2. The facial nerve, or cranial nerve (CN) VII, is the nerve
of facial expression. The pathways of the facial nerve
are variable, and knowledge of the key intratemporal
and extratemporal landmarks is essential for accurate
physical diagnosis and safe and effective surgical
intervention in the head and neck.
3. The facial nerve is composed of approximately 10,000
neurons, 7,000 of which are myelinated and innervate
the nerves of facial expression. Three thousand of the
nerve fibers are somatosensory and secretomotor and
make up the nervus intermedius. The course of the
facial nerve and its central connections can be roughly
divided into the segments listed
4. SupranuclearCerebral cortexNA
BrainstemMotor nucleus of facial nerve, superior salivatory
nucleus of tractus solitariusNA
Meatal segmentBrainstem to internal auditory canal
(IAC)13-15mm
Labyrinthine segmentFundus of IAC to facial hiatus3-4mm
Tympanic segmentGeniculate ganglion to pyramidal
eminence8-11mm
Mastoid segmentPyramidal process to stylomastoid
foramen10-14mm
Extratemporal segmentStylomastoid foramen to pes
anserinus15-20mm
5. Embryology of facial n.
By the third week of gestation, the fascioacoustic
primordium gives rise to CN VII and VIII. During the
fourth week, the chorda tympani can be discerned
from the main branch. The former courses ventrally
into the first branchial arch and terminates near a
branch of the trigeminal nerve that eventually
becomes the lingual nerve. The main trunk courses
into the mesenchyme, approaching the epibranchial
placode.
6. The geniculate ganglion, nervus intermedius, and
greater superficial petrosal nerve are visible by the
fifth week. The second branchial arch gives rise to the
muscles of facial expression in the seventh and eighth
week. To innervate these muscles, the facial nerve
courses across the region that eventually becomes the
middle ear. By the eleventh week, the facial nerve has
arborized extensively. In the newborn, the facial nerve
anatomy approximates that of an adult, except for its
location in the mastoid, which is more superficial.
7. Central Connections
Crosby and DeJonge, along with Nelson, have provided 2 of
the most complete descriptions oCortex and internal
capsule
The voluntary responses of the facial muscles (eg, smiling
when taking a photograph) arise from efferent discharge
from the motor face area of the cerebral cortex. The motor
face area is situated on the precentral and postcentral gyri.
The facial motor nerves are represented on the
homunculus diagram below with the forehead uppermost
and the eyelids, midface, nose, and lips sequentially
located more inferiorly.
8. Discharges from the facial motor area are carried
through fascicles of the corticobulbar tract to the
internal capsule, then through the upper midbrain to
the lower brainstem, where they synapse in the
pontine facial nerve nucleus. The pontine facial nerve
nucleus is divided into an upper and a lower half,
bilaterally.
9. The corticobulbar tracts from the upper face cross and
recross en route to the pons; the tracts to the lower face
cross only once.
In 1987, Jenny and Saper performed an extensive study of
the proximal facial nerve organizations in a primate model
and found evidence that in monkeys, upper facial
movement is relatively preserved in upper motor neuron
injury, because these motor neurons receive relatively little
direct cortical input.
10. In contrast, the lower facial muscles are more severely
affected, because their motor neurons depend on
significant cortical innervation. The authors believe
these observations also explain similar findings in
humans.
11. In their study, Jenny and Saper found that the
descending corticofacial fibers in monkeys innervated
the lower facial motor nuclear region bilaterally but
with contralateral predominance. The upper facial
motor nuclear regions received scant direct cortical
innervation on either side of the brain.
12. The deficits observed with unilateral ablation of the
corticobulbar fibers reflect the fact that upper facial
motor neurons do not receive significant cortical
innervations and that lower facial motor neurons
contralateral to the lesion, which have functional loss,
are dependent on direct contralateral cortical
innervation, with the remaining ipsilateral cortical
projections being insufficient to drive them.
13. These findings may explain why a focal lesion in the
facial area on 1 side of the motor cortex in humans
spares eyelid closure and forehead movement but
results in paralysis of the lower face.
