Bell's palsy, also known as facial palsy and idiopathic peripheral facial palsy, is a paralysis or weakness that affects the muscles of the face. It usually develops on one side of the face.

Bell's palsy occurs when the seventh cranial nerve, the facial nerve, is compressed or swollen, but the exact cause of why it happens is not clear. Bell’s palsy may occur after a viral infection, such as herpes, adenovirus, and influenza B virus.

Bell's palsy can affect people of any age, most commonly those over 65 years old. Factors that may increase risk of developing Bell's palsy include pregnancy, upper respiratory infection, diabetes, and other factors.

The compression or swelling of the facial nerve can restrict blood flow to nerve cells, and lead to the facial weakness or paralysis that is characteristic of Bell's palsy.

Symptoms of Bell’s palsy start suddenly, but it may take two to three days before symptoms develop. Symptoms do not progress, meaning that they do not become more severe after they start. Signs and symptoms include weakness or paralysis in one side of the face, eye dryness or increased tear production on the affected side, earache or pain underneath the ear on the affected side, an altered sense of taste and drooling due to lack of control over the muscles of the face.

Bell's palsy can be diagnosed by a physical exam, electromyography, and nerve conduction test, in addition to other tests to exclude other conditions. Medications (corticosteroids, antiviral drugs), physical therapy, plastic surgery and eye care are treatment options for Bell's palsy.

Depending on the extent of nerve damage, several complications can occur, such as eye drying leading to corneal ulceration that may lead to infections and cause vision loss, irreversible damage to facial nerve, and swelling in facial muscles due to loss of nerve function.

Most people with Bell's palsy start to get better within two weeks after the initial onset of symptoms, even without treatment. Most individuals have a complete recovery within three to six months. Some patients, however, may have symptoms that last longer.  The symptoms may never completely disappear, in a few cases. In rare cases, the disorder may recur, either on the same or the opposite side of the face.

An article published in the Spanish journal Revista De Neurologia reported that 11 to 40 cases of Bell's palsy per 100,000 inhabitants per year occur worldwide.

Bell's palsy is the most common disorder affecting the facial nerves. It results from compression or swelling of the nerve and causes weakness or paralysis of the muscles in face, mostly on one side of the face. 

The exact cause of Bell's palsy is unknown. In most cases, it often follows a viral infection, so there may be a link. Some viruses associated with Bell’s palsy include: 

• Herpes simplex virus, which causes cold sores and genital herpes.
• Influenza B virus, which causes the flu.
• Herpes zoster virus, which causes chickenpox and shingles.
• Adenovirus, which causes respiratory illnesses.
• Rubella virus, which causes German measles.
• Mumps virus, which causes mumps.

Other conditions that may cause Bell's palsy include:

• HIV/AIDS infection.
• Lyme disease (bacterial infection caused by ticks).
• Middle ear infections.
• Sarcoidosis - inflammation of the lymph nodes, lungs, liver, eyes, skin, or other tissues. 
• Syphilis. 

The risk of developing Bell’s palsy increases due to several factors that include:

• Age

Bell's palsy can affect people of any age, but it is more common among those over 65 years old. 

• Pregnancy and birth

Pregnancy can increase the risk of developing Bell's palsy, especially during the third trimester. The risk also increases during the first week after giving birth. 

• Upper respiratory infections

People with an upper respiratory infection such as the flu or a cold are at an increased risk of developing Bell's palsy. 

• Diabetes

The risk of developing Bell’s palsy is higher in people with diabetes. 

• Genetics

Some people may be genetically predisposed to have Bell's palsy and it occurs in recurrent attacks, but this is rare. 

The facial nerve supplies small muscles of the face used to control facial expressions such as smiling and frowning. It also supplies muscles that control the eyelids, and have branches that carry taste sensations from the tongue to the brain. 

There is a facial nerve on either side of the face, each coming out from the brain and passing through a gap in the bone near the upper jaw. Signals sent from the brain to the muscles supplied by the facial nerves can be interrupted if the nerve is compressed or swollen. The vast majority of Bell’s palsy cases occur on one side of the face, i.e. only one of the two facial nerves is affected.

