Topical anesthesia of the tympanic membrane : an experimental animal study

Myringotomy plays an important role in otological therapy. The procedure requires an efficient anesthesia, which can be obtained without general anesthesia. However, the use of local anesthetics on the tympanic membrane (TM) has been abandoned in many places, as general anesthesia has been readily available. In the present study the effects of some commonly used topical anesthetics on the TM structure and inner ear were tested in an animal model (rat and guinea pig).Four different anesthetic compounds—Xylocaine®, Bonain's liquid, phenol and Emla®—were applied to the TMs of the animals, which were sacrificed 10 minutes to 5 months after application. Morphological effects regarding time after treatment and number of applications were elucidated. At sacrifice the tissue was fixed and the TMs analysed by light microscopy (LM) and transmission electron microscopy (TEM). In nine animals phenol, Xylocaine® Spray or Emla® was applied to the round window niche and ABR recordings were made at 24 h to 6 months after exposure. After the final ABR evaluation the animals were sacrificed and the cochleae prepared for LM and scanning electron microscopy (SEM).On the TM phenol and Bonain's liquid caused instant destruction of the keratinizing stratified squamous epithelium followed by long-lasting hyperplasia of this epithelium and the underlying connective tissue. A pronounced hyperplasia of these two layers was also noted for the Xylocaine® Spray group, but without immediate destruction of the keratinizing epithelium. The extent of structural changes differed in relation to the extent of spreading of the agent. Emla® showed little, if any, sign of epithelial reaction and had no effect on the connective tissue. Regarding the inner ear Emla®, Xylocaine® Spray and phenol induced significantly impaired ABR thresholds mainly affecting the higher frequencies. However, the impaired ABR thresholds were reversible and at the end of the experiment there was no significant impairment compare
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