Weakness of the seventh cranial nerve, also known as the facial nerve, results in an inability to move the muscles of facial expression on the side of the disease. Bell's palsy is currently defined as an unexplained weakness of the facial nerve. Although the facial appearance can be similar to that caused by Lyme disease, it is believed to be a completely different disease mechanism.
Viral Infection
Although physicians do not know exactly what causes Bell's palsy, it is suspected that it may be a viral infection of the tissues around the facial nerve. The source of the infection can be a reactivation of a prior viral infection, such as the herpes simplex virus. The infection may be of the actual facial nerve itself but is more likely in the tissues around the nerve, affecting the nerve during the inflammatory response.
Swelling of the Facial Canal
The facial nerve travels from the brain, through the ear, on the way to the face, through a very narrow canal. That canal is lined with a thin layer of soft tissue. It is believed that Bell's palsy may be caused by swelling of the tissues around the facial nerve. According to the journal Laryngoscope, there is evidence that swelling around the facial nerve affects the function of the nerve, and surgically relieving that swelling may return the nerve's function.
Facial Nerve Inflammation
Although the mechanism of Bell's palsy is unknown, people with Bell's palsy sometimes have signs of inflammation around the facial nerve visible on an MRI. According to the journal Clinical Pediatrics, the inflammation may be of the nerve itself or the soft tissues in the narrow canal surrounding the nerve. Another clue to this mechanism is that people who receive anti-inflammatory treatment with steroids tend to have a better chance of recovery from Bell's palsy.
References
- Clinical Pediatrics; Facial Nerve Palsy in the Pediatric Population; Josef Shargorodsky, May 2010
- Laryngoscope; Surgical Management of Bell's Palsy; Bruce Gantz, November 1999
- European Archives of Otolaryngology; Management of Peripheral Facial Nerve Palsy; John Finsterer; January 2008
