Abstract

Rabbits can now be provided with the same standard of diagnostic investigation as other companion species. As diagnostic imaging commonly plays a key role in clinical investigation, practitioners should now consider computed tomography (CT) invaluable particularly if respiratory tract, dental, or ear disease is involved. Availability, cost, and requirements for anesthetics have now made it accessible to the pet practitioner. In addition, many rabbits can be scanned without the need for a general anesthetic or sedation, making it an advantageous investigation tool for the both the patient and the clinician.

Introduction

With continued advancements in rabbit medicine and surgery, it is now expected that rabbits should be provided with the same standard of patient care, diagnostic investigation and treatment options as other companion species. Diagnostic imaging commonly plays a key role in clinical investigation, with ultrasonography and radiography the most commonly used imaging modalities in general practice. Computed tomography (CT) scanning supersedes both for many disease investigations, including respiratory tract disease, dental disease and ear disease. It has also been used to assess abdominal as well as musculoskeletal disease. Availability, cost and requirement for anaesthetic have previously made it inaccessible to many. Now, CT scanning is more widely available, not only in referral institutes, but also in many privately owned clinics.

Rabbits are a prey species that can be skilled at masking clinical signs of disease. Subtle changes, indicative of disease, are often overlooked by owners. It is not uncommon therefore for a rabbit to present for veterinary assessment only once significantly debilitated. The ability to perform conscious CT scans, without need for a general anaesthetic or sedation, is therefore advantageous for both the patient and the clinician.

Respiratory Tract

In general practice, radiography is the routine imaging modality used to assess the respiratory system, particularly the lower airways. In rabbits, however, radiography could be considered to have low sensitivity for accurate diagnosis of non-advanced disease, as it is common for early pathological changes to be overlooked.

Respiratory disease is a major cause of morbidity and mortality in the rabbit patient. Rabbits are obligate nasal breathers; therefore, the presence of upper respiratory exudates or obstructions can be very serious. Chronic rhinitis can lead to destruction of the ventral nasal concha (turbinates). The turbinates are mucosa lined and contain the vomeronasal organ and olfactory epithelium, which gives rabbits their acute sense of smell. CT offers an excellent level of accuracy for diagnosing sinusitis, including localisation of the recesses involved. While endoscopy can be used to visualise the nasal passages, patient and scope size can be limiting factors. Endoscopy is likely to be more rewarding when guided by CT findings to a defined location. Conventional radiography is highly likely to miss subtle disease of the upper respiratory tract.

Previously, for suspected tracheal pathology, direct visualisation with endoscopy performed under general anaesthetic was required. CT offers a non-invasive option for assessment of tracheal lumen size. Tracheal diameter is dependent on weight of the rabbit, (reference ranges of 4.7–5.9 mm reported). For cases with suspected tracheal stenosis or stricture, CT can offer a helpful pre-cursor step, to identify if endoscopy is required or if pathology elsewhere in the respiratory tract is causing the clinical presentation.

The thoracic cavity is small, relative to the size of the animal.¹ Breathing is mainly diaphragmatic and resting respiratory rates, (30–60 breaths per minute), are faster than those of dogs and cats. The lung lobes are not divided into lobules and, as such, pneumonia is rarely localised as it may be in other species. Subclinical bronchopneumonia appears to be a common CT finding.² Intrathoracic fat is often present, particularly in the mediastinum and can be difficult to differentiate from the thymus which persists into adulthood in rabbits. Use of an intravenous contrast agent is a necessity for clearer definition of soft tissue structures. CT offers a fast and effective way of diagnosing lung pathology in this species.

The level of accuracy offered with the use of CT, in regards to location and severity of respiratory disease, effectively guides the clinician with their treatment strategy and case management. It can also help manage owner expectation, particularly when treatment may not be curative and ongoing treatment interventions are likely required.

