Craniofacial Muscles

15 Facial Nerve Innervation and Facial Palsies
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two-thirds of the tongue), the taste fi bers from the soft palate via the palatine and
greater petrosal nerves, and the preganglionic parasympathetic innervation to the
lacrimal, submandibular, and sublingual glands. The lesser petrosal nerve carries
secretomotor fi bers to the parotid gland. There is also a small cutaneous sensory
component arising from the posterior auricular area.
The facial nerve has the longest bony course of any nerve as it traverses the
facial canal of the petrous temporal bone and then the lateral canal of the mastoid
bone, some 20–30 mm in all. This extended encased section is vulnerable to both
swelling through edema or in fl ammation, and fractures. The labyrinthine segment
in particular is especially narrow and lacks arterial cascades making it susceptible
to ischemia. Between the labyrinthine and tympanic segments lies the geniculate
ganglion where the petrosal branches are given off. Lesions prior to the geniculate ganglion
result in more severe ocular complications due to the lack of lacrimal
secretions (Mavrikakis 2008 ) . In the tympanic segment, the nerve passes behind the
cochleariform process against the medial wall of the cavum tympani, above and
posterior to the oval window. The bony wall is commonly thin or dehiscent here,
and the nerve may lie directly against the middle ear mucosa making it particularly
vulnerable to iatrogenic injury and middle ear infections. Between the external
auditory meatus and the horizontal semicircular canal the nerve makes a second
turn into the lateral canal of the mastoid bone. Three branches exit in this segment:
the chorda tympani, the nerve to stapedius, and the nerve from the auricular branch
of the vagus nerve (CNX).
Base of skull fractures can result in a facial palsy as a result of transection or
tension accompanied by entrapment of the nerve in its bony course. The presence of
a facial palsy in the setting of head and neck trauma is an indication that urgent
further assessment is essential with CT and MRI scans.
The facial nerve exits the skull through the stylomastoid foramen, supplies a
branch to the posterior auricular muscle, passes between and innervates the posterior
belly of digastric and the stylohyoid muscle before entering the parotid gland. In the
substance of the parotid, it divides into upper and lower trunks, which subdivide
into fi ve main branches: temporal, zygomatic, buccal, marginal mandibular, and
cervical.
The temporal branch innervates the upper eyelid and forehead. Paralysis results
in a ptosis of the brow and upper lid that in the elderly can be severe enough to
obscure vision. In addition, the upper eyelid is responsible for blinking and distributing
the watery tear fi lm that protects the cornea. Ulceration of an unprotected
cornea is a serious risk, and assessment of an adequate Bell’s re fl ex is an essential
part of any examination. Xeropthalmia (i.e., dry eye syndrome) is extremely uncomfortable
for patients and a dry eye can paradoxically elicit excessive tear production
that overwhelms the lacrimal duct resulting in epiphora.
The zygomatic and buccal branches innervate the muscles of the midface and
experience signi fi cant cross-innervation. Paralysis results in ptosis of the lower
eyelid and increased scleral show, exacerbating the eye problems alluded to above.
In combination with the two inferior branches, they control the movements of the
mouth and are responsible for oral continence, smiling, and speech.