C. Hain, MD. • Page last modified:
August 22, 2020
Bell's palsy is an acquired weakness of one side of the face, due to an injury to the facial nerve. The symptoms on the affected side typically include inability to close the eye, to smile, wrinkle the forehead and whistle. Speech may be mildly slurred. Tearing occurs because the eye does not close completely. Taste sensation may be diminished on the front half of the tongue. Sounds may appear louder on the affected side (hyperacusis) -- this may be caused by paralysis of the stapedius muscle but also occurs independently. Papillitis may be seen of the fungiform papillae of the affected side. Bell's palsy usually develops over hours to days. The peak involvement usually happens within several days. Mild pain behind the ear is common at onset, as is a subjective sensation of "numbness" of the affected side. Usually it is first noticed when a persons observes it in a mirror, or on eating because food tends to collect between the cheek and gums.
How common is Bell's palsy ?
About 25/100,000 persons per year develop Bell's palsy. The incidence increases slightly with age. There are only minor differences in rates between the sexes and among persons of different race. There is a slightly higher incidence in the winter.
Bells palsy has numerous potential causes. It is presently thought that most cases arise from herpes simplex virus infections (the same one that gives you cold sores in your mouth as well as other variants). In support of this idea is the observation that HSV-1 DNA is detected in 86% of facial nerve ganglia of persons who have had Bells palsy, compared to only 43% of controls (Linder et al, 2005). However, there are a wide variety of other possibilities including diabetes, Lyme disease, sarcoid, HIV infection, and various cancers.
Individuals at increased risk for idiopathic lower motor facial palsy (i.e. facial weakness due to injury to the nerve or facial nerve nucleus in the brainstem) include pregnant women, diabetics, those who have had recent episodes of influenza or respiratory infection, and those with family history of the disease.
Other causes of facial palsy include trauma, CNS disorders such as stroke and neoplasms. Facial palsy is rarely caused by middle ear infection, where it is felt that facial nerve dehiscence may expose the nerve and make it more vulnerable. In acute cases, parenteral antibiotics and myringotomy is generally the treatment. Surgical treatment may be undertaken in patients with epidural abscess, cholesteatoma or mastoiditis.
Differential diagnosis of facial nerve paralysis (adapted from Marzo et al, 2002)
Idiopathic (Bell's Palsy)
- Herpes Zoster (Ramsay Hunt)
- Necrotizing external otitis
- Skull base osteomyelitis
- Lyme Disease
- Basilar skull fracture
- Birth injury
- Penetrating temporal bone trauma
- Parotid injury (as after surgical removal of a parotid tumor)
- Facial nerve neuroma
- Facial nerve hemangioma
- Jugular glomus
- Acoustic neuroma
- Metastatic lesion (from cancer)
- Wegeners granulomatosis
Diagnosis is based on history, findings on physical examination, and the results of laboratory tests. On physical examination, acutely, the face on the affected side is weak and eye closure is incomplete or absent. Sparing of the forehead would suggest a central facial paralysis rather than Bell's palsy. The naso-labial fold is flattened. Whistling is usually impossible. The tympanic membrane is normal unless there is a herpes infection of the ear (see Ramsey-Hunt syndrome). In peripheral facial weakness (Bell's palsy), after several weeks there is usually a characteristic rolling up of the eye when eye closure is attempted - -this is called Bell's Phenomenon (see below for a movie).
See the movie listing page for more movies.
Other cranial nerves may be affected too-- There may be inflamed circumvallate papillae (area supplied by 9), a decreased gag reflex (sensory 9), and palatal weakness (10th). Hearing may be affected due to involvement of the stapedius reflex. On recovery the face may show evidence of misrouting of nerve fibers (see section on prognosis).
An MRI scan will be performed if there is any possibility of a stroke or brain tumor. MRI scan is generally not felt to be necessary acutely in all persons with a new Bell's palsy, but it is used in persons who do not recover or who are getting worse rather than better. The usual pattern seen on MRI imaging of Bells is enhancement of the nerve within the meatal, geniculate, and sometimes tympanic segments, which may last for 4 months. Enhancement of the facial nerve may be seen in viral infections also (Suzuki et al, 2001).
Tumors are particularly likely if the facial paralysis has gradually evolved over weeks or more, if there is a history of previous cancer, or if there are masses that can be seen in the ear or parotid gland area of the face. Imaging is generally done if there is hearing loss, or slow onset of paralysis (over 6 mo.), or if surgical treatment is planned. on MRI, there may be enlargement of the nerve or enhancement in only one segment. Nevertheless, MRI often cannot differentiate between tumor and Bells palsy, and a normal MRI does not always exclude a malignant facial nerve lesion. It has been suggested that lack of recovery at 6 months suggests a malignancy (Marzo et al, 2002).
Depending on the situation, tests for diabetes, Lyme disease, sarcoidosis, myasthenia gravis, AIDs, and Guillain Barre syndrome, are occasionally performed, especially in persons with weakness on both sides of the face. Some believe that Lyme is the most common cause of facial paralysis in children, but this probably varies according to exposure risk (i.e. where in the country the person lives). Lumbar puncture may be necessary (spinal tap) for many of these possibilities.
