Bell's palsy is the most common cause of acute idiopathic peripheral facial nerve paralysis, constituting 60-75% of all peripheral facial paralysis cases. Annual incidence is 15-30 per 100,000. Although the exact etiology is unknown, Herpes Simplex Virus type 1 (HSV-1) reactivation is the most accepted theory — latent infection in the geniculate ganglion leads to nerve edema and ischemia through viral reactivation. The disease typically occurs in adults aged 15-60 with no gender difference. Risk is increased in pregnancy and diabetic patients. Clinically characterized by unilateral peripheral facial paralysis (entire half of face including forehead) developing within 48-72 hours. 85% of patients recover spontaneously within 3 weeks to 3 months. CT is usually normal — imaging is primarily performed to exclude tumoral causes (schwannoma, parotid tumor) and inflammatory/infectious causes (Ramsay Hunt). Enhancement of intratemporal facial nerve segments (especially labyrinthine, tympanic, and/or mastoid) on post-contrast T1 MRI is the typical finding.
Age Range
15-60
Peak Age
40
Gender
Equal
Prevalence
Common
The pathophysiology of Bell's palsy is based on viral neurotropism theory. HSV-1 (or less commonly VZV) resides latently in the geniculate ganglion — immune suppression, stress, or hormonal changes (pregnancy) trigger viral reactivation. The reactivated virus spreads along the facial nerve and initiates inflammation in the nerve sheath. The labyrinthine segment is the narrowest portion of the Fallopian canal (diameter: 0.68 mm) — edema in this narrow canal causes nerve compression (compartment syndrome-like mechanism). Compression impairs axonal transport and vascular perfusion → ischemia → Wallerian degeneration begins. Enhancement on MRI is the result of this inflammatory process: viral edema disrupts blood-nerve barrier → gadolinium leaks around the nerve → appears as bright enhancement on T1. Normal facial nerve geniculate ganglion and distal intratemporal segments may show mild enhancement (normal venous plexus) — but the labyrinthine segment does not normally enhance. Enhancement in the labyrinthine segment is very specific for Bell's palsy. Enhancement intensity and extent correlate with prognosis — extensive enhancement predicts worse prognosis.
Enhancement of the labyrinthine segment of the facial nerve — this segment does not normally enhance (unlike geniculate ganglion). Labyrinthine segment enhancement is the most specific MRI finding for Bell's palsy and with clinical correlation establishes the diagnosis.
Linear smooth enhancement of labyrinthine, tympanic, and/or mastoid segments of the facial nerve on post-contrast T1. Geniculate ganglion enhancement is the most prominent area. Enhancement is most prominent in the acute phase of paralysis and regresses with recovery.
Report Sentence
Linear smooth enhancement of the labyrinthine/tympanic/mastoid segments of the facial nerve on the ___ side is observed on fat-suppressed post-contrast T1 sequences, consistent with Bell's palsy; no nodular thickening or mass is detected.
Temporal bone CT is normal — Fallopian canal intact, no bone destruction, middle ear and mastoid normal. This negative finding is very important in excluding tumoral cause.
Report Sentence
Fallopian canal is intact on temporal bone CT with no bone destruction or mass identified; tumoral cause is excluded.
Mild signal increase in the facial nerve on T2-weighted images as a sign of edema. This finding is not always prominent and is less reliable than enhancement finding. CISS/FIESTA sequences better demonstrate nerve edema.
Report Sentence
Mild signal increase in the facial nerve on T2-weighted sequences is observed, consistent with acute neuritis/edema.
Prominent enhancement of the geniculate ganglion — most common and earliest enhancing site in Bell's palsy. CAUTION: Normal geniculate ganglion may also show mild enhancement (normal venous plexus) — pathological enhancement is distinguished by clinical correlation and labyrinthine segment involvement.
Report Sentence
Prominent enhancement of the geniculate ganglion is observed, together with labyrinthine segment enhancement consistent with Bell's palsy.
No diffusion restriction on DWI — important negative finding distinguishing Bell's palsy from cholesteatoma and abscess/collection. Cholesteatoma shows prominent diffusion restriction.
Report Sentence
No diffusion restriction is detected in the temporal bone region on DWI, excluding cholesteatoma.
Criteria
Acute onset (<72 hours), unilateral, peripheral type. Recovery within 3 months. Most common form.
Distinct Features
Early steroid treatment (prednisolone 1 mg/kg, 10 days) accelerates recovery. MRI usually unnecessary — clinical diagnosis sufficient. 85% complete recovery.
Criteria
Slow progression (>2 weeks), bilateral, recurrent, or no recovery for >6 months. MRI mandatory.
Distinct Features
Schwannoma, parotid tumor, lymphoma, sarcoidosis, Lyme disease must be excluded. Biopsy if nodular thickening or mass suspicion on MRI. If bilateral, sarcoidosis and Lyme investigation.
Criteria
VZV (Varicella-Zoster Virus) reactivation. Facial paralysis + vesicular rash around ear (herpes zoster oticus) + hearing loss/tinnitus.
Distinct Features
More severe course than Bell's palsy. Lower recovery rate (50-70% vs 85%). Requires antiviral treatment (valacyclovir) + steroid. Facial nerve + vestibulocochlear nerve enhancement on MRI.
Distinguishing Feature
Metastasis shows bone destruction and nodular nerve thickening; Bell's palsy has normal CT and linear/smooth enhancement. Known malignancy history supports metastasis.
Distinguishing Feature
LCH shows lytic bone destruction and mass effect; CT is completely normal in Bell's palsy. LCH more common in children, Bell's palsy in adults.
Distinguishing Feature
Fibrous dysplasia shows expansile ground-glass bone lesion; bone structures are completely normal in Bell's palsy. Facial paralysis in FD from bone compression, in Bell's from viral neuritis.
Distinguishing Feature
Congenital atresia shows EAC bony plate and ossicular malformation; all structures normal in Bell's palsy. Atresia present from birth, Bell's palsy has acute onset.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
3-monthBell's palsy is a clinical diagnosis and imaging is generally unnecessary in typical presentations (acute onset, unilateral, peripheral type). Early steroid treatment (prednisolone started within first 72 hours) improves recovery rate — NNT (number needed to treat) = 10. Benefit of antiviral treatment (acyclovir/valacyclovir) alone is not proven but combination with steroid is debated. MRI indications: (1) atypical presentation — slow progression, bilateral, recurrent; (2) no recovery for >6 months — tumoral cause exclusion mandatory; (3) patients with accompanying findings — hearing loss, vestibular symptoms, other cranial nerve palsies. Prognosis is generally good — 85% complete recovery, 10% partial recovery, 5% permanent sequelae (synkinesis, contracture).
Bell's palsy is generally self-limiting with 85% of patients recovering within 3 weeks to 3 months. Early steroid treatment (prednisolone) improves recovery rate. In atypical presentations (bilateral, slow progression, recurrent, no recovery >6 months), tumoral cause must be excluded with MRI.