How facial functions are affected
The intracranial segment of the facial nerve (the section within the brain and skull), is responsible for many discrete bodily functions that we may not think about from day to day.
Saliva, tears, and taste
The parasympathetic nerve fibres control the following glands in the head and neck:
- The submandibular salivary gland, just under the jaw, and the sublingual salivary gland, under the tongue, help keep our mouth moist so we can speak and eat.
- The mucous glands in our nose and throat which keep the passages in our mouth and nose moist. This moistens the air that we breathe.
- The lacrimal glands or tear glands which keep our eyeballs lubricated so we can blink and close our eyes easily and also to protect the surface of our eye and therefore our vision. The facial nerve is also involved in the involuntary blink reflex and this is why some people with facial palsy may lose the involuntary blink reflex of the affected eye.
- The specialist sensory fibres arising from the Chorda Tympani control specialist sensory functions, such as taste sensation to the anterior 2/3 of the tongue. This is why people who damage the intracranial segment of their facial nerve may experience altered taste or sometimes a tingling tongue. It also controls sensation around the ear which is why some people may experience pain and tenderness in this area.
Sound sensitivity
Finally, the intracranial segment of the facial nerve gives rise to the stapedius nerve which controls the ‘stapedius muscle’ in our ear. It is the smallest muscle in the body measuring approximately 1mm in length. It is located in the middle ear just behind our eardrum. It helps to reduce the vibration in our ear when we hear a very loud sound, for example, a siren or a fire alarm. It dampens down the sound to limit the damage to our hearing. This is why some people with facial palsy may find that some sounds are much louder than normal because the facial nerve has been injured in this area. The medical term for this is ‘hyperacusis’ whereby normal sounds seem louder and we become hypersensitive to all the sounds around us.
Facial movement
The facial nerve exits the skull entirely and this section of the nerve is termed ‘extracranial’ meaning outside the brain and skull.
The extracranial section of the facial nerve is solely responsible for facial movement and expression. It comes out onto the face in front of the ear and then travels downwards to the parotid gland the largest of the salivary glands in our mouth. It is in the parotid gland that the facial nerve ends by splitting into five branches as follows:
- Temporal branch
- Zygomatic branch
- Buccal branch
- Marginal Mandibular branch
- Cervical branch
These branches supply the muscles of facial expression.
The Temporal Branch is responsible for the following:
- Pulling the ear forward and raising the ear and moving the scalp forward although the ability to do these actions varies from person to person.
- Frowning
- Raising the eyebrows/looking surprised
The Temporal and Zygomatic branches together are responsible for:
- Closing the eyelids.
- Scrunching up the skin around our eyes.
The Zygomatic and Buccal branches together are responsible for:
- Lifting the corners of the mouth for smiling.
The Buccal branch is responsible for:
- Lifting the upper lip.
- Smiling
- Pulling the corner of the mouth backwards.
- Protruding the lips.
- Closing and compressing the lips.
- Flaring and compressing the nostrils.
The Buccal and Marginal Mandibular branches together are responsible for:
- Pulling the corner of the mouth downwards.
- Pulling the lower lip downwards.
The Marginal Mandibular Branch is responsible for:
- Pulling the skin of the chin upwards
The Cervical Branch is responsible for:
- Pulling the corners of the mouth down.
The following symptoms can arise as a result of damage to the facial nerve:-
- Difficulty eating and drinking.
- Difficulty speaking.
- Dribbling when the lips cannot seal properly.
- Reduced saliva which can cause problems with dry mouth, oral hygiene and can increase the risk of tooth decay and gum disease.
- Difficulty closing the eyes for sleep, blinking or scrunching our eyes up to protect ourselves from bright sunlight.
- Reduction in tear production which causes the eye to become dry which combined with problems with blinking can be hazardous. It can result in dry patches over the surface of the eye which if left untreated can form ulcers and increase the risk of loss of vision.
- Hypersensitivity to sound.
- Loss or change in taste.
- Pain and tenderness around the ear.
- Loss or weakness of facial expression on the affected side of the face.
- Difficulty breathing through the nose on the affected side of the face.
Any of these physical symptoms can have a negative impact on a person’s mood and self-esteem. Depression and anxiety are common symptoms experienced by people living with facial palsy. Many people may try to mask their facial weakness by inhibiting their facial expression, especially smiling. This has the result of suppressing emotion and may be linked to low mood.
Hearing Loss and Balance
The nerve responsible for hearing is called the vestibular nerve. It follows the intracranial course of the facial nerve very closely and often injury, swelling or pressure of the facial nerve may impact on the vestibular nerve and vice versa. However, hearing loss is predominantly a symptom of injury or disease of the vestibular nerve which is often a gradual process and may go undetected for many months or even years. The most common cause of hearing loss associated with facial palsy is due to a benign tumour on the vestibular nerve. This is commonly called an acoustic neuroma or vestibular schwannoma.
Facial pain
The facial nerve is not involved in facial sensation except for some pain sensation around the ear. The trigeminal nerve is the main sensory nerve for the face. It transmits sensory information to the skin, sinuses and the mucous membranes (the skin inside our mouth, nose and throat), and also teeth and gums. Facial pain is not a symptom of facial nerve injury. Facial pain is often referred to as Trigeminal Neuralgia and can be both intermittent and constant.
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