Advanced diagnostic techniques are not indicated in all cases of otitis externa. Where otitis media is suspected then further tests become more important. Video otoscopy, which has become widely available to primary care veterinary surgeons as well as within specialist institutions, is useful to visualize the canal and also the tympanic membrane. Other diagnostic modalities that may be used include radiography, CT and MRI. Brainstem auditory evoked responses (BAER) are useful to assess an animal’s hearing; auditory tympanometry can help assess the tympanic membrane and the middle ear.
3.1 VIDEO OTOSCOPY
Video otoscopy is now widely available in primary care veterinary practice. The video otoscope has several advantages over the hand-held otoscope. A conventional hand-held otoscope produces a bright light that is directed down a conical tube; light from the highlighted object is passed back to the user and viewed through a magnifying lens at the eyepiece at the end of the otoscope cone. The two major disadvantages of this type of system are that (1) much of the light is absorbed by the otoscope cone, meaning illumination of the object is often poor, and (2) insertion of instruments down the cone in the form of forceps or flushing tubes obscures the view for the observer.
Video otoscopes (Fig. 3.1) overcome the shortcomings of the hand-held instruments by providing excellent illumination and high levels of magnification that facilitate examination, cleaning and drying of the ear as well as performing minor surgical procedures (Fig. 3.2). All modern video otoscopes have working channels which accept a range of different instruments including flushing catheters, grabbing forceps, fine-bore needles and laser tips (Fig. 3.3). Insertion of a feeding tube or urinary catheter down the working channel allows efficient irrigation and suction of the canal and the middle ear. Similarly, such tubes can be used to undertake myringotomy as part of the investigation of otitis media. Grabbing forceps can be used to remove foreign material including ceruminoliths and take pinch biopsies of lesions in the canal. They can also be used to grasp and remove nasopharyngeal polyps in cats. Fine-bore needles can be used to inject pharmaceuticals such as glucocorticoids into discrete lesions and the wall of the canal to treat hyperplastic change. Laser tips inserted down the canal can be used to remove lesions from the canal or destroy overactive glandular tissue and also to perform myringotomy. Video otoscopes also have the ability to store information. Images from the ear canal can be captured, stored and then printed out (Fig. 3.4). This has the advantage that owners and referring veterinarians can be shown the condition of the ear canal before therapy and the changes in the ear as therapy progresses. Most instruments also have the capability to record video footage which again can be useful to show to clients and also may be used for instruction as a teaching aid.
Radiography is a useful tool in the investigation of ear disease in the dog and cat, but it is not as sensitive a diagnostic tool as CT and MRI. To use it to its full potential accurate positioning of the patient is essential and a comparison of the two sides helps in assessing for abnormalities (Table 3.1). Radiography can be used to assess both the ear canal and the tympanic bulla. The most useful views are the dorsoventral, rostrocaudal (open mouth) and lateral oblique views. Subtle changes in soft tissue density in the middle ear or in the canals can be identified by comparing left and right sides. Bony change in the wall of the bulla such as sclerosis and thickening or lysis, or calcification of the canal, can be identified often without the need for comparison. Although the dorsoventral view can be taken in a sedated animal all of the views are better if the animal is anaesthetized.
The dorsoventral (DV) view is taken with the animal in ventral recumbency (Figs 3.5–3.6). The animal should lie symmetrically with the interpupillary line parallel to the film. The hard palate should be parallel to the table and the base of the skull should be as close to the plate as possible. The beam should be centred at the intersection of two imaginary lines: the first a sagittal line and the second, which should be at right angles to it, a lateral line drawn through the estimated position of the two tympanic membranes.
The ventrodorsal (VD) view is taken with the animal in dorsal recumbency (Figs 3.7–3.8). As in the previous view the animal should be positioned symmetrically with the hard palate parallel to the plate. Animals in this position are generally more unstable than those in the DV view and usually need tape around their jaw behind the mandible to position the head. The beam should be positioned as the DV view.
The lateral oblique (LO) view is taken with the animal in lateral recumbency with the head positioned parallel to the plate (Figs 3.9–3.13). The jaw should be closed and the bulla to be radiographed should be closest to the film. To avoid superimposition of the two bullae the animal’s head is rotated along the long axis until the sagittal plane is 20% to the horizontal. The beam should be centred on the base of the ear.