[Morphological changes in the stria vascularis and hair cells after mastoid-vibration using a cutting bur]

Severe sesorineural deafness develops occasionally after tympanoplasty. Since a vibration induced by the drill during surgery is thought to be one of the cause, we examined its effect on the inner ear in this study. Five guinea pigs aged about 1 month were used. The right ear of each animal was used as the control, and the left ear underwent stimulation with the drill induced vibration. The vibration was applied to the temporal bone for 60 seconds using a cutting bur. After the vibration, the stria vascularis and hair cells in the cochlea were examined. For the investigation of the permeability of the stria vascularis, horseradish peroxidase (HRP) was injected intravenously as a tracer, and the tissue was observed with a light and an electron microscopes. Three sections of the stria vascularis from each of the superior turn (the third turn) and the inferior turn (the basal turn) of bilateral cochleae were randomly prepared, and they were classified into Types I to IV according to the degree of HRP leakage. When a hair cell had even one vacuole inside, it was classified as a vacuole-positive cell. After stimulation with the drill-induced vibration, about 10% of the blood vessels of the stria vascularis showed severe leakage of HRP (Types III, IV). In the intermediate cells, partial degeneration of mitochondria was found. No significant difference in the permeability of the blood vessels was found between the turns. The route of leakage of HRP from blood vessels of the stria vascularis was neither the transport via pinocytotic vesicle nor passage through the intercellular space of endothelial cells. The leakage through a tubular-like structure in the endothelial cells was observed. Since no HRP was found in this tubular-like structure ordinarily, a channel might have been opened by injury due to the vibration. The leakage from the blood vessels of the stria vascularis in the control side could scarcely be observed. Many hair cells in the drill-stimulation side in both the first and the third turns showed vacuoles with loss of intracellular structure. Slight loss of auditory hairs was found in this study, suggesting that the development of a vacuole was the first reaction to the drill-stimulation. Since the sensitivity toward stimulation of the drill-induced vibration was different among individual animals, it appears possible that the inner ear is injured in some cases without direct touch to the auditory ossicles owing to technical failure.