Bell’s palsy is a partial, or total, paralysis or sudden weakness of the muscles on one side of the face. It makes half of the face appear droopy, therefore a smile is one-sided, and the eye on that side resists closing. It is temporary for most people and, whilst it usually only affects nerves on one side of the face, it can affect both sides in rare cases. Bell’s palsy is named after Sir Charles Bell, a 19th century Scottish surgeon who was the first to describe the condition. It is the most common cause of facial paralysis, accounting for 60% to 75% of all cases of unilateral facial paralysis.
The facial nerve, or 7th cranial nerve, enters the skull through a small canal in the petrous temporal bone beneath the ear called the Fallopian canal. It delivers nerve impulses to the muscles of the face, ear, salivary glands, tear glands, and the tongue. Each nerve provides muscle control on that side of the face for activities that include blinking and closing eyes, and facial expressions such as smiling and frowning. The facial nerve also transmits sensations of taste from the tongue.
In Bell’s palsy, the facial nerve is swollen and inflamed, interrupting the transmission of nerve impulses from the brain to the facial muscles. It usually resolves by itself within a few months.
Figure 1. 7th Cranial (Facial) Nerve. Image by Patrick J. Lynch, medical illustrator (Patrick J. Lynch, medical illustrator) [CC BY 2.5], via Wikimedia Commons.
Due to the complexity and multiple functions of the facial nerves, many symptoms result from damage or disruption in nerve function. Symptoms of Bell’s palsy are variable in each individual, and the severity may range from mild weakness to total paralysis. The classical symptom of Bell’s palsy is a sudden weakness or paralysis on one side of the face. Other symptoms may include:
- Droopy appearance of the eyelid and the corner of the mouth
- Difficulty closing the eye, smiling or making facial expression
- Excessive tearing or a dry eye
- Dryness of the mouth
- The face feels heavy
- Numbness on one side of face
- Pain or discomfort around the jaw or in or behind the ear
- Ringing in one or both ears
- Decreased ability to taste
- Increased sensitivity to sound
- Impaired speech
- Difficulty eating or drinking
These symptoms usually occur suddenly and peak within 48 hours.
The exact cause of Bell’s palsy is unknown, however there are two primary hypotheses:
1. Viral hypothesis
One hypothesis proposes that a viral infection causes the swelling and inflammation; increasing pressure within the Fallopian canal that leads to ischaemia of nerve cells. In mild cases where patients recover rapidly, only the myelin sheath of the nerve is damaged. The myelin sheath is the fatty covering which acts as an insulator on nerve fibres in the brain.
There are many reports of association between facial paralysis and infection with viral pathogens. Neurotropic viruses, such as herpes simplex and varicella zoster, are known to establish latent infections in the peripheral nervous system, and can reactivate to cause neurological and mucocutaneous disorders. Furthermore, infections of varicella zoster virus that involve the facial nerve near the inner ear can cause a condition similar to Bell’s palsy known as Ramsay Hunt syndrome.
However, there is also evidence that the bacteria that causes Lyme disease, Borrelia burgdorferi, causes Bell’s palsy.
2. Immunological hypothesis
Some evidence implies that autoimmune mechanisms are involved in Bell’s palsy. The association between facial paralysis and Guillain–Barré syndrome, a cell mediated autoimmune neuritis, demonstrate the following similarities:
- The lymphocytes of both Bell’s palsy and Guillain–Barré syndrome patients show similar sensitisation to peripheral human basic protein P1L.
- Bell’s palsy and Guillain–Barré syndrome are both acutely demyelinating of the peripheral nervous system.
- T suppressor cells are significantly reduced and T helper cells are normal in both Bell’s palsy patients and acute Guillain–Barré syndrome patients.
- Viral infection may cause both these conditions, as viruses can initiate an autoimmune reaction against the myelin of nerves, to result in demyelination of the facial nerve.
