Segmental herniation of the facial nerve after epineural incision is regarded as proof that idiopathic Bell's palsy is due to elevated pressure caused by anoxia secondary to local vasospasm. However, epineural biopsies—taken at decompressions—have failed to reveal edema or other lesions. The nerve itself was observed to bulge through its entire intratympanic course. Idiopathic Bell's palsy often shows epidemiologic features and sometimes follows viral infections. Diabetes, especially latent diabetes, and arteriosclerosis are often associated with Bell's palsy—and could account for hemorrhages reported at the geniculate ganglion region. Bell's palsy is probably not one pathological entity—among its plausible causes are viral infections and vascular assaults both above the tympanic part of the facial nerve. The classical entrapment theory still awaits confirmation especially as myelin bulging is a normal phenomenon which becomes more pronounced in reaction to various injuries which are not necessarily local intratympanic ones.
Sadé J. Pathology of Bell's Palsy. Arch Otolaryngol. 1972;95(5):406–414. doi:10.1001/archotol.1972.00770080642003