Computed Tomographic Grading of Ear Diseases in Guinea Pigs ( <i>Cavia porcellus</i> )

Ear disease affecting the external and middle ears is a significant health concern in guinea pigs, which can be clinically silent and, if left untreated, can lead to severe and irreversible complications. Computed tomography (CT) is the primary imaging modality used to diagnose otitis in other species, particularly otitis media (OM). This study aimed to develop a CT-based grading scale to assess the severity of CT changes in guinea pigs with otitis. As a prerequisite to this, a detailed, subjective CT-based anatomical characterization of the normal guinea pig ear was conducted to establish a foundational reference framework essential for the construction and application of the grading system. The study included head CT of 60 guinea pigs from 2 institutions. After reviewing normal CT anatomy, a novel grading system was created for otitis externa (OE) and OM, with a scale of I-II for OE and I-V for OM, and applied to the guinea pig population by five reviewers. The level of agreement among reviewers was statistically assessed, revealing an excellent intra-observer agreement and a moderate inter-observer agreement between evaluators, with higher reliability for OM (κ = 0.84) than OE (κ = 0.75). This novel CT-based grading scale provides a systematic approach for evaluating the severity of ear disease in guinea pigs. However, the complexity of guinea pig ear anatomy and the variability in the structure of the tympanic bullae among healthy individuals can make grading particularly challenging.