A 7-year-old spayed female Miniature Rex European rabbit (Oryctolagus cuniculus) was evaluated by its primary care veterinarian because of scratching at the right ear. Physical examination revealed purulent exudate in the right ear, but the left ear appeared unaffected. Cytologic examination of the exudate revealed inflammatory cells and rod-shaped bacteria, with no evidence of parasitic or fungal organisms. Radiographic evaluation of the skull and microbial culture and susceptibility testing of the exudate were recommended but declined by the owner. The ears were cleaned by the primary care veterinarian, and the owner was shown how to clean the ears at home. Enrofloxacin drops^a (2 to 3 drops in the right ear canal, q 12 h) were prescribed by the primary care veterinarian for 1 month along with daily ear cleanings. Clinical signs resolved; however, 1 month after discontinuation of treatment, the rabbit began scratching at the right ear again. Radiography of the skull (dorsoventral, right lateral oblique, and left lateral oblique views) revealed periosteal reactions in both tympanic bullae. Aerobic and anaerobic microbial cultures of samples obtained from the right ear canal yielded a moderate growth of Pseudomonas aeruginosa susceptible to marbofloxacin, amikacin, and gentamicin and intermediately susceptible to enrofloxacin. Marbofloxacin^b (7 mg/kg [3.2 mg/lb], PO, q 12 h) and enrofloxacin drops (2 to 3 drops in the right ear, q 12 h) were prescribed for 1 month. After the treatment period, purulent exudate was still present in the right ear canal. Marbofloxacin treatment was discontinued because of the cost and its apparent ineffectiveness, although administration of enrofloxacin drops was continued.

One month later, the owner reported signs of pain in the rabbit. A blood sample was obtained for clinicopathologic analysis. Results of a CBC were within reference limits, and serum biochemical analysis revealed mildly increased alkaline phosphatase activity (79 IU/L; reference range, 4 to 20 IU/L) and increased glucose concentration (235 mg/dL; reference range, 80 to 150 mg/dL) with mildly decreased phosphorus (2.7 mg/dL; reference range, 4.4 to 7.2 mg/dL) and potassium (4.0 mEq/L; reference range, 4.3 to 5.8 mEq/L) concentrations. Radiography of the skull was repeated, and no changes were detected via comparison with the previously obtained radiographs. Aerobic microbial culture of samples obtained from the right ear canal at this time yielded an abundant growth of P aeruginosa, which was resistant to enrofloxacin and intermediately susceptible to marbofloxacin.

Because the otitis had not responded to medical management and the owner had been treating the ears for > 4 months, meloxicam^c (0.1 mg/kg [0.045 mg/lb], PO, q 24 h) was prescribed for the rabbit and referral was made to the University of Illinois College of Veterinary Medicine Veterinary Teaching Hospital for discussion of surgical treatment.

On physical examination, the patient had a body condition score of 4.5 on a scale of 1 to 5 and purulent exudate was readily visible in the right ear canal. The tympanic membrane could not be seen because of inflammation of the external ear canal and exuberant exudate. All other physical examination findings were within anticipated limits. Evaluation of the head by means of CT, with and without contrast medium, was recommended to determine the extent of bulla involvement and to evaluate for potential causes such as neoplasia.

The rabbit was given medetomidine^d (0.2 mg/kg [0.09 mg/lb], IM) and glycopyrrolate^e (0.02 mg/kg [0.009 mg/lb], IM) as preanesthetic medications. Anesthesia was induced and maintained with isoflurane^f in oxygen via face mask. Computed tomography of the skull was performed, and images were obtained before and after use of contrast enhancement. Iopromide^g (980 mg/kg [445 mg/lb], IV) was administered for the contrast-enhanced images. Following the procedure, medetomidine effects were reversed with atipamazole^h (1 mg/kg [0.45 mg/lb], IM), and recovery from anesthesia was uneventful.

