sensory branch—the sensory auricular nerve—to the external acoustic canal. You can imagine, in cases of a parotid tumor, the remaining functions of the seventh nerve would be compromised with the exception of the sensory auricular nerve. So parotid tumors, unless extremely large, should spare sensory dysfunction of the skin inside the ear.

That summarizes the clinically relevant anatomy of the facial nerve, but it leaves us with plenty of gaps in nervous system disease. For the remainder of the episode, we cover hemifacial spasm, blepharospasm, facial myokimia, and pathologic sialorrhea as it pertains to neurodegenerative disease. There is plenty more about the facial nerve that we didn’t cover today, but I think this is a good start.

 

[Jim Siegler]


REFERENCES

Gilden DH. Clinical practice. Bell’s Palsy. The New England journal of medicine. 2004;351:1323-31.

Sweeney CJ and Gilden DH. Ramsay Hunt syndrome. Journal of neurology, neurosurgery, and psychiatry. 2001;71:149-54.

Gaio E, Marioni G, de Filippis C, Tregnaghi A, Caltran S and Staffieri A. Facial nerve paralysis secondary to acute otitis media in infants and children. J Paediatr Child Health. 2004;40:483-6.

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