Conditions: Nerves of the Face
There are two main nerves of the face, the facial nerve and the trigeminal nerve. A problem with the facial nerve may create weakness or paralysis of one or more muscles of the face—a situation with many potential causes and which may result in significant problems. Numbness or inappropriate sensation of the face (including pain), on the other hand, is due to an alteration in the a different nerve of the face, the trigeminal nerve, and has its own various causes.
Anatomy and function
The facial nerve and facial muscles.
Each side of the face has seventeen muscles innervated by branches of the 7th cranial nerve, called the facial nerve. Problems with this nerve may lead to weakness of one or more of these muscles. Muscles involved in chewing (tongue and jaw muscles) as well as sensation in the face go through the trigeminal nerve, also known as the fifth cranial nerve. Problems with this nerve may lead to decreased sensation, altered sensation, or pain.
• Short animation demonstrating the trigeminal nerve and its branches.
The trigeminal nerve (fifth cranial nerve).
Facial nerve weakness or paralysis
The left or right facial nerve may become weak or paralyzed due to trauma, infection, or stroke. Branches of the facial nerve allow for blinking and closing of the eyelids, moving the lips for speech and eating, as well as for facial expression. If the eyelid is unable to close well, the eye (especially the cornea) may dry out and cause irritation or even loss of vision.
SMELL (olfaction)
Our sense of smell comes from nerve branches of the olfactory nerve passing through tiny holes in the skull base into the nose. Any blockage of airborne particles from reaching these nerve endings, inflammation of these nerve endings, or a problem with the nerve may cause an alteration of smell, such as decreased detection of smells, alteration of the quality of the smell, or complete inability to detect odors. Most of flavor is actually due to smell, because flavor is the combination of taste and smell. Taste comes from the tongue and is only four distinct senses: sweet, bitter, sour, salty, and umami.
The chorda tympani nerve, the main taste nerve, is shown in yellow.
Taste
Taste is a special sense of the tongue that is different from smell and different from the sense of touch. The only tastes we appreciate are bitter, sour, salty, sweet, and umami (which is reminiscent of chicken broth). All other aspect of flavor come from smell, mediated by the olfactory nerve. The main taste nerve, the chorda tympani nerve, is a special branch of the facial nerve that runs from the taste buds to the brain along a very circumtuitous route, hitchhiking with two different cranial nerves and passing through parts of the middle ear on its way. The base of the tongue contributes to taste via the glossopharyngeal nerve, and parts of the throat also contribute to a lesser degree via the vagus nerve. Alterations in taste may arise from problems with this nerve, but also from dry mouth, inflammation, radiation, medications, or injury of the tongue.
Herpes zoster (shingles) affecting only the first branch of the trigeminal nerve, called herpes zoster ophthalmicus..
Shingles (herpes zoster)
Shingles is an infection caused by the varicella zoster virus, which the same virus that causes chicken pox. The first time someone is infected with this virus, the individual develops chicken pox. The body recovers, but the virus is not eliminated from the body. Instead, the virus lays dormant in nerves and can be reactivated later in life, and when it does so, it travels down one nerve branch to the specific area of skin served by that nerve. The skin develops a rash with very distinct boundaries matching the boundaries of that nerve branch. The infected skin might break down, allowing a bacterial infection to set up in addition to the virus.
Ramsay Hunt or herpes zoster oticus.Source: Worme M, Chada R, Lavallee L
Herpes zoster oticus (ramsay hunt syndrome)
As with shingles, when an individual who previously had chicken pox (varicella zoster virus) develops reactivation of this virus through the facial nerve, the face can become weak or paralyzed on one side and blisters (vesicles) may form in the bowl of the ear due to a minor branch of the facial nerve serving a small patch of skin in that part of the ear. Other symptoms may include pain in the ear, jaw, or neck, loss of taste (tongue, not smell), and a rash (vesicles) on the tongue or palate. If the balance and hearing nerve (vestibulocochlear nerve) is also involved, symptoms may include hearing loss, tinnitus, or vertigo. Early symptoms include intense pain in one ear, the jaw on one side or the neck on one side which may precede the acute facial paralysis by a week or more. This infection has some similarities to Bell’s palsy, but is different.
Bell’s palsy
Bell's palsy is one type of facial nerve weakness or paralysis that results in a temporary inability to move the facial muscles on the the entire affected side of the face. In most cases, the weakness is temporary and significantly improves over weeks. The facial weakness may be mild or complete paralysis. The time from initial symptoms to maximal symptoms occurs within 72 hours, or else the problem is not Bell’s palsy. Bell's palsy can trigger an increased sensitivity to sound.
The cause of Bell's palsy is unknown and it can occur in any age, but risk factors include a recent upper respiratory infection, diabetes, and pregnancy. Not all facial weakness is due to Bell’s palsy. Other conditions that can cause facial weakness include brain tumor, stroke, Ramsay Hunt syndrome, trauma, and Lyme disease.
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