This compression or swelling can cause restriction of blood flow to nerve cells, and lead to the facial weakness or paralysis that is characteristic of Bell's palsy. 

The signs and symptoms of Bell’s palsy appear suddenly and develop quickly to reach their peak within 48 hours. They include:

• Weakness or paralysis in one side of the face.
• Irritation of the eye on the affected side, marked by dryness or increased tear production.
• Earache or pain underneath the ear on the affected side.
• Increased sensitivity to sound in the affected ear.
• An altered or reduced sense of taste.
• Drooling due to the loss of muscle control in the face.
• Mouth dryness.
• Pain around the jaw.
• Ringing in one or both ears.
• Dizziness.
• Food falling out of one side of the mouth.
• Speech problems. 
• Difficulty closing one eye.
• Drooping of the eyelid or the corner of the mouth.
• Problems making facial expressions, such as smiling.
• Twitching or weakness of the muscles in the face.
• Headache if there is an infection such as Lyme disease.

There is no specific test that can confirm Bell’s palsy, but several diagnostic measures can be followed, including: 

Physical exam

The doctor will check the movement of facial muscles by asking the person to perform several movements, such as closing eyes, lifting brows, showing teeth and frowning. 

Electromyography

Electromyography, EMG, is used to check the health of nerves used to control the facial muscles. 

Nerve conduction test

Nerve conduction tests are used to measure the speed with which electrical signals move through a nerve. 

Diagnosis by exclusion

The doctor may perform some tests to exclude other conditions that may be causing the symptoms, such as:

• Stroke.
• Lyme disease.
• A tumor.
• Moebius syndrome (rare, congenital neurological condition).
• Middle ear infection.
• Cholesteatoma (an abnormal collection of skin cells in the middle ear).
• Head injury. 

Often, no treatment is needed to relieve symptoms of Bell’s palsy, but sometime treatment may help. Treatment options include:

Medications

Corticosteroids are used to reduce the swelling around the facial nerve, which may help reduce symptoms. Antiviral medications may be used in combination with corticosteroids in patients with severe Bell’s palsy, despite that there is no evidence that antiviral drugs are beneficial for treating Bell’s palsy. 

Physical therapy

Physical therapy may help prevent permanent contractures of facial muscles caused by their shrinkage. 

Surgery

Plastic surgery may be required to fix facial nerve problems that persist. It may also help restore muscle strength and improve appearance. 

Eye care

Bell’s palsy may cause difficulties in closing the eye, which can cause tears to evaporate, leaving the eye dry and prone to infection. Eye drops or eye ointments may be used to prevent this. 

Botulinum toxin (Botox) injections

Some people with long-term Bell's palsy may benefit from Botox injections. Botox is injected into either the affected side of the face to help relax tight muscles and reduce twitching, or into the unaffected side of the face to reduce muscle activity if they have become overactive. 

Several complications can occur as a result of Bell's palsy depending on the extent of nerve damage. These include: 

• Irreversible damage to facial nerve. 
• Corneal ulceration can occur due to prolonged dryness of the eye, which lead to infections and cause vision loss. 
• Synkinesia due to the facial nerve growing in an altered way. Synkinesia is the involuntary movement of a part when another part is moved voluntarily, e.g. the eye closes when the person smiles. 
• Swelling in facial muscles due to loss of nerve function.
• Rarely, tearing when eating, a condition known as 'crocodile tears'. 

There is no known way to prevent Bell’s palsy. 

The outlook for those with Bell's palsy is generally very good, as most patients show improvement within two weeks of the onset of symptoms, and completely recover and restore normal function within three to six month, even without treatment. 

For some, however, the symptoms may last longer than six months, or may never go away.  Bell’s palsy may recur, but this is rare.

A study published in Revista De Neurologia revealed that an estimated 11 to 40 cases of Bell's palsy per 100,000 inhabitants occur every year in the world. Variations in the appearance of Bell's palsy in the world depend on factors, such as the climate, racial factors and the prevalence of predisposing diseases.