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Bell’s Palsy

September 11, 2013 14 comments

I figured it is about time to write something about the blog’s title, bell’s palsy, since I’ve seen there are a lot of people searching for this topic about facial paralysis.

Bell’s palsy is a common condition that presents with an acute onset of lower motor neuron (LMN – could say like  peripheral nerves) facial weakness affecting the muscles on one side of the face.

People of all ages may be affected, including children, although it is most common in patients aged 30-50 years. The exact cause in not known, but there are certain conditions known to be responsible.

If we take a look in pathogenesis, how the condition evolves, we see that there is segmental demyelination in a local conduction block proximally. And what this actually means? Every nerve has its sheath around that is made of substance called myelin. This myelin sheath does not cover the whole nerve, but in segments, leaving tiny gaps, called Nodes of Ranvier (take a look at the picture below). And these gaps are like capacitors that stores electrical charge and allows bursts of electric impulses (know as action potentials) to move along the neuron in a saltatory fashion. Think of it as the action potential would jump from one of these gaps to the next one. This actually prevents loosing the charge. Now imagine that you widen this gap. This slows the nerve conduction, spread it even more and you get a block, conduction block. Now you cannot get the full use of that nerve leaving you weakness.

Since this conduction block happens proximally (closer to its origin), it allows a relatively rapid and complete recovery in about 85% of cases. This is because myelin rapidly regenerates. In others, axonal degeneration occurs, which will produce a severe paralysis. Again, think of it as the whole neuron is cut, and that is why it is so severe. Often this is then followed by incomplete recovery associated with aberrant re-innervation, that is, fibers from the periocular muscles my regenerate and supply the mouth, and vice versa. Such faulty re-innervation may lead to “jaw-winking”, and even hemifacial spasm. Where axonal degeneration has occurred, electromyography of the facial muscles will show fibrillation and features of denervation, although these changes may not appear until some 10 days after the onset. In some instances the pathogenesis is a mixture of axonal degeneration and demyelination.

Ok, let’s leave this aside and let’s carry on and look how this condition actually presents clinically. Normally patients may present with pain in or behind the ear preceding or appearing with the development of facial weakness. There is inability to close the eye or move the lower face and mouth on one side of the face.

The lack of blinking leads to tears spilling out of the eye, which waters to cause complaints of blurred vision. The cheek is flaccid and saliva and fluids may escape from the corner of the mouth. The weakness commonly progresses over 24-72 hours to reach a maximum. In many patients there are complaints of numbness in the affected side of the face, although trigeminal sensation is spared and there should be no weakness of jaw movement, since it is supplied by the motor root of the trigeminal nerve. Trigeminal sensation is sensation on the skin of the face, because sensory innervation of it comes from fifth cranial nerve – trigeminal nerve. Hence the name.

About 40-50% of patients are aware of disturbed taste on the ipsilateral anterior part of the tongue (ipsilateral means it is on the same side as the lesion). This points to a lesion in the distal part of the facial nerve below the geniculate ganglion, but above the origin of the chorda tympani (branch of facial nerve that is responsible for taste). Many patients also notice hyperacusis (over-sensitivity to certain frequency ranges of sound and difficulty tolerating everyday sounds, some of which may seem unpleasantly loud) because the stapedius muscle (very small muscle that moves the smallest bone in the human body and makes us hear) is supplied by a branch of the facial nerve, which leaves the nerve in the facial canal proximal to the chorda tympani.

If a zoster infection is responsible, there will be herpetic vesicles on the pinna (the visible part of the ear that resides outside of the head, also called auricle or auricula) or in the external auditory canal on the affected side. Ramsay Hunt described a herpetic infection of the geniculate ganglion with the development of an acute facial palsy – Hunt’s syndrome. In some of these patients the 8th cranial nerve (auditory nerve, responsible for hearing) may also be infected, producing acute vertigo, deafness and tinnitus (ringing of the ears). A few patients may show a bilateral (on both sides) facial palsy of lower motor neuron (LMN) pattern; this may appear as part of a Guillain-Barre syndrome, from Lyme disease, from sarcoidosis or even carcinomatous meningitis.

About 85% of patients show signs of improvement within some 3 weeks of the onset. About 70% of patients recover normal function in the face but some 16% are left with asymmetry, signs of aberrant re-innervation and some weakness. An incomplete palsy at the onset or signs of recovery starting within 3-4 weeks usually are good prognostic features for recovery. This is mirrored in the electrophysiological findings. In the more severely affected, where axonal degeneration has taken place, recovery is slower and often incomplete. Recurrent facial palsies require more intensive investigation to exclude any compressive lesion on the middle ear or skull base, and to look for any systemic upset such as sarcoidosis, hypertension, diabetes.

Normally when there is suspicion of Bell’s palsy several investigations are made. Such as blood tests. Full blood count, erythrocyte sedimentation rate (ESR), fasting glucose levels, tests for Borrelia. Of course there is imaging; in selected patients MRI and/or CT scanning, chest X-ray. Electromyography (EMG) studies as these may assess the severity of damage and help in prognosis; they may also indicate a more widespread neuropathy. ENT examination…

Once all the tests are conclusive and the Bell’s palsy diagnosis is made, patients can undergo treatment. There appears to be little difference in outcome between patients treated with steroids and those who are not. Many doctors believe that a short intensive course of steroids given within 5-7 days of the onset of palsy may reduce the swelling of the facial nerve and so prevent axonal degeneration. Prednisolon 40 mg daily for 5 days and then tapered off over the next week is a typical regimen. It has been suggested that such a course should be given to all patients seen acutely with a complete palsy at the time of consultation or with impaired taste.

Because of the possible infective causation by herpes virus, acyclovir has also been used in treatment of an cute facial palsy. This certainly should be given if a zoster infection (Hunt’s syndrome) is suspected. The combination of acyclovir with steroids in those patients with complete facial palsies has also been used. Surgical decompression of the facial nerve has had its supporters over the years, although there has been no rigorous controlled trial to indicate benefit and as over 70% of patients will make a full recovery with no treatment it is hard to justify the surgical risks.

Care of the eye is always important if there is incomplete lid closure but as the cornea is not anesthetic, the patient will be aware of any intruding foreign body. Occasionally it may be necessary to suture the lids partially together, a tarsorrhaphy, to protect the eye.

In those patients left with marked residual weakness or asymmetry, a number of surgical measures may be used to try to improve their appearance. These include plastic surgery with implants of soft tissues to restore the contours. Such measures will improve the symmetry of the face at rest but are by no means a ‘cure’.