Conditions: Facial Weakness or Paralysis

A problem with one specific nerve to the face, the facial nerve may create weakness or paralysis of one or more muscles of the face—a situation with many potential causes.

 

Anatomy and function

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, causing inability to blink adequately, smile or make other facial expressions, move the lips for speech and eating. Sensation in the face goes through a different nerve, the trigeminal nerve. The facial nerve starts in the brainstem, travels through the skull close to ear structures, and then exits the skull by the earlobe and passing through the parotid salivary gland in the cheek as it branches to reach individual muscles. A small branch of the facial nerve called the chords tympani branches away from the main facial nerve trunk in the middle ear and travels to the tongue to provide sensation of taste. Each side of the face has its own facial nerve and each nerve controls only that side (except in the forehead, where there is some crossover.)

Facial nerve weakness in general

The left or right facial nerve as a whole may become weak or paralyzed due to trauma, infection, or stroke. Individual 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. Weakness of one or more individual branches affects only the specific muscles the affected branch(es) control. The zygomatic branch of the facial nerve controls the eyelids; if the eyelid is unable to close well, the eye (especially the cornea) may dry out and cause irritation. If untreated, a dry eye can lead to loss of vision in the affected eye.

Example of complete Right facial nerve paralysis (all branches) under different expressions and efforts:

Eye closure

Eyes opened

Smile

Mouth wide open

Full face contraction

 

individual branch(es) of the facial nerve

One or more branches of the facial nerve may be dysfunctional, leading to specific muscular weaknesses. While variable from individual to individual, the facial nerve branches are conceptualized as follows: temporal, zygomatic, buccal, marginal mandibular, and cervical.

  1. The temporal branch innervates the frontalis muscle, which raises the eyebrows and furrows the brow.

  2. The zygomatic branch innervates the orbicularis oculi muscle, which closes the eyelids, and hold the lower eyelid snugly against the eyeball.

  3. The buccal branches, which innervate the muscles of the mid-cheek, including those used in smiling, squeezing the cheeks inwards toward the teeth, dilating the nostril, and moving the upper lip.

  4. The marginal mandibular branch, which innervates muscles controlling the lower lip used to hold the lower lip up against the teeth, pursing the lips as if to blow out a candle, smiling, and making a water tight seal of the lips when eating and drinking.

  5. The cervical branch, which innervates the platysma muscle in the neck. This muscle is less important than others innervated by the facial nerve and causes neck bands that may be considered cosmetically undesirable.

Bell’s Palsy

The most common cause of facial nerve weakness, but by no means the only cause, is Bell’s Palsy. Other conditions that can cause facial weakness include brain tumor, stroke, Ramsay Hunt syndrome, trauma, and Lyme disease.

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.

Tumor

A growth, particularly a cancerous growth, that compresses, invades, or otherwise injures the facial nerve may cause facial weakness. The location of an offending tumor determines which facial muscles are at risk of becoming weak. A characteristic of tumor caused facial nerve weakness that is distinct from Bell’s Palsy is that tumors tend to cause progressive facial weakness over weeks to months, whereas Bell’s Palsy must progress for no more than 72 hours. Tumors that may cause facial weakness include parotid salivary gland growths, skin cancers that may spread along nerve branches, tumors of the nerve sheath (such as acoustic neuroma or facial neuroma), and brain tumors.

bacterial Infection

Because the facial nerve passes through the middle ear space (behind the ear drum) before exiting the skull, a bacterial infection in that area (otitis media) may cause the facial nerve on that side to become weak. This scenario is notable for hearing loss, ear pain, and possible drainage of pus from the ear canal on the side of the facial weakness.

Cholesteatoma

Cholesteatoma is a destructive and expanding growth consisting of skin cells (keratinizing squamous epithelium) in the middle ear and/or mastoid. Cholesteatomas are not cancerous as the name may suggest, but can cause significant problems because they expand in size and erode through nearby structures. This can result in the destruction of the bones of the middle ear (ossicles), as well as growth through the skull base into the brain. They often become infected and can result in chronically draining ears. Treatment almost always consists of surgical removal.

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.

Photo by Scott Bauer., Public domain, via Wikimedia Commons

Lyme disease

Lyme disease is a caused by the Borrelia bacterium, which is spread by deer ticks. The most common sign of infection is an red rash that expands in size over time, and which appears at the site of the tick bite about a week afterwards. The rash is typically neither itchy nor painful. Other early symptoms may include fever, headaches and fatigue. If untreated, symptoms may include facial weakness on one or both sides of the face, joint aches, headaches, neck stiffness, and heart palpitations. Not everyone with Lyme disease has all of the symptoms and many of the symptoms are not specific to Lyme disease but can occur with other diseases as well.

Stroke (cerebrovascular accident, CVA)

A stroke, or cerebrovascular accident, is an emergency resulting from lack of blood flow to brain tissue. Facial weakness is one potential way a stroke may manifest, though other signs may include difficulty speaking (finding the right words or having slurred speech), weakness of an arm or leg, visual impairment, headache.

 

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