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Facial Nerve ( VII ) Seventh Cranial Nerve

There are 12 cranial nerves in our body which are broadly classified as motor (control movements) and sensory (sense pressure, touch, pain and temperature). They are numbered from I to XII using Roman numerals. The anatomy of the facial nerve is very complex. Originating from the brain stem, the VIIth cranial nerve (Facial nerve) enters the bone of the ear (temporal bone) through a small bony tube (the internal auditory canal) which has very close association with the hearing and balance nerves. Along its inch-and-a-half course through a small canal (Fallopian canal) within the temporal bone, the facial nerve winds around the three middle ear bones, behind the eardrum, and then through the mastoid (the bony area of the skull behind the ear that is palpable), passes through the stylomastoid foramen near the mastoid process and enters the parotid gland (major salivary gland in the cheek). Here it divides into its main branches inside the parotid gland. These branches then further divide into 8000 smaller nerve fibers that reach into the face, which supply the various facial muscles, neck, salivary glands and the ear. It also controls the perceived sound volume and our balance. They also stimulate secretions from the tear glands of the eye and the salivary glands in the front of the mouth. Taste sensations from the anterior 2/3 of the tongue (through the cordae tympani) and nerve sensation to the muscle of the stirrup bone in the middle ear (the stapes) are also carried by this nerve.
The face has many muscles, each with its own unique functions, most of which are controlled by the Facial Nerve (VII cranial nerve). Information passing along the fibers of this nerve allows us to laugh, cry, smile, or frown, hence the name: "the nerve of facial expression". Unlike other muscles, the facial muscles insert directly into the skin of the face. Contraction of the muscles causes the skin to move resulting in the different facial expression. Signals from the complex array of nerves to the various muscles instruct the muscles to move in combinations as well as individually.

What is Bell's palsy?
Bell's palsy is a form of facial paralysis resulting from damage to the VIIth (facial) cranial nerve. The condition is named for Sir Charles Bell, a Scottish surgeon who studied the nerve and its innervations of the facial muscles 200 years ago. Since the function of the facial nerve is so complex, many symptoms may occur when the fibers of the facial nerve are disrupted. A disorder of the facial nerve may result in twitching, weakness or paralysis of the face, dryness of the eye or the mouth, in disturbance of taste etc.
      Bell's Palsy temporarily prevents the nerve from transmitting signals to the muscles, causing weakness or paralysis. When half or one side of these individual nerve fibers are interrupted, hemi-facial weakness occurs. If these nerve fibers are irritated, then movements of the facial muscles appear as spasms or twitching.
     The muscles that close the eyelid are controlled by VII nerve, but the muscles that control other eye movements and the ability to focus are not. There will be difficulty in closing the eyelids, but other functions and movement of the eye is not affected. The sense of taste is affected, but tongue motion is not. Skin sensation may be affected near the ear, but sensation over the rest of the face usually remains normal. Chewing and swallowing are usually not affected.

What are the causes for Bellís palsy?
The specific cause of Bell's Palsy is unknown. It can strike almost anyone at any age. A number of things can damage the facial cranial nerve and lead to Bell's palsy. Several systemic diseases can cause facial paralysis and are sometimes misdiagnosed as Bell's palsy.
      Most Bells Palsy are designated as idiopathic (unknown), but it is said to be caused by an inflammation within a small bony tube called the fallopian canal, through which the nerve passes before exiting from the stylomastoid foramen. The canal is an extremely narrow area, and an inflammation within it is likely to exert pressure on the nerve. The nerve has not yet divided into its several branches thus resulting in impairment of all functions controlled by the VIIth nerve.

What are the symptoms?
The 7th cranial nerve has both motor and sensory functions. Its motor functions include shutting the eye, lifting the eyebrow, and supply the muscles that move the mouth and lips. Its sensory functions include tasting on the front of the tongue and dampening the level of the sound we hear. So the symptoms of Bell's palsy include any abnormalities involving these various muscles. Many people describe feeling a pain behind their ear or near the jaw a few days before the other symptoms develop. The most common symptom of Bell's palsy is weakness on one entire side of the face. A person may not be able to close one eye, or they may have difficulty shutting their eye completely. The forehead doesn't wrinkle when a person tries to lift their eyebrow. The lower part of the face may droop down. Patients aren't able to lift their mouths to smile or fill their cheeks with air. They may drool from the mouth. Some people may feel a tingling or numbness in the face. In addition to one-sided facial paralysis with possible inability to close the eye, symptoms of Bell's palsy may include pain, tearing, drooling, hypersensitivity to sound in the affected ear, and impairment of taste.
      One of the functions of the eyelid is to protect the eye from injury, and our eyes usually shut very quickly when something is likely to hit or fly into them. Sometimes people with Bell's palsy get eye injuries as a result of the eye not shutting completely in defense.
      Sensory Functions may be affected. Some people may not have any taste on the front of their tongue. People may have an increased sensitivity to sound in the ear on the affected side, and so things sound louder than normal.

