What is the cause of Bell's palsy?

Cause of Bell's Palsy

The American Academy of Otolaryngology-Head and Neck Surgery explicitly states that currently, no cause for Bell's palsy has been identified, making it a diagnosis of exclusion where all other potential etiologies must be ruled out. 1

Established Medical Position

Bell's palsy is defined as an acute unilateral facial nerve paresis or paralysis with onset in less than 72 hours without an identifiable cause. 1, 2 This is the official position of the American Academy of Otolaryngology-Head and Neck Surgery, which emphasizes that the diagnosis can only be made after careful elimination of all other potential causes of facial paralysis. 1, 2

Proposed Mechanisms (Unproven)

While the exact cause remains unknown, several theories have been proposed in the research literature, though none are definitively established:

Viral Reactivation Theory

• Herpes simplex virus (HSV) type 1 reactivation within the geniculate ganglion has been proposed as a potential mechanism, followed by inflammation and entrapment of the nerve in the bony foramen. 3, 4
• Varicella zoster virus (VZV) reactivation has also been suggested as a possible trigger. 4
• However, these remain hypotheses only and are not confirmed as the definitive cause. 5

Autoimmune Mechanism Theory

• Some research suggests Bell's palsy may represent an autoimmune demyelinating cranial neuritis, potentially a mononeuritic variant of Guillain-Barré syndrome. 5
• A cell-mediated autoimmune mechanism against myelin basic protein has been proposed. 5
• The theory suggests that viral infection or reactivation may provoke an autoimmune reaction against peripheral nerve myelin components. 5

Alternative Theories

• Bacterial infection has been proposed in some cases based on clinical observations showing neutrophil elevation and response to antibiotics, though this remains highly speculative. 6

Clinical Implications of Unknown Etiology

The lack of a confirmed cause means Bell's palsy remains a diagnosis of exclusion. 1, 2 Before making this diagnosis, you must systematically exclude:

• Stroke (distinguished by forehead sparing in central lesions) 1
• Brain tumors or parotid gland tumors 1
• Infectious diseases: Lyme disease, herpes zoster (Ramsay Hunt syndrome), sarcoidosis 1
• Trauma or temporal bone fractures 1
• Cancer involving the facial nerve 1
• Postsurgical complications 1

Pathophysiological Location

While the cause is unknown, the anatomical site of pathology is well-established: Bell's palsy affects the facial nerve after it exits the brainstem, typically within the narrow temporal bone canal where inflammation causes compression. 1 This peripheral location explains why all ipsilateral facial muscles lose innervation, producing complete hemifacial weakness including the forehead. 1

Treatment Rationale Despite Unknown Cause

Despite the unknown etiology, treatment with oral corticosteroids within 72 hours is strongly recommended based on proven efficacy in improving recovery rates (83% recovery at 3 months with prednisolone vs 63.6% with placebo). 2 The mechanism by which steroids work—whether through anti-inflammatory effects on viral-induced inflammation or immunosuppressive action in an autoimmune process—remains unclear but does not diminish their proven clinical benefit. 5, 2

Important Caveat

Recurrent facial paralysis is NOT Bell's palsy and requires identification of an underlying etiology, as true Bell's palsy is by definition an acute condition without identifiable cause occurring for the first time. 7 Recurrence should trigger comprehensive workup including MRI, Lyme serology, diabetes screening, and evaluation for sarcoidosis. 7