Bell's Palsy

Populations and Risk Factors

• Affects all age groups; peak incidence between ages 15 and 45
• Equal prevalence between sexes overall, though pregnant women (especially in the third trimester) have approximately 3 times the population risk
• Diabetes mellitus increases risk by 2–4 times, likely due to pre-existing microvascular compromise of the vasa nervorum supplying CN VII
• Upper respiratory tract infection in the preceding 1–2 weeks increases risk — viral reactivation theory
• Immunocompromised individuals at higher risk (HIV, organ transplant recipients)
• Recurrence rate approximately 7–12%; bilateral cases are rare (< 1%) and should raise suspicion for systemic causes (sarcoidosis, Lyme disease, Guillain-Barre)
• Family history: approximately 4–14% of cases have a familial component, suggesting a genetic predisposition to narrow facial canal anatomy

Causes and Pathophysiology

CN VII Anatomy and Course

• CN VII (facial nerve) is predominantly a motor nerve controlling the muscles of facial expression, the stapedius (middle ear), and the posterior belly of digastric. It also carries parasympathetic fibers to the lacrimal and salivary glands and a small sensory component (taste from the anterior two-thirds of the tongue via the chorda tympani).
• The nerve exits the brainstem at the cerebellopontine angle, enters the internal acoustic meatus, and takes a tortuous course through the facial canal (fallopian canal) within the temporal bone — this is the longest intraosseous course of any cranial nerve, spanning approximately 30 mm.
• The nerve exits the skull through the stylomastoid foramen, passes through or adjacent to the parotid gland, and then branches into five terminal motor divisions (remembered by the mnemonic "Two Zebras Bit My Cat"):
• Temporal branch: frontalis, orbicularis oculi (upper), corrugator supercilii
• Zygomatic branch: orbicularis oculi (lower), nasalis
• Buccal branch: buccinator, orbicularis oris (upper), zygomaticus major/minor
• Marginal mandibular branch: depressor anguli oris, depressor labii inferioris, mentalis
• Cervical branch: platysma

Compression Mechanism

• The facial canal within the temporal bone is a rigid bony tube that cannot expand. When CN VII becomes inflamed (from viral reactivation, autoimmune response, or ischemia), the resulting edema compresses the nerve against the walls of this unyielding canal.
• The meatal foramen (narrowest segment of the facial canal, approximately 0.68 mm diameter) is the primary compression point — even mild swelling here produces significant nerve conduction compromise.
• This compression produces a neuropraxia (conduction block without axonal disruption) in mild cases, or axonotmesis (axonal degeneration with intact endoneurium) in moderate-to-severe cases. The degree of compression determines the clinical severity and recovery timeline.

Etiology — Viral Reactivation Theory

• HSV-1 reactivation (herpes simplex virus type 1) is the leading etiologic hypothesis. HSV-1 lies dormant in the geniculate ganglion (a sensory ganglion of CN VII within the temporal bone) and reactivates under conditions of stress, immunosuppression, or local trauma, producing inflammation and edema of the nerve.
• Other viral associations: varicella-zoster virus (Ramsay Hunt syndrome — a related but distinct condition with vesicular eruption on the ear), Epstein-Barr virus (EBV), cytomegalovirus (CMV), mumps, influenza
• Non-viral causes: Lyme disease (Borrelia burgdorferi — bilateral facial palsy should raise suspicion), sarcoidosis, autoimmune neuritis
• When a specific cause is identified, the condition is technically no longer "Bell's palsy" (which is idiopathic by definition), but the clinical presentation and MT approach are similar

LMN vs. UMN Facial Paralysis — The Critical Distinction

• Bell's palsy is an LMN lesion. The nerve damage occurs at or below the facial nerve nucleus in the pons. Because all motor signals to the ipsilateral face pass through this single pathway, the entire half of the face is paralyzed — forehead, eye, cheek, mouth, and chin.
• Stroke produces a UMN lesion. The upper face (frontalis, orbicularis oculi) receives bilateral cortical innervation — motor cortex signals reach both the left and right facial nuclei. When one cortical pathway is destroyed by stroke, the upper face still receives input from the intact contralateral cortex. Therefore, only the lower face (below the eye) is paralyzed. The patient can still wrinkle their forehead and close their eyes.
• Clinical rule: If the forehead is involved (cannot wrinkle forehead, cannot close eye fully), it is an LMN lesion (Bell's palsy, Ramsay Hunt, or other peripheral cause). If the forehead is spared, it is a UMN lesion (stroke, tumor, or other central cause) — this is a medical emergency requiring immediate referral.