Specific tests are available to quantify the degree of weakness, but these are not generally felt to be helpful to patient care. ENOG (Electroneurography) is a method of electrically stimulating the nerve. It is not useful during the first 72hrs. At 3 weeks, patients with < 92% degeneration have a uniformly good outcome. The Hilger stimulator is a related device. Emerging tests are magnetic nerve stimulation. It may provide similar information as ENOG, but without as much potential for pain (from the electrical stimulation).
In 75% of patients, no cause for Bell's palsy is established. It is speculated that a viral infection is the cause of Bell's Palsy in this situation. Two kinds of herpes viruses have been associated with Bell's palsy. Classically, when a herpes eruption is present on the side of facial weakness, then Ramsay Hunt syndrome is diagnosed. In this case, the facial weakness is attributed to an infection with the varicella zoster virus. In addition to the rash, hearing loss is much more common in Ramsay Hunt than most other causes of Bells. Also herpes simplex virus type 1 (HSV-1) has been implicated in the pathogenesis of Bells palsy. In recent years it has been possible to document virus in saliva using PCR (Suzuki et al, 2001), but this information has so far not been useful in treatment. Varicella zoster in the geniculate (facial) ganglion can also be reactivated during a bout of infectious mononucleosis (Grose et al, 2002).
Mild injury causes "neuropraxia". The nerve is still there, it just is slowed down. There is decreased impulse conduction, and prognosis is good. Moderate injury -- may cause interruption of axoplasmic flow and axonotmesis. Wallerian degeneration occurs over 2-3 weeks. Full recovery generally occurs within 2 months. Severe injury is called "neurotmesis". Wallerian degeneration occurs over 3-5 days, misdirected axon regeneration occurs and patients experience prolonged recovery and end up with synkinesis.
Fragmentation, swelling and degeneration is often seen in axon cylinders. There may be lymphocytic infiltration of nerve bundles. Viruses may be found -- HSV, Epstein Barr, mumps, CMV and influenza.
75% of patients with Bell's palsy experience complete recovery, most within 2 to 3 weeks. An additional 15% experience satisfactory recovery, but may have persistent facial asymmetry. 5 to 10% of patients have poor recovery at 4 months with persistent neurologic impairment and cosmetic disfigurement. Prognosis is thought to correlate with severity of injury. In neuropraxia, there is a conduction block but the nerve itself is intact and the nerve can be stimulated distal to the block. In axonotmesis, the nerve tube is intact but the axon within dies. Usually recovery is 100% here also. In neurotmesis, the axon and tubule are lost and prognosis is variable.
Many persons with Bells will develop synkinesis. This means that when they blink, the corner of the mouth may twitch slightly. It is caused by a misrouting of facial nerve fibers as it grows back to innervate the facial muscles. Some persons may have "crododile tears", which is tearing when they eat. This is caused by a mix up in autonomic fibers carried by the facial nerve. Others may have "sweating" of the ear when they eat, caused by a similar mechanism.
All patients with Bell's palsy need to take precautions against drying of the eye on the side of facial weakness. This will generally include use of artificial tears during the day, and use of "lacrilube" jelly at night. Eye patches are often counterproductive because the eyelid easily gets dislodged from the patch, allowing the eye to brush against the patch causing discomfort and potential damage. A moisture chamber can be used as an alternative to frequent use of eye drops during the day. The moisture chamber keeps the cornea from drying. In persons who have persistent redness or visual obscuration, a ophthalmologist's help must be enlisted. In some instances, the lid must be sewed shut until facial movement improves. Gold weights can also be placed in the lid to keep it closed.
Prednisone treatment is thought to speed recovery and reduce the frequency of a bad result. Prednisone must be given within the first week of facial weakness, in order to be effective. It is generally reserved for persons with complete facial paralysis. Usually a dose of about 60 mg is given per day in a single morning dose. This dose is usually continued for about a week, and then tapered off to nothing at about 10 days. In persons at risk, blood pressure, blood glucose and electrolytes should be monitored.
Antiviral treatment for herpes simplex may improve prognosis (Adour, 1996). The protocol used involved acyclovir 400 mg 5 times daily for 10 days. However, recent improvements in antivirals suggest that famciclovir 500 mg tid may be a more effective choice. Valacyclovir (Valtrex) has the same effect.
Surgery is not needed in most cases of idiopathic Bell's palsy, as 90% of patients recover spontaneously. In cases where there has been trauma, facial nerve decompression may be justified. This area is controversial -- the surgical literature is more optimistic about the benefits of surgery than the medical literature. The surgery involves opening the nerve sheath in the narrowest portion of the fallopian (facial) canal. The only reliable surgical approach is via the middle fossa -- meaning that it is essentially a neurosurgical procedure.
Physical therapy is not generally thought to be helpful although it probably doesn't hurt. In persons with severe residual impairments at least 1 year after onset, several surgical procedures are available to improve cosmetic appearance.
Patients with Bell's palsy due to non-herpetic viral infection, sarcoid, diabetes or cancer are treated for these conditions if treatment is available, in addition to the general treatments outlined above.
About 7% of patients have recurrent Bells palsy. The mean recurrence interval is 10 years. Recurrent Bells tends to cluster in families as well as diabetics.