During regrowth, nerves are generally able to track the original path to the right destination—but some nerves may sidetrack leading to a condition known as synkinesis. For instance, regrowth of nerves controlling muscles attached to the eye may sidetrack and also regrow connections reaching the muscles of the mouth. In this way, movement of one also affects the other. For example, when the person closes the eye, the corner of the mouth lifts involuntarily. Around 9% of people have some sort of ongoing problems after Bells palsy, typically the synkinesis already discussed, or spasm, contracture, tinnitus or hearing loss during facial movement or crocodile-tear syndrome. This is also called gustatolacrimal reflex or Bogorads syndrome and results in shedding tears while eating. This is thought to be due to faulty regeneration of the facial nerve, a branch of which controls the lacrimal and salivary glands. Gustatorial sweating can also occur. Epidemiology
The number of new cases of Bells palsy ranges from about one to four cases per 10,000 population per year. The rate increases with age. Bells palsy affects about 40,000 people in the United States every year. It affects approximately 1 person in 65 during a lifetime. A range of annual incidence rates have been reported in the literature: 15, 24, and 25–53 (all rates per 100,000 population per year). Bells palsy is not a reportable disease, and there are no established registries for people with this diagnosis, which complicates precise estimation. Frequency
About 40,000 people are affected by Bells Palsy in the United States every year.
There may also be an association with migraines.In December 2020, the U.S. FDA recommended that recipients of the Pfizer and Moderna COVID-19 vaccines should be monitored for symptoms of Bells palsy after several cases were reported among clinical trial participants, though the data were not sufficient to determine a causal link. Genetics
A meta-analysis of genome-wide association study (GWAS) identified the first unequivocal association with Bells palsy. Pathophysiology
Bells 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 Bells 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. 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 Bells palsy through endoneurial fluid sampling.