Bells Palsy

Introduction

Sir Charles Bell described facial nerve paralysis in the 1800s. Bell’s palsy is the label used to describe the relatively sudden onset of weakness of the face due to a dysfunction of the facial nerve that supplies the muscles of the face. It is to be distinguished from the facial weakness seen in, for example, a stroke when the defect is not in the nerve but in the brain. The label works quite well, but hides the fact that the physician must carefully consider a number of other possible causes of the nerve problem before diagnosing “Bell’s” which really means “of unknown cause.” Nowadays, based on a few pathological specimens, and blood tests for antibodies, it is believed that there is a strong association with herpes simplex virus, which may be the cause in many cases previously labeled idiopathic, which means “of unknown cause”

Anatomy

Just under the skin of the face are a number of muscles that allow for movement of the lips, the brow, and closure of the eye. In order for any muscle to contract, it needs to  be stimulated and that stimulation is the work of the facial nerve. Thus, at will, a normal person can smile, purse his/her lips, close the eyelids, or elevate the brow.

The nerves that supply the head are called “cranial nerves” and, by convention, not only does each nerve have a name, but they are also numbered 1 through 12. The facial nerve is labeled  “7” and Bell’s palsy could also be called a seventh nerve palsy or weakness.

A nerve can be likened to an electrical wire. Its core is the nerve fiber, or axon (like the copper of the electrical wire), and around the fibers is an insulating layer of fatty tissue called myelin (like the rubber insulation used to cover the core copper of an electrical wire). The myelin allows for fast conduction of electricity in the nerve, and in its absence conduction is very much slowed or may fail altogether. As in an electrical cable, there are numerous individual nerve fibers (wires) gathered together to form the complete nerve (cable).

The facial nerve contains not only motor fibers which innervate the facial muscles, but also nerve fibers which are part of the autonomic nervous system. These autonomic nerves supply the lacrimal gland in the eye and stimulate it to make tears, and also the salivary glands under the tongue and jaw bone which, when activated, secrete saliva.  Furthermore nerves carrying taste sensation from the tongue join the seventh nerve and travel retrograde with it to terminate in the brain stem.

A branch of the 7th nerve called stapedius innervates a muscle attached to the ear drum and acts as a sort of damper of the eardrum - it contracts when a loud noise is heard, thus, hopefully protecting the eardrum from damage.

Epidemiology 

The annual incidence of Bell’s palsy is about 13 to 34 cases per 100,000 population, which makes it a fairly common problem. It affects about 40,000 people each year in the Unites States. The risk is increased three-fold during pregnancy, and although Bell’s is usually a single or “one shot” occurrence, 8-10% of patients will experience a second attack at some time in the future. Diabetes is present in 5-10% of patients with Bell’s palsy.

About half of all patients with facial palsy qualify for the label “Bell’s,” implying no cause found.

Pathology 

The basic pathology is an inflammation of the facial nerve within the narrow facial canal. Inflammation causes swelling and there is no space to accommodate the swelling so that pressure on the nerve causes more damage. Microscopically the nerve cover or perineurium is thickened and inflammatory cells (lymphocytes) can be seen inside the nerve around the fiber bundles and around the small blood vessels that supply the nerve. There is degeneration of the myelin insulation around the individual nerve fibers. Postulated mechanisms of damage to the nerve include simple inflammation, pressure, and also lack of sufficient blood supply to keep the nerve functioning.

Clinical Features 

Onset of facial weakness in Bell’s palsy is fairly rapid. It can occur over hours, and may progress for a few days. Slurry speech is secondary to weak lips. There be may some drooling of saliva. The patient notices facial asymmetry when looking in the mirror. Rarely, because of a droopy lower eyelid there may be some excessive tearing on that side. The astute patient may pick up on decreased taste on that side and sounds may appear louder in the ear on the affected side.

Patients often complain of a peculiar sensation or numbness of the face on the side of paralysis. This is usually attributable to faulty feedback from the paralyzed muscles, which feels strange to the patient.

It is not uncommon to experience pain behind the earlobe for a few days before the paralysis sets in. Sometimes this spreads to become a headache, and it can be persistent.

The physician evaluates facial weakness by asking the patient to show the teeth, smile, and whistle. Brow elevation will be weak, and it is possible to pry open the “screwed up” eye because the muscle that causes eye closure is weak. In severe paralysis the patient is unable to close the eyelid at all, and one can see the sclera (white of the eye) and perhaps part of the pupil even though the patient is attempting to close the eye. In these circumstances the eyeball rolls upwards so that the pupil can not be seen and that sign is referred to as “Bell’s Phenomenon.”

It is important to try to discern whether the damage to the nerve has completely destroyed it, which carries a poor prognosis, from just a very severe lesion. In severe paralysis, one looks very carefully to detect even a flicker of movement of facial muscle activity which indicates that the nerve is still in continuity although badly damaged. An incomplete lesion carries a much better prognosis.

Further testing may reveal loss of taste in the tongue on that side. A pledget of cotton is dipped in a sugar solution and painted on to the protruded tongue. (If the tongue is not protruded the sugary solution will spread to the normal side and the sign will be missed).

Hearing can be tested by finger rub close to the ear. In Bell’s palsy hearing acuity may be increased on the weak side because stapedius is paralyzed.

Loss of taste and hyperaccusis (increased hearing) are indicators of the severity of the problem rather than help with diagnosis.

The physician always inspects the ear itself and on occasion may see small blisters as one might see in chicken pox, which makes the diagnosis of a kind of shingles called “Geniculate Herpes Zozter” or “Ramsay Hunt  Syndrome.” The virus invades a group of nerve cell bodies which are clumped together along the course of the nerve in the facial canal to form the Geniculate Ganglion.

Investigations 

The patient may be referred to the electrodiagnostic laboratory for a nerve conduction study (NCS) and electromyography (EMG).

In NCS the facial nerve is stimulated through the skin close to the ear with mild intermittent electrical shocks and the response in the muscle innervated by that nerve, is a “muscle twitch,” technically called a “compound muscle action potential,” or CMAP, which is recorded using an oscilloscope. At about 10 days after onset of Bell’s palsy the size of the CMAP, which tends to become smaller with damage to the nerve, yields an estimate of the severity of the damage to the nerve.

The EMG can be used to assess the prognosis for recovery. If no activity at all is seen on volitional contraction of the facial muscles, a very severe lesion with a poor prognosis for recovery is diagnosed. If just a bit of activity is seen the prognosis is better even though the clinician cannot see any movement - the EMG is more sensitive. The principle is that if no conduction at all occurs the nerve has been irreversibly cut or damaged, as though rendered asunder. If even a very slight amount of muscle electrical activity is seen on volition, the nerve is still in continuity and a few of the axons are conducting. If the nerve is still in continuity, the healing process will cause axons to bud from the area of damage and grow down the distal part of the nerve to ultimately reinnervate the muscles.

The physician may request imaging of the nerve:  

High resolution Computed Axial Tomography (CT) is excellent for demonstrating boney detail and can be used to image the facial canal. In simple Bell’s palsy the canal should be normal.

Magnetic Resonance Imaging (MRI) is better for showing soft tissues and can be used with an intravenous dose of contrast medium (gadolinium) to get even better detail. If the structures being imaged soak up the contrast medium the radiologist refers to “enhancement.” Normally, there is mild enhancement of the nerve at the geniculate ganglion, and in the distal part of the nerve near the eardrum. More significant or pathological enhancement is seen in the nerve in anything from 50-100% of patients with Bell’s palsy, and lack of enhancement is regarded as a good prognostic sign.

Differential Diagnosis  

The wise physician always considers a number of possibilities when making a diagnosis.

While Bell’s palsy is, by definition without causeone should at least wonder about the cause of inflammation or about structural abnormalities affecting the nerve.

Herpes Simplex 1:

Based on blood tests for antibodies, the possibility of direct infection or an immune response to this virus was considered for many years. Nowadays, given modern technology and DNA testing, simplex reactivation is considered the likely cause of Bell’s palsy in most cases. The virus has been identified in the fluid around and in the nerve when decompression surgery was used as a treatment modality.

Herpes Zoster:

Although more rare, Geniculate Zoster is the second most common viral cause of facial nerve palsy and has been described above. In one series of 1701 cases, zoster was the cause in 116.

Simplex and Herpes virus infections therefore account for the bulk of patients diagnosed with Bell’s palsy.

Other rare infections:

Lyme Disease, a spirochetal infection, deserves special mention. Facial palsy occurs in 50-60% of patients with Lyme, and can be bilateral. In general, patients with facial palsy due to Lyme almost always have other signs or clues to the diagnosis. There may or may not be a history of a tick bite, but painless and non tender swelling and redness of the face preceding the palsy are common. There may be other features of Lyme infection such as a history of a typical “bull’s eye” skin rash, arthritis, heart block, dizziness, or hearing loss. If antibodies to Lyme are looked for in the blood, the IgM antibody peaks in the third to sixth week after infection, and the IgG antibody rises from the fourth week. If a spinal tap is done, inflammatory cells in the cerebro spinal fluid are suggestive, and specific IgG antibodies are sensitive and specific for diagnosis.

