Abstract

Ear diseases are common in rabbits. In many of these diseases, medical treatment is not sufficient, and surgery is required. Surgeries of the pinna (otectomy, ear hematoma surgery) are quite similar to the dog, but surgeries of the ear canal (partial resection of the ear canal with or without bulla osteotomy) are different due to the particular rabbit anatomy. This presentation will describe in detail these anatomical particularities and the different surgical techniques.

Introduction

Ears are an important organ and an anatomical characteristic of rabbits. During artificial selection and domestication, man has created rabbits with larger ears than the wild rabbits (Belgium hare breed), with very small ears (dwarf Polish breed), or even droopy ears (lop breeds). They can represent up to 12% of the surface of the body in some breeds.

The rabbits’ ears allow them to perceive the finest sounds and to anticipate the aggression of predators. Rabbits do not have sweat glands and cannot thermoregulate by sweating or panting, as dogs do. Ears play a major role in the thermoregulation¹ of the rabbit, their important vascularity allowing them to evacuate excessive heat by a counter-current system.

Anatomy

Figure 1. Rabbit ear anatomy

The ear comprises three parts.² The outer ear consists of the pinna and the ear canal, which extends distally to the tympanic membrane. The ear cartilage is the supporting structure of the pinna and gives its shape. Unlike in dogs, the ear canal does not consist of a vertical and a horizontal portion. The shape of the ear canal in rabbits is like a “J” and there is no histological difference between the vertical and the curved part of the ear canal. The first part of the ear canal’s shape (vertical part) is given by the tragus (Figure 1 #21), the helix (Figure 1 #20) and the scutiform cartilage (Figure 1 #18), and the second part (curved), by the annular cartilage (Figure 1 #17). The arterial network of the pinna comes from the caudal auricular artery, a branch of the external carotid artery. It divides successively to give lateral, intermediate, and medial auricular branches. These branches feed the outer face of the pinna. The superficial temporal artery, also derived from the external carotid artery, gives rise to the rostral atrial artery that supplies the rostral part of the pinna. The venous network is a satellite of the arterial network, the rostral atrial vein emerges in the superficial temporal vein and the caudal auricular vein flows into the maxillary vein.

The tympanic membrane of rabbits is elliptical and consists of distinct portions, the larger pars tensa located ventrally and the smaller pars flaccida located dorsally. It is located just deep to the annular cartilage making it difficult to visualize in upright-eared breeds and impossible in lop-eared breeds, without sedation or general anesthesia. The tympanic bulla is proportionally larger in rabbits compared with dogs and cats. The bone is thicker on the lateral and rostral aspects of the bullae and thinner ventrally. The facial nerve lies very close to the tympanic bulla.^1,2 The nerve enters the facial canal on the medial aspect of the bulla and exits the skull immediately caudal to the bulla, through the stylomastoid foramen coursing along the ventral surface of the tympanic bullae.

The inner ear is at the level of the petrous part of the temporal bone. It includes the cochlear and vestibular apparatus. The bony labyrinth consists of the semicircular ducts, the cochlea, the utricle, and the vestibule. The bony walls delimit a space filled with a liquid called perilymph, and a membrane within this space forms the membranous labyrinth, which contains the endolymph. The vestibular apparatus comprises the semicircular ducts connected to the utricle by the ampoule, and then to the saccule. The vestibular ramp communicates with the cochlear ramp by a foramen called helicotrema. The organ of Corti, located along the cochlea canal, is the organ that allows the transformation of the vibrations transmitted by the fluid of the inner ear into electrical information transmitted to the branch of the vestibulocochlear nerve (nerve VIII).

Otectomy

Indications

A rabbits’ ear can be seriously injured during a fight between mates, followed by bites and other trauma. Lacerations of the ear are possible following an ear stuck in the animal’s cage. If the cartilage is severely affected or if the loss of skin is too great, an otectomy has to be considered. Some cases of serious frostbite have also been reported in animals living outside. In this case, the purpose of the otectomy is to prevent any spread of necrosis. The presence of a tumor is also an indication for an otectomy. Squamous cell carcinoma and lymphoma have been described in laboratory rabbits.