Wallerian Degeneration and Recovery

• In mild cases (neuropraxia), recovery begins within 2–3 weeks as edema resolves and conduction resumes across intact axons
• In moderate cases (axonotmesis), Wallerian degeneration occurs distal to the compression site — the axon degenerates but the endoneurial tube remains intact, guiding regeneration at approximately 1 mm/day
• Synkinesis occurs when regenerating axons enter incorrect endoneurial tubes, producing inappropriate muscle co-contraction during recovery (e.g., eye closes when patient smiles; mouth twitches when patient blinks). This is a sign of nerve recovery but indicates disordered reinnervation.
• Most recovery occurs within 3–6 months. Cases showing no improvement by 6 months have a poor prognosis for full recovery.

Signs and Symptoms

Acute Phase (Onset to 2–3 Weeks)

• Sudden onset of unilateral facial weakness, typically reaching maximal paralysis within 48–72 hours
• Forehead: cannot wrinkle the forehead or raise the eyebrow on the affected side
• Eye: cannot fully close the eye (lagophthalmos); Bell's phenomenon — the eye rolls upward and outward when the patient attempts to close it, exposing the sclera; reduced blink reflex leads to corneal drying, irritation, and risk of corneal scarring
• Mouth: drooping at the corner; inability to smile symmetrically, pucker lips, whistle, or show teeth; drooling from the affected side; difficulty with articulation of labial sounds (p, b, m)
• Ear: hyperacusis (increased sensitivity to sound) on the affected side due to paralysis of the stapedius muscle, which normally dampens vibration of the ossicular chain
• Taste: altered taste on the anterior two-thirds of the tongue (chorda tympani involvement) — less common; suggests a more proximal lesion
• Pain: retroauricular or periauricular pain may precede the paralysis by 1–2 days; pain is from inflammation within the facial canal, not from muscle spasm
• Emotional impact: significant psychological distress from sudden facial disfigurement; difficulty eating, drinking, and speaking affects social interaction

Recovery Phase (3 Weeks to 6 Months)

• Gradual return of facial muscle function, typically progressing from upper face to lower face
• Synkinesis — inappropriate co-contraction of muscles during voluntary movement (e.g., involuntary eye closure when smiling, mouth movement when blinking); indicates regenerating axons have entered incorrect endoneurial tubes
• Crocodile tears (gustatory lacrimation) — tearing when eating; parasympathetic fibers regenerate into lacrimal gland pathways instead of salivary gland pathways
• Possible residual weakness, asymmetry, or contracture on the affected side
• Facial muscle contracture on the affected side from fibrotic changes during the paralytic period if the muscles are not maintained
• Compensatory hypertonicity on the unaffected side from overuse during the paralytic phase — the healthy side works harder during eating, speaking, and facial expression

