To elucidate the pathology of facial palsy due to a paralyzed facial nerve of the corresponding severity, an experimenal rat model was prepared by compression of the facial nerve with a pean's forceps for 30 seconds. The electrophysiological and histological changes of the facial nerve in this model were examined. ENoG was performed under general anesthesia 3, 5, 14, 28 and 56 days after induction of facial palsy. The buccinator ramus of the facial nerve proximate to the parotid gland was excised and immediately fixed with glutaraldehyde and osmium tetrodoxide. After dehydraion, it was embedded in epoxy resin for light and electron microscopy.
The mean ENoG value was 13% on the 3rd day, 18% on the 5th day, 27% on the 14th day, 32% on the 28th day and 78% on the 56th day, indicating recovery with time. Light microscopy revealed a reduction of residual myelinated nerve fibers in the beginning, which were reduced on the contralateral side to 33% on the 3rd day, to 16% on the 5th day and to 5% on the 14th day; however, they increased to 28% on the 28th day and 51% on the 56th day. The ratio of myelinated fibers' increased in parallel with the ENoG value but with a 10 day delay. This delay was due to the 10mm distance between the electrically stimulated region and the excised region. Electron microscopy revealed no distinct evidence of degeneration on the 3rd day, but on the 5th day a high density area within the axis cylinder appeared, and degenerative changes such as disappearance of microfilaments and microtubules were observed. On the 14th day, the nerves had almost completely gone with the exception of only myelinated fibers and Schwann cells. By the 28th day these degenerative changes had reversed and myelinated fibers had increased. A numerous myelinated fibers were visible on the 56th day.
These findings indicate that this experimental model of facial palsy corresponding to approximately 13% ENoG is a pathological condition from perfectly which complete recovery can be made. Reinnervation in possible because the surrounding connective tissue in preserved, despite Waller's degeneration.