Surgical Technique

To perform an otectomy, also called pinnectomy, the animal is anesthetized, then the ear is totally shaved.³ The cutaneous incision is previously marked with an indelible marker to ensure a harmonious contour of the ear. The ear is surgically prepared. The vascularization of the ear can be visualized using a transilluminator. The ear vessels are marked to prevent any accidental section of these during the surgery. To prevent bleeding, ligations of the most important vessels are performed using a PDS 4.0 (Figure 2a). The skin is incised on each side following the predefined line. The cutaneous bleeding is stopped by pressure using gauze soaked with a few drops of epinephrine. The skin is dissected five millimeters to the base of the ear to separate it from the cartilage (Figure 2b). The cartilage is cut five millimeters back from the skin using scissors. The skin is then sutured using an interrupted pattern with PDS 3.0 (Figure 2c).

Figure 2a

Figure 2b

Figure 2c

Post-Operative Care

A bandage is made to protect the sutures. Gauze sponges are rolled on themselves and placed inside the ear. The ear is “wrapped” around the gauzes. The entire ear is then wrapped in gauze bandage material and fixed with tape.

The animal is discharged with an analgesic (meloxicam 1 mg/kg/q 12 h for one week). A wound check and a bandage change are performed on day 7 and day 14. The stitches are removed on day 14 if they have not resorbed.

Complications

The wound can dehisce if the cartilage has not been shortened enough from the skin. In this case, the cartilage puts too much pressure on the surgical wound which prevents good healing.

Ear Hematoma

Etiology

The othematoma or ear hematoma seems infrequent in the rabbit.⁴ The cause is most often due to self-injury when the animal scratches or shakes his ears. The underlying causes may be an inflammatory process, a parasitic infection, or a presence of a foreign body. These causes must be treated to avoid any recurrence. The hematoma usually appears on the inner surface of the ear.

The branches of the caudal auricular artery that penetrate inside the cartilage are often the source of the bleeding. Bleeding continues as long as there is no pressure balance between blood pressure and pressure within the hematoma. This painful process causes an increase in pruritus and discomfort, causing the rabbit to shake the head and traumatize the ear further, thereby exacerbating the hematoma. Eventually, the pressure within the hematoma may increase and stop the bleeding. In this mature hematoma, fibrin is lysed leading forming a sero-hemorrhagic fluid. If the othematoma is not treated, it can lead to thickening and significant deformation of the ear.

Surgical Technique

On an anesthetized animal, the ears are shaved and then surgically prepared^3,4 (Figure 3a). The blood vessels are visualized with a transilluminator and marked with a sterile pen. The skin on the side of the hematoma is incised along its entire length. Sero-hemorrhagic fluid and fibrin are removed. The inside of the wound is rinsed with warm saline fluid. If needed, a flap of skin is excised along the incision so that the edges can properly face each other when they are affixed to the cartilage. Single points crossing the ear through the cartilage (through and through) are made using 2-0 diameter non-resorbable sutures (Figure 3b). The stitches are made parallel to the major axis of the ear over the entire area that has been separated from the cartilage (Figure 3c). The incised skin is left open in order to drain any new collection of blood or serum.

Figure 3a

Figure 3b

Figure 3c

Post-Operative Care

A bandage similar to that used during the otectomy is made. The bandage is changed daily until there is no more sero-hemorrhagic fluid production and then the bandage is changed every 3 days. The bandage and the stitches are removed 14 days after the surgery. The rabbit is sent home with NSAIDs and opioids.

Complications

A perforation of the vessels of the ear can happen when the stitches are placed. If the underlying cause of the othematoma has not been treated or if the number of stitches is not enough to be able to flatten the skin on the cartilage, then, the othematoma may recur.