15. Temporal and zygomaticOrbicularis oculiCloses eyelids and contracts skin
around eye
Zygomatic and buccalZygomaticus majorElevates corners of mouth
BuccalZygomaticus minorElevates upper lip
Levator labii superiorisElevates upper lip and midportion nasolabial fold
Levator labii superioris alaeque nasiElevates medial nasolabial fold and nasal
ala
RisoriusAids smile with lateral pull
BuccinatorPulls corner of mouth backward and compresses cheek
Levator anguli orisPulls angles of mouth upward and toward midline
OrbicularisCloses and compresses lips
Nasalis, dilator narisFlares nostrilsNasalis,
compressor narisCompresses nostrils
16. Buccal and marginal mandibularDepressor anguli
orisPulls corner of mouth downward
Depressor labii inferiorisPulls lower lip downward
Marginal mandibularMentalisPulls skin of chin
upward
CervicalPlatysmaPulls down corners of mouth
17. Caution is advised in using preservation of forehead
function to diagnose a central lesion. Patients may
have sparing of forehead function with lesions in the
pontine facial nerve nucleus, with selective lesions in
the temporal bone, or with an injury to the nerve in its
distribution in the face.
18. An accurate neurologic diagnosis is best made by
examining deficits in conjunction with "the company
they keep." A cortical lesion that produces a lower
facial deficit is usually associated with a motor deficit
of the tongue and weakness of the thumb, fingers, or
hand on the ipsilateral side.
19. Nerve fibers influencing emotional facial expression
are thought to arise in the thalamus and globus
pallidus. Supranuclear pyramidal lesions spare
movements of the face initiated as emotional
responses and reflexes. With nuclear and infranuclear
lesions, loss of involuntary and voluntary facial
movement occurs.
20. The facial nerve nuclei also receive afferent input from
other brainstem nuclei. Input from the trigeminal
nerve and nucleus form the basis of the
trigeminofacial reflexes; eg, the corneal reflex. Input
from the acoustic nuclei to the facial nerve nucleus
forms part of the stapedial reflex response to loud
noises.
21. Extrapyramidal system
The extrapyramidal system consists of the basal ganglia
and the descending motor projections other than the fibers
of the pyramidal or corticospinal tracts. This system is
associated with spontaneous, emotional, mimetic facial
motions. The interplay between the pyramidal and
extrapyramidal systems accounts for resting tone and
stabilizes the motor responses.
22. The masked facies associated with Parkinsonism are
known to be the result of destruction of the
extrapyramidal pathways. The facial dystonia seen in
Meige syndrome is thought to be due to basal ganglion
disease.
23. Lower midbrain
A lesion in the lower midbrain above the level of the facial
nucleus may cause contralateral paresis of the face and
muscles of the extremities, ipsilateral abducens muscle
paresis (due to effects on the abducens nerve), and
ipsilateral internal strabismus. If the lesion extends far
enough laterally to include the emerging facial nerve
fibers, a peripheral type of ipsilateral facial paralysis may be
apparent.
24. Pons
The facial motor nucleus is located in the lower third of the
pons, beneath the fourth ventricle. The neurons leaving
the nucleus pass around the abducens nucleus as they
emerge from the brainstem. Involvement of the facial nerve
nucleus and VI nerve nucleus are suggestive of a lesion
near the fourth ventricle. A lesion near the ventricle at the
level of the superior salivatory nucleus may result in a dry
eye in addition to a peripheral facial paralysis and abducens
paresis.
25. Cerebellopontine Angle and the Internal Auditory Canal
The facial nerve emerges from the brainstem with the nerve of
Wrisberg, ie, the nervus intermedius (see the image below). The
nervus intermedius gained its name from its position as it courses
across the cerebellopontine angle (CPA) between the facial nerve and
the vestibulocochlear nerves (ie, CN VII, CN VIII). The average distance
between the point where the nerves exit the brainstem and the place
where they enter into the internal auditory canal (IAC) is
approximately 15.8 mm. The facial nerve and the nervus intermedius
lie above and slightly anterior to CN VIII.