Dentition

Rabbits have elodont (open-rooted) dentition, with growth rates of 3 mm per week recorded for the incisor teeth and 3 mm per month for the cheek teeth (premolar and molar teeth). The teeth have no true ‘root’ (c) and are long crowned. The visible part of the tooth within the oral cavity is known as the ‘clinical crown’, while the part of the tooth beneath the gingiva is the ‘reserve crown’. The incisors are chisel-shaped, designed for chopping and cutting food. The vestigial second pair of upper incisors are located directly behind the first pair and are known as ‘peg teeth’. The term ‘cheek teeth’ refers to both the premolars and molars which are functionally identical. The cheek teeth grind the food into a soft paste, ready to be swallowed. The maxillary cheek teeth are set wider apart (anisognathism) than the mandibular cheek teeth, such that only one side of the cheek teeth are in occlusion at any one time. The mandibular cheek teeth grow faster than the maxillary teeth.³

The oral commissure is small and the oral cavity long. Cheek folds across the diastema make visualisation of the cheek teeth only possible with the aid of an otoscope in the conscious animal. This may be achieved effectively and otoscopic examination of the oral cavity should be considered a staple part of every clinical examination. A thorough oral examination however may need to be done under sedation or anesthesia, particularly if abnormalities or disease are present. The tongue is large and has a mobile rostral portion and a relatively fixed thicker caudal portion (torus).⁴

There are four pairs of salivary glands: parotid, submaxillary, sublingual and zygomatic. In some breeds, skull morphology can lead to an increased risk of disease, e.g., dwarf breed with mandibular prognathism/maxillary brachygnathism which results in incisor malocclusion that can lead to secondary cheek teeth disease. In addition, breeds with a foreshortened skull appear predisposed to nasolacrimal duct and dental disease.⁴

CT evaluation of dentition allows assessment of each individual tooth, the lamina dura (tooth socket) for assessment of alveolar bone and both the mandibular and maxillary bone of the jaw. Swellings or abscesses within the soft tissue structures can be identified, along with local lymphadenopathy.³ Dentition can be assessed for overall structure and integrity, degree of curvature, evidence of fractures or lysis, reserve crown abscess involvement, elongation of the reserve crown with resulting secondary pathology, degree of osteomyelitis present, etc.

Where dental extractions are required, CT provides accurate data, allowing for precision planning and execution of tooth removal. Grading of the level of dental pathology present allows the management of owner expectations and considerations of patient welfare.

Ear Disease

Ear disease, involving the middle ear and external ear canal, is a common problem in domestic rabbits. Although it can occur in any rabbit, lop-eared breeds appear predisposed. In the UK, lop-eared rabbits are popular, with nine recognised breeds, varying in size from the mini lop to the English lop. Altered ear anatomy, with folding of the pinna and stenosis of the distal external ear canal, predisposes to ceruminous accumulations. Initially asymptomatic, disease can progress to otitis externa (OE) with headshaking, ear scratching and head shy behaviour. Clinical findings may include the presence of bacteria and inflammatory cells, inflammation of the mucosal wall, lateral pouching to the distal external ear canal and bulging or rupture of the tympanum. Material from the external ear canal may enter the tympanic bulla, when rupture of the tympanum occurs, causing secondary otitis media (OM).⁵

Treatment for OE is either by a medical or surgical route depending on the severity of disease present. For otitis externa cases, without pouching of the ear canal at the ear base, endoscopic or otoscopic guided cleaning or flushing is recommended +/- medicated ear drops. Where marked pouching of the ear bases is present, a surgical stoma can be created to facilitate drainage of the wax and debris, preventing pressure on the tympanic membrane which could lead to rupture and development of an OM. As well as a descending route from the external ear canal, OM can also occur via ascending infection from the Eustachian tubes, secondary to respiratory disease. Otitis media is commonly asymptomatic in rabbits, therefore diagnosis by conventional methods can be challenging. Chronic OM can lead to loss of hearing, which has been reported in geriatric rabbits, however, this can be difficult to assess in a clinical setting. Even severe OM with bone lysis can be overlooked during physical examination, as integrity of the lateral tympanic bulla wall is difficult to evaluate by palpation, due to the position of the mandible. Severe, progressive disease, however, has been associated with clinical signs of ipsilateral facial contracture, soft tissue swelling and abscessation lateral to the tympanic bulla. In some cases, this can progress to otitis interna with nystagmus, head tilt and other associated neurological signs. Prompt diagnosis and treatment of ear disease is therefore of critical importance. CT scanning is the most accurate diagnostic method for determining the presence of fluid in the tympanic bulla of rabbits; ultrasonography and radiography are less accurate for diagnosing early to intermediate disease.

