Bell's palsy is a common neurologic disorder characterized by peripheral paralysis of the seventh cranial nerve (also known as the facial nerve), resulting in muscle weakness on one side of the face.

The condition is named after Sir Charles Bell, a young surgeon from London who described a case of facial palsy in 1828, along with the detailed anatomy and function of the seventh cranial nerve. Today it is thought to be the most common cause of facial paralysis, responsible for up to 75% of unilateral mononeuropathies (disorders which involve only one nerve). Epidemiological studies reveal that Bell’s palsy affects 15–30 individuals per 100.000 persons each year, with peak incidence mostly in the range between 20 and 40 years of age.

In contrast to other potential causes of facial paralysis such as stroke, tumors or middle ear lesions, this condition is often referred to as idiopathic, which means a specific cause cannot be pinpointed. Still, herpes simplex virus (HSV) has become accepted as the likely cause of Bell's palsy. Reactivation of a dormant virus when body's immune defenses are low leads to swelling and inflammation of the facial nerve, resulting in its damage and subsequent palsy.

The classic sign of Bell's palsy is a weakness or complete paralysis of muscles on one side of the face with an abrupt onset, usually within 48 hours. The condition involves the forehead and lower aspect of the face, and patients are often unable to keep an eye closed or to retain food in the affected side of the mouth. Inappropriate reaction to loud noises, taste disturbances and decreased tearing also represent some of the concomitant symptoms of this disease.

The most important step in diagnosis is to determine whether facial paralysis occurred due to a problem in the central nervous system (as in stroke) or is it actually Bell's palsy. Only the lower facial muscles are affected in stroke patients, while Bell's palsy is characterized by the paralysis of both the upper and lower parts of the face. Apart from such facial paralysis, symptoms arise within several days and usually subside within 6 weeks. A prolonged and gradual course with persistent relapses and no recovery suggest that a tumor might be an underlying cause. Since Bell's palsy is essentially a diagnosis of exclusion, patients should be referred to a neurologist as soon as possible to rule out more serious conditions.

Corticosteroids are often used to treat Bell's palsy, as they proved helpful in reducing inflammation of the nerve and shortening symptom duration; the key is to use them shortly after onset to attain the desired effect. Antiviral medications are sometimes added to the therapy regimen, and surgical nerve decompression is pursued in more challenging cases. Although a majority of affected individuals recover completely and spontaneously over time, some patients can be left with lifelong consequences.