What are the problems associated with Bellís palsy?
    • Muscle weakness or paralysis
    • Overall droopy appearance
    • Forehead wrinkles disappear
    • Lower eyelid droop
    • Brow droop
    • Nose runs or is constantly stuffed
    • Difficulty in speaking
    • Difficulty in eating and drinking
    • Cannot blow or whistle
    • Sensitivity to sound (hyperacusis)
    • Excess or reduced salivation
    • Sensitivity to light
    • Facial swelling
    • Diminished or impairment of taste
    • Pain in or near the ear
    • Drooling from the corners of the mouth
    • Vertigo
    • Blisters in ear or other areas
EYE RELATED
    • Excessive tearing
    • Eye closure difficult or impossible
    • Lack of tears
    • Tears fail to coat cornea
    • Impossible or difficult to blink
    • Eye irritation or itching

What is Residual Effects (Synkinesis)?
People recover at different rates, but generally the regeneration of nerves will be complete in the three months after onset. If recovery is delayed you may begin to notice movements in areas of the face that you are not even trying to move, this is referred to as Residual Effects. Residual effects can be present in cases where recovery from Bell's palsy is delayed beyond the 3 months point. Generally, the longer the recovery takes beyond the initial three months, the more severe the residual effects. For example, when you smile the eye may close or twitch or when you close you eye the corner of your mouth may pull up or out to the side. This condition is known as synkinesis. It is characterized by uncoordinated or unsynchronized facial movements that occur along with normal movements. Synkinesis varies in severity from mild to severe. In its worst form it can result in uncontrollable movement of the facial muscles on the affected side during any attempted expression. The affected side of the face may feel tight as the result of the uncontrolled muscle contractions (spasms).

Synkinesis is theorized to be the effect of abnormal nerve regeneration. Some of the healing facial nerve fibers can actually implant themselves into the wrong muscles. The facial nerve is like the telephone cable. Within that strand are between 6000-7000 different nerve fibers that conduct the electrical signal from the brain to the facial muscles causing them to contract. They are very delicate. Inflammation from the Bell's palsy can harm, or "break" some of these very frail fibers. In time the damaged fibers heal. They regenerate at the rate of about 1-2mm per day. But there's no mechanism that directs these fibers back into their original muscles. The brain sends the signal for the muscle to contract thinking the nerve fiber is still connected to the original muscle, but instead, the nerve may be lodged in an entirely different muscle.

Treatment for residual synkinesis can be effective at any time after it is noticed. The focus of the treatment is on re-coordinating the various muscles rather than stimulating them. We know that when synkinesis is present the facial muscles are viable, or "alive". Even an abnormal movement is still a movement. It is possible to restore more normal movement patterns and expression with specific, appropriate training.

Some Facts about Bell's palsy

  1. It is an "equal opportunity" disease. The percentage of left or right side cases is approximately equal, and remains equal for recurrences.
  2. The incidence of Bell's palsy in males and females, as well as in the various races is also approximately equal.
  3. Older people are more likely to be afflicted, but children are not immune to it. Children tend to recover well.
  4. Diabetics are more than 4 times more likely to develop Bells Palsy than the general population.
  5. Conditions that compromise the immune system such as HIV increase the odds of facial paralysis occurring and recurring.
  6. Bilateral Bell's palsy is rare, accounting for less than 1% of cases.
  7. The degree of paralysis should peak within several days of onset, usually never longer than 2 weeks (3 weeks max for RHS).
  8. The nerve regenerates at a rate of approximately 1 millimeter per day.
  9. Approximately 50% of Bells Palsy patients will have essentially complete recoveries in a short time. Another 35% will have good recoveries in less than a year.
  10. The nerve can continue to regenerate for 18 months, probably even longer. Improvement of appearance can continue beyond that time frame.
  11. The possibility of recurrence had been thought to be as high as 10 - 20%. These figures have been lowered as more has been learned about the types of facial paralysis that are now known to be other than Bell's palsy. Estimates of the rate of recurrence still vary widely, from around 4 - 14%.
  12. The muscles that close the eyelid are controlled by VII nerve, but the muscles that control other eye movements and the ability to focus are not. There will be difficulty in closing the eyelids, but other functions and movement of the eye is not affected. The sense of taste is affected, but tongue motion is not. Skin sensation may be affected near the ear, but sensation over the rest of the face usually remains normal. Chewing and swallowing are not affected.
  13. Various viral infections (herpes, Lyme disease, mumps, tuberculosis, HIV, etc), tumors, bony abnormalities, a skull fracture or any other trauma, surgery, neurological dysfunction (diabetes), various neurological disorders (Guillain-Barre syndrome, myasthenia gravis, etc), micro-circulation problems, etc can lead to facial palsy.
  14. Bell's palsy and Ramsey Hunt syndrome can be bilateral, but it's rare. Mononucleosis, flu, Guillain-Barre Syndrome, Leukemia, Lyme disease, Sarcoidosis and Heerdfort's Syndrome can be potential triggers of bilateral palsy.
  15. Residual effects can be present in cases where recovery from Bell's palsy is delayed beyond the 3 months point.

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Dr Antony George, Trichur Institute of Head And Neck Surgery (TIHANS),
Shornur Road,Trichur,Kerala-680001,India.
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updated Aug2002.