Serology results show elevated levels of cytokines in Bell’s palsy patients, suggesting a cell-mediated component. A breakdown of peripheral tolerance in patients with hepatitis C virus infection as a result of interferon-alpha therapy, and the appearance of Bell’s palsy in children after vaccination also support the immunological hypothesis.
The annual incidence of Bell’s palsy is 15 to 30 cases per 100,000 people. Men and women are equally affected by Bell’s palsy, however there is an increase in prevalence to 43 cases per 100,000 in pregnant women, with the highest risk during the third trimester or in the first week after labour. Although it can occur at any age, it is less common in people below 15 or over 60 years of age. The peak of incidence is between 15 to 45 years. People with diabetes or upper respiratory infection, such as influenza or a cold, have a greater risk of developing Bell’s palsy. People who have a family history of recurrent attacks are also more prone to recurrent attacks, suggesting that there may be a genetic predisposition.
Approximately 70% of facial nerve palsies are considered Bell’s palsy. A Bell’s palsy diagnosis is based on clinical presentation (acute-onset unilateral facial paresis or paralysis), and by excluding other possible causes of facial paralysis, as there is no specific laboratory test to confirm diagnosis of the disorder. A comprehensive history and physical examination should assess all symptoms; underlying medical problems; and otologic, ocular, oral and neurologic function. Assessment of cranial nerves with characterisation of facial movement may help identify central or peripheral pathology. For example, the forehead is bilaterally innervated, so sparing of the forehead muscles is suggestive of a central lesion (e.g. stroke).
Other disorders which may also cause facial nerve palsies include, but are not limited to: Guillain-Barré syndrome, Melkersson-Rosenthal syndrome, multiple sclerosis, sarcoidosis, Ramsay Hunt syndrome, otitis media, Lyme disease, bone fractures, carcinomas of the head and neck, meningitis, diabetes and stroke. These conditions usually have additional distinguishing signs and symptoms. Hence, features that are atypical for Bell’s palsy, including gradual symptom onset and bilateral facial nerve paralysis, may warrant additional investigation.
Blood tests can sometimes be helpful to rule out other disorders, such as blood glucose to check for diabetes, serologic tests for Lyme disease, and serum angiotensin-converting enzyme to check for sarcoidosis. An MRI or CT scan can eliminate other structural causes of pressure on the facial nerve, such as tumour or fracture. An electromyography test can confirm the presence of nerve damage and determine the severity and extent of nerve involvement. However, it is not necessary to perform diagnostic imaging routinely and electrodiagnostic testing is only recommended for Bell’s palsy patients with complete facial paralysis.
The House-Brackmann grading system categorises symptoms on a scale of one to six and is a useful tool for determining the severity of Bell’s palsy and assessing recovery:
Table 1. House-Brackmann facial nerve grading scale.
|1||Normal||Normal symmetrical function|
|2||Mild||Slight weakness. Complete eye closure. Slight asymmetry of smile. Synkinesis barely noticeable, contracture or spasm absent.|
|3||Moderate||Obvious weakness. May not be able to lift eye brow. Complete eye closure. Asymmetrical mouth movement. Obvious synkinesis, mass movement or spasm.|
|4||Moderately severe||Obvious disfiguring weakness. Inability to lift eye brow. Incomplete eye closure. Asymmetry of mouth. Severe synkinesis, mass movement and spasm.|
|5||Severe||Motion barely perceptible. Incomplete eye closure. Slight movement corner mouth. Synkinesis, contracture and spasm usually absent.|
|6||Total paralysis||No movement. Loss of tone. No synkinesis, contracture or spasm.|
Goals of treatment for Bell’s palsy include hastening recovery time and improving the recovery of facial nerve function. The Therapeutic Guidelines: Neurology recommends the use of prednis(ol)one, at a dose of 1mg/kg (up to 100mg) orally, daily in the morning for five days, which should be initiated within 48 hours of symptom onset. Antiviral drugs are not recommended for facial nerve palsy, unless vesicles are present in the ear, which indicates that varicella zoster infection of the facial nerve (Ramsay Hunt syndrome) is the likely cause. If symptoms have been present for less than 72 hours, antiviral therapy may be added to prednis(ol)one at doses equivalent to those recommended for shingles.
