Radiography

The superimposition of structures in skull radiographs makes this region difficult to assess. Good positioning of the head is essential to enable interpretation, therefore general anaesthesia and the use of positioning aids is recommended for radiography of the head. Radiographs should be repeated until good positioning is achieved. Diagnosis of disease should not be made on the basis of a subtle finding observed on a malpositioned radiograph.

The dorsoventral (or ventrodorsal) and rostrocaudal open-mouth views are extremely useful because they enable comparisons between left and right, which can aid recognition of abnormalities. As not all animals are symmetrical, it is not uncommon to observe one bulla that is larger than the other. In each of these views, the tympanic bullae are superimposed on other parts of the skull, which complicates the interpretation. In case of ambiguous findings, additional projections that minimise superimposition are the left 20-degree ventral-right dorsal and the right 20-degree ventral-left dorsal oblique views. In the cat, the rostro-10-degree ventral-caudodorsal projection (closed mouth) is an alternative to the rostrocaudal (open mouth). Some consider it easier to acquire and it is useful in an animal that cannot open its mouth. A lateral view of the skull should always be included when examining the ears, being particularly useful to examine the nasopharynx.

Computed X-Ray Tomography

Computed tomography (CT) is based on the same physical principles as radiography and produces images that resemble radiographs of sections of the body. A major advantage of cross-sectional images is the ability to examine structures without the confusing effect of superimposition. CT images of the ears are usually acquired in the transverse plane using thin contiguous slices (1.5 mm) and a high-resolution reconstruction algorithm. Patients are usually placed in sternal recumbency with the head supported in a padded trough that facilitates stable and symmetrical positioning. Patients for head CT are either sedated or anaesthetised.

Magnetic Resonance Imaging

Magnetic resonance (MR) imaging is based on radio signals emitted by hydrogen atoms in the body under the influence of a powerful external magnetic field. MR imaging can detect biochemical and biophysical changes secondary to disease, therefore, produces images very different from radiography or CT. Patients for MR imaging are usually positioned in dorsal recumbency and the images are acquired in the transverse, dorsal and sagittal planes. A basic MR imaging protocol for an animal with suspected ear disease would include T1-and T2-weighted transverse images of the head, including the brain, plus additional T1-weighted images after intravenous administration of gadolinium, which acts as a contrast agent.

Ultrasonography

Ultrasonography is a novel method for imaging the ear. A 6.5 MHz curvilinear transducer is suggested for this particular region. For studies of the external auditory canal, the transducer may be placed on the lateral aspect of the head; for examining the bulla, it is placed on the ventral aspect with the patient in sternal recumbency. In a normal animal, the air within the external and middle ear effectively blocks ultrasound transmission.

Imaging Findings in Ear Disease

In animals with otitis externa there may be accumulation of secretions and exudate in the ear canal and thickening of the aural mucosa; these lesions may be visible using any of the imaging modalities. Chronic otitis externa may result in dystrophic calcification or ossification of the external ear cartilages; however, because calcification is an irreversible process, its presence does not necessarily indicate that there is active inflammation. Uncomplicated otitis externa does not usually require any imaging, unless there is some doubt about the condition of the tympanic membrane and middle ear.

When it is not possible to examine the tympanic membrane adequately by otoscopy, it may be assessed radiographically by infusing contrast medium into the external ear canal. This technique, known as canalography, involves dorsoventral and rostrocaudal (open-mouth) views of the skull after infusion of 2-5 ml of non-ionic water-soluble iodinated contrast medium per ear. To reduce the number of false-negatives, the ear canals should be cleaned with saline solution prior to canalography. Finding contrast in the bulla proves that the membrane is ruptured. It is easier to recognise small amounts of contrast medium in the tympanic bulla if a comparison with the survey radiographs is made. Canalography is a simple contrast study that is particularly useful in animals with suspected otitis media but no survey radiographic signs of middle ear disease.

In mild or acute cases there may be no imaging signs of disease, but when otitis media becomes chronic, there is usually accumulation of exudate or granulation tissue within the tympanic bulla and/or thickening of the bulla wall. These abnormalities may be visible using any of the available imaging techniques. Radiography is used most often for examining the middle ear in dogs and cats; however, it is relatively insensitive--up to 25% of patients with otitis media have no radiographic signs of disease. CT and MR imaging are both more sensitive than radiography for examining the middle ear. A recent study indicates that the combination of radiography and ultrasound can provide a more accurate assessment of the bulla than either of them alone. Ultrasonography is an attractive technique for clinical use because it is relatively quick, non-invasive and may be used on conscious patients; however ultrasonographic findings are very dependent on the skill of the operator. In experienced hands, ultrasonography was inferior to radiology for evaluation of the middle ear; in inexperienced hands, this technique is considered unreliable.

