Which cranial nerve is most commonly affected by Bell's palsy?

Explanation

## Most Common Cranial Nerve in Bell's Palsy **Key Point:** The facial nerve (CN VII) is affected in >95% of cases of Bell's palsy, making it the most common cranial neuropathy in clinical practice. ### Epidemiology of Bell's Palsy **High-Yield:** Bell's palsy accounts for approximately 60–75% of all acute facial paralysis cases. The annual incidence is 15–30 per 100,000 population, with peak incidence in the 3rd–4th decades. ### Why CN VII is Affected 1. **Anatomical vulnerability**: The facial nerve has the longest intracranial course and passes through the narrow facial canal (Fallopian canal), making it susceptible to compression during inflammation or edema. 2. **Viral etiology**: Reactivation of herpes simplex virus-1 (HSV-1) or varicella-zoster virus (VZV) within the geniculate ganglion causes inflammation and swelling in the confined space. 3. **Ischemia mechanism**: Edema within the rigid bony canal compromises blood supply, leading to ischemic nerve injury. ### Clinical Presentation of CN VII Palsy | Feature | Finding | |---------|----------| | **Onset** | Sudden (hours to 1–2 days) | | **Distribution** | Unilateral facial weakness (forehead, eye, mouth) | | **Eye sign** | Bell's sign (eye rolls upward on attempted closure) | | **Taste** | Loss of taste anterior 2/3 tongue (chorda tympani involvement) | | **Hearing** | Hyperacusis (stapedius paralysis) | | **Prognosis** | 70–85% complete recovery without treatment | **Clinical Pearl:** The presence of taste loss and hyperacusis indicates proximal CN VII involvement (above the chorda tympani and stapedius branch), which is typical of Bell's palsy. ### Differential Diagnosis of Facial Paralysis | Nerve | Condition | Frequency | |-------|-----------|----------| | **CN VII** | Bell's palsy | Most common (60–75%) | | **CN VII** | Ramsay Hunt syndrome (VZV) | 5–10% of acute facial paralysis | | **CN V** | Trigeminal neuropathy | Rare; usually sensory | | **CN IX** | Glossopharyngeal neuropathy | Very rare; isolated | | **CN XI** | Accessory nerve palsy | Rare; usually traumatic | **Mnemonic: FACIAL** — **F**acial nerve, **A**cute onset, **C**ranial nerve VII, **I**diopathic (Bell's), **A**ffects anterior 2/3 tongue taste, **L**oss of forehead wrinkles, **–** ### Management Approach 1. **Corticosteroids**: Prednisone 1 mg/kg/day (max 80 mg) × 7 days, then taper (most effective if started within 72 hours). 2. **Eye care**: Artificial tears, lubricating ointment, eye taping, protective glasses (critical to prevent corneal ulceration). 3. **Antivirals**: Acyclovir or valacyclovir may be considered, especially if started early. 4. **Imaging**: MRI or CT if atypical features, recurrent palsy, or bilateral involvement (to exclude tumor, stroke, or infection). **High-Yield:** The key to Bell's palsy management is early recognition and aggressive eye care to prevent corneal complications, which can occur in up to 10% of untreated cases.