26. The nervus intermedius conveys (1) afferent taste
fibers from the chorda tympani nerve, which come
from the anterior two thirds of the tongue; (2) taste
fibers from the soft palate via the palatine and greater
petrosal nerves; and (3) preganglionic
parasympathetic innervation to the submandibular,
sublingual, and lacrimal glands.
27. The fibers for taste originate in the nucleus of the
tractus solitarius (NTS), and the fibers to the lacrimal,
nasal, palatal mucus, and submandibular glands
originate in the superior salivatory nucleus. Fibers to
the lacrimal gland are carried with the greater
superficial petrosal nerve until it exits the skull, at
which point the fibers branch off as the Vidian nerve.
28. Nervus intermedius and vestibulocochlear nerve
The nervus intermedius also has a small cutaneous sensory
component from afferent fibers originating from the skin of the
auricle and postauricular area.
The close anatomic association between the facial nerve, the
nervus intermedius, and the vestibulocochlear nerve at the level
of the CPA and in the IAC may result in disturbances in tearing,
taste, salivary gland flow, hearing, balance, and facial function as
a result of lesions at this level.
29. Common examples are the symptoms of tinnitus,
unilateral hearing loss, and balance disturbances often
associated with acoustic schwannomas. Large acoustic
schwannomas may progress to involve the facial nerve
and even CN V, CN IX, CN X, and CN XI.
30. The facial nerve and the nervus intermedius enter the
IAC with the vestibulocochlear nerve. The gross and
microscopic anatomic relationships among the
locations of CN VII, CN VIII, and the nervus
intermedius are of surgical importance. The
vestibulocochlear nerve enters the IAC inferiorly
(caudad).
31. The facial nerve runs superiorly (cephalad) along the roof
of the IAC. A useful mnemonic for remembering this
relationship is "Seven-up over Coke." At the fundus of the
IAC, the falciform crest (crista falciformis) divides the IAC
into superior and inferior compartments. The facial nerve
passes along the superior part of the ledge, separated from
the superior vestibular nerve by a vertical bony ridge
named the Bill bar (after the esteemed Dr William House).
32. Intratemporal Course of the Facial Nerve
The facial nerve travels through the petrous temporal
bone, as shown in the image below, in a bony canal called
the fallopian canal (after Gabriel Fallopius). No other nerve
in the body travels such a long distance through a bony
canal. Because of this bony shell around the nerve,
inflammatory processes involving the central nervous
system (CNS) and the facial nerve or traumatic injuries to
the temporal bone can produce unique complications.
33. Labyrinthine (proximal) segment
The labyrinthine segment of the facial nerve lies beneath
the middle cranial fossa and is the shortest segment in the
fallopian canal (approximately 3.5-4mm in length). In this
segment, the nerve is directed obliquely forward,
perpendicular to the axis of the temporal bone, as shown
above. The facial nerve and the nervus intermedius remain
distinct entities at this level.
34. The term labyrinthine segment is derived from the
location of this segment of the nerve immediately
posterior to the cochlea. The nerve is posterolateral to
the ampullated ends of the horizontal and superior
semicircular canals and rests on the anterior part of
the vestibule in this segment.
35. The labyrinthine segment is the narrowest part of the
facial nerve and is susceptible to compression by
means of edema. This is the only segment of the facial
nerve that lacks anastomosing arterial cascades,
making the area vulnerable to embolic phenomena,
low-flow states, and vascular compression.
36. After traversing the labyrinthine segment, the facial nerve
changes direction to form the first genu (ie, bend or knee),
marking the location of the geniculate ganglion. The geniculate
ganglion is formed by the juncture of the nervus intermedius
and the facial nerve into a common trunk. Additional afferent
fibers from the anterior two thirds of the tongue are added to the
geniculate ganglion from the chorda tympani. Three nerves
branch from the geniculate ganglion: the greater superficial
petrosal nerve, the lesser petrosal nerve, and the external
petrosal nerve.
37. Petrosal nerves
The greater petrosal nerve emerges from the upper portion
of the ganglion and carries secretomotor fibers to the
lacrimal gland. The greater petrosal nerve exits the petrous
temporal bone via the greater petrosal foramen to enter the
middle cranial fossa. The nerve passes deep to the
Gasserian ganglion (ie, trigeminal ganglion) to the foramen
lacerum, through which it travels to the pterygoid canal.