Partial Resection of the Ear Canal

Indications

Ablation of the first portion of the ear canal is indicated for otitis externa not responding to medical treatment, particularly in the case of severe ear infections with a large accumulation of pus which is often difficult to extract.⁵ The diagnosis of otitis externa is made using an otoscope or an endoscope. Because of their particular anatomy, lop breeds have more chance to develop this kind of complication. Indeed, in these rabbits, the auricular pinna obstructs the opening of the ear canal, the air circulation and the evacuation of cerumen is more difficult. The presence of a mass (neoplasia) obstructing the auditory canal may also require the completion of this surgery.

Surgical Technique^6,7

The rabbit is anesthetized and placed in lateral decubitus. The base of the ear is shaved just behind the lateral canthus of the eye and up to the end of the pinna. This area is prepared aseptically. A surgical drape is placed in order to have access to the opening of the ear canal, as well as to the base of the ear. Two parallel skin incisions are made from the base of the ear along the long axis of the pinna (Figure 4 #1). The subcutaneous tissues of this flap are dissected, separated from the cartilage, and then retracted rostrally (Figure 4 #2). Scissors are used to create two parallel incisions of the ear canal, thus cutting the scutiform cartilage (Figure 4 #3). Special attention is paid to avoid cutting the rostral and caudal auricular vein during this step. The cartilage flap is repulsed rostrally, then incised at its base at the opening of the vertical part of the ear canal. The curved part of the ear canal is then highlighted by the visualization of the annular cartilage. The cutaneous flap is then incised and removed (Figure 4 #3). The skin, as well as the subcutaneous tissue, are sutured using (PDS 4.0) at the edge of the annular cartilage marking the opening of the equivalent of the horizontal canal. The remaining ear canal is then cleaned and rinsed with warm saline to remove purulent debris. The free edge of the vertical portion of the ear remaining canal is sutured to the skin using a simple interrupted pattern using the same suture as above. During this step, it is important to check that the remaining part of the scutiform cartilage does not exert too much tension on the sutures. If necessary, it is dissected, and a thin flap is removed. At the end of the surgery, the base of the ear canal (the curved part of the J) is in direct communication with the external environment, reducing the accumulation of debris and thus allowing easier cleaning.

Figure 4

Complications

The wound facing the cartilage can dehisce if it has not been cut short enough and exerts too much pressure on the sutures. There is a risk of hemorrhage if the rostral or caudal auricular veins are punctured. If pressure hemostasis is not possible, ligation of these vessels may result in necrosis of the ear. Therefore pressure or the use of hemostatic sponges is preferred.

Postoperative Care

Postoperative care consists of analgesia, systemic and local antibiotic therapy. The remaining part of the ear canal is rinsed using a fine syringe to remove the crusts and the remaining pus.

The animal needs to be seen in consultation one week after surgery to check the healing of the surgical wound and to remove, after moistening with warm saline, the accumulation of crusts around the sutures. The sutures can be removed 2 weeks after the surgery if they have not resorbed themselves.

Ear Canal Ablation and Lateral Bulla Osteotomy

Indications

Otitis externa can progress and reach the middle ear. The other etiology of a middle ear infection is a contamination of the tympanic bulla from the upper respiratory system through the eustachian tube. This infection includes the eardrum and the tympanic bulla. Otitis media is considered very painful for rabbits like for other animals. The complaint could be only depression and anorexia. Palpation of the base of the ear may indicate acute pain. Rabbits could present a vestibular syndrome (meaning that the inner ear is also affected), but that is not always the case. The diagnosis of otitis media could be made by endoscopic examination. In this case, the tympanic membrane is visualized, and pus may be seen beyond it due to its transparency. The tympanic membrane could also be ruptured and direct visualization of pus in the tympanic bulla could be seen. But sometimes, because of the thickness of the pus, and the inflammation of the ear canal, this examination is impossible. A CT scan of the skull is the diagnostic test of choice to allow the visualization of the bulla.⁸ Medical treatment is often disappointing, requiring the use of surgery to treat otitis media.

Surgical Technique: Partial Ear Canal Ablation and Bulla Osteotomy (PECALBO)^7,9

As this is a painful surgery, a proper analgesic protocol must be performed. The use of a continuous rate infusion of fentanyl (0.5 µg/kg/h), lidocaine (0.4 mg/kg/h), and ketamine (0.3 mg/kg/h) usually provides suitable analgesia.

The animal is anesthetized and placed in lateral decubitus. The base of the ear is shaved and surgically prepared. The beginning of the surgery technique follows the previously described technique of the partial resection of the ear canal, up to the exposure of the curved part of the J-shaped ear canal. Then, the remaining part of the ear canal is dissected minutely until it joins the tympanic bulla (Fig 5 #5). This operation requires patience and skill because the facial nerve, the rostral and caudal auricular arteries and veins run in this area. It is recommended to use Lone Star Retractors® in order to retract the skin and to facilitate the visualization of the tissues to dissect. The remaining part, the auditory canal, is separated at its base. The acoustic meatus is visualized and enlarged by performing an osteotomy using a fine bone rongeur forceps (Figure 5 #6). Once the tympanic bulla has been sufficiently opened, it is gently cleaned with a fine Volkmann curette to remove as much pus as possible. The tympanic bulla is then gently flushed with warm saline. The cutaneous and subcutaneous tissue surrounding the vertical part of the ear canal is then sutured with an interrupted pattern (PDS 4.0). Tissues surrounding the opening of the tympanic bulla are marsupialized with an interrupted pattern (PDS 4.0) (Figure 5 #6).

Figure 5

Alternative Surgical Techniques

Closed Surgical Wound

Another slightly PECALBO technique has been described.¹⁰ In this case series, the surgeon made a single cutaneous vertical incision over the base of the ear canal. The surgical technique is then the same as presented previously up to the flushing of the tympanic bulla. Following this, the tissues over the bulla are closed with resorbable sutures. The mucosa of the distal incision in the ear canal is closed with a resorbable suture ensuring the mucosa was accurately apposed. A single, horizontal mattress suture is placed across the incised cartilage of the vertical canal to close it. The remaining soft tissues are opposed, and the skin is closed using a subcuticular pattern.

The main difference with the previously described technique is that the skin is closed on the surgical site. According to the literature in dogs and cats, the main complication is a dehiscence of the surgical wound due to some remaining infected tissues left in the surgical site.¹¹ It realistically is very complicated in rabbits to ensure removal of all the affected tissue when doing an ear canal surgery. The risk of having some remaining infected tissue is high. Some authors recommend the placement of polymethylmethacrylate (PMMA) beads mixed with antibiotics,¹² but because the result of the culture and the sensitivity may not be available at the time of surgery, there is a possibility that the bacteria could be resistant to the selected antibiotic. By using marsupialization and flushing the wound daily, anaerobe bacteria could be killed, and the flush can mechanically clean the wound from debris.

Total Ear Canal Ablation and Lateral Bulla Osteotomy (TECALBO)

Some authors described a total ear canal ablation technique in addition to the lateral bulla osteotomy similar to what was described in dogs and cats.^11,13

An AT-shaped skin incision is made with the horizontal component parallel and ventral to the dorsal edge of the tragus. A vertical incision is made from the midpoint of the horizontal incision extending ventral to the tympanic bulla. The horizontal incision was extended circumferentially around the opening of the external ear canal. Loose connective tissue is bluntly dissected until the lateral aspect of the vertical canal is exposed. The cartilage of the acoustic meatus and the scutiform cartilage are dissected from surrounding tissues. The canal attachment to the bony acoustic duct is excised, and the entire external ear canal with cartilage plates is removed. Soft tissues are bluntly dissected and elevated from the tympanic bulla. Lempert rongeurs are 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 a curette are used to remove the caseous debris and epithelial lining within the bulla. PMMA beads mixed with antibiotics are placed together in the bulla and surrounding soft tissues to fill the surgically created dead space. The subcutaneous and subcuticular layers are closed with 3-0 polydioxanone suture in a simple continuous pattern.

