Ultrasound accuracy for diagnosing ear infections in cats

Abstract

BACKGROUND: Otitis media (OM) is diagnosed via imaging or, in some cases, otoscopic evaluation, by detecting fluid in the tympanic bulla (TB). In cats, the bulla septum divides the TB into ventromedial (VMC) and dorsolateral (DLC) compartments, with ultrasound restricted to imaging the VMC only. HYPOTHESIS/OBJECTIVES: To evaluate the accuracy and feasibility of ultrasound in diagnosing naturally occurring OM in cats, using computed tomography (CT) or magnetic resonance imaging (MRI) as the reference standard. ANIMALS: Thirty-two privately owned cats (64 ears) with and without OM were enrolled in the study. MATERIALS AND METHODS: In this cross-sectional study, CT or MRI confirmed fluid (OM) or air (normal) in the TB, while ultrasound imaged the VMC for air or fluid. Performance statistics for ultrasound in diagnosing OM were calculated. RESULTS: Bulla ultrasound took an average of 3.5 min to complete and 23 cats were awake. Ultrasound detected air in the VMC in 41 ears, fluid in 22 ears and acoustic shadowing in one ear owing to TB wall thickening, precluding the detection of gas or fluid. Thirty-nine middle ears were air-filled and 25 ears had fluid based on CT/MRI. Two false negatives resulted from undetectable scant fluid lines. Ultrasonographic data of 63 ears (ear with acoustic shadow was excluded) showed the following: sensitivity (92%), specificity (100%), positive predictive value (100%), negative predictive value (95%) and accuracy (97%). CONCLUSIONS AND CLINICAL RELEVANCE: Ultrasound was rapid, well-tolerated and reliably differentiated fluid from air in the VMC, diagnosing OM in most cats. False negatives arose from scant fluid. Acoustic shadowing may represent chronic OM.