Bell's Palsy is a paralysis of the facial nerve which results to an inability to control facial muscles in the affected side. There are several conditions that can cause facial paralysis: brain tumor, stroke, and Lyme disease. However, if no specific cause can be traced to the paralysis, the condition is called Bell's Palsy. It is named after the Scottish anatomist Charles Bell, who first described it. Bell's palsy is the most common acute mononeuropathy, or diseases involving only one nerve, and is the most common cause of acute facial nerve paralysis. Bell's Palsy is defined as an idiopathic unilateral facial nerve paralysis, and is usually self-limiting. Its trademark is the rapid onset of partial or complete palsy, usually in a single day.

Bell's palsy is diagnosed through exclusion, by eliminating other reasonable possibilities. Therefore, no specific cause can be ascertained. Bell's palsy is commonly referred to as idiopathic or cryptogenic, meaning its causes are unknown. Being a disease that belongs to the residual diagnostic category, the Bell's Palsy diagnosis likely spans different conditions which the current level of medical knowledge cannot distinguish. Studies show that 45% of patients are not referred to a specialist, which suggests that Bell's palsy is considered by physicians to be a diagnosis that is easy to manage. There is a significant number of cases that have been misdiagnosed. This is unsurprising from a diagnosis of exclusion, which depends on a thorough investigation of all the symptoms of the patient.

Treatment depends on the severity of the problem. In patients with incomplete facial palsy, where the prognosis for recovery is very good, treatment may be unnecessary. However, patients with complete paralysis, marked by an inability to close the eyes and mouth on the affected side, are usually treated with anti-inflammatory corticosteroids. If infection is found to be the cause, then an antibiotic to fight bacteria or antiviral agents (to fight syndromes caused by viruses) may be used. If simple swelling is believed to be responsible for the disorder, then steroids are often prescribed to the patient. In certain circumstances, however, surgical removal of the bone around the nerve, or decompression, may be appropriate.

It is thought that an inflammatory condition leads to the swelling of the facial nerve known as nervus facialis. This nerve travels through the skull in a narrow bone canal beneath the ear. Nerve swelling and compression in the narrow bone canal are thought to lead to nerve damage, inhibition or death. There are no readily identifiable causes for Bell's Palsy, but clinical and experimental evidence suggests that herpes simplex type 1 infection may play a role.

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