Objective: To understand the anatomy of Eustachian tube and analyze Micro-Computerized Tomography (Micro-CT) images of tree shrew temporal bone and develop valid animal model for otitis media with effusion.
Methods: To study anatomy of Eustachian tube 10 Tree shrews were dissected under the aid of surgical microscope. General gross and microscopic anatomy of Eustachian tube was studied and only 6 of those Tree shrews were scanned through Micro-CT examination. To develop the animal model for Otitis Media with Effusion (OME) 16 Tree shrews without any inducing factor electro cautery was done on the right pharyngeal orifice of Eustachian tube. All the experimental animals left ear was considered as control group.
Results: Anatomy of Eustachian Tube: Eustachian tube of Tree Shrew is divided into three parts: the membranous part, cartilaginous part and bony portion. Muscles responsible for functioning Eustachian tube found to be two groups; Tensor Veli Palatini muscle (TVP) and Levetor Veli Palatini muscle (LVP). Narrowest portion of the Eustachian tube is at the junction between cartilaginous and bony portion. TVP is composed of 2 groups of muscle fibers, originating from skull base and Eustachian tube. Two groups of muscles form a tendentious intersection hook around the pterygoid hamulus and insert into the soft palate near the posterior border of hard palate. LVP originates from medial lamina of Eustachian tube cartilage near the junction between cartilage and bone, and finally insert into the posterior portion of soft palate. Micro CT scan can clearly show the bony structure of the skull, palate, turbinates, temporal bulla, auditory ossicle, cochlear structures, etc. But for soft tissue display is poorer.
Animal Model of Otitis Media: Otomiscroscopic, tympanometry， pathology and Micro-CT examination showed presence of middle ear effusion in those animals whose right Eustachian tube pharyngeal orifice was blocked permanently using electro cautery .No abnormalities found on the examination of the left ear.
Conclusion：We developed easy way to induce otitis media with effusion in animal model .We proved OME is caused due to the obstruction in the Eustachian tube. Through our anatomical study we found tree shrew Eustachian tube divided into three parts; membranous, cartilaginous and bony parts. The normal function of Eustachian tube depends on two muscles namely TVP and LVP. Substantial similarities were seen between tree shrew and human ears. It would be helpful for researchers to locate structures in nasopharynx and middle ear in further experiments with tree shrew.