Evidence for treatment recommendations
These recommendations are informed by recent randomised controlled trials and Cochrane reviews and are supported by the American Academy of Neurology’s Evidence Based Guideline Update: Steroids and antivirals for Bell palsy, published in 2012, and by the American Academy of Otolaryngology’s Clinical Practice Guideline: Bell’s palsy, published in 2013.
A Cochrane review published in 2010 assessed the effects of corticosteroid therapy for Bell’s palsy. A key finding from meta-analysis was that participants allocated to corticosteroids were significantly less likely to suffer incomplete recovery of facial function after at least six months from randomisation, compared with those allocated to control (no effective treatment).
Another Cochrane review published in 2015 assessed the effects of antiviral therapy alone or in combination with other therapies. No significant benefit was found from the addition of antivirals to corticosteroids compared with corticosteroids alone or with placebo for patients with Bell’s palsy. However, for patients with severe Bell’s palsy (House-Brackmann scores of five to six), there was a small significant benefit in the rate of complete facial recovery after six months for antivirals plus corticosteroids compared with corticosteroids alone.
The American Academy of Otolaryngology strongly recommends that “Clinicians should prescribe oral steroids within 72 hours of symptom onset for Bell’s palsy patients 16 years and older” and “should not prescribe oral antiviral therapy alone for patients with new-onset Bell’s palsy”. They provide an option that “Clinicians may offer oral antiviral therapy in addition to oral steroids within 72 hours of symptom onset”.
Analgesics such as aspirin, paracetamol, or ibuprofen may relieve pain associated with facial palsy.
If eye closure is impaired, it is helpful to cover the eye on the affected side with protective glasses, goggles, or a patch in windy or dusty surroundings, and to instil lubricating eyes drops during the day and gels or ointments at bedtime. Ophthalmological review is often useful if the facial palsy is severe.
Physical therapy includes heat, massage, and facial exercises including emotional expressive exercises. The aim is to stimulate the facial nerve and to help maintain muscle tone. However, the evidence to support physical therapies is derived from flawed trials.
Other proposed therapies which aim to restore nerve function include relaxation techniques, acupuncture, electrical stimulation, biofeedback training, and vitamin therapy (including vitamin B12, B6, and zinc). Again, there is little evidence to support these therapies.
Decompression surgery for Bell’s palsy, which aims to relieve pressure on the nerve, is controversial and seldom recommended. On rare occasions, cosmetic or reconstructive surgery may be used to lessen deformities and correct damage, such as an eyelid that will not close or a crooked smile.
The emotional impact of this condition cannot be underestimated. Patients often struggle with the facial disfigurement. Clinicians can provide support to patients by providing realistic expectations of recovery, and the duration of symptoms.
Generally, the prognosis for patients with Bell’s palsy is very good. The extent of recovery is determined by the severity of the lesion, with clinically incomplete lesions usually recovering well. Improvement is gradual and recovery times can vary between individuals. Regardless of treatment, most patients begin to improve within two weeks of symptom onset and recover completely within three to six months.
In a few cases, the symptoms may last longer or never completely disappear. Up to 30% of patients with Bell’s palsy fail to recover facial function completely. In rare cases, the disorder may recur, either on the same or opposite side of the face.
Following recovery from severe facial nerve lesions, abnormal regeneration of the facial nerve may lead to synkinesis of the facial muscles; for example there may be twitching of the angle of the mouth on blinking, and the eye may close or wink on smiling. Involuntary tearing of the eye (crocodile tears) on the affected side or gustatory sweating can occur upon eating in response to a salivary stimulus by misdirected autonomic fibres.
Bell’s palsy patients with new or worsening neurologic findings, ocular symptoms or incomplete facial recovery after three months may benefit from referral to a facial nerve specialist.
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