Myringotomy in dogs: Contamination rate from the external ear canal - a pilot study

Myringotomy for sample collection from the middle ear cavity for cytology and bacterial culture is considered a routine method to diagnose otitis media in dogs. The objective of this study was to determine the rate of contamination of middle ear aspirates with material from the external ear canal obtained by video-otoscopic guided myringotomy. In canine cadavers (n = 17) free from otitis externa the external ear canals were flushed under video-otoscopic control and a fluorescent dye was instilled. After removal of residual fluid a myringotomy was performed. If air was aspirated, 1 mL of saline was instilled through the same myringotomy needle into the middle ear cavity and re-aspirated. Contamination from the external ear canal was demonstrated by positive fluorescence of the aspirate. Bacterial cultures and cytological examinations of the external ear canals and middle ear cavities were performed. Data from 28 ears under investigation were included. In 19 of 28 middle ear aspirates (67.9%), clear yellow fluorescent fluid was obtained, indicating a contamination from the external ear canal. Microorganisms were detected in 4 of 26 middle ear samples (15.4%) and in 15 of 26 external ear canals (57.7%). Sample collection by myringotomy in this study was associated with a high contamination rate, implying that the suitability of this method for detection of otitis media in patients with concurrent otitis externa is questionable. Furthermore, the potential for iatrogenic spread of pathogenic microorganisms into the middle ear cavity needs to be considered.