What is the etiology of Bell's (Bell's) palsy?

From the Guidelines

Bell's palsy is primarily caused by inflammation and swelling of the facial nerve, resulting in facial muscle weakness or paralysis on one side of the face, with a suspected viral etiology, although the exact mechanism is currently unknown. The condition is characterized by acute unilateral facial paresis or paralysis, with onset in less than 72 hours and without identifiable cause 1. The facial nerve carries nerve impulses to muscles of the face and also to the lacrimal glands, salivary glands, stapedius muscle, taste fibers from the anterior tongue, and general sensory fibers from the tympanic membrane and posterior ear canal.

Key Points

• The exact etiology of Bell's palsy remains unclear, but viral infections are believed to be the most common trigger 1.
• Risk factors include diabetes, pregnancy, upper respiratory infections, and immunocompromised states.
• The condition typically presents with sudden onset of unilateral facial weakness, difficulty closing the eye, drooping of the mouth corner, decreased tear production, hyperacusis, and altered taste sensation.
• Most cases (about 70-85%) resolve spontaneously within 3-6 months, though some patients may experience residual symptoms or incomplete recovery 1.

Diagnosis and Treatment

• The diagnosis of Bell's palsy is made by excluding other causes of facial paresis or paralysis through history and physical examination 1.
• Clinicians should prescribe oral steroids within 72 hours of symptom onset for Bell’s palsy patients 16 years and older 1.
• Clinicians should implement eye protection for Bell’s palsy patients with impaired eye closure 1.

Prognosis

• The prognosis for Bell's palsy is generally good, with most patients experiencing complete recovery within 3-6 months 1.
• However, some patients may experience residual symptoms or incomplete recovery, highlighting the importance of prompt diagnosis and treatment 1.

From the Research

Etiology of Bell's Palsy

The etiology of Bell's palsy is believed to be related to the reactivation of a latent microorganism, such as herpes simplex virus type 1 (HSV-1) 2, 3, 4.

• Studies have suggested that HSV-1 may be the causative agent of Bell's palsy, with evidence from laboratory studies and clinical trials supporting this theory 5, 3, 4.
• The detection of HSV DNA in geniculate ganglia and the presence of HSV-specific latency-associated transcripts in control patients provide further evidence for the latent state of HSV in the geniculate ganglia 3.
• Reactivation of HSV genomes from the geniculate ganglia is thought to be the most important cause of Bell's palsy, leading some to suggest that "Bell's palsy" be renamed as "herpetic facial paralysis" 3, 4.

Clinical Evidence

Clinical trials have investigated the efficacy of antiviral agents, such as acyclovir and valacyclovir, in combination with corticosteroids, in the treatment of Bell's palsy 2, 6, 5.

• The results of these trials have been mixed, with some studies showing a significant improvement in outcomes with antiviral therapy 2, 6, while others have found no added benefit 5.
• Early treatment with antiviral agents and corticosteroids, within 3 days of onset, may be necessary for effective therapy and to prevent nerve degeneration 6.

Herpes Simplex Virus and Bell's Palsy

The link between HSV and Bell's palsy has been extensively studied, with evidence from animal experiments and human clinical studies supporting an etiological relationship 3, 4.

• The application of molecular biological techniques, such as polymerase chain reaction (PCR), has provided ample evidence for a causal relationship between Bell's palsy and HSV 3, 4.