Analysis of CT images indicated that the right tympanic bulla was enlarged, had an abnormal shape, and was completely filled with isoattenuating material (Figure 1). After contrast administration, there was no enhancement of this material consistent with an exudate. Isoattenuating material was also present in the external ear canal. Thinning of the osseous wall of the bulla was evident, but no obvious osteolysis was detected. The osseous portion of the bulla appeared to be intact. The left tympanic bulla was filled with air and appeared normal. No dental abnormalities were apparent. These findings were consistent with an infectious, rather than neoplastic, process, and chronic otitis media and externa was considered the most likely diagnosis.

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Axial (A) and coronal (B; reconstructed) CT images of the tympanic bullae of a 7-year-old spayed female Miniature Rex European rabbit referred to a veterinary teaching hospital for evaluation of severe chronic otitis media and externa of > 4 months' duration that was nonresponsive to medical management. Notice the enlarged and misshapen right tympanic bulla filled with isoattenuating material (arrow). Images were obtained prior to IV administration of contrast medium. R = Right.

Citation: Journal of the American Veterinary Medical Association 239, 2; 10.2460/javma.239.2.228

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Because of the chronic, nonresponsive nature of the otitis and the evidence of otitis media as well as otitis externa detected in CT images, the owner elected to pursue total ear canal ablation with lateral bulla osteotomy. Meloxicam treatment was continued, and a total ear canal ablation and lateral bulla osteotomy were performed the following week. Buprenorphineⁱ (0.03 mg/kg [0.014 mg/lb], IM), midazolam^j (1 mg/kg, IM), and ketamine^k (10 mg/kg [4.5 mg/lb], IM) were administered as preanesthetic medications. Anesthesia was induced with isoflurane in oxygen via face mask, and the rabbit was intubated. Anesthesia was maintained with isoflurane in oxygen. The patient was placed in left lateral recumbency, and a T-shaped skin incision was made with the horizontal component parallel and ventral to the dorsal edge of the tragus. A vertical incision was made from the midpoint of the horizontal incision extending ventral to the right tympanic bulla. The horizontal incision was extended circumferentially around the opening of the external ear canal. Loose connective tissue was bluntly dissected until the lateral aspect of the vertical canal was exposed. The cartilage of the acoustic meatus and the scutiform cartilage were dissected from surrounding tissues. The canal attachment to the bony acoustic duct was excised, and the entire external ear canal with cartilage plates was removed. Caseous exudate was present at the base of the external ear canal and within the tympanic bulla.

Soft tissues were bluntly dissected and elevated from the tympanic bulla. Lempert rongeurs were used to remove the lateral aspect of the bony acoustic duct and to perform an osteotomy at the lateral aspect of the bulla. Cotton-tipped applicators and an angled 000 curette were used to remove the caseous debris and epithelial lining within the bulla. A sample of the debris was obtained with a swab and submitted for microbial culture and susceptibility testing. The bulla was irrigated with 60 mL of sterile saline (0.9% NaCl) solution.^l Polymethylmethacrylate beads^m were prepared according to the manufacturer's instructions; a portion was mixed with gentamicinⁿ (50 mg/g of cement), and a portion was mixed with cefazolin^o (100 mg/g of cement). These were placed together in the bulla and surrounding soft tissues to fill the surgically created dead space. The subcutaneous and subcuticular layers were closed with 3–0 polydioxanone suture^p in a simple continuous pattern. A lidocaine patch^q was applied adjacent to the incision site.

The effects of midazolam were reversed with flumazenil^r (0.08 mg/kg [0.036 mg/lb], IV), and buprenorphine (0.03 mg/kg, IV, q 8 h) was administered for treatment of postoperative pain. The rabbit received lactated Ringer's solution^s with KCl (16 mEq/L) added (2 mL/kg/h [0.9 mg/lb/h], IV) as well as marbofloxacin (7 mg/kg, PO, q 12 h) and cisapride^t (0.5 mg/kg [0.23 mg/lb], PO, q 12 h). A liquid diet^u (6 mL, q 4 h) was fed via syringe, and artificial tears^v were applied to both eyes every 2 hours. Postoperatively, the patient's right ear had a slight droop and the right eye did not have a palpebral reflex.

