Disorders of the facial nerve are usually quite obvious, as facial expression is reduced or absent. This is not easy to overlook, although some disorders are more severe than others. Most disorders result in weakness or complete absence of movement, although much less commonly, some result in increased movement or spasm of the facial muscles.
This section will give a brief overview of facial nerve anatomy, testing for facial nerve disorders, causes of facial nerve disorders and ways of rehabilitating facial nerve dysfunction.
The facial nerve serves several functions, although its predominant function is to move the muscles of the face. It also carries nerve fibers that innervate salivary and tear glands. It also carries fibers that give taste and sensation to the tip of the tongue. The large bulk of the nerve is devoted to innervating the facial muscles. These are the motor fibers.
The motor fibers originate in the brainstem and travel a circuitous route to the muscles of the face. The nerve exits the brainstem and travels across a reservoir of spinal fluid contained in an area called the cerebellopontine angle. It enters a bony canal in the temporal bone of the skull called the internal auditory canal (IAC). It exits the IAC and makes a sharp turn to enter the middle ear space. It then makes a second sharp turn to enter the mastoid bone of the skull. The nerve traverses the mastoid bone and exits the skull in front of the ear where is splits into several branches. It is embedded in the parotid gland, which is a large salivary gland in the cheeks, in front of the ear. It then exits the parotid gland, after branching into five main divisions and many smaller divisions, to finally reach the facial muscles. Even though the facial nerve makes many turns in its course from the brain to the face, this course is predictable and usually very uniform.
The most important step in making a diagnosis of facial nerve disorders is observing the symmetry and movement of the face. All branches of the facial nerve need to be tested and degree of weakness noted. It is useful to have the patient raise their eyebrows, close their eyes, wiggle their nose, smiling widely and grimacing. These maneuvers will essentially test all branches of the facial nerve.
Several scales have been developed to describe facial movement. When there is a disorder that involves the main trunk of the nerve, the House-Brackmann scale is most often used. This scale has been adopted as the standard reporting scale by the American Academy of Otolaryngology Head and Neck Surgery. When individual branches are affected, such as after a trauma or surgery, this scale is not used, and a more subjective description is employed. Injuries to the facial nerve between the brainstem and the parotid gland are characterized by weakness in all branches of the facial nerve. When there is a lesion of the facial nerve in the brain, there is only weakness of the lower face below the eyes. This is the result of the upper face getting innervation from both sides. Therefore, a patient that has had a stroke that affects the brain in areas that provide input to the facial nerve will only have weakness of the face below the eyes.
It is important to determine the severity of a facial nerve injury. This will guide prognosis and treatment. The most commonly accepted method of classifying the degree of injury is based on microscopic examination of the nerve. There are five degrees of injury in the Sunderland classification. This is not to be confused with the House-Brackmann scale which categorizes facial function. First degree injuries will recover with no weakness and fifth degree injuries will have no recovery. Second thru fourth degree injuries will have some recovery.
Testing of the Facial Nerve
EMG Electromyography or EMG is a technique that amplifies and records the spontaneous evoked and voluntary muscle electrical responses. Needle electrodes are placed into the muscles with minimal discomfort to the patient. Electrical activity is recorded. Various types of injuries, give different recordings which can sometimes help determine degree of injury and chances for recovery. It is most useful in determining facial nerve regeneration.
Electrical Testing The purpose of this testing is to determine the degree of nerve injury and chances for spontaneous recovery. It can also help determine if surgery is warranted. It is only used in patients that have no evidence of facial movement on physical examination. A commonly used test is called the maximal stimulation test (MST). A device called the Hilger monitor is used. This consists of a stimulus generator with a probe that has two balls on the end. The balls are placed on the skin over selected branches of the facial nerve. A small electrical current is passed through the balls and facial movement is observed. The MST is performed by determining the maximal tolerated current on the normal side. The affected side is then stimulated with the same current. Facial movement is graded as normal, reduced, barely perceptible or absent. It is suggested that patients with barely perceptible or absent movement would benefit from surgical decompression under certain circumstances.
Electroneuronography This test, also called ENOG, is the recording of muscle electrical activity in response to an electrical stimulus placed across the skin. It is similar to EMG in that electrical activity in the muscles is recorded and similar to MST in that the current is placed across the skin and not with needles. The two sides are measured, and should be within 3% in normal people. If the weak side falls below 90% of the normal side, surgical decompression of the facial nerve is considered.
