What is Bell's palsy?

Bell's Palsy Overview

Bell's palsy is an acute unilateral facial nerve paralysis or paresis of sudden onset (within 72 hours) without identifiable cause, resulting from inflammation and edema of the facial nerve within the narrow temporal bone canal. 1

Definition and Diagnostic Criteria

Bell's palsy is fundamentally a diagnosis of exclusion that requires careful elimination of other causes of facial weakness before the diagnosis can be made. 1 The condition is defined by three essential features:

• Acute onset occurring in less than 72 hours 1
• Unilateral facial nerve paresis or paralysis affecting the ability to move facial muscles 1
• No identifiable cause after appropriate workup to exclude other etiologies 1

A critical diagnostic caveat: bilateral facial palsy is extremely rare in true Bell's palsy and should immediately trigger investigation for Guillain-Barré syndrome, Lyme disease, or sarcoidosis rather than accepting it as idiopathic. 2, 3

Pathophysiology

The mechanism involves inflammation and edema of the facial nerve causing compression within the narrow temporal bone canal, leading to disrupted nerve impulse conduction. 1, 4 While a viral etiology (particularly herpes simplex virus type 1) is suspected, the exact mechanism remains unknown. 1, 4

The neural edema develops within 72 hours, causing mechanical compression that disrupts impulses to:

• Facial expression muscles (causing paralysis) 4
• Lacrimal and salivary glands (causing dry eye or mouth) 4
• Stapedius muscle (causing hyperacusis) 4
• Taste fibers from anterior two-thirds of tongue (causing taste disturbance) 4
• General sensory fibers from tympanic membrane and posterior ear canal 4

Epidemiology and Risk Factors

Incidence ranges from 11.5 to 53.3 per 100,000 person-years across different populations, with peak incidence in the 15-45 year age group. 1, 2 Both sexes are equally affected, though some studies show slightly higher rates in females. 1

Key risk factors include:

• Diabetes mellitus (increases risk through ischemic and neuropathic mechanisms) 4, 2
• Pregnancy (especially third trimester) 1, 4, 2
• Upper respiratory tract infections (supports viral hypothesis) 4, 2
• Compromised immune systems 4, 2
• Obesity and hypertension 1

Clinical Presentation

Patients present with unilateral facial weakness affecting both upper and lower face (distinguishing it from central causes like stroke which spare the forehead). 4, 5

Common symptoms include:

• Inability to close the eyelid (lagophthalmos) leading to risk of corneal injury 4
• Oral incompetence causing difficulty eating and drinking 4
• Dryness of eye or mouth 1, 4
• Taste disturbance or loss 1, 4
• Hyperacusis (excessive sensitivity to sound) 1, 4
• Sagging of eyelid or corner of mouth 1
• Ipsilateral pain around ear or face (common presenting symptom) 1, 4

Differential Diagnosis - Critical Exclusions

Before diagnosing Bell's palsy, you must exclude the following identifiable causes: 2

Central Causes

• Stroke (central facial weakness spares the forehead due to bilateral cortical innervation of upper facial muscles) 2
• Brain tumors 2

Peripheral Structural Causes

• Parotid gland or infratemporal fossa tumors 2
• Cancer involving the facial nerve 2
• Trauma or temporal bone fractures 2

Infectious Causes

• Herpes zoster (Ramsay Hunt syndrome) - look for vesicles in ear canal or on palate 2, 3
• Lyme disease - particularly in endemic areas with tick exposure history 2, 3

Systemic Diseases

• Sarcoidosis - can cause recurrent facial nerve involvement 2, 3
• Guillain-Barré syndrome - especially if bilateral presentation 2, 3

Natural History and Prognosis

Most patients show some recovery within 2-3 weeks after onset and completely recover within 3-4 months. 1 The prognosis varies by severity:

• Approximately 70% of patients with complete paralysis will recover fully within 6 months without treatment 1
• Up to 94% of patients with incomplete paralysis will recover fully 1
• However, approximately 30% of patients do not recover completely and may experience permanent facial weakness with muscle contractures 1, 4

Complications

Short-term

• Incomplete eyelid closure with resultant dry eye and risk of corneal injury 4, 5

Long-term

• Permanent facial weakness with muscle contractures in up to 30% of patients 1, 4
• Synkinesis (involuntary facial movements) 1
• Psychological burden from facial asymmetry and impaired interpersonal relationships 1

Recurrent Bell's Palsy

When facial paralysis recurs, it is no longer considered idiopathic Bell's palsy and requires identification of an underlying etiology. 3 Recurrent cases mandate:

• MRI imaging to exclude structural lesions 3
• Laboratory testing including glucose/HbA1c, Lyme serology if geographically appropriate, and consideration of ACE levels and chest imaging if sarcoidosis suspected 3
• Exclusion of herpes zoster, sarcoidosis, Lyme disease, and diabetes as these can cause recurrent episodes 3

Key Clinical Pitfalls

• Do not diagnose Bell's palsy without excluding other causes - it is a diagnosis of exclusion 1, 2
• Bilateral facial palsy is NOT Bell's palsy - investigate for Guillain-Barré, Lyme disease, or sarcoidosis 2, 3
• Recurrent facial palsy requires full workup - it is not idiopathic 3
• Central vs peripheral distinction is critical - stroke spares the forehead, Bell's palsy does not 2
• Eye protection is paramount - lagophthalmos can lead to corneal injury 4