Occult otitis media in dogs with chronic otitis externa – magnetic resonance imaging and association with otoscopic and cytological findings

BACKGROUND: Identification of perpetuating factors, such as otitis media (OM), is important for the successful management of canine chronic otitis externa (OE). HYPOTHESIS/OBJECTIVES: Occult OM can occur in cases of chronic OE; a focused magnetic resonance imaging (MRI) examination is a useful tool in their management. ANIMALS: One hundred twenty one client-owned dogs presented for investigation and treatment of chronic OE between 2009 and 2018. METHODS AND MATERIALS: Mixed retrospective (74 dogs) and prospective (47 dogs) study of chronic OE cases without neurological signs, describing the MRI, otoscopic and cytological findings; comparing cases with and without MRI evidence of OM. RESULTS: A total of 123 MRI studies were analysed (two dogs scanned twice). A short, focused MRI scan allowed detection of inflammation of the mucosal bulla lining as well as excellent discrimination between avascular material and vascularised soft tissue in the tympanic cavity. OM was found in 41 of 197 (21%) ears with chronic otitis externa. On otoscopy, the tympanic membrane was intact in six of 41 ears (15%), ruptured in 16 of 41 (39%) and not visible in 14 of 41 (34%) [no data in five of 41 (12%)]. Analysis of cytological findings showed that the presence of rods was only associated with an increased likelihood of OM when found together with inflammatory cells. CONCLUSIONS AND CLINICAL IMPORTANCE: Occult OM is a not uncommon finding on MRI of dogs with chronic OE. A targeted MRI study ("bulla mini-scan") may be useful as part of the clinical investigations.