Surgical considerations (Proceedings)

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Seminar Notes

General Considerations and Pre-operative Preparation

Since rabbits are a highly sensitive prey species, a very conscientious effort needs to be made to reduce stress, pain and any other factors that could be a detriment to their health in the peri-operative period. The surgical principles are the same for rabbits as in other species, but there are some unique anatomical, physiological, postural, and behavioral aspects that need to be considered when dealing with the rabbit as a surgical patient.

One of the most common responses of a rabbit body to stress is ileus. Because rabbits have a very tight oesophageal sphincter, they cannot vomit. Therefore, they do not need to be fasted prior to an anaesthetic episode. In fact, fasting a rabbit can predispose them to ileus.

Rabbits have very thin skin and fine hairs. Great care needs to be taken not to nick the rabbit's skin as the surgery site is prepared. Setting aside one very sharp blade that is not to be used for shaving heavily soiled or rough coated animals and using it on rabbits only is useful. Shaving the hair off is best accomplished by gently stretching the skin over the area to be shaved, and clipping slowly.

For general peri-operative pain management, buprenorphine can be given 30 minutes prior to anaesthesia induction at a dose of 0.05-0.10mg/kg SC (Bradley, 2001). Good pre-anaesthetic sedation can be obtained with either ketamine at 10-15mg/kg IM or medetomidine at 100-250mcg/kg IM. Once the sedative has taken effect, anaesthesia can be induced via mask with isoflurane. Once anaesthesia is fully induced, tracheal intubation can be accomplished. If an endoscope is available, it can be used to assist in tracheal intubation. If not, the following technique can be used: The rabbit is positioned on a table between the restrainer and the intubator. Gauze strips or a mouth gag are used to hold the mouth open. The restrainer extends the head forward with the maxilla slightly extended more forward than the mandible. The tongue is gently pulled out by the intubator, and a Miller -0- laryngoscope blade is inserted into the mouth on it's side with the side that has the light bulb on it towards the roof of the mouth and the other side along the tongue. The blade is gently inserted to the back of the mouth then pushed down on the back of the tongue. With the neck extended forward the pharynx can be visualized. It is common to see the epiglottis above the slightly translucent soft palate. The soft palate can be gently pushed with the end of the endotracheal tube to flip the epiglottis down onto the back of the tongue. At this point, the glottis can be well visualized and 0.05mL of injectible lidocaine can be sprayed on it to prevent spasm when intubation is attempted. Most rabbits require a 2.5-3.0mm uncuffed tracheal tube. It is useful to use a stylet for the actual intubation process. Since the opening into the rabbit's airway is located very caudally and curves downward at an angle, it is useful to bend the distal 7-8mm of the tip of the stylet at an approximately 30 degree angle. Care needs to be taken to be sure the end of the stylet does not extend past the tip of the tracheal tube. Once intubated, the tube is secured with gauze strips and fastened behind the neck. Since the tracheal tubes are so small, the gauze should be secured around the adaptor rather than the tube itself. The intubated rabbit can usually be maintained at 2-2.5% isoflurane with an oxygen flow rate of 2 litres/min. If tracheal intubation is unsuccessful, the rabbit can be maintained on a mask and usually requires a rate of 3-3.5% isoflurane with an oxygen flow rate of 2 litres/min. Rabbits can also be maintained with a tube that is positioned in the nasal passageway. To use this method, a topical anaesthetic is applied to the nasal cavity and a 1.0-1.5mm tube is passed medially and ventrally. This technique is useful for very small rabbits. They can be maintained with the tube in the nasal passageways or the tube can be pushed in further to enter the trachea through the nasal passages (Harcourt-Brown, 2002).

Due to its small size, a rabbit has a relatively high surface area to body mass ratio. Several methods can be used to supply supplemental heat to keep the patient warm during the procedure. These include the convective heaters (Orcutt, 2000), a circulating warm water pad, or 1 litre bags filled with either warm water or sand.

