The facial nerve is a small structure that does a remarkable amount of work. It animates the entire face — the difference between a person and a mask. It carries taste from the front of the tongue, dampens loud sound at the stapedius muscle, supplies the parasympathetic fibers that water the eye and the mouth, and reports back a slim band of skin sensation from the ear canal. When CN VII is injured, the resulting weakness is visible across the room. When the injury is at the right place, it also leaves a constellation of less obvious deficits — taste loss, hyperacusis, dry eye — that together localize the lesion along its course with almost geographic precision. Few cranial nerves reward careful examination so reliably.

The single most important distinction in any patient with facial weakness is whether the lesion is upper motor neuron (central, sparing the upper face) or lower motor neuron (peripheral, involving the upper face). That distinction takes about ten seconds to make and triages the entire differential.

Functional Anatomy

The facial nerve has four functional components:

• Branchial motor — innervates the muscles of facial expression (derived from the second pharyngeal arch), the stapedius muscle in the middle ear, the posterior belly of the digastric, and the stylohyoid.
• Parasympathetic (visceromotor) — supplies the lacrimal gland (via the greater superficial petrosal nerve), the submandibular and sublingual salivary glands (via the chorda tympani), and the mucous glands of the nose and palate.
• Special sensory (taste) — carries taste from the anterior two-thirds of the tongue via the chorda tympani.
• General sensory — a small contribution to the skin of the external auditory canal and the concha of the auricle, via the nervus intermedius and a branch usually called the auricular branch.

The motor nucleus sits in the lower pons. The nerve loops dorsally over the abducens nucleus (forming the facial colliculus on the floor of the fourth ventricle), then exits the brainstem at the pontomedullary junction with the nervus intermedius — the small bundle that carries the parasympathetic, taste, and sensory fibers — and the nearby CN VIII. From there it traverses the cerebellopontine angle, enters the internal auditory meatus, and runs an extraordinarily complex course through the temporal bone before exiting at the stylomastoid foramen.

Inside the temporal bone, the nerve passes through a series of named segments and branches:

1. The labyrinthine segment, ending at the geniculate ganglion, where the greater superficial petrosal nerve branches off to supply the lacrimal gland.
2. The tympanic (horizontal) segment, which crosses the medial wall of the middle ear.
3. The mastoid (vertical) segment, from which the nerve to the stapedius branches off, followed by the chorda tympani, which separates carrying taste and parasympathetics.
4. The extracranial portion, beyond the stylomastoid foramen, which courses through the parotid gland and divides into the temporal, zygomatic, buccal, mandibular, and cervical branches that supply the face.

Each branch point matters clinically, because a lesion above a branch produces the deficits supplied by that branch, while a lesion below spares them. This is the localizing power of the facial nerve exam: by combining the pattern of facial weakness with findings of taste, lacrimation, and hyperacusis, you can place the lesion within a centimeter or two of its actual location along the nerve.

The Crucial Distinction: Upper vs Lower Motor Neuron Weakness

The frontalis muscle and the orbicularis oculi receive bilateral corticobulbar input, while the lower face muscles receive predominantly contralateral input. A unilateral cortical or corticobulbar tract lesion therefore weakens the contralateral lower face but spares the forehead and (mostly) the eye closure — a pattern called upper motor neuron (central) facial weakness. A peripheral facial nerve lesion weakens the entire half of the face, including the forehead — lower motor neuron (peripheral) facial weakness.

The bedside test is to ask the patient to wrinkle their forehead by looking up or raising their eyebrows. If the forehead furrows symmetrically and the weakness is confined to the lower face, the lesion is central. If one side of the forehead does not move and the eye cannot close, the lesion is peripheral. This single observation distinguishes a stroke from a Bell palsy almost every time.

Two caveats:

• Emotional movements may dissociate from voluntary ones. A patient with a central facial weakness from a stroke may still smile symmetrically with genuine amusement, because emotional facial movements are driven by extrapyramidal pathways that bypass the motor cortex. Conversely, a patient with a thalamic or basal ganglia lesion may have emotional facial paresis — preserved voluntary movement but asymmetric smiling — which is a localizing sign in its own right.
• The eye closure rule has nuances. Mild lower face weakness from a cortical lesion may sometimes accompany a barely perceptible orbicularis oculi weakness. The rule is “no significant forehead weakness in central lesions,” not “no weakness at all.”

The Examination

Inspection at Rest

Before asking the patient to do anything, look at the face at rest. The unaffected side bears the entire tone of the facial muscles; the affected side hangs slightly lower. Look for:

• Asymmetry of the nasolabial fold — flattened on the weak side.
• Drooping of the corner of the mouth.
• A widened palpebral fissure on the affected side from loss of orbicularis tone — sometimes prominent enough to make the patient look “wider-eyed” on one side.
• Tearing onto the cheek, or eye redness, from incomplete eye closure (lagophthalmos).
• Synkinetic twitches if the patient is recovering from a previous palsy.

