Neurodegeneration of the mastoid segment of the human facial nerve associated with senescence

OBJECTIVE: To describe and analyze the microscopic morphometry of the mastoid segment of the human facial nerve in the region of the stylomastoid foramen, in different age groups of elderly individuals. METHODS: Eighteen blocks of temporal bones from cadavers aged between 63 and 94 years old were used and divided into 3 age groups: 60-70, 71-80 and over 80-years-old. The nerve was collected, fixed in a glutaraldehyde and paraformaldehyde solution and then subjected to dehydration and resin embedding processes. Semi-thin cross-sectional sections (1 μm) were obtained using an ultramicrotome and stained with toluidine blue. Histological images were captured and analyzed using Zen 2.6 software. The variables were described by mean and standard deviation and compared between age groups (SPSS; p < 0.05). RESULTS: The Cross-Sectional Area (CSA) of the nerve, diameter of the myelinated fiber, CSA of the myelinated fiber, thickness of the myelin sheath and percentage of total area occupied by myelinated fibers were lower in the groups with older ages, with significant differences among all age groups (p < 0.001). The total number of myelinated fibers and density remained constant and did not present significant differences between age groups. CSA and axon diameter decreased at older ages, with significant differences only between the second and third groups (p < 0.001). The degree of myelination (g ratio) and the percentage of area occupied by the endoneurium, unmyelinated fibers and degenerating tissue were higher in the older age groups (p < 0.001). In the older age groups, there was an increase in the percentage of myelinated fibers with lower CSA and a reduction in the percentage of myelinated fibers with higher CSA. CONCLUSION: The results indicate that the facial nerve presents morphometric variations suggestive of neurodegeneration, associated with aging. LEVEL OF EVIDENCE: Level V.