Computed Tomographic Features of the Osseous External Ear Canal, Tympanic Membrane, and Tympanic Bulla in Clinically Normal Horses

• The wave-like tympanic membrane coursed in a flat angle from dorsolateral to ventromedial.
• The three-dimensional shape of the osseous external ear canal (OEEC) was an oval, truncated cone.
• Visibility of small structures was improved with 0.6-mm slice thickness.
• Volume of the OEEC may be influenced by breed and body weight.
• Determined reference values may aid diagnosis of aural diseases and adjustment of audiometric devices.

Only sparse anatomic and morphometric information exists concerning the equine acoustic organ. The aim of the present study was, therefore, to describe the computed tomographic (CT) anatomy and determine the CT dimensions of the osseous external ear canal (OEEC), tympanic membrane (TM), and tympanic bulla (TB) in clinically normal horses. Computed tomography of the head was performed in 23 horses, presented for reasons unrelated to the auditory organ, with a 4-slice (group 1) or a 40-slice (group 2) CT scanner, respectively. The following measurements of the OEEC were performed: volume, dorsal, ventral and central length, halfway of central length, and inner diameter at four different locations. Length of the TM, width of the hypotympanon and the largest width and height of the external acoustic porus (EAP), and the TB were also recorded. The OEEC in all ears was of an oval, truncated cone shape running in a dorsolateral to medioventral direction. The TM coursed at a flat angle from dorsolateral to medioventral and had a wave-like appearance on transverse CT images. Significantly lower width and height of the EAP, inner diameter of the OECC at its narrowest point, and OEEC volume were found in group 1. A significant positive correlation between OEEC volume and head length or body weight, respectively, was found in group 2. Results of the present study may serve as reference values and aid the diagnosis of aural abnormalities and disease conditions. Moreover, they provide a basis for the adjustment of audiometric testing devices to horses.