Assessment Profile

Subjective Presentation

• Chief complaint: "I woke up and one side of my face wasn't working" or "I noticed I couldn't close my eye / my mouth was drooping" — onset is typically overnight or develops over several hours; the patient may first notice difficulty drinking (liquid dribbling from the affected side) or asymmetry in the mirror
• Pain quality: retroauricular or postauricular ache on the affected side (present in approximately 50% of cases); not sharp or radiating; hyperacusis — everyday sounds on the affected side feel uncomfortably loud; headache is not typical and should raise suspicion for central causes
• Onset: sudden (hours to 1–2 days to maximal paralysis); may be preceded by viral illness, stress, cold exposure, or dental procedure; no trauma; no seizure or altered consciousness (these suggest stroke)
• Aggravating factors: cold wind or drafts on the affected side; attempting facial expressions (frustrating and tiring); eating and drinking (functional difficulty, not pain)
• Easing factors: warm compress to the affected side provides comfort; eye patching reduces corneal irritation; symptoms do not vary with body position, exertion, or time of day (unlike UMN lesions which may have diurnal variation)
• Red flags: Bilateral facial paralysis → suspect Lyme disease, Guillain-Barre, or sarcoidosis — medical referral. Forehead sparing (can wrinkle forehead and close eye but lower face is paralyzed) → suspect stroke — FAST screen, emergency referral; do not treat. Progressive weakness beyond 3 weeks without improvement → suspect tumor or other structural cause — medical referral.

Observation

• Local inspection: facial asymmetry visible at rest — flattening of the nasolabial fold on the affected side, drooping of the mouth corner, widened palpebral fissure (eye appears wider because orbicularis oculi is flaccid and cannot maintain tone); during voluntary movement, asymmetry becomes more pronounced; Bell's phenomenon visible when attempting eye closure; possible tearing of the affected eye from impaired drainage (lacrimal pump requires orbicularis function)
• Posture: generally unremarkable; some clients may tilt the head toward the unaffected side or hold a hand near the affected eye protectively; no trunk or limb postural compensation (this is a cranial nerve condition)
• Gait: normal — Bell's palsy does not affect the limbs, trunk, or balance; abnormal gait in the presence of facial paralysis strongly suggests a central lesion (stroke, tumor, MS) rather than Bell's palsy

Palpation

• Tone: Flaccid hypotonia of all facial muscles on the affected side — frontalis, orbicularis oculi, zygomaticus, buccinator, orbicularis oris, mentalis, platysma. Muscles feel soft, thin, and non-responsive. In the recovery phase with synkinesis, tone may be uneven with areas of reinnervation showing increased activity. Compensatory hypertonicity on the unaffected side — particularly the muscles of mastication (masseter, temporalis) and the contralateral facial muscles from overuse during eating, speaking, and expression.
• Tenderness: mastoid process and retroauricular area on the affected side (inflammation site); parotid gland region may be tender if swollen; no referred path tenderness (this is a motor nerve condition — sensation is intact; any deep pain is local, not referred)
• Temperature: affected side may feel cooler from reduced muscle activity and vasomotor changes; no active inflammation visible on the skin surface; warmth over the mastoid may suggest ongoing nerve inflammation
• Tissue quality: affected side — facial muscles feel atrophic, thin, and inelastic with reduced bulk compared to the unaffected side; during recovery, palpable fasciculations may be present as reinnervation occurs; unaffected side — masseter and temporalis may develop hypertrophic changes and trigger points from compensatory overuse during chewing

Motion Assessment

• AROM: facial expression testing performed bilaterally — raise eyebrows (frontalis), close eyes tightly (orbicularis oculi), smile showing teeth (zygomaticus, risorius), pucker lips (orbicularis oris), puff cheeks (buccinator), depress lower lip (depressor labii); on the affected side, these movements are absent (acute phase), weak, or produce synkinetic co-contraction (recovery phase); cervical AROM is typically full and pain-free
• PROM / end-feel: not directly applicable to facial muscles in the traditional sense; however, passive manipulation of the affected facial tissues reveals reduced elastic recoil (flaccid muscles do not spring back); in later stages, fibrotic changes may produce increased tissue resistance
• Resisted testing: not applicable to facial muscles in the standard resisted testing format; the facial expression tests above serve as the functional equivalent; neck and shoulder resisted testing is normal (any limb weakness suggests a central or systemic lesion, not Bell's palsy)

Special Test Cluster

Stroke vs. Bell's palsy — the #1 differential: If the patient can wrinkle their forehead on the affected side, it is NOT Bell's palsy — it is an UMN lesion. Apply the FAST screen immediately. Do not proceed with treatment until stroke has been ruled out. This distinction must be demonstrated in any exam scenario involving facial paralysis.

Differential Assessment