Facial nerve paralysis is a condition characterized by paralysis of one or all of the branches of the facial nerve. Bell’s palsy is unilateral weakness of the muscles of the face. It occurs when there is an injury to the seventh cranial nerve leading to drooping of one half of the face. It is also known as idiopathic facial paralysis.
The symptoms are usually acute appearing within 48 hours and they include lower facial paralysis on one side of the face, reduced secretion of tears, posterior auricular pain, increased sensitivity to sound (hyperacusis), disorders in taste, pain in the ear, inability to close the eye lids, facial muscle weakness, eye pain, blurred vision and epiphora .
The diagnosis of Bell’s palsy is usually made based on history and physical examination. However, investigations may be needed to exclude other conditions.
Other investigations that can be carried out include EMG, nerve conduction tests, blepharokymography and histology.
Most patients who suffer from this condition often recover spontaneously without treatment. However, treatment might be needed in some patients and also to accelerate recovery. Medical treatments include the use of corticosteroids, antiviral agents and a combination of the two.
The eye should also be protected with the use of artificial tears and ointments. They should be applied to make up for the lack of lacrimation. The use of external eyelid weights have also proven to be beneficial.
There is a place for physical therapy like facial exercises, neuromuscular retraining, and acupuncture in the treatment of Bell’s palsy . Surgeries to manage this condition, though not commonly advocated, include facial nerve decompression and eyelid implants.
Close to 90% of patients recover in 3 months or less without any apparent disfigurement. There are three groups that the patients fall into based on their prognosis. The first group are those who recover completely from the lesion without any long-term complications and this is the group most patients fall into. The second group does not recover completely but they appear completely recovered to the inexperienced eye, most people in this group have had axonotmesis with interruption of the axons. The last group are those with permanent neurological damage, visible even to the inexperienced eye, and this group is made up mostly by the elderly .
Factors associated with poor prognosis include advanced age, complete paralysis and reduced taste or salivation on the affected side. Other factors are pain in the posterior auricular area and decreased lacrimation.
The cause of Bell’s palsy has not been fully understood, however, some factors have been associated with increased likelihood of developing the disease. The most implicated factor is exposure to cold like in chilly weathers and air-conditioned environments. Others include infection with herpes simplex virus, herpes zoster, Lyme disease, HIV, Epstein-Barr virus, cytomegalovirus, syphilis and mycoplasma. It could also result from microvascular disease like diabetes and hypertension and from other inflammatory causes, viral diseases and autoimmune diseases .
Bell’s palsy has an annual incidence of between 0.015% and 0.03%. It is most commonly seen during the winter months. It is the commonest cause of unilateral facial paralysis and it occurs more on the right side of the face. Bell’s palsy affecting both sides of the face is rare. It also has a higher incidence in people who have underlying medical conditions like diabetes, HIV and preeclampsia.
The incidence of Bell’s palsy increases with advancing age with the highest incidence seen in patients 65 years and over. It affects both male and female sexes equally, and there is no clear difference in the incidence for different races .
The exact way by which this condition occurs is still unclear. However, it is mostly agreed that due to ischemia and edema, there is compression of the facial nerve within the facial canal. Thee facial canal is a small hole in the temporal bone which the facial nerve goes through during its course. The exact way by which this edema or inflammation comes about is unclear.
The commonest site of compression of the facial nerve is at the meatal foramen which has a diameter that is less than 0.7mm. The labyrinthine segment, the first portion of the facial nerve goes through this foramen. Due to its constricted limits, it is rational that processes such as inflammation, compression, ischemia and demyelination will weaken nerve conduction at this site .
Facial nerve injury in Bell’s palsy usually occurs near or at the geniculate ganglion, peripheral to the nerve’s nucleus. There is associated autonomic and gustatory abnormalities accompanying the motor paralysis if the injury occurs proximal to the geniculate ganglion. The same effects are seen if they occur between the geniculate ganglion and the origin of the corda tympani, but the lacrimal glands are spared. If it occurs at the stylomastoid foramen, there is only facial paralysis.
The cause of Bell’s palsy is unclear but the following factors have been known to contribute to development of the condition. They are exposure to cold, herpes simplex virus, herpes zoster, HIV, cytomegalovirus, sarcoidosis, Lyme disease and Epstein–Barr virus.
Signs and symptoms
Symptoms of Bell’s palsy mainly affect one half of the face and they include drooping of the face, drooping of the eyelid, altered taste sensation, lack of tears in the eye, increased sensitivity to sound, drooling, inability to make facial expressions like smiling .
Diagnosis of Bell’s palsy is mainly made one the bases of the presenting symptoms, although some tests might be needed to exclude other conditions. Such tests include blood tests for infection, glucose, HIV and syphilis. Imaging tests like CT scan and MRI may also be required. Some nerve conduction tests may also be carried out.
Treatment involves the use of corticosteroids, antivirals, artificial tears and ointments. Physical therapies to train the muscles of the face are also beneficial. In some cases, surgery on the facial nerve or the eyelids would be beneficial  .