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Bell’s palsy: pathophysiology, symptoms, causes, risk factors, diagnosis and treatment. How to differentiate Bell’s palsy from stroke.
Bell’s palsy is a form of facial muscle weakness or paralysis, typically on one side of the face. It results from dysfunction of the facial nerve, also known as the seventh cranial nerve. The facial nerve has many branches and diverse functions. It controls the muscles of facial expression, including those involved in eye blinking and closing; it carries nerve impulses to tear glands, salivary glands; and conveys taste sensations from the anterior two-thirds of the tongue. There are two facial nerves, one on each side of the face. Typically, only one nerve, and hence one side of the face, is affected. The malfunction of the facial nerve is thought to result from its inflammation. The swollen nerve is compressed as it exits the skull within a narrow bony canal. Symptoms develop suddenly, usually within a couple of days, and can range from mild weakness to total paralysis of face muscles. Other symptoms may include drooping of mouth, drooling, inability to close one eye, facial pain or abnormal sensation, distorted sense of taste, and intolerance to loud noise. By definition, Bell’s palsy is idiopathic, meaning it has no known cause, but it has been associated with certain viral infections. In particular, reactivation of a dormant virus, triggered by stress, trauma or minor illness, is often thought to be the culprit. Risk factors include diabetes, hypertension, obesity, pregnancy, and upper respiratory infections. Diagnosis is based on clinical presentation after other possible causes of facial paralysis are excluded. Patients usually present with rapid development of symptoms, reaching a peak in severity around 72 hours from the time of onset. In most cases, muscle weakness can be observed with both upper and lower facial muscles, including the forehead, eyelid, and mouth. If forehead muscle strength is not affected, a central cause, especially stroke, should be suspected. This is because the upper facial muscles, unlike the lower ones, receive nerve impulses from both hemispheres of the brain, so a lesion in one side will not affect their function. An electromyography test can be used to confirm nerve damage and determine the extent of severity. Imaging studies can help rule out structural causes, such as a tumor or skull fracture. Because Bell’s palsy impairs the eyelid’s ability to close and blink, the affected eye is exposed to drying and potential injury. Patients must keep the eye moist with lubricating eye drops, and protect it from injury with an eye patch, especially at night. Without treatment, Bell’s palsy resolves spontaneously in about 2 thirds of patients. Symptoms usually start to improve after a few weeks, and complete recovery is achieved in about six months. Corticosteroids, when started early, can reduce inflammation and improve recovery. Some patients may benefit from physical therapy or facial massage. Decompression surgery to relieve pressure on the nerve is rarely needed and not usually recommended. Severe cases may take longer to resolve. A small number of patients with complete paralysis may continue to have some symptoms for life.
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