A 35-year-old woman presents with sudden-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 this presentation?

Explanation

## Most Common Cause of Acute Facial Nerve Palsy **Key Point:** Bell's palsy accounts for 60–75% of all cases of acute facial nerve paralysis, making it the most common cause by far. ### Clinical Features of Bell's Palsy - Sudden onset (hours to days) - Unilateral facial weakness affecting all branches of the facial nerve - Loss of forehead wrinkles (distinguishes from central lesions) - Inability to close the eye on affected side (Bell's sign) - Hyperacusis (due to stapedius paralysis) - Loss of taste on anterior 2/3 of tongue (chorda tympani involvement) ### Why Bell's Palsy is Most Common | Feature | Bell's Palsy | Parotid Tumour | CVA | Otitis Media | |---------|--------------|----------------|-----|-------------| | **Onset** | Sudden (hours) | Gradual (weeks) | Sudden | Gradual/Acute | | **Frequency** | 60–75% | <5% | 5–10% | <5% | | **Forehead involvement** | Yes (always) | May spare | No (spared) | Variable | | **Associated features** | Hyperacusis, taste loss | Palpable mass, pain | Contralateral signs | Ear discharge, fever | **High-Yield:** The **absence of forehead wrinkles** on the affected side is the clinical hallmark that confirms peripheral (facial nerve) involvement rather than central (upper motor neuron) pathology. **Clinical Pearl:** Bell's palsy is idiopathic in most cases, though viral reactivation (especially HSV-1 and VZV) is increasingly recognized as a contributing factor. Corticosteroids within 72 hours of onset improve recovery rates. **Mnemonic — Causes of Facial Nerve Palsy (BELL'S PALSY):** - **B**ell's palsy (idiopathic) — most common - **E**ar infections (otitis media, mastoiditis) - **L**esions of parotid (tumour, trauma) - **L**ower motor neuron (stroke affecting brainstem) - **S**urgical trauma - **P**aralysis from Ramsay Hunt syndrome - **A**cute otitis media - **L**yme disease (Borrelia burgdorferi) - **S**arcoidosis (Heerfordt syndrome) **Warning:** Do NOT confuse Bell's palsy with a central facial nerve lesion (stroke). In central lesions, the **forehead is spared** because the upper facial muscles receive bilateral innervation from the motor cortex.