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64 DERMATOLOGY DETAILS PEER REVIEWED cases; therefore, it is important to remember that ear cytology or a single swab submitted for C/S may not reveal the total population of organisms truly present in the ear canal. This might explain why, in some cases, sensitivity results (in vitro) and response to topical therapy (in vivo) do not always correlate. This study poses questions regarding the true benefit of C/S for selecting antimicrobials for otic infections; therefore, clinicians should be careful and critical when interpreting otic cytologic and culture results. Clinicians should sample the middle ear separately if otitis media is confirmed when the tympanic membrane is intact. The types and sensitivity pattern of bacteria isolated from the middle ear may differ from those of bacteria isolated from the external canal. 8 In one study, different organisms were cultured from the middle and external ear, and even when Pseudomonas species were cultured twice from the same ear, different strains were suspected on the basis of the sensitivity pattern exhibited. 8 Culture results should be interpreted with caution because mixed bacterial flora and light commensal and contaminant bacteria might be present and may not be relevant as pathogens. Diagnostic Imaging Techniques Dogs with chronic, recurrent, and severe otitis and those with neurologic signs (eg, vestibular signs or facial nerve paralysis), para-aural swelling, or pain on opening the mouth usually require diagnostic imaging to help identify contributing problems, such as middle ear disease (eg, otitis media, neoplasia) and otitis interna, that cannot be identified with regular otoscopy ( Table 2 ). Patients with an apparently normal tympanic membrane may also have otitis media. Although otitis interna is uncommon in dogs with chronic otitis externa, otitis media is common, with a reported incidence of 50% to 88.9%. 8 In dogs with recurrent ear infections of 6 months or longer, up to 89% may have concurrent otitis media; about 70% have an intact but abnormal tympanic membrane. 8 TABLE 2 Advantages and Disadvantages of Diagnostic Imaging Techniques DIAGNOSTIC IMAGING TECHNIQUES ADVANTAGES DISADVANTAGES Radiography • Can detect mineralization and neoplasia in the ear canal and bony changes in the bulla wall • Usually does not require sedation, general anesthesia, or intravenous contrast material • Has limited value in identifying soft tissue changes • Cannot rule out otitis media • Cannot distinguish between fluid and tissue in the middle ear • Not as sensitive as CT and MRI for predicting presence and severity of middle ear disease, such as otitis media Ultrasonography • Relatively quick and noninvasive • Can detect thickening and fluid in tympanic bulla • Usually does not require sedation or general anesthesia • Cannot distinguish between fluid and tissue in the middle ear CT • Provides excellent images of bony structures and can differentiate bony changes in bullae from soft tissue reactions • Can detect presence of fluid in tympanic bulla (ie, otitis media) • Can detect otitis interna, tumors, and meningitis • Requires general anesthesia and administration of intravenous contrast material • Not as sensitive for identifying otitis interna, tumors, and meningitis compared with MRI • Expensive MRI • Better for assessing soft tissue structures of external ear, inner ear, adjacent neural structures, and brain • Can detect fluid in tympanic bulla (ie, otitis media) and otitis interna • Can detect tumors and their specific location, as well as meningitis • May not identify mineralization of the external canals unless it is severe • Requires general anesthesia and administration of intravenous contrast material • Expensive CT, computed tomography; MRI, magnetic resonance imaging.