Cause
The cause of Bell's palsy is unknown.
[1] Risk factors include
diabetes, a recent
upper respiratory tract infection, and
pregnancy.
[1][5]
Some viruses are thought to establish a persistent (or
latent) infection without symptoms, e.g., the
varicella zoster virus[15] and the
Epstein–Barr virus, both of the
herpes family.
Reactivation of an existing (dormant) viral infection has been suggested as a cause of acute Bell's palsy.
[16] This new activation could be triggered by trauma, environmental factors, and metabolic or emotional disorders.
[17]
Familial inheritance has been found in 4–14% of cases.
[18] There may also be an association with
migraines.
[19]
Pathophysiology
Bell's palsy is the result of a malfunction of the
facial nerve (
cranial nerve VII), which controls the
muscles of the face. Facial palsy is typified by inability to move the muscles of facial expression. The paralysis is of the infranuclear/lower motor neuron type.
It is thought that as a result of
inflammation of the facial nerve, pressure is produced on the nerve where it exits the skull within its bony canal (the
stylomastoid foramen), blocking the transmission of neural signals or damaging the nerve. Patients with facial palsy for which an underlying cause can be found are not considered to have Bell's palsy
per se. Possible causes of facial paralysis include
tumor,
meningitis,
stroke,
diabetes mellitus,
head trauma and inflammatory diseases of the cranial nerves (
sarcoidosis,
brucellosis, etc.). In these conditions, the neurologic findings are rarely restricted to the facial nerve. Babies can be born with facial palsy.
[20] In a few cases, bilateral facial palsy has been associated with
acute HIV infection.
In some research, the
herpes simplex virus type 1 (HSV-1) has been identified in a majority of cases diagnosed as Bell's palsy through endoneurial fluid sampling.
[21] Other research, however, identified, out of a total of 176 cases diagnosed as Bell's palsy, HSV-1 in 31 cases (18 percent) and
herpes zoster in 45 cases (26 percent).
[16]
In addition, HSV-1 infection is associated with
demyelination of nerves. This nerve damage mechanism is different from the above-mentioned—that edema, swelling and compression of the nerve in the narrow bone canal is responsible for nerve damage. Demyelination may not even be directly caused by the virus, but by an unknown
immune response.
Facial nerve: the facial nerve's nuclei are in the
brainstem (represented in the diagram by "θ"). Orange: nerves coming from the left hemisphere of the brain, yellow: nerves coming from the right hemisphere. Note that the forehead muscles receive innervation from both hemispheres (yellow and orange)