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

Explanation

## Most Common Cranial Nerve Affected in Bell's Palsy **Key Point:** The facial nerve (CN VII) is the most commonly affected cranial nerve in Bell's palsy, accounting for approximately 60–75% of all acute peripheral facial palsies. ### Epidemiology & Pathophysiology Bell's palsy is an acute, idiopathic facial paralysis that results from inflammation and compression of the facial nerve within the temporal bone. The exact etiology remains unclear, though viral reactivation (especially HSV-1) is strongly suspected. ### Clinical Features of CN VII Involvement - **Motor deficits:** Complete unilateral facial paralysis affecting muscles of facial expression - **Inability to close the eye** (loss of orbicularis oculi function) — most clinically significant sign - **Drooping of mouth corner** on affected side - **Loss of nasolabial fold** on affected side - **Hyperacusis** (increased sensitivity to sound) due to stapedius paralysis ### Why CN VII is Most Vulnerable 1. **Narrow temporal bone canal:** The facial nerve passes through a bony canal in the petrous temporal bone with minimal space for swelling. 2. **Anatomical constraint:** Unlike other cranial nerves, CN VII has limited room to expand if inflamed, leading to ischemia from compression. 3. **Long intratemporal course:** The nerve takes a complex path with three turns (labyrinthine, tympanic, mastoid segments), increasing vulnerability. **Clinical Pearl:** The presence of **Bell's sign** (upward deviation of the eye when attempting to close the eyelid) is pathognomonic for CN VII palsy and helps differentiate peripheral from central facial weakness. **High-Yield:** Approximately 80% of Bell's palsy cases recover completely within 3–6 months with or without treatment; corticosteroids within 72 hours of onset improve outcomes. [cite:Harrison 21e Ch 381]