It is common to observe material, usually fluid, in the middle ear of dogs with no clinical signs of otitis. Accumulation of fluid in the middle ear can occur as a result of Eustachian tube dysfunction, secondary infection of middle ear effusion then leads to otitis media. Brachycephalic dog breeds are predisposed to fluid accumulation in the middle ear, probably because these dogs have some degree of pharyngeal and/or Eustachian tube dysfunction. Eustachian tube dysfunction can also occur in dogs with lesions affecting the trigeminal nerve and dogs with cleft palate, which predisposes them to otitis media. Although otitis media often occurs by direct extension of otitis externa through a ruptured tympanic membrane, some affected dogs have an intact tympanic membrane.

Low-grade chronic inflammation stimulates a relatively orderly remodelling of the bone forming the wall of the bulla, which frequently results in fairly uniform thickening and sclerosis. Another possible cause of thickening of the bulla is temporomandibular osteopathy, which may be localised to the bullae rather than the mandible.

Otoliths appear as well defined, round or ovoid, mineral opacities within the tympanic bulla, sometimes associated with thickening of the bulla wall. Their aetiology is unknown, but their presence may be a sign of current or previous chronic otitis media. Hypothetically they may represent dystrophic mineralisation of inflammatory polyps or heterotopic bone formation.

In cats with signs of otitis media, it is important to examine the nasopharynx for possible nasopharyngeal polyp, which typically appears as a rounded intraluminal soft tissue mass that displaces the soft palate ventrally.

The inner ear, which is located within the petrous temporal bone, is not visible radiographically as a distinct structure because of superimposition of the surrounding structures. Diagnosis of otitis interna cannot be made on the basis of survey radiographs or ultrasonography. CT and MR imaging enable more detailed examination of the inner ear. Of these, MR imaging is probably most useful in animals with vestibular syndrome because it enables optimal examination of both the inner ear and adjacent neural structures. Fibrosis associated with chronic otitis interna may obliterate the spaces in the membranous and bony labyrinths, hence diminishing the MR signal from endolymph and perilymph--T2-weighted images are used for this. Acute labyrinthitis may produce contrast enhancement within the membranous labyrinth in post-gadolinium T1-weighted images.

Central nervous system complications of otitis media/interna are recognised as a serious problem in animals. Alteration in extracranial soft tissue structures, nerves, meninges and brain parenchyma are less frequently identified in CT images and may exist in the presence of normal images. MR imaging with intravascular contrast administration is the modality of choice when intracranial extension of otitis media/interna is suspected.

Neoplasms arising from aural structures are usually locally invasive and should be suspected when bone destruction and periosteal reaction are observed affecting the tympanic bulla or adjacent structures, including the temporomandibular joint. Obliteration or displacement of the external ear canal and soft tissue swelling of the adjacent regions are other signs that suggest the presence of an aural or para-aural soft tissue mass.

Trauma affecting the tympanic bullae is relatively uncommon because of their protected position, but direct trauma can result in fracture.

Summary

Radiography remains a popular and useful method for examining dogs and cats with signs of ear disease, but increasing experience in CT and MR imaging has shown that these cross-sectional techniques have advantages. CT appears to be the optimal technique for examining the middle ear, but it has not been directly compared to MR. MR imaging is the most sensitive imaging modality for examining the inner ear and adjacent neural structures.

References

1.  Benigni L, Lamb CR. Diagnostic imaging of ear disease in the dog and cat. In Practice 2006; 28: 122-130.

2.  Bischoff MG, Kneller SK. Diagnostic imaging of the canine and feline ear. Veterinary Clinics of North America: Small Animal Practice 2004; 34: 437-458.

3.  Doust R, King A, et al. Assessment of middle ear disease in the dog: a comparison of diagnostic imaging modalities. Journal of Small Animal Practice 2007; 48: 188-192.

4.  Garosi LS, Dennis R, Schwarz T. Review of diagnostic imaging of the ear diseases in the dog and cat. Veterinary Radiology & Ultrasound 2003; 44: 137-146.

5.  Rohleder JJ, Jones JC, et al. Comparative performance of radiography and computed tomography in the diagnosis of middle ear disease in 31 dogs. Veterinary Radiology and Ultrasound 2006; 47(1): 45-52.