Common Causes of Bell's Palsy
Bell's palsy is, by definition, idiopathic—meaning it has no identifiable cause. 1
Understanding the Diagnostic Definition
Bell's palsy is diagnosed only when no other medical etiology can be identified as the cause of acute unilateral facial nerve paresis or paralysis. 1 This is a diagnosis of exclusion, not a disease with known causes. 1
The American Academy of Otolaryngology-Head and Neck Surgery explicitly states: "Currently, no cause for Bell's palsy has been identified." 1
Suspected Pathophysiologic Mechanisms (Not Proven Causes)
While the exact mechanism remains unknown, several theories exist about what may trigger Bell's palsy:
Viral Hypothesis
- Herpes simplex virus (HSV) reactivation within the geniculate ganglion is the most widely suspected mechanism, though not definitively proven. 2, 3
- Herpes zoster virus has also been implicated as a potential trigger. 2
- The proposed mechanism involves viral reactivation leading to inflammation and edema of the facial nerve within the narrow temporal bone canal, causing nerve compression. 1
Autoimmune Theory
- A cell-mediated autoimmune mechanism against myelin basic protein has been proposed, with some researchers suggesting Bell's palsy may represent a mononeuritic variant of Guillain-Barré syndrome. 2
- This theory posits that viral infection or reactivation may provoke an autoimmune reaction against peripheral nerve myelin components. 2
Other Proposed Mechanisms
- Ischemic mechanisms have been suggested as potential contributors. 4
- Bacterial infection has been hypothesized in some cases based on neutrophil elevation and response to antibiotics, though this remains controversial. 5
Critical Distinction: What Bell's Palsy Is NOT
Other conditions that can cause facial paralysis must be excluded before diagnosing Bell's palsy: 1
- Stroke (central facial weakness spares the forehead)
- Brain tumors
- Parotid gland or infratemporal fossa tumors
- Cancer involving the facial nerve
- Infectious diseases: Lyme disease, herpes zoster (Ramsay Hunt syndrome), sarcoidosis 1
- Trauma or fractures
- Postsurgical complications
Clinical Pearls
- Bilateral Bell's palsy is rare and should prompt investigation for alternative diagnoses such as Lyme disease, sarcoidosis, or Guillain-Barré syndrome. 1
- The diagnosis requires rapid onset (<72 hours) of unilateral facial weakness involving the forehead. 1, 6
- Routine laboratory testing and imaging are NOT recommended for typical presentations of Bell's palsy. 6
Common Pitfall
The most critical error is failing to recognize that Bell's palsy is a diagnosis of exclusion. If you can identify a cause for the facial paralysis, it is not Bell's palsy by definition—it is facial paralysis due to that specific identified cause. 1