The day after surgery, administration of meloxicam (0.1 mg/kg, PO, q 24 h) was continued. The rabbit began eating leafy greens and hay and was discharged to the owner's care. The owner was instructed to continue administration of marbofloxacin and meloxicam PO at the prescribed doses in addition to buprenorphine (0.03 mg/kg, PO, q 8 h) for 2 weeks, to continue cisapride administration and syringe feeding^u (20 to 30 mL, q 6 h) until the rabbit's appetite returned to normal, and to apply artificial tears^v to the right eye every 6 hours until the palpebral response returned.

Histologic examination of tissue samples from the external ear canal revealed changes consistent with severe chronic otitis externa. Microbial culture of fragments of the osseous tympanic bulla revealed moderate growth of Pseudomonas spp susceptible to gentamicin.

At suture removal 2 weeks after surgery, the rabbit's palpebral reflex was present but weak on the affected side and other physical examination findings were normal. Instructions were given to apply artificial tears to the right eye every 12 hours until the palpebral response returned to normal. No further follow-up was available.

Discussion

Otitis externa in rabbits can be caused by bacteria, yeast, or parasites.¹ Additionally, neoplasia, foreign bodies, canal hypertrophy, or congenital conformations may predispose a rabbit to ear infections.¹ For instance, lop-eared rabbits have long hanging ear pinnae that cause flexion of the auricular cartilage, predisposing them to stenotic ear canals.² Clinical signs of otitis externa in rabbits are similar to those in other animals and may include head shaking, ear scratching, and signs of aural pain. If the middle or inner ear is affected, vestibular signs may be evident.¹

If the tympanic membrane is intact and visible, a myringotomy may be performed; however, the tympanic membrane lies deep in the bony acoustic duct (Figure 2), and specialized equipment such as endoscopy or microscopy may be necessary for visual evaluation.² In addition, exudate within the external ear canal and ear canal stenosis characteristic of certain rabbit breeds can make evaluation of the tympanic membrane difficult. In the rabbit of the present clinical report, inflammation of the external ear canal and exudates were present that prevented visual assessment of the tympanic membrane.

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Illustrations of rabbit external ear canal anatomy. In panel A, notice the multiple cartilaginous plates and bony acoustic duct overlying the tympanic membrane. Panel B depicts the surgical site after excision of cartilage plates and lateral bulla osteotomy.

Citation: Journal of the American Veterinary Medical Association 239, 2; 10.2460/javma.239.2.228

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Exudate from the external ear should be evaluated microscopically for the presence of ear mites (Psoroptes cuniculi), yeast, and bacteria. Medical treatment of bacterial otitis externa consists of ear cleaning under anesthesia and appropriate antimicrobial treatment on the basis of microbial culture and susceptibility testing; however, medical treatment can be difficult because rabbits produce an exudate that is caseous and difficult to eliminate.¹ Also, aggressive ear cleaning may result in irritation of the ear canal or damage to the tympanic membrane. In the rabbit of the present report, the tympanic membrane could not be seen and enrofloxacin drops, which are reported to be safe in animals with a ruptured tympanic membrane,³ were used. The mild derangements in serum biochemical values were not considered to be clinically relevant or to contraindicate surgery. When medical treatment fails, as it did in this patient, surgical intervention is warranted.¹

