Short-term effects of local anaesthetic agents on the structure of the rat tympanic membrane.

Four different local anaesthetic agents were tested on the tympanic membrane of the rat in order to detect any structural changes. Specimens were obtained for examination under the light microscope 10 min, 24 h, 1 week, and 2 weeks after exposure to each anaesthetic. The histological changes observed were, in general, rather similar for all agents tested, although certain differences were noted. The initial change observed was damage of the outer epithelial layers, later also accompanied by changes in the corresponding endothelial cover. No perforations were observed macroscopically. The healing process was characterized by hyperplasia of the connective tissue layers of the drum. However, the most prominent feature of this process was the hyperplastic epithelium, ingrowing from the periphery of the membrane to cover denuded areas. During certain stages of the healing process the thickness of the membrane amounted to 40 times that of the normal drum. The initial changes appeared severe both with the clinically more potent and effective anaesthetics (phenol and Bonain's liquid), and with Xylocaine spray.