Voluntary Movement

Walk the patient through the major muscle groups, comparing sides at each step:

• Forehead (frontalis): “Raise your eyebrows” or “look up at the ceiling.”
• Orbicularis oculi: “Close your eyes tightly — don’t let me open them.” Try gently to pry the lids apart. A weak side opens easily.
• Nasolabial and buccinator: “Show me your teeth.” “Puff out your cheeks — don’t let me press the air out.”
• Orbicularis oris: “Pucker your lips like you’re going to whistle.”
• Platysma: “Grimace, pull the corners of your mouth down and tense your neck.”

Subtle weakness can be brought out by the Bell phenomenon: when the patient tries to close the eyes, the eye on the weak side rolls upward and out, exposing the white sclera. This is normal physiology that becomes visible because the eye does not close to hide it.

Branch-Specific Findings That Localize

This is where the facial exam becomes a true localizer.

• Taste on the anterior two-thirds of the tongue is carried by the chorda tympani. To test it, ask the patient to protrude the tongue and apply a small amount of sweet, salty, or sour solution to one side, then the other. The tongue must remain protruded during testing — withdrawing it spreads the solution. Loss of taste on the anterior tongue localizes the lesion above the chorda tympani branch (mastoid segment or higher).
• Hyperacusis — a sense that sounds, especially of moderate intensity, are uncomfortably loud — reflects loss of the stapedius muscle, which damps the ossicular chain. A patient with hyperacusis on the side of facial weakness has a lesion above the branch to stapedius.
• Loss of lacrimation on the affected side is documented with the Schirmer test (a strip of filter paper hooked into the lower lid) but can sometimes be asked about: “Is the eye on that side drier than the other?” Loss of tears localizes the lesion above the geniculate ganglion (labyrinthine segment).
• Sensation in the external auditory canal and concha: tap or stroke the canal entrance gently and ask if it feels normal. The classical Hitselberger sign is hypoesthesia of the posterior wall of the external auditory canal — a delicate finding suggesting a CPA mass, especially vestibular schwannoma.

The Corneal Reflex (Efferent Limb)

The efferent limb of the corneal reflex is the facial nerve’s branch to orbicularis oculi. If the patient does not blink when one cornea is touched, but they do blink consensually when the other cornea is touched (and they confirm they felt the touch), the lesion is on CN VII rather than CN V₁.

Patterns of Facial Nerve Dysfunction

Localization Along the Course

Bell’s Palsy

Idiopathic acute peripheral facial palsy — Bell’s palsy — is the most common cause of unilateral facial weakness. It evolves over hours to days, often with retroauricular pain preceding the weakness, and reaches its nadir within seventy-two hours. The pattern is a complete or near-complete peripheral palsy with all the features above (taste loss, hyperacusis, dry eye) reflecting a lesion at or near the geniculate ganglion. Reactivation of HSV-1 in the geniculate ganglion is the leading proposed cause, although the evidence remains circumstantial.

The diagnosis is clinical, but with two requirements. First, the weakness must be a complete peripheral pattern. A patient with forehead sparing has a stroke until proven otherwise, not Bell’s palsy. Second, the weakness must be isolated: associated hearing loss, severe ear pain with vesicles, other cranial nerve findings, slow progression beyond a week, or failure to improve within three to four months should all prompt imaging and a search for an alternative diagnosis.

Treatment with oral corticosteroids (started within seventy-two hours) significantly improves recovery. Antivirals are sometimes added in severe cases, with smaller incremental benefit. Eye protection — artificial tears by day, ointment and patching at night — is essential to prevent corneal exposure. About seventy percent of patients recover completely; the rest have varying degrees of residual weakness or synkinesis.

Ramsay Hunt Syndrome (Herpes Zoster Oticus)

Reactivation of varicella-zoster virus in the geniculate ganglion produces facial weakness, often more severe than Bell’s, accompanied by vesicles in the external auditory canal, on the auricle, on the tympanic membrane, or on the anterior tongue. Pain is prominent. Hearing loss, vertigo, and tinnitus often accompany the palsy because of close anatomical involvement of CN VIII. Treatment is high-dose antivirals plus steroids; recovery is less complete than after Bell’s, and synkinesis is more common.

Bilateral Facial Palsy

Simultaneous or rapidly sequential bilateral facial weakness is uncommon and has its own differential, almost none of which is Bell’s. Consider:

• Guillain-Barré syndrome — particularly the Miller Fisher variant or pharyngeal-cervical-brachial variant.
• Lyme disease — bilateral facial palsy is a hallmark of early disseminated Lyme neuroborreliosis, especially in endemic areas.
• Sarcoidosis — Heerfordt syndrome (uveoparotid fever) classically includes bilateral CN VII palsy.
• Basal meningitis — tuberculous, fungal, carcinomatous.
• Brainstem stroke — bilateral pontine lesions are uncommon but possible.
• Möbius syndrome — congenital bilateral CN VI and VII palsies from agenesis of brainstem nuclei.

A patient with bilateral facial palsy needs a more aggressive workup than Bell’s: MRI of the brain, lumbar puncture, Lyme serology, ACE/calcium screening for sarcoid, and consideration of GBS evaluation including nerve conduction studies.

Hemifacial Spasm

Involuntary, intermittent, painless contractions of the facial muscles on one side — usually starting around the eye and spreading down the face over months to years. The mechanism is usually a vascular loop (anterior inferior cerebellar artery is classical) compressing CN VII at the root entry zone, producing ectopic activity and ephaptic transmission between fibers. The clinical pearl is to distinguish hemifacial spasm from blepharospasm, in which the spasms are bilateral and dystonic in character, and from facial myokymia, which is a finer rippling movement seen in multiple sclerosis and pontine glioma. Treatment is botulinum toxin or microvascular decompression.

Synkinesis

Aberrant regeneration after CN VII injury produces involuntary movements in one part of the face when another part is moved voluntarily. Eye closure may pull the corner of the mouth up; smiling may close the eye; the classical “crocodile tears” syndrome (lacrimation while eating) reflects regenerating salivary fibers reaching the lacrimal gland. Synkinesis is a sign of incomplete recovery after a severe peripheral facial palsy.

🔍 Did You Know?

Emotional facial paresis — preserved voluntary facial movement but asymmetric smiling in response to a joke or emotional stimulus — is a localizing sign. The pathway for emotional facial movement runs through the limbic system and basal ganglia rather than the motor cortex, and lesions of the thalamus, internal capsule, or insula can dissociate the two. A patient who can show their teeth on command but whose face becomes lopsided when they laugh has a lesion sparing the corticobulbar tract but involving the extrapyramidal facial pathway.

Pitfalls and Pearls

• Look at the forehead first. If the upper face moves symmetrically, you are looking at a central lesion. If it does not, you are looking at a peripheral lesion. This single observation triages most facial weakness within seconds.
• Bell’s palsy must be a complete peripheral pattern. Forehead sparing makes the diagnosis wrong. Look again.
• Check for vesicles in the ear and on the auricle. Ramsay Hunt is missed if you only look at the face. Lift the helix and peer into the canal in every patient with peripheral facial palsy.
• Bilateral simultaneous facial palsy is not Bell’s. Work it up.
• The Bell phenomenon is normal physiology becoming visible. Its presence does not indicate disease; its absence on the affected side in a patient with apparent eye closure suggests the patient is also weak in the upward gaze pathway or has supranuclear pathology.
• Test taste on the protruded tongue. Letting the tongue retract spreads the test substance across midline and ruins the test.
• The patient with peripheral facial palsy needs eye protection. The cornea is the organ most at risk in the first weeks. Artificial tears every two hours by day and ointment with taping at night.
• Slowly progressive peripheral facial weakness is a parotid tumor or a CPA mass until proven otherwise. Bell’s palsy reaches its worst within three days, not three months.
• Recurrent facial palsy on the same side — especially with a fissured tongue and facial swelling — raises the possibility of Melkersson-Rosenthal syndrome, a rare but distinctive entity.

References

1. Campbell WW. DeJong’s The Neurologic Examination. 7th ed. Philadelphia: Lippincott Williams & Wilkins; 2013. Chapter 16.
2. Gronseth GS, Paduga R. Evidence-based guideline update: Steroids and antivirals for Bell palsy. Neurology. 2012;79(22):2209-2213.
3. Sullivan FM, Swan IR, Donnan PT, et al. Early treatment with prednisolone or acyclovir in Bell’s palsy. N Engl J Med. 2007;357(16):1598-1607.
4. Baugh RF, Basura GJ, Ishii LE, et al. Clinical practice guideline: Bell’s palsy. Otolaryngol Head Neck Surg. 2013;149(3 Suppl):S1-S27.
5. Halperin JJ. Facial nerve palsy associated with Lyme disease. Muscle Nerve. 2003;28(4):516-517.
6. Sweeney CJ, Gilden DH. Ramsay Hunt syndrome. J Neurol Neurosurg Psychiatry. 2001;71(2):149-154.
7. Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 7th ed. Philadelphia: Wolters Kluwer; 2017.