A 35-year-old woman presents with acute-onset unilateral facial weakness, inability to close the right eye, and drooping of the right corner of the mouth. On examination, the forehead wrinkles are absent on the right side. What is the most common cause of peripheral facial nerve palsy in this patient?

Explanation

## Most Common Cause of Peripheral Facial Nerve Palsy **Key Point:** Bell's palsy accounts for 60–75% of all cases of peripheral facial nerve paralysis, making it the most common cause by far. ### Clinical Features of Bell's Palsy **High-Yield:** Bell's palsy presents with: - Acute onset (hours to 2–3 days) - Unilateral facial weakness affecting all muscles of facial expression - Inability to close the eye on the affected side (loss of orbicularis oculi) - Loss of forehead wrinkling (upper facial muscles are affected) - Hyperacusis (due to stapedius paralysis) - Altered taste on anterior 2/3 of tongue (chorda tympani involvement) ### Differential Diagnosis of Peripheral Facial Nerve Palsy | Cause | Frequency | Key Features | | --- | --- | --- | | **Bell's palsy** | 60–75% | Idiopathic, acute, complete recovery in 80–90% | | Parotid tumour | 5–10% | Gradual onset, palpable mass, may have skin changes | | Otitis media/mastoiditis | 2–5% | History of ear infection, conductive hearing loss | | Stroke (central) | 2–5% | Forehead sparing, other neurological signs | | Ramsay Hunt syndrome | 5–10% | Vesicles in ear, severe pain, worse prognosis | | Lyme disease | <1% (endemic areas) | Bilateral in 25%, history of tick exposure | **Clinical Pearl:** The **forehead wrinkles are absent** in this patient, which indicates upper facial muscle involvement — a hallmark of peripheral (not central) facial nerve palsy. Central lesions spare the forehead because of bilateral cortical innervation of the upper facial muscles. **Warning:** Do not confuse peripheral facial nerve palsy with central facial weakness (stroke). In central lesions, the forehead is spared and only the lower face is weak. ### Why Bell's Palsy is Most Common 1. Idiopathic aetiology (no identifiable structural lesion) 2. Associated with viral reactivation (HSV-1, VZV) 3. Affects all age groups but peak incidence 40–60 years 4. Excellent prognosis with corticosteroids and eye care [cite:Harrison 21e Ch 379]