Morphometrics of the facial nerve following fractionated external irradiation: an experimental study in rats.

UNLABELLED This study provides morphometric data on radiation-induced alterations of the facial nerve in female Wistar rats. The facial nerves were explanted 3 to 4 or 7 to 9 months after completion of a fractionated external irradiation of the left side of the neck (5 days per week, 6 weeks, total: 60 Gray). Both facial nerves were investigated in order to identify possible effects of scattered irradiation on them. A total of 54 animals were investigated and 11,193 measurements were obtained. The computer-assisted image analysing system CUE-3 Color Image Analyzer (Olympus, Japan) was used. Facial nerves from untreated animals served as controls. RESULTS Three to four months after irradiation, the ratio of axon area and total area of the cross-sectioned nerve was shifted towards the axon area. Seven to 9 months after completion of irradiation, both the myelin sheath and the axon were reduced in diameter. The loss of substance affected predominantly the myelin sheath. Peripheral nerves are radiosensitive. Seven to 9 months after irradiation there was an increase of the axon area compared to the total area, both in absolute values and in relative terms. Radiosensitivity of peripheral nerves can be measured morphometrically in suitable animal models. The measurements disclosed temporal and spatial patterns of radiation response of the nerves. These results may partly explain the increased radiosensitivity of the peripheral nervous system observed in the long-term follow-up after completion of radiotherapy of head and neck in humans.