Radiography and CT of the skull aid in determining whether there is middle ear involvement. If the tympanic cavity has increased density but there is no evidence of otitis externa, the rabbit should be evaluated for respiratory signs, as rabbits commonly develop otitis media from spread of bacteria from the nares through the Eustachian tubes.⁴ The patient should be evaluated for pasteurellosis, although various bacteria can cause otitis media by extension from the nares.⁵ If medical management fails and there is no evidence of otitis externa, a ventral bulla osteotomy may be indicated to allow removal of debris, collection of diagnostic samples, placement of antimicrobial-impregnated PMMA beads, and ventral drainage.⁶ In the rabbit in the present report, there was evidence of otitis externa and otitis media and therefore ventral bulla osteotomy was not indicated. If there are signs of otitis externa but no abnormalities of the bulla are detected on radiographs or CT images, a lateral ear canal resection may be beneficial.² It has been suggested that this procedure allows drainage of exudate and improves ventilation of the ear canal; however, in dogs with otitis externa treated with lateral ear canal resection, many owners believed that improvement was unsatisfactory.⁷ Lateral ear canal resection is no longer recommended for managing dogs with severe otitis externa.^3,8 Further studies are necessary to determine whether this procedure is truly beneficial in rabbits with chronic otitis externa. Many animals with severe otitis externa have evidence of otitis media, and in dogs and cats, a total ear canal ablation and lateral bulla osteotomy are indicated for otitis externa that is not responsive to medical management.^3,8 In the authors' experience, this is also the treatment of choice for rabbits with chronic, refractory otitis externa with or without otitis media.

The anatomy of the ear in rabbits is different from that of dogs and cats^9,10 (Figure 2). Rabbits do not have a horizontal ear canal. The vertical ear canal extends ventrally to the external acoustic meatus without changing direction. In rabbits, the external ear canal is composed of multiple cartilage plates. The cartilage of the auricle, the cartilage of the acoustic meatus, and the scutiform cartilage should be identified and removed when performing a total ear canal ablation in this species. Failure to remove all of these cartilage plates may result in abscess or fistula formation.¹¹ The external acoustic meatus lies deep within the bony acoustic meatus, compared with the anatomic location in dogs and cats, making it difficult to visually assess the tympanic membrane. The bony acoustic duct is a bony tube, the opening of which is the bony acoustic meatus. The lateral wall of this tube must be removed along with the cartilages before performing the lateral bulla osteotomy. The tympanic bulla is also thick at the lateral aspect but thin at the medial and ventral aspects. This can make the osteotomy difficult to perform with rongeurs, and in the authors' experience, it may be necessary to use a pneumatic burr, although it was not required for this rabbit.

Most sources do not recommend placement of antimicrobial-impregnated beads for local antimicrobial treatment when performing a total ear canal ablation and lateral bulla osteotomy in dogs and cats^3,11; however, rabbits produce a caseous exudate, and it is difficult to completely eliminate infections, making recurrent infection common.¹ Placing antimicrobial-impregnated PMMA beads is recommended in rabbits because of the increased risk of recurrence or abscess formation. In the rabbit of the present report, gentamicin-impregnated PMMA beads were used on the basis of microbial culture and susceptibility test results, and cefazolin-impregnated PMMA beads were used to increase the antimicrobial spectrum to include gram-positive and anaerobic bacteria. Studies^12,13 have shown that when only gentamicin or cefazolin is added to PMMA at the described concentrations, each elutes above the minimum inhibitory concentration for target bacteria for > 30 days; however, coelution of antimicrobials in the same PMMA beads results in a significantly shorter duration of elution.¹³ Therefore, beads containing cefazolin and beads containing amikacin or gentamicin in equal numbers are placed into the bulla and used to loosely fill any dead space created during tissue dissection and removal of caseous debris.

Cisapride is a prokinetic agent that is used in several species to treat gastrointestinal stasis, which is a common postoperative complication in rabbits.¹⁴ Cisapride may also stimulate appetite in rabbits,¹⁵ which is important during the postoperative period in this species. Cisapride can cause QT-interval prolongation in humans and has been withdrawn from the market in many countries, including the United States. However, cisapride does not appear to have adverse effects in veterinary patients.¹⁶ Cisapride can be compounded, which is regulated by state and federal authorities. In this case, cisapride was compounded from bulk powder by a pharmacist and prescribed under a valid veterinarian-client-patient relationship.

Before performing a total ear canal ablation and lateral bulla osteotomy in rabbits, appropriate diagnostic tests should be performed and medical treatment should be attempted. It is important to be familiar with the ear anatomy and physiology of rabbits, as there are several unique features in rabbits that can affect the surgical procedure and outcome.