The relationship between Bell's palsy and morphometric aspects of the facial nerve.

Epidemiological data of Bell's palsy (BP) have been reported. For example, the annual incidence of BP is 15-30 per 100,000 persons, with equal numbers of men and women affected, and there is no predilection for either side of the face. However, details of the relationship between BP and morphometric aspects of the facial nerve have not been available in textbooks. We performed a morphometric analysis of human facial nerve fibers and estimated the total number of myelinated axons (TN) and average transverse area of myelinated axons (ATA). The facial nerve showed a significant decrease of TN with increasing age (r = -0.77; p < 0.01), but showed no significant changes of ATA with age (r = -0.01; p = 0.96). We supposed that the TN decrease with age was a factor in the delayed recovery from BP seen in the elderly. Moreover, the TN and ATA showed no significant differences between female and male specimens (p < 0.05), or between the right and left side specimens (p < 0.05). Our present results seem to explain the absence of significant sex and affected side differences in BP.