Facial palsy has been described with infections with a number of other agents. These include cytomegalovirus, Epstein Barr virus, mumps, influenza B, and coxsackie virus. Rickettsial infection, too, has been implicated.

HIV:

HIV is a rare cause of facial palsy; the palsy occurs early on, at the time of sero-conversion.

Bacterial Infection:

Simple inspection of the ear, and eardrum will suggest the diagnosis.

A yellow creamy fluid or pus issuing from a ruptured ear drum in a patient with facial palsy suggests the diagnosis of cholesteatoma, an infected collection of debris and pus in the bone adjacent to the facial canal.

Other Systemic Inflammatory Conditions:

Sarcoidosis, a chronic inflammation may be associated with bilateral facial palsy, and Sjogren’s, an autoimmune syndrome, is an unusual cause.

The Melkersson-Rosenthal syndrome consists of recurrent facial palsies with lip or facial swelling and a fissured tongue. It is caused by a tuberculoid granuloma or chronic inflammation and is of unknown cause and treatment.

Multiple Sclerosis: 

Because the facial nerve pursues a fairly lengthy course within the brain stem, it is sometimes caught up in a plaque of demyelination and may look superficially like a Bell’s palsy. Always there will be other signs or symptoms to suggest the diagnosis and one should not jump to thoughts of multiple sclerosis in a patient with simple, uncomplicated Bell’s palsy.

Structural Nerve Compression:

A tumor anywhere along the length of the facial nerve could cause a facial nerve palsy, but always will declare itself by way of other signs or symptoms.

Prognosis 

The prognosis of Bell’s Palsy is related to the severity of the nerve damage. Clinically incomplete lesions tend to recover. Various grading scales have been created to measure the severity of the deficit and are sometimes used. One such is the House-Brackmann Scale (Table 1).

Without treatment, about 85% of patient show improvement within three weeks. Overall, about 70% of patients will have a complete recovery. Patients with an incomplete lesion have a 94% chance of return to normal function. Treatment is designed to increase the salvage rate beyond recovery related to natural history.

The palsy of geniculate zoster is associated with more severe paralysis and an accordingly worse prognosis.

Synkinesis:

In patients who have very severe lesion, but in whom the nerve sheath remains in continuity, there is outgrowth of nerve fibers to reinnervate the facial muscles. Growing nerve fibers proceed down the still viable nerve sheath at a rate of about 1mm a day. Normally, as described above, each muscle is discretely innervated and can move in isolation from the other facial muscle. In patients with severe palsy and reiinervation, this ability is lost, and a mass action of all the facial muscles occurs simultaneously even when one is attempting to move only a single muscle. Thus, on blinking there is twitching of the corner of the mouth, and on smiling, the eye closes. This mass action is called synkinesis or “moving together.”

The same phenomenon can involve the autonomic fibers, causing the lacrimal and salivary glands to secrete simultaneously. When the patient tastes something very tart or sour, salivation is triggered, but the lacrimal gland is also stimulated, resulting in overproduction of tears which may spill over the lower lid. This has been called the syndrome of “crocodile tears” (remember Peter Pan?)

Management

Imaging: Do all patients need to be imaged?

Imaging is warranted if the physical signs and symptoms are atypical, or if there is slow progression beyond three weeks.

If there is no improvement at two to three months it is wise to image for pathology other than a simple Bell’s palsy.

NCS & EMG: Do all patients need to have electrical studies?

The test is basically a good way to estimate the degree of nerve damage and is used mainly for prognostic purposes. However, these tests are not mandatory, because whatever the result,it wont change treatment

Eye Care:

Because of poor lid closure and sometimes poor tear production, the cornea is at risk and may be scratched or abraded. If the patient cannot close the eye, artificial tears should be dropped in every hour during the day, and an ophthalmic ointment should be use at night. Protective glasses or goggles are worth considering and at night the eye can be taped shut, taking care that the tape is not placed directly on the eyelid because it could slip and abrade the cornea.

In very severe and prolonged cases the ophthalmologist may consider stitching the outer parts of the eyelids together to keep the eye partially closed (tarsorraphy).

