Bell's palsy

Overview

Most common cause of acute unilateral facial weakness - a diagnosis of exclusion. Caused by idiopathic inflammation of CN VII (likely HSV-1 reactivation), causing oedema within the bony facial canal.

Presentation

•Rapid onset - maximum weakness within 72 hours

•Post-auricular pain - often earliest symptom, 24-48 hours before weakness

•Unilateral facial weakness - entire ipsilateral face including forehead (LMN pattern)

•Lagophthalmos - inability to fully close eye; risk of corneal injury

•Loss of taste - anterior two-thirds of tongue (chorda tympani involvement)

•Hyperacusis - stapedius muscle weakness

Investigations

•Bell's palsy is a clinical diagnosis - no routine investigations required for typical presentation

•Blood glucose - exclude diabetes mellitus

•Lyme serology - in endemic areas or relevant exposure

•MRI with gadolinium (gold standard) - reserved for atypical, progressive, or non-resolving presentations to exclude structural cause

Management

•Treatment must be started within 72 hours of symptom onset for maximum benefit

🥇 First-line

•prednisolone 50 mg orally once daily for 10 days - reduces nerve inflammation/oedema; NNT ~10 to prevent one incomplete recovery; reduces synkinesis risk

•Eye care (all patients): lubricating drops (day) + lubricating ointment (night); tape eye closed at night if lagophthalmos; refer ophthalmology if corneal injury

•Second-line (severe/complete palsy - House-Brackmann Grade IV-VI): add aciclovir 400 mg five times daily for 10 days (or valaciclovir) to corticosteroids - reduces late sequelae; NNT ~100 in mild-moderate palsy so not routinely added

⚠️

Antiviral monotherapy is NOT recommended - antivirals alone show no clear benefit over placebo. Antivirals should only ever be added to corticosteroids, never used instead of them.

Complications

•Incomplete recovery - ~15-30% without treatment; reduced to ~17% with corticosteroids

•Synkinesis - involuntary co-contraction during movement (e.g. eye closure when smiling); aberrant nerve regeneration; corticosteroids reduce risk

•Crocodile tears - aberrant reinnervation causing tearing when eating

•Corneal exposure keratopathy - from lagophthalmos; can cause permanent visual loss

Prognosis

•~70-85% achieve full/near-full recovery, beginning within 3-6 weeks

•Poor prognostic features: complete paralysis (Grade VI), age >60, diabetes, severe post-auricular pain, no recovery at 3 weeks, Ramsay Hunt syndrome

•Recovery when it occurs may take up to 12 months

Key Anatomy - UMN vs LMN

•LMN lesion (Bell's palsy) - entire ipsilateral face affected including forehead; motor nucleus/nerve itself is damaged

•UMN lesion (e.g. stroke) - forehead spared because bilateral cortical representation allows contralateral hemisphere to compensate

🎯

Forehead involvement is the key finding that confirms an LMN pattern and supports Bell's palsy over a central (UMN) cause.

Red Flags - Exclude Alternative Diagnoses

🚨

Refer urgently if: forehead sparing (UMN - stroke?), bilateral palsy (sarcoidosis, Guillain-Barré, Lyme), vesicles in ear/oropharynx (Ramsay Hunt syndrome), parotid mass or lymphadenopathy (malignancy), no recovery by 3 months.

House-Brackmann Grading (Severity)

Grade	Description
I	Normal function
II	Slight weakness; complete eye closure with minimal effort
III	Obvious weakness; complete eye closure with effort; good forehead movement
IV	Disfiguring weakness; incomplete eye closure; no forehead movement
V	Barely perceptible movement; incomplete eye closure
VI	Complete paralysis; no movement