Procedures

Ovariohysterectomy

Ovariohysterectomy is indicated in the pet rabbit to prevent unwanted pregnancies and pseudopregnancies, to modify territorial behaviour, and to prevent or remove neoplasias of the reproductive tract. An incidence of uterine neoplasias as high as 80% has been reported in rabbits greater than 4 years of age (Paul-Murphy, 1997). The rabbit has a bicornate uterus, two cervices and a very large mesometrium that is used to store large amounts of fat. Upon opening the abdomen of a mature female rabbit, the uterus can be found located caudally and ventrally. In an immature rabbit, the uterus is found in a more dorsal location. It is not uncommon to find a small to moderate amount of free fluid in the abdomen. The suspensory ligaments are fairly lax compared to other species, and the ovaries are relatively easy to exteriorise. The fallopian tubes often are very long and extend quite far cranially. Care needs to be taken to exteriorise them and remove them in their entirety. They can be quite friable. The ovarian arteries are ligated in a routine manner with an absorbable suture material. The lateral uterine ligaments (round ligaments) are then broken down to allow the entire uterus to be exteriorised. The vaginal body is relatively large and flaccid. It is possible for urine to reflux up into the vaginal vestibule. If this happens, the urine can be "milked" down and a routine cerclage ligature can be placed approximately 1-2cm distal to the cervices. The body wall is then closed in a routine manner. Even in over weight rabbits, there is very little subcutaneous tissue. A separate subcutaneous closing layer is not routinely necessary. A subcuticular skin closure holds more than adequately and is well tolerate by most rabbits.

Rabbits have a high tendency for forming adhesions. They are so good at it that they are used as models for adhesion formation and prevention in human medicine. A protocol of 200mcg of verapamil SC q8h for a total of nine doses was shown to reduce adhesion formation in rabbits which had one uterine horn traumatized by ligation followed by burns induced with thermocautery. The suggested mechanisms of action by which calcium channel blockade mediates this process is that the drug (verapamil) may be 1- intervening at sequential loci of the adhesion formation cascade, 2-reducing vasoactive/inflammatory mediator production during the acute inflammatory response, 3-inhibit irreversible platelet aggregation, 4-protect against acute granulocyte-mediated tissue injury, 5-reduce microvascular permeability which results in decreased exudation of fibrin-rich plasma as substrate for clot formation and 6-inhibit fibroblast penetration into clot matrices (Steinleitner, 1990).

It is not uncommon for female rabbits to become inappetent after being spayed. This can result in a dangerous situation as the inappetance can lead to ileus and hepatic lipidosis. Administration of cyproheptadine at a dose of 1 mg PO BID is a useful appetite stimulant in rabbits. A post-operative dose of metoclopromide at 0.5mg/kg is helpful for encouraging intestinal motility post surgically. Another dose of buprenorphine can be given 6-8 hours post operatively as needed for additional pain management. Some individuals seem to be more sensitive to the procedure than others. It is often beneficial to send the owner home with 1-2 doses of meloxicam (0.2mg/kg) to be given orally q24h for the first 24 to 48 hours post op.

Castration

Indications for castration are population control as well as behaviour modification. Intact male rabbits exhibit distinct urine marking behaviour. Although neoplasia of the testicles does occur, it is relatively uncommon and tends to manifest only in older rabbits (>8 years). Rabbits have relatively large inguinal rings that remain patent throughout their lives. Many practitioners castrate rabbits via a scrotal approach in a manner similar to feline castration and report rarely having problems with abdominal organs herniating through the canals. Since closing them is not difficult or time consuming it is best to take the precaution of closing the inguinal canals at the time of surgery. One method of castration is as follows: a 2cm pre-scrotal incision is made through the skin. The testicles easily slip back up into the abdominal cavity when pressure is applied to the scrotum in order to push them up to the incision. It is helpful to occlude the inguinal canal by applying pressure to the pubic bone with an index finger while pushing the testicle up to the incision with the thumb. Once exteriorised, the procedure can be completed with either an open or closed castration technique. The vaginal tunic covering the testicle is torn away from the scrotum. The spermatic cord is then be ligated in a routine manner with absorbable suture material. The inguinal rings can be somewhat vague; this is especially true in older or obese rabbits. They are visualized most easily by grasping the cranial ventral border of the ring with a thumb forceps and pulling that border up towards the surgeon (as the rabbit is positioned in dorsal recumbency). In a smaller rabbit (<2-2.5kg) it is possible to ligate the rings with one cruciate suture that encompasses the lateral aspect of the right ring, the medial aspect of both rings and the lateral aspect of the left ring. In a larger rabbit, it is often necessary to close each ring separately.