Bells palsy and Ramsay Hunt Syndrome
Historically, Bells palsy is considered an idiopathic facial paralysis. It can only be diagnosed after all other causes of facial weakness have been eliminated. Fairly strong evidence now points to herpes virus reactivation in the facial nerve as the cause. Ramsay Hunt syndrome (RHS) is also caused by a herpes virus. It is caused by herpes zoster, which is the virus that causes chicken pox, and is similar to the common herpes reactivation disease commonly known as shingles . The herpes viruses live in nerves even after the initial infection is over. For unknown reasons, they can be reactivated and cause injury to the nerve in which they live. If the virus lives in the facial nerve, it will cause a facial paralysis
The medical history and physical exam gives the physician sufficient information to make a diagnosis. Both Bells palsy and RHS have similar complications that relate to the ear. These include facial paralysis to varying degrees, and patients can also have hearing loss, vertigo and taste changes. Patients with RHS often have more severe vertigo and ear pain. It is important for the physician to exclude conditions that can mimic these two disorders. This is done my documenting that all branches of the facial nerve are weak, there are no middle ear conditions that can be causing the weakness, and there are no masses in the parotid gland. Patients with RHS will have blisters in the ear canal, and this finding will distinguish RHS from Bells palsy. The onset of facial weakness usually progresses and reaches its maximum severity at about two weeks. Patients often complain about pain around the ear that can radiate to the face, and the ear pain is usually much worse in RHS patients. Taste can often be disturbed, and many patients complain that sounds seem unpleasant or too loud. It is also common for patients to have inflammation of the taste buds on the tongue.
Approximately 10% of patients will have a family history of Bells palsy. The incidence of Bells palsy is about 20 cases per 100,000 people annually. It affects men and women equally, however, the incidence of Bells palsy is higher in pregnant women than the general population, about 40 cases per 100,000. Bells palsy comes back about 10% of the time. People who have had one recurrence are more likely to have a second recurrence, and people who have had three or more recurrences have a 50% chance of it coming back again.
Treatment for Bells palsy and RHS All patients with Bells palsy or RHS should be given a course of oral steroids. The physician should not wait to see how bad the weakness will get because waiting until there is complete paralysis may cause irreversible damage to the nerve. The dose of steroids varies between clinicians, but a common dosing regimen is 30mg of prednisone twice a day for a week and then taper it off. It is well established that Bells palsy and RHS are caused by the herpes virus, and we recommend giving patients anti-viral medications like Acyclovir, or one of the newer medicines like Famciclovir.
MRI is often obtained in patients with Bells palsy to exclude tumors as the cause of the facial weakness. This is somewhat controversial. Electrical testing is only necessary in patients with a complete paralysis. Patients that do not become completely paralyzed at the maximum of their disease will not need surgical intervention. Patients that do become completely paralyzed may benefit from surgical decompression of the facial nerve if the ENOG deteriorates on the affected side to less than 90% of the normal side, the ENG show appropriate muscle activity and they have not improved with steroids. Surgical intervention is only appropriate within the first two weeks from the onset of the weakness, so it is very important for patients to see their doctor immediately if facial weakness occurs.
Generally, good return of facial function can be expected in patients with Bells palsy. Patients with incomplete weakness at the maximum of the disease have a greater than 90% chance of excellent return of function. Patients with complete paralysis, but still have good electrical testing, can also expect good return of function, and patients with complete paralysis and loss of electrical activity have approximately 50% chance of return of normal function. Patients with RHS have worse return of function than Bells palsy patients.
Other Inflammatory Causes of Facial Weakness
Otitis Media Facial paralysis or weakness can occur from acute or chronic ear infections. This is a more significant finding in patients with chronic ear infections caused by cholesteatoma. It implies that the bony canal through which the facial nerve traverses the middle ear is eroded by the disease. When it occurs in an acute ear infection, it is usually because there is a congenital opening in the bony canal. It is important to get a CT scan of the temporal bones which house the ear in these patients. Medical therapy includes antibiotics, often intravenously administered, as well as steroids. Surgical intervention is often warranted to drain the infection. Ear tubes are sometimes sufficient, but it is often necessary to perform a mastoidectomy to remove infection and cholesteatoma.
Lyme Disease Lyme disease is caused by a bacteria that is carried by certain species of ticks. The disease has many symptoms which occur at different time periods. The bacteria enter the blood stream and can cause problems with many different nerves. The facial nerve is the most commonly affected cranial nerve. Facial weakness can be on one side or both. Diagnosis of Lyme disease as the cause of facial paralysis can be made with a blood test that looks for antibodies to the bacteria. When there is nerve involvement, the recommended therapy for Lyme disease is intravenous antibiotics for 3-4 weeks.
Traumatic Facial Disorders
Trauma may occur to the facial nerve from blunt or penetrating trauma to the temporal bone. Facial nerve injuries can also occur as the result of surgery. There are many opinions about the best way to manage facial nerve injuries. The management is determined by the site and degree of injury. The nerve can be injured by several different mechanisms, most commonly gunshots, stabbings, car accidents and other blunt injuries to the face or temporal bone. Patients with paralysis from these types of accidents usually have several other more urgent injuries. It is important to stabilize all life threatening injuries before addressing the facial nerve problems. The timing of repairs is therefore dictated by the extent of the other injuries. CT and MRI are often obtained to help diagnose the site of the injury. When the nerve is completely cut, the best outcome occurs with sewing the two ends together. This isnt always possible, so sometimes it is necessary to place a nerve graft. The donor nerve can come from several areas including, the ankle, neck or arm. Good return of function is expected with these procedures. If there is not enough nerve left to place a nerve graft, it is possible to redirect the hypoglossal nerve (the nerve that moves the tongue) to the facial nerve. This gives excellent return of facial function, and does not affect swallowing and talking as long as there is a functioning hypoglossal nerve on the other side.
Tumors of the Facial Nerve
Facial schwannomas These are the most common tumor of the facial nerve. They are benign neoplasms that arise from the Schwann cells and can occur anywhere along the course of facial nerve. They sometimes can extend intracranially, making it difficult to differentiate between other tumors. Facial schwannomas can also occur in the setting of neurofibromatosis type 1 or 2.
Vascular tumors - Improved imaging technology has made the diagnosis of these rare tumors easier. Hemangiomas are the most common vascular tumor of the facial nerve. They are benign, slow growing lesions that arise from the rich network of vessels that surround the facial nerve. The geniculate ganglion is the most common site of origin on the nerve.
Diagnosis and Management of facial nerve tumors The presenting signs for a facial nerve tumor is often facial weakness in about 75% of patients. This loss may occur suddenly, and mimic a Bells palsy. The only way to distinguish it is to obtain an MRI. Anyone with no return of function after a Bells palsy most likely has a tumor somewhere along the course of the facial nerve. Hearing loss is the second most common finding in patients with facial nerve tumors. It can be caused by compression of the cochlear nerve with intracranial extension, or it can cause a conductive hearing loss if the tumor is present in the middle ear. It is imperative for the surgeon to resist a biopsy as this can cause facial paralysis. Facial tumors may be diagnosed when a middle ear mass is seen. Pain is rare, and may be a sign of malignancy. Some tumors can grow to very large size before they cause any symptoms
Patients with a suspected facial tumor should undergo a complete examination including a hearing test. The facial function needs to be documented. Radiological testing is the most useful. CT scans give the best picture of the bones surrounding the facial nerve and MRI gives the best information about the nerve itself. These two tests are used in conjunction to give a complete description of the nerve and tumor.
The ideal goal of surgery for facial tumors is complete removal of the tumor with preservation or restoration of facial function and conservation of hearing. These goals are often difficult to realize. Facial preservation can sometimes occur with vascular lesions, but is much more difficult to achieve with schwannomas. The timing of surgery is controversial. Young patients with poor facial function should have surgery with some form of facial reanimation. This can be achieved with nerve grafting or hypoglossal to facial nerve grafts.
It is less clear how to manage patients with normal facial function. Some people argue for early resection with reanimation, while others suggest waiting. The argument for early resection is that it is easier and less dangerous for the patient to remove a tumor while it is smaller. Another argument is that if the tumor extends intracranially, it will be more difficult to preserve hearing. Others argue that it is reasonable to wait until the facial tumor causes facial weakness and then perform surgery. These are slow growing tumors, and it may take many years for there to be weakness, if at all. Others suggest that surgery be performed to decompress the tumor and remove all bone around it. This will allow it to grow larger with less likelihood of causing weakness. The best outcome that can be expected from a facial nerve graft is grade III with eye weakness and abnormal movement of the face, although the face looks normal at rest. Some surgeons will wait until there is a grade III function before operating.
Surgical approaches to the facial nerve are determined by the location of the tumor and must be tailored to the individual needs of the patient. The approaches include a middle cranial fossa approach, transmastoid approach, translabyrinthine approach, or combinations of these. The technical aspects of the surgeries are quite detailed, and requires an extensive knowledge of the temporal bone anatomy.