The Bite Wound Patient: A Potential Surgical Emergency
Dr David Holt, BVSc, Diplomate ACVS
The initial appearance of many bite wounds is deceptive, because the majority of tissue damage occurs below the skin. On limbs, the teeth of the attacking animal often fracture bones. Bites located over the thorax and abdomen frequently penetrate these body cavities; in addition, the teeth crush and lacerate large areas of muscle and subcutaneous fat and create large areas of dead space, whilst leaving only small puncture wounds in the skin.
In general terms, all bite wounds should be considered at least "Contaminated"; that is they are severely contaminated with bacteria. These wounds can also contain foreign debris and a substantial amount of nonviable tissue. Bacteria rapidly proliferate and invade the tissues, creating a "Dirty" or "Infected" wound. Once the bite wound patient is stabilized, the surgeon must focus on: 1. Assessing the extent of the injuries, 2. Minimizing the bacterial population in the wound through aggressive wound debridement and lavage, 3. Repairing fractures and defects in the thoracic and abdominal walls, and 4. Deciding between immediate and delayed wound closure.
In a survey of bite wounds presented to The Emergency Service at the Veterinary Hospital, University of Pennsylvania, the most common sites for bite wounds were: 1. The limbs, 2. The head, 3. The neck, 4. Regions over the thoracic and abdominal cavities, and 4. The perineal region. Viability of an affected limb can be clinically assessed by the temperature and color of the leg, and by cutting a toenail if systemic blood pressure is adequate. Measurement of toe web temperature and selective angiography may be necessary in some cases. The possibility of normal function should be determined after careful orthopedic and neurologic examination of the leg. Animals with head and neck wounds should have careful examinations focusing on possible damage to the central, peripheral nervous systems and the upper airway, and fractures of the skull, mandible, and cervical vertebrae. The possibility of penetration into the thoracic and abdominal cavities can be assessed by radiographs and either thoracocentesis or peritoneal lavage. Blunt probing of these wounds to determine their extent should be avoided as the results are often misleading; if there is any doubt about cavity penetration, the wound should be thoroughly explored. Perineal wounds are often severely contaminated even if the rectum is not perforated, as bacteria contaminating the perineal skin can colonize the entire wound rapidly.
Stabilization of the patient before anesthesia is imperative; many of these animals present suffering from poor tissue perfusion due to shock. In the majority of cases, this can be corrected by aggressive therapy with intravenous crystalloid solutions, with colloid solutions such as blood, plasma or synthetic colloids given if necessary. However, in those animals presenting with severe septic shock, complete stabilization may not be possible because of infection of necrotic tissue in the bite wounds. This can result in a vicious cycle, as hypovolemic shock has been shown to contribute to increased wound infection rates in humans. Broad spectrum bacteriocidal antibiotics should be administered perioperatively. A penicillin should be included in the regimen to kill anaerobic bacteria. Postoperative antibiotic therapy should be based on the results of aerobic and anaerobic culture and sensitivity tests.
Once the animal is anesthetized, a sterile, water soluble gel should be placed in the wounds to prevent any further contamination during preparation. A extremely large area is clipped around the wounds and prepared for aseptic surgery. Debridement of all necrotic and infected tissue is vital for successful wound management. Any remaining necrotic tissue will harbor bacteria and prevent effective phagocytosis. Debridement begins superficially and progresses deeper into the wound. Tissue viability can be judged at surgery by bleeding. Viable muscle will twitch when gently pinched. Loss of function rarely occurs after even extensive muscle removal. Debridement is best performed with sharp dissection. The use of electrocautery should be kept to a minimum. Vital structures such as nerves and tendons should be preserved wherever possible. In some cases, the surgeon must recognize that preservation of a limb is not possible and be prepared to perform an amputation.
During exploration, the wounds should be copiously lavaged. In the early post-wounding period, bacteria can be dislodged by lavage delivered at a pressure of eight pounds per square inch; later, the beneficial effect of lavage seems to be derived from loosening and removal of debris, necrotic tissue and blood clots from the wound. Volume lavage alone is ineffective; the appropriate pressure for lavage can be generated by using a 35cc syringe and a 19 gauge needle. Higher lavage pressures may be deleterious by forcing bacteria deeper into a wound and opening deeper tissue planes. The author prefers warm sterile saline solution for lavage.
Fractures should be explored and small avascular fragments removed. Larger fragments may be useful for reconstruction. Opinion is divided over immediate repair. Depending on the fracture and the extent of surrounding soft tissue injuries, some surgeons may choose to repair the fracture at the initial surgery. Others may immobilize the limb in a cast or splint for 24 to 48 hours with the objective of having healthy soft tissues surrounding the fracture at the time of definitive fracture repair. Multidimensional external fixateurs and bone plates provide the necessary rigid fixation for definitive repair. A large autogenous cancellous bone graft should be placed in and around the fracture after repair and lavage.
The decision on closure of a bite wound after debridement and lavage is dependent on several factors. These include the wound's location, the degree of remaining contamination, adequacy of blood supply, wound tension, and the status of the patient. In the absence of quantