Named after the anatomist Sir Charles Bell, who first described the facial nerve (cranial nerve [CN] VII), Bell palsy is characterized by acute, idiopathic, peripheral facial paralysis. Bell palsy affects CN VII, a mixed sensory and motor nerve that carries fibers involved in taste, lacrimation, salivation, and sensation of the ear while also innervating the muscles of facial expression. The condition is a common neurologic complaint in both men and women, with an annual incidence of approximately 20-30 cases per 100,000 population.[2,3,4]
The facial paralysis seen in Bell palsy is typically unilateral, with either the right or left side of the face affected with equal frequency. Complete palsy is present in most cases. Bilateral paralysis occurs in less than 1% of patients. Although Bell palsy is thought to be a clinically benign diagnosis that typically resolves over time, as many as 30% of patients fail to recover full functionality. In addition, the temporary facial paralysis of Bell palsy can significantly affect quality of life. It can be disfiguring, affect the ability to eat, and even lead to permanent eye injury owing to an inability to completely close the eye.
Risk factors for Bell palsy include diabetes, hypertension, immunocompromised status, obesity, pregnancy, and recent upper respiratory viral infection. Although the exact cause of Bell palsy remains unclear, potential etiologies include anatomical factors, viral infection, ischemia, autoimmune/inflammatory demyelination, genetic predisposition, and even acute cold exposure.
The anatomical theory posits that CN VII is susceptible to trauma and compression, owing to its long course from the brainstem that abuts many bony structures. Specifically, CN VII emerges from the pons to join the vestibulocochlear nerve (CN VIII) through the internal auditory meatus. CN VII then travels 20-30 mm in the facial canal, exiting through the stylomastoid foramen and passing through the parotid gland, where it divides into its terminal branches that innervate the muscles of facial expression. This course is longer than that of any other cranial nerve, making CN VII particularly vulnerable to anatomical changes.
An etiology that is widely accepted is that the edema and inflammation is caused by the reactivation of viruses that target peripheral neurons, such as herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) and varicella zoster virus.[9,10] Because these viruses can establish latent infection within nerve ganglia, reactivation and subsequent viral-mediated axonal degradation can be incited by immune modulation or immunosuppression. Of note, an inactivated intranasal influenza vaccine has been linked with Bell palsy, possibly owing to reactivation of HSV.[12,13]
Another postulated mechanism for Bell palsy is ischemia. Ischemic insults, such as diabetic microangiopathy or cerebral venous thrombosis, are thought to compromise the vasa nervosum of the facial nerve.[14,15] This may lead to vasospasm, increased capillary permeability, edema, and inflammation, with an end result of perivasculitis and endarteritis. Fibrotic nerve sheath scarring left from this process may result in refractory facial paralysis and may require surgical decompression.
Proposed inflammatory mechanisms for Bell palsy are numerous, including the idea that it may be a mononeuritic variant of Guillain-Barré syndrome, in which the peripheral nerve myelin is attacked by a cell-mediated immune reaction. In addition, Bell palsy is seen frequently during pregnancy and with severe preeclampsia, suggesting that pregnancy-related fluid retention, perineural edema, and resulting inflammation are possible mechanisms.
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Cite this: James Lee, Stephanie Oh, Gaurav Gupta. A 44-Year-Old With a Headache, Photophobia, and Phonophobia - Medscape - Nov 13, 2019.