CT scanning can therefore be thought of as the gold standard imaging modality for assessment of otitis media in rabbits.^6,7 Disease can be graded in terms of severity which can help classify, monitor and guide towards appropriate treatment protocols. Table 1 lists a grading system for evaluating the external ear canal. Table 2a provides a grading system for OM with 2b providing a schematic.

Table 1. Material within the external ear canal—CT grading scale: descriptive reference

Table 2a. Otitis media CT grading reference—descriptive

Table 2b. Otitis media CT grading reference—schematic and CT images

Gastrointestinal

While radiography is widely used for assessment of gaseous distension within the gastrointestinal tract and ultrasonography known to be more sensitive for assessment of gastrointestinal wall thickening, CT can be used effectively as a complementary modality. For complex recurring gastrointestinal stasis cases, the use of CT to rule out underlying pathology can be very helpful when first principle investigations have been exhausted. CT evaluation of the appendix and sacculus rotundus should be evaluated in all patients undergoing post-contrast CT imaging.^8,9

Musculoskeletal

The rabbit skeleton is light, making up only 7–8% of bodyweight whereas the muscles can account for over 50% of bodyweight. The front limbs are short, in contrast to the long and powerful hind limbs. Osteoporosis development has been linked to a lack of exercise in rabbits, historically seen in animals confined to small hutches.

The forelimb has five digits and the hind limb four. The gait is plantigrade at rest and digitigrade when running. The spine is naturally curved, and the ribs flattened. The vertebral formula of the rabbit is C7, T12(–13), L7, S4, C15–16.

Diseases of the skeleton are seen with some frequency in pet rabbits. Due to an increasing aging population of pet rabbits, age-related disease is becoming more common. Osteoarthritis (OA) is thought to be one of the most common age-related complaints in rabbits and ‘incidental’ OA seen during CT assessment for another condition is commonplace. Diagnosis of OA should result in a thorough focused patient evaluation with evaluation of mobility, potential pain and management of secondary issues e.g., urine scald from inappropriate urination posture. Long-term analgesia and ongoing monitoring and management should be considered in these cases.¹⁰

Computed Tomography Scan Technique

Helical scanning has revolutionized both the quality and speed of obtaining CT images in clinical practice. Helical scanning is where the patient moves through the scanner, through a rotating x-ray beam and detector set. The technology allows for three-dimensional data sets to be obtained. Using advanced imaging software, 3D image reconstruction, also known as multi-planar reconstruction (MPR), can be used to evaluate anatomical structures and identify pathology. In MPR, images can be viewed in one of three axes without superimposition. Scanning can be performed in both soft tissue and bone settings, with the option of high resolution ‘slice’ size (machine dependent but can be as small as 0.6 mm). This can provide an excellent level of detail and definition which can be particularly useful for structures. Post-contrast images should also be obtained with contrast appearing hyperattenuating on the images.

Preparations for Scanning

All patients undergoing scanning have an intravenous catheter placed in the marginal ear vein (author’s preference). This facilitates the administration of the intravenous contrast agent.

Intravenous Catheter Placement in the Marginal Ear Vein

1.  The fur should be clipped over the marginal ear veins of both ears and EMLA cream applied. Clipping both ears allows for a second attempt to be made at catheter placement should there be a complication with one vein.

2.  Ten to 20 minutes post EMLA application, use alcohol to clean the skin surface. Note: When necessary the patient should be wrapped in a towel to prevent struggling.