This technique is more invasive than the previous one and can lead to the risk of more trauma to the surrounding tissues such as blood vessels and nerves. It also increases the difficulty of managing pain medically. In non-lop breeds, the total ablation of the ear canal could also induce a bending or “flopping” ear after the surgery. Another main difference is that the surgical wound is closed at the end of the surgery with the same limitation as discussed previously.

Post-Operative Care

Pain treatment must be continued and reevaluated daily until the animal eats by himself. The continuous infusion of fentanyl, lidocaine, ketamine is gradually decreased during one to two days, and if the animal is comfortable, buprenorphine could be rotated into the treatment program. Meloxicam is added to the regimen post-operatively. Broad-spectrum antibiotics such as penicillin (SC) or azithromycin (PO) are used while waiting for the culture and sensitivity results. The surgical wound is gently flushed with saline twice daily. The average time of hospitalization is 3 days.¹⁴

The rabbit is discharged, with pain medications, antibiotics, and surgical wound flush, when he is eating spontaneously and comfortable. Weekly rechecks are made to monitor surgical wound healing.

Complications

Multiple complications are described after this type of surgery. It is important to take the time to discuss the possibilities of this with the owner prior to the surgery.

The risks related to this surgery are damage to the facial nerve, leading to a hemiparalysis of the face, a lesion of the vestibular apparatus during the curettage of the tympanic bulla leading to a vestibular syndrome, and hemorrhage by damaging and ligation of the rostral veins or auricular arteries leading to necrosis of the ear. If a head tilt is already present, there is also a risk that the head tilt may not fully improve, resolve, or does not improve at all. In the author’s experience, the longer the delay between the beginning of the clinical signs and the surgery, the less likely the animal is to fully recover from head tilt. Overall, the rate of complication is lower than in dogs.¹⁴

Ventral Bulla Osteotomy (VBO)^15,16

The indication for a VBO is an otitis media without otitis externa, and with no osteolysis of the tympanic bulla.

A VBO is performed in dorsal recumbency with the neck fully extended. Because of the presence of a prominent semicircular mandibular angle in rabbits, the bulla cannot be directly palpated until the overlying muscles are partially dissected. A 4–5 cm skin incision is made parallel and medial to the mandible. The incision is continued through the platysma muscle medial to the mandibular salivary gland.

The digastricus muscle is dissected from the hyoglossus and styloglossus muscles taking care to avoid the hypoglossal nerve coursing lateral to the hyoglossus muscle. Lone StarRetractors® are used to provide exposure of underlying tissues throughout the procedure. The bulla is palpated between the jugular process of the skull and mandibular angle. Using a Freer periosteal elevator, blunt dissection is continued until the ventral surface of the bulla is exposed. The bulla is entered on the ventral aspect with a Steinmann pin and hand chuck. The osteotomy is extended circumferentially with rongeurs or a pneumatic burr to allow adequate access and drainage. The bulla is carefully curetted to remove any debris as well as the epithelium lining the middle ear. Samples for culture and susceptibility and histopathology are obtained as necessary and the site lavaged with sterile saline. Antibiotic-impregnated PMMA beads are placed in the surgical field.¹⁶ The surgical site is closed in a routine fashion using 4-0 or smaller absorbable suture material for the muscle and subcutaneous tissue and 4-0 or smaller nonabsorbable suture material for the skin. An alternative to the use of PMMA is to place a drain and rinse the bulla using the drain.⁹

Myringotomy¹⁷

In case of otitis media without otitis externa, an endoscopic examination of the tympanic bulla could be performed. Depending on the size of the rabbit, a 5 mm otoscope with a 2.7 mm 30° or a 1.9 mm 30° optical view with an operating channel sheath could be used. The external ear canal needs to be thoroughly cleaned and flushed with sterile saline. The tympanic membrane is visualized and is perforated with a myringotomy needle. The exudate is aspirated and sampled for bacterial culture and sensitivity. The tympanic bulla is flushed with sterile saline. A large incision of the tympanic membrane is often necessary to allow adequate lavage of debris and exudate from the middle ear. As the rabbits’ pus often present in the middle ear is thick, it can be challenging to fully clean the tympanic bulla during the first procedure. The author recommends repeating this procedure once a week until the tympanic bulla is perfectly clean.

charly.pignon@vet-alfort.fr

References

1.  O’Malley B. Rabbits. In: O’Malley B, ed. Clinical Anatomy and Physiology of Exotic Species, Structure and Function of Mammals, Birds, Reptiles, and Amphibians. St. Louis, MO; Elsevier et Saunders WB; 2005:163–261.

2.  Popesko P, Rajtova V, Jindrich HJ. Rabbit, guinea pig. In: A Colour Atlas of Anatomy of Small Laboratory Animals. Volume 1. London: Saunders; 1992.

3.  Handerson R, Horne R. Pinna. In: Slatter D, ed. Textbook of Small Animal Surgery. 3rd edition. New York: Saunders; 2002:1737–1746.

4.  Manjunatha D, Mahesh V, Ranganath L. Surgical management of aural hematoma in Russian grey giant rabbit. Int J Agric Sc Vet Med. 2014;2(3):37–38.

5.  Harcourt-Brown F. Otitis externa. In: Textbook of Rabbit Medicine. Oxford, UK: Butterworth; 2002:232–233.

6.  Krahwinkel D. External ear canal. In: Slatter D, ed. Textbook of Small Animal Surgery. 3rd edition. New York: Saunders; 2002:1746–1756.

7.  Capello V. Treatment of the otitis externa and media in pet rabbits. Exotic DVM. 2004;6.3:15–21.

8.  de Matos R, Ruby J, Van Hatten RA, et al. Computed tomographic features of clinical and subclinical middle ear disease in domestic rabbits (Oryctolagus cuniculus): 88 cases (2007–2014). J Am Vet Med Assoc. 2015;246(3):336–343.

9.  White R. Middle ear. In: Slatter D, ed. Textbook of Small Animal Surgery. 3rd edition. New York: Saunders; 2002:1757–1766.

10.  Eatwell K, Mancinelli E, Hedley J, et al. Partial ear canal ablation and lateral bulla osteotomy in rabbits. J Small Anim Pract. 2013;54(6):325–330.

11.  Smeak D. Management of complications associated with total ear canal ablation and bulla osteotomy in dogs and cats. Vet Clin North Am Small Anim Pract. 2011;41:981–994.

12.  Chow EP, Benett RA, Whittington JK. Total ear canal ablation and lateral bulla osteotomy for treatment of otitis externa and media in a rabbit. J Am Vet Med Assoc. 2011;239(2):228–232.

13.  Chow EP. Surgical management of rabbit ear disease. J Exot Pet Med. 2011;20(3):182–187.

14.  Pignon C, Hyunh M, Coquelle M, et al. Assessment of tympanic bulla osteotomy for treatment of middle ear disease in 8 domestic rabbits. In: Proceedings from the ICARE. Wiesbaden, Germany; 2013:191.

15.  Chow EP, Benett RA, Dustin L. Ventral bulla osteotomy for treatment of otitis media in a rabbit. J Exot Pet Med. 2009;18(4):299–305.

16.  Csomos R, Bosscher G, Mans C. Surgical management of ear disease in rabbits. Vet Clin North Am Exot Anim Pract. 2016;19(1):189–204.

17.  Jekl V, Hauptman K, Knotek Z. Video otoscopy in exotic companion mammals. Vet Clin North Am Exot Anim Pract. 2015;18(3):431–445.