38. In the pterygoid canal, the greater petrosal nerve joins
the deep petrosal nerve to become the nerve of the
pterygoid canal. Axons from this nerve synapse in the
pterygopalatine ganglion; postganglionic
parasympathetic fibers, which are carried via branches
of the maxillary (V2) divisions of the trigeminal nerve
(CN V), innervate the lacrimal gland and mucus
glands of the nasal and oral cavities.
39. The external petrosal nerve is an inconstant branch
that carries sympathetic fibers to the middle
meningeal artery; however, it is not as well known.[5]
The lesser petrosal nerve carries secretory fibers to the
parotid gland. This nerve carries parasympathetic
contributions from the tympanic plexus (from CN IX)
and the nervus intermedius.
40. Tympanic (horizontal) segment
The tympanic segment extends from the geniculate ganglion to
the horizontal semicircular canal and is 8-11mm in length. The
nerve passes behind the cochleariform process and the tensor
tympani. The cochleariform process is a useful landmark for
finding the facial nerve. The nerve lies against the medial wall of
the cavum tympani, above and posterior to the oval window. The
wall can be very thin or dehiscent in this area, and the middle ear
mucosa may lay in direct contact with the facial nerve sheath.
41. The fallopian canal has been reported to be dehiscent
in the area of the oval window in 25-55% of
postmortem specimens. Always anticipate finding a
dehiscent or prolapsed facial nerve in its tympanic
segment, especially in patients with congenital ear
deformities.
42. The distal portion of the facial nerve emerges from the
middle ear between the posterior wall of the external
auditory canal and the horizontal semicircular canal.
This is just distal to the pyramidal eminence, where
the facial nerve makes a second turn (marking the
second genu).
43. The most important landmarks for identifying the
facial nerve in the mastoid are the horizontal
semicircular canal, the fossa incudis, and the digastric
ridge. The second genu of the facial nerve runs
inferolateral to the lateral semicircular canal. This is a
relatively constant relationship.
44. In cases in which the lateral canal is difficult to
identify (eg, cholesteatoma, tumor), the use of other
landmarks, along with cautious exploration, is advised.
The digastric ridge points to the lateral and inferior
aspect of the vertical course of the facial nerve in the
temporal bone. In poorly pneumatized temporal
bones, the digastric ridge may be difficult to identify.
45. The distal aspect of the tympanic segment can be
surgically located via a facial recess approach. The
chorda tympani nerve and the fossa incudis can be
used to identify the nerve when performing a facial
recess approach,
46. The long process of the incus points toward the facial
recess. The chorda tympani nerve serves at the lateral
margin of the triangular facial recess. The chorda
tympani nerve can be exposed along its length and can
be followed inferiorly and medially to its takeoff from
the main trunk of the facial nerve. In practice,
surgeons most likely employ cues from all these
landmarks in respecting the integrity of the facial
nerve.
47. Mastoid segment
The second genu marks the beginning of the mastoid
segment. The second genu is lateral and posterior to the
pyramidal process. The nerve continues vertically down the
anterior wall of the mastoid process to the stylomastoid
foramen. The mastoid segment is the longest part of the
intratemporal course of the facial nerve, approximately 10-
14mm long. During middle ear surgery, the facial nerve is
most commonly injured at the pyramidal turn.
48. The 3 branches that exit from the mastoid segment of the
facial nerve are (1) the nerve to the stapedius muscle, (2)
the chorda tympani nerve, and (3) the nerve from the
auricular branch of the vagus. The auricular branch of the
vagus nerve arises from the jugular foramen and joins the
facial nerve just distal to the point at which the nerve to the
stapedius muscle arises. Pain fibers to the posterior
auditory canal may be carried with this nerve.
49. The chorda tympani is the terminal branch of the
nervus intermedius. The chorda runs laterally in the
middle ear, between the incus and the handle of the
malleus. The nerve crosses the middle ear cavity and
exits through the petrotympanic fissure (ie, canal of
Huguier) to join the lingual nerve. The chorda
tympani nerve carries preganglionic secretomotor
fibers to the submaxillary and sublingual glands.
50. . The chorda also carries special sensory afferent fibers
(ie, taste fibers) from the anterior two thirds of the
tongue and fibers from the posterior wall of the
external auditory canal responsible for pain,
temperature, and touch sensations.
51. The facial nerve exits the fallopian canal via the
stylomastoid foramen. The nerve travels between the
digastric and stylohyoid muscles and enters the
parotid gland. A sensory branch exits the nerve just
below the stylomastoid foramen and innervates the
posterior wall of the external auditory canal and a
portion of the tympanic membrane.
52. Extratemporal Facial Nerve
A number of useful landmarks are used to locate the facial
nerve. Topographic landmarks, shown in the image below,
can serve as guides for locating the course of the facial
nerve and its branches. For example, a line drawn between
the mastoid tip and the angle of the mandible can serve as
a useful landmark for the superior limits of a neck
dissection. Removal of parotid tissue inferior to this line
can be performed relatively safely.
53. The topographic trajectory of the frontal and/or
marginal branches should be identified during a
rhytidoplasty, submandibular gland excision, and/or
neck dissection. The frontal branch can be roughly
located along a line extending from the attachment of
the lobule (approximately 5mm below the tragus),
anterior and superior to a point 1.5cm above the lateral
aspect of the ipsilateral eyebrow
54. Surgical landmarks to the facial nerve include the
tympanomastoid suture line, the tragal pointer, and
the posterior belly of the digastric muscle. The
tympanomastoid suture line lies between the mastoid
and tympanic segments of the temporal bone and is
approximately 6-8mm lateral to the stylomastoid
foramen.
55. The main trunk of the nerve can also be found midway
between (10mm posteroinferior to) the cartilaginous
tragal pointer of the external auditory canal and the
posterior belly of the digastric muscle. The nerve is
usually located inferior and medial to the pointer.
56. During surgical dissection, the surgeon may encounter
a branch from the occipital artery that lies lateral to
the facial nerve. Brisk bleeding at this time may be a
sign that the nerve is in close proximity; hemostasis
should be obtained using bipolar electrocautery, and
further dissection should proceed cautiously. The
styloid process is deep to the main trunk of the nerve.
57. In the infant and young child, these landmarks are not
applicable because of differences in the rate of
anatomic development of the parotid gland and
mastoid. The modified Blair incision most commonly
used in adults is often avoided in children because the
facial nerve is located more superficially, and the risk
of injury is increased with elevation of the skin flaps.
58. Many textbooks on pediatric otolaryngology provide
detailed descriptions of the safe placement of surgical
incisions for exposing the facial nerve and its branches
in children.
Once it has exited the fallopian canal at the
stylomastoid foramen, the facial nerve gives off several
rami before it divides into its main branches.
59. Below the stylomastoid foramen, the posterior auricular nerve
leaves the facial nerve and innervates the postauricular muscles.
Two small branches innervate the stylohyoid muscle and
posterior belly of the digastric muscle.
The facial nerve crosses lateral to the styloid process and
penetrates the parotid gland. The nerve lies in a fibrous plane
that separates the deep and superficial lobes of the parotid
gland. In the parotid gland, the nerve divides at the pes
anserinus into 2 major divisions; ie, the superiorly directed
temporal-facial and the inferiorly directed cervicofacial
branches.
60. After the main point of division, 5 major branches of
the facial nerve exist, as follows:
Temporal (ie, frontal)
Zygomatic
Buccal
Marginal mandibular
Cervical
61. The facial nerve innervates 14 of the 17 paired muscle
groups of the face on their deep side. The 3 muscles
innervated from other sources are the buccinator, levator
anguli oris, and mentalis muscles. Frequent connections
between the buccal and zygomatic branches exist. The
temporal and marginal mandibular branches are at highest
risk during surgical procedures and are usually terminal
connections without anastomotic connections.
62. Superficial musculoaponeurotic system
The superficial musculoaponeurotic system (SMAS) is
a superficial fascial layer that extends throughout the
cervical facial region. In the lower face, the SMAS
invests the facial muscles and is continuous with the
platysma muscle. Superiorly, the SMAS ends at the
level of the zygoma because of attachments of the
fascial layers to the zygomatic arch.
63. The temporoparietal fascia is not continuous with the
SMAS, but they are most likely embryologic
equivalents. The temporoparietal fascia extends from
the zygomatic arch as an extension of the deep
temporal fascia. In the temporal region, the frontal
branch of the facial nerve crosses the zygomatic arch
and courses within the superficial layer of the deep
temporal fascia (temporoparietal fascia).
64. In the scalp, the equivalent of the SMAS is the galea
aponeurotica, which splits to ensheathe the frontalis,
occipitalis, procerus, and some of the postauricular
muscles. In the upper face, the neurovascular
structures exit their bony foramina and penetrate the
SMAS to run within its superficial aspects or on its
surface.
65. The SMAS encloses all of the facial muscles and is their
only attachment to the overlying dermis, thus transmitting
contractions of the facial muscles to the overlying skin. A
conceptual understanding of the anatomy of the SMAS is
important to the surgeon. In the lower face, the facial nerve
always runs deep to the platysma and SMAS and innervates
the muscles on their undersurfaces (except for the
buccinator, levator anguli oris, and mentalis muscles).
66. The SMAS also helps the surgeon to identify the
location of the facial nerve during dissection toward
the midline of the face, where the nerve can be found
running on top of the masseter muscle just below the
SMAS.
67. Temporal branches
The relationships of the temporal branch are complex and only
briefly described in this article. Refer to Larrabee and Makielski
for a more complete anatomic description.[7] The temporal
branch of the facial nerve exits the parotid gland and runs within
the SMAS over the zygomatic arch into the temple region. The
frontal branch enters the undersurface of the frontalis muscle
and lies superficial to the deep temporalis fascia. To avoid injury
to the frontal branch during elevation of facial flaps, the surgeon
should elevate either in a subcutaneous plane or deep to the
SMAS.
68. Marginal branches
The mandibular (or marginal) division lies along the body
of the mandible (80%) or within 1-2cm below (20%). This is
a critical landmark in head and neck surgery. The marginal
branch lies deep to the platysma throughout much of its
course. It becomes more superficial approximately 2cm
lateral to the corner of the mouth and ends on the
undersurface of the muscles. Injury to the marginal branch
results in paralysis of the muscles that depress the corner
of the mouth.
69. Facial Nerve Paralysis
The spectrum of facial motor dysfunction is wide, and
characterizing the degree of paralysis can be difficult.
Several systems have been proposed, but since the mid-
1980s, the House-Brackmann system has been widely used.
In this scale, grade I is assigned to normal function, and
grade VI represents complete paralysis. Intermediate
grades vary according to function at rest and with effort.
70. GradeDescriptionCharacteristics
INormalNormal facial function in all areas
II Mild dysfunction - Slight weakness noticeable on close
inspection; may have very slight synkinesis.
III Moderate dysfunction - Obvious, but not disfiguring,
difference between 2 sides; noticeable, but not severe,
synkinesis, contracture, or hemifacial spasm; complete eye
closure with effort y slight synkinesis
71. House Brackmann grading
IV-Moderately severe dysfunction -Obvious weakness
or disfiguring asymmetry; normal symmetry and tone
at rest; incomplete eye closure
V -Severe dysfunction - Only barely perceptible
motion; asymmetry at rest
VI -Total paralysis --No movement
72. Vascular Supply of the Facial Nerve
The cortical motor area of the face is supplied by the
Rolandic branch of the middle cerebral artery. Within the
pons, the facial nucleus receives its blood supply primarily
from the anterior inferior cerebellar artery (AICA). The
AICA, a branch of the basilar artery, enters the internal
auditory canal (IAC) with the facial nerve. The AICA
branches into the labyrinthine and cochlear arteries.
73. The superficial petrosal branch of the middle
meningeal artery is the second of 3 sources of arterial
blood supply to the extramedullary (ie, intrapetrosal)
facial nerve. The posterior auricular artery supplies the
facial nerve at and distal to the stylomastoid foramen.
Venous drainage parallels the arterial blood supply.