3.  With an assistant raising the vein by occluding the base of the ear, the ear should be held in one hand, with the catheter held in the other. Note: The catheter should be pre-flushed with heparinized saline to reduce the risk of blood clotting in the catheter.

4.  The vein is very superficial and when the bevel of the catheter is through the skin, the angle of the catheter should be almost parallel to the ear.

5.  Once in the vein, the stylet and catheter should be advanced approximately 0.3 cm before the catheter only is advanced further. Flashback of blood is not always seen due to the small vessel size. Holding a finger under the ear prevents the ear from bending while the catheter is advanced.

6.  Once the catheter is completely in the vein, the stylet is removed, and a plug or injection cap fastened to the catheter.

7.  Tape is used to secure the catheter. It is often helpful to use a rolled-up swab in the external ear canal to support the catheter and maintain the natural shape of the ear.

8.  The catheter is flushed to double-check correct positioning and patency. The fluid can often be seen running through the vessels of the ear or can be palpated entering the vessel at the level of the catheter.

9.  Further bandaging material can then be added, for security—a single layer of soft bandage followed by vet wrap.

(Editor’s note: use of the marginal ear vein can sometimes lead to pathology, particularly if there is extravasation of material. Alternative sites for IV catheter placement include the cephalic and lateral saphenous veins. Owners need to be informed of the site to be used and potential problems following venous access.)

Conscious restraint for CT can be facilitated using a VetMouseTrap plexiglass tube, (Universal Medical Systems, Solon, Ohio, USA), with the patient in sternal recumbency.¹¹ Tube dimensions are 40 cm length with 18 cm diameter. Flow-by oxygen should be provided throughout the procedure, with the VetMouseTrap acting as an oxygen chamber for those patients with respiratory compromise. Visual stimulation is reduced with the reduction of light levels in the room and placement of a blanket over the restraint device. Environmental noise should also be minimised. A towel to sit on, and one in a U-shape around the rabbit is inserted into the VetMouseTrap to improve comfort and reduce risk of movement. Owing to the prey instinct of the rabbit and the fact that investigation is often occurring in a debilitated patient, movement artefact risk is generally very low. The images take on average three minutes to achieve, and while there is some stress involved for the rabbit, the avoidance of anesthesia means recovery from the procedure is almost instantaneous, with patients able to return to their kennels in the rabbit ward immediately post scanning. This eliminates anaesthetic risk, reduces risk of post-anaesthetic gastrointestinal stasis and ensures staff time can be utilised elsewhere with no need for anaesthetic recovery monitoring. Figure 1 illustrates a rabbit being restrained for CT scan using the VetMouse Trap system.

Figure 1

Rabbits were positioned on a towel in sternal recumbency in a plexiglass tube device, with a rolled towel in a U-shape around them (A). The device was secured for patient safety (B), and a blanket was used to reduce visual stimulation while the device was secured to the CT table (C). The front of the tube remained uncovered, allowing provision of supplementary oxygen and observation of the patient from the CT control room (D).

Intravenous Contrast

Unless renal disease is suspected or confirmed, an intravenous contrast agent can be administered, and post-contrast images acquired within 15 seconds after contrast injection. A bolus of iodinate non-ionic contrast medium (370 mg iodine/kg, Iopamiro, Bracco, Manno, Switzerland) at a concentration of 2 ml/kg, followed by 1.5 ml of saline solution flush can be used. Administration of contrast is standardised by means of a power injector at flow rate of 0.8 ml/second.

Summary

Computed tomography scanning should be considered the gold standard option for investigation of dental, middle ear and respiratory disease in the pet rabbit. Due to the high incidence of subclinical or concurrent disease, whole body scans, with full evaluation of the images (including post-contrast) should be performed for thorough patient evaluation. A thorough understanding of normal anatomy is key to be able to identify disease related abnormalities.

The ability to perform CT scans in conscious patients can revolutionise the standard of veterinary medicine we can offer our rabbit patients and the speed at which a diagnosis can be achieved.

References

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