In patients with permanent weakness a small gold weight can be inserted into the upper lid to allow it to close by gravity.

Drug therapy  

Corticosteroids:

The decision to use any form of drug therapy is dependent on controlled clinical trials which assess the benefit or lack thereof of  any particular drug.

The most recent careful prospective trial was conducted in Scotland and the results were published in the New England Journal of Medicine in 2007. The conclusion was that prednisolone significantly improves the chances of complete recovery at 3 and 9 months. Of 496 patients treated, 357 had recovered at 3 months and of the remainder, another 80 recovered at 9 months. That translates to 96.1% recovery at 9 months in those treated with steroid, compared to 85.2% of those treated with placebo complete recovery . Another way of looking at the data is to say that 9 patients would need to be treated to achieve one additional full recovery. The dose of prednisolone was 25mg twice a day. Treatment should begin as early as possible and it’s not likely that the medication will work after 72 hours from onset; there are no data about late treatment.

In the US, the usual corticosteroid used is prednisone given at a dose of 1mg/kilogram of body weight for 7 to 10 days.

Antiviral Agents:

If Herpes Simplex is a common cause of Bell’s palsy it makes sense to use a drug active against this virus. In the trial described above, one experimental wing was to test the effect or lack thereof of added acyclovir which is active against Simplex. The addition of acyclovir was not effective.

A newer drug is valacyclovir, it is a  precursor of the active form of acyclovir and is converted  to acyclovir in the body. Acyclovir has increased bioavailability.

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Surgical Decompression 

Surgical decompression of the facial nerve in Bell’s palsy is not currently recommended.

The Quality Standards Subcommittee of the American Academy of Neurology concluded that there is insufficient evidence to make recommendations regarding surgical decompression of the facial nerve in Bell’s palsy.

With surgery there is a risk of deafness, cerebrospinal fluid leakage, and more facial nerve injury.

Electrical Stimulation of the Facial Nerve

Electrical stimulation has been used to try to promote nerve recovery. In the absence of controlled trials and given the lack of safety data this is not a viable option for treatment and is mentioned only for completeness.

Psychological Support

Facial palsy can be very disturbing to the patient. In one large series, half of the patients surveyed reported considerable psychological distress and restriction of social activities as a consequence of Bell’s palsy. Accordingly patients should be supported and encouraged, and one good way of supporting them is to review the figures for prognosis. In general this is a benign condition.

Peitersen E. The natural history of Bell’s palsy. Am J Otol 1982; 4: 107-11

House JW, Brackmann DE. Facial nerve grading system. Otolaryngol Head Neck Surg 1985; 93: 146-7

Gilden DH. Bell’s palsy. N Eng J Med 2004; 351: 1323-31

Ronthal M. Bell’s palsy. UpToDate 2007; http://www.uptodateonline.com

Baringer JR. Herpes simplex virus and Bell’s palsy. Ann Intern Med 1996; 124:63

Murakami S, Mizobuchi M, Nakashiro Y et al. Bell’s palsy and herpes simplex virus: identification of viral DNA in endoneurial fluid and muscle Ann Intern Med 1996; 124: 27-30

Theil D, Arbusow V, Derfuss T et al. Prevalence of HSZ-1 LAT in human trigeminal, geniculate and vestibular ganglia and it implication for cranial nerve syndromes. Brain Pathol 2001; 11: 408-13

Stjernquist-Desatnik A, Skoog E, Aurelius E. Detection of herpes simplex and varicella-zoster viruses in patients with Bell’s palsy by the polymerase chain reaction technique. Ann Otol Rhinol Laryngol 2006; 115:306-11

Kawaguchi K, Inamura H, Abe Y, et al. Reactivation of of herpes simplex virus type 1 and varicella zoster virus and therapeutic effects of combination therapy with prednisolone and valacyclovir in patients with Bell’s palsy. Laryngoscope 2007; 117: 147-56

Sullivan FM, Swan IRC, Donnan PT et al. Early treatment with prednisolone or acyclovir in Bell’s palsy. N Engl J Med  2007; 357: 1598-607

Weir AM, Pentland B, Murray J et al. Bell’s palsy: the effect on self image, mood  state and social activity. Clin Rehabil 1993; 7: 88

Grogan PM, Gronseth GS. Practice parameter: Steroids, acyclovir and surgery for Bell’s palsy (an evidence based review): report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 2001; 56:830

TABLE 1

The House Brackmann Scale