Vulvaplasty

Rabbits that have become obese commonly develop a large flap of skin that folds over the genital area. This flap can be so extensive that large amounts of faeces and debris can get caught up in there and cause severe infections. Even after the diet is corrected and the rabbit loses weight, the flap often remains and needs to be addressed surgically. The area is prepared surgically as well as possible. The skin of these flaps is often very irritated from the alkaline urine and is very friable. A semicircular incision is made around both sides of the flap and a half moon shaped piece of skin is removed. The remaining wound edges are sutured together with an absorbable 4-0-suture material.

Orthopedic procedures

An epidural of lidocaine provides excellent perioperative analgesia for rabbits undergoing orthopedic procedures of the rear limbs. The lumbar area is prepped in the usual manner. The space between the seventh lumbar vertebra and the sacrum is palpated. A 25 gauge needle is inserted. The needle is judged to be correctly placed if there is no resistance to the infusion of a small amount of air into the epidural space. The dose of lidocaine for use in an epidural is 1ml/7kg body wt.

Fracture repair

A rabbit's skeleton comprises only 8% of its body mass (Cruise, 1994). Their bones are small and fragile and quite prone to fracture. Standard principles for fracture repair are applied, but modifications for placement of external fixators and incisions need to be considered for application in the rabbit. If a foreleg is fractured the surgical plan is adjusted depending on whether or not the fracture is oblique or transverse. In most cases in the author's experience the forelimb fractures tend to be transverse and have been quite amenable to fixation with an external fixator technique tie-in technique (Aron, 1997 ). This method affords additional stability especially when the ideal number of cortices cannot be secured with external fixator pins on either side of the fracture site. The intramedullary pin is bent at a 90-degree angle as it exits the joint and is tied in with the other fixator pins to an external connecting bar. Partial interface pins are useful as external fixator pins. Traditional connecting bars and clamps are often too large to use on rabbits. A connecting bar can be fashioned by placing a Penrose drain over the ends of the external fixator pins, clamping off one end of the Penrose drain with a haemostat and then filling the tubing with methacrylate. As the hardening of the methacrylate is an exothermic process, care needs to be taken to prevent the methacrylate filled drain from coming into direct contact with the rabbit's skin. The methacrylate filled drain becomes very hot as it is hardening. In the foreleg, a Type II fixator is well tolerated. A soft padded bandage should be applied to prevent the rabbit from catching it on objects or cage wire as it moves around post op.

Tibial fractures can be repaired by intramedullary pinning with an added cerclage wire as indicated if it is an oblique fracture. Transverse fractures can be repaired with a Type I external fixator using the tie-in technique. Due to the rabbit's hunched posture of its rear legs and its need for coprophagy, a Type II fixator would be predicted not to be as well tolerated as a Type I fixator.

Luxations

The two most common joints to be luxated are the elbow and coxofemoral joints. If the elbow is luxated, it can be reduced successfully if the injury is addressed promptly. The rabbit is put under general anaesthesia, and the elbow luxation is reduced by flexing the joint and rotating the displaced proximal aspect of the ulna medially. The leg should then be splinted in extension for 7-14 days. Traumatic luxation of the coxofemoral joint is often more difficult to permanently reduce without surgical intervention. Traumatic luxation of this joint usually involves a complete tear of the ligament of the head of the femur. Due to a rabbit's normal hunched posture of their rear legs and their poor tolerance of such a wrap, an Ehmer sling is not apt to be fruitful for resolution of this condition. A femoral head osteotomy is often indicated and well tolerated in this species. The coxofemoral joint is approached dorsally. The joint capsule is incised and the femoral head is exteriorised by rotating the stifle medially and dorsally. The femoral neck is severed using a gigli wire. Ronguers can be used to clip off any rough edges left behind. The joint capsule is then closed with 4-0 or 5-0 absorbable suture material. The muscle is closed in a routine manner and the skin is closed with a subcuticular closure.

It is not uncommon for a rabbit to present with a severely infected open fracture. It is extremely difficult to achieve good resolution by means of surgical repair of the fracture if infection is already present. If indicated, amputation of either a front limb or rear limb are well tolerated by most rabbits. Rear leg amputation is performed at the level between the proximal and middle third of the femur. A small piece of bone wax is then applied to the end of the bone. It is beneficial to make the skin incision such that the medial flap of skin is left significantly longer than the lateral aspect of the incision. This results in the final closure being up off the ground on the lateral aspect of the stump instead of being on the ventral, weight-bearing surface. The muscles of the thigh are dissected away from the bone at their insertion points on the stifle. Dissecting off the muscles so much further distal than the level that the bone is cut ensures a very well padded stump. Since rabbits use their rear legs to remove wax from their ears, the caretaker will need to be instructed to gently remove the wax from the rabbit's ear that is on the same side as the amputated limb in the future.

If the injury or infection of the foreleg involves only the paw, it can be amputated just proximal to the carpus. The amputation is made high enough on the bone so that a good pad of muscle can be used to cover the stump. If the injury is far enough proximal to merit amputation of the whole limb, there are pros and cons to consider regarding the removal of the scapula or disarticulation and amputation at the scapulohumeral joint. Amputation including removal of the scapula may be more cosmetic as there will be significant muscle wasting over the scapula after the amputation due to misuse. However, leaving the scapula may afford a protective barrier to the chest wall (Kapatkin, 1997).

Wound Management

Rabbits are prone to injuries that often require extensive wound management. The wounds are often deeply infected, and because of the caseous nature of rabbit pus, it can be very difficult to keep the area clean and resolve the infection. Prior to the initiation of treatment a culture of the wound should be submitted. These wounds require staged debridement, systemic antibiotics, and pain management as needed. A useful technique for establishing a clean granulation bed involves the use of 50% dextrose dressings. The principle behind using the 50% dextrose is that the dextrose is too hyperosmotic for any bacterial growth (Kelleher, 2000). The hyperosmotic dressing is used for as long as needed until the wound no longer shows evidence of infection when it is examined at the time of the daily bandage changes. Usually after the first 7-10 days, the dressing can be changed to a non-adherent dressing such as silver sulfadiazine cream covered with a non-adherent pad.

Depending on where the wound is located, creative bandaging techniques are often required. If the wound is on an area of the body that is not conducive to a wrap-around bandage, it is useful to place loops of suture material in the healthy skin around the periphery of the wound. The bandage can then be held on by placing an occlusive layer of bandage material over the wet to dry dressing and securing it to the patient by lacing it through the suture loops.

If there is a significant amount of bone exposed at the site of the wound holes can be drilled through the cortex of the bone to encourage granulation tissue to cover the bone. After the holes are drilled, the wound is dressed with a topical antibacterial agent and a non-adherent pad. This bandage is then changed daily. When changing the dressing, care should be taken not to disturb any clot that is over the bone (Swaim, 1997).

Abscesses

Rabbit abscesses can be quite the nemesis to veterinary practitioners. Rabbit's abscesses are not amenable to the simple lancing and drainage that often resolve abscesses in dogs and cats. Rabbits lack the lysosomal enzyme that breaks down dead cells into a liquid form (Brown, 2001). The abscesses tend to have thick capsules around them and often have finger like projections that extend into surrounding tissues. In order to resolve an abscess in a rabbit, aggressive surgical resection of all affected tissue is often required in addition to systemic antibiotic therapy.

Eye Surgeries

Enucleation

Enucleation is indicated in cases where the eye is too badly damaged by trauma to repair, severe, intractable infection within the globe, end stage glaucoma that is no longer responsive to medical management, and for surgically addressing retrobulbar abscesses. In the rabbit, enucleation should be done using an open technique whereby only the globe is removed versus a technique which involves keeping the eyelids closed and excising around them. This is because there is a large venous sinus that is located dorsomedially in the orbit that the surgeon needs to visualize and avoid. Severe haemorrhage can result if the integrity of this venous sinus is breached. If the sinus is compromised and bleeding occurs it will usually stop with the application of direct pressure for several minutes.

Corneal occlusion syndrome

This is a syndrome seen in rabbits where the conjunctiva grows down over the cornea (Kirschner, 1997). This usually occurs 360 degrees around the eye and does not adhere to the cornea. It doesn't appear to cause any discomfort or impaired vision, as it does not tend to grow over the pupil. Rabbits can live for years with this condition with no apparent ill effects. If any conjunctivitis or irritation does occur, it can be addressed surgically. The excess conjunctiva is cut off and the fornix is then reconstructed by suturing the remaining sides of the conjunctiva back together with 7-0 absorbable suture material.

Entropion Correction

It is not uncommon for rabbits to be afflicted with tiny abscesses along the margin of the eyelids. After they resolve small puckers of scar tissue can form and cause the lid to roll in and abrade the corneal surface. A wedge-shaped incision is used to remove these from the margin of the eyelid.

Gastrointestinal Surgeries

Surgery of the GI tract should be carefully considered in the rabbit. Trichobezoars are often overdiagnosed. It is not uncommon to see a gas shadow around the ingesta in the rabbit's stomach on radiographs. This is especially true if the radiographs were taken while the rabbit was under anaesthesia while the radiographs were being taken. Some rabbits become highly stressed when being induced with gas anaesthesia and swallow air. Since the ingesta in the stomach is fairly thick, it sticks together and the result is a shadow of air around the ingesta. Also, these rabbits are often having abdominal radiographs taken because they have presented with some sign of GI disease. If they are at all dehydrated, the GI tract contents will reflect this better than peripheral assessment of mucous membranes and skin turgor. When rabbits are dehydrated, they tend to pull water out of the GI tract to keep the rest of the body hydrated. This results in a very doughy, thick consistency of the contents. This doesn't necessarily indicate a true hairball that requires surgical intervention. As a result of their normal grooming behaviour, rabbits always have hair in their intestinal tract. Pathological hairballs do exist, but are not terribly common. What is usually happening is that there is a motility disorder. Rehydration and judicious use of motility modifying drugs such as metoclopromide and cisapride as well as increasing the bulk fibre in the diet are more fruitful in resolving these types of disorders. If a true hairball does exist in the stomach, gastrotomy should be performed in the routine manner.

The most common places for rabbits to have an obstruction in their GI tract are the distal duodenum and the ileocolic junction. Survey radiographs are useful for making the diagnosis and with timely surgical intervention rabbits usually recover from enterotomies quite well. Rabbits normally have a very slow GI transit time and thus oral administration of barium for contrast studies is often not useful. Since ileus is so common in rabbits, the main objective when evaluating abdominal radiographs is to distinguish between a case of intestinal obstruction versus ileus. In the case of ileus the gas pattern tends to extend all the way to the colon.

By taking care to understand unique anatomical, physiological and behavioural aspects of the rabbit patient, the veterinary surgeon can make appropriate accommodations for per-operative and surgical care of these increasingly popular pets.

References:

Aron, D., Redig, P., Continuing Education Seminar Notes on Fracture Management in Avian & Exotic Patients, University of Georgia, College of Veterinary Medicine, 1997.

Bradley, T., ICE proceedings, 2001.

Kelleher, S., Wound and Abscess Management in Rabbits, Exotic DVM, 2.3, pp. 49-51, 2000.

Brown, S.A., Abscesses in Rabbits. In: The Small Mammal Health Series. Veterinary Information Network. March 2001.

Harcourt-Brown, F. Textbook of Rabbit Medicine. Oxford: Butterworth Heinemann, 2002

Kirschner, S. E., In: Hillyer, E.V., Quesenberry, K.E., eds. Ferrets Rabbits and Rodents – Clinical Medicine and Surgery. Philadelphia: W.B. Saunders, 1997; pp 339.

Orcutt, C., Use of Convective Heaters witgh Exotic Patients, Exotic DVM 2.3, pp47-48.

Paul-Murphy, J., Reproductive and Urogential Disorders, In: Hillyer, E.V., Quesenberry, K.E., eds. Ferrets Rabbits and Rodents – Clinical Medicine and Surgery. Philadelphia: W.B. Saunders, 1997; pp 202.

Steinleitner, A., Lambert, H., Kazensky, C, Sanchez, I., Sueldo, C., Reduction of Primary Postoperative Adhesion Formation under Calcium Channel Blockade in the Rabbit. J. Surg. Res., 48., 1990. 42-45.

Swaim, S.F., Henderson, R.A., Small Animal Wound Management. Baltimore: Williams & Wilkins, 1997. 305-306.