Imaging of the Tympanic Bulla

Radiographic assessment of the bullae can be very helpful in determining the extent of bony involvement and the presence of increased tissue or fluid filling the bullae (see Chapter 2).

The first radiograph is taken using an open-mouth, rostrocaudal view with the x-ray beam directed through the pharynx. The tongue should be pulled rostrally to remove soft tissue that overlies the bullae. If an endotracheal tube is in place, it should be temporarily removed for this view. The procedure requires angling the mandible and maxilla to an angle of 10 to 15 degrees away from perpendicular while maintaining symmetry of the skull.

In a dog with minimal bony changes, the bullae appear as normal, eggshell-thin circular structures medial to the mandibular rami on the rostrocaudal view. The cortical outline is thin and the middle of the bullae radiolucent because the bullae are filled with air. The cat has an air-filled, two-chambered tympanic bulla separated by a bony septum.

When the bulla is chronically affected, either the intraluminal or extraluminal bone shows new bone production or remodeling. Bone lysis may be evident. The cartilage of the external canal may also be calcified and easily visible on a radiograph. Dystrophic mineralization of soft tissue surrounding the ear canal may be present. Often an entire bulla appears radiopaque because of large volumes of thick exudate or tissue growths (neoplasm, polyp, or choleasteatoma) filling the air space. If lytic lesions are present, there may be radiolucent areas or the bone constituting the bulla may be absent, as in squamous cell carcinoma or osteomyelitis. Dogs or cats with recent ventral bulla osteotomy may be identified by the absence of a segment of bone; however, bone may regrow after bulla osteotomy.

If large volumes of flushing solution are infused into the ear canal of a dog with a ruptured eardrum before radiographic assessment, a misreading of the radiograph can occur because the bullae become filled with the flushing fluid and appear radiopaque on the radiograph. One limitation of radiographic evaluation is that old sclerotic lesions in the bulla of aged animals cannot be differentiated from a more current proliferative otitis media lesion.

Computed axial tomography (CAT scan) of the tympanic bullae, where available, may aid in differentiating bony lesions in the bulla from soft tissue reactions. Many universities and teaching hospitals have access to CAT scanners. In the United States, specialty referral centers are acquiring older CAT scanners from human hospitals and may be able to provide this type of radiographic examination.

These radiographs provide views in the horizontal, vertical, and sagittal planes. CAT scans provide a number of views in these planes taken at various distance inter­vals, each successive view slightly farther from the previous. Each successive view may be considered as one slice of bread removed from an entire loaf, with each slice being derived from a different part of the whole.

Even when the ear canal is stenotic and otoscopic examination is impossible, CAT scans are able to give clear impressions of the status of the ear canal distal to the stenosis as well as clues to the pathology in the middle ear. Bony lesions of the bulla can be differentiated from soft tissue lesions using CAT scans. Three­dimensional measurement of the size of a lesion may be estimated by calculating the distance between successive views in different planes.

Magnetic resonance imaging (MRI) of the ear is also being done to visualize the middle and inner ears. MRI can use such fluids as the endolymph within the cochlea and semicircular canals to provide contrast to the examination. Patients with neurologic signs relating to middle- or inner-ear disease can be seen. Extension of infection into the cerebrospinal fluid space and meninges can be detected by MRI. The computer-generated images can be viewed in three planes, as with the CAT scan. At the present time, this technology is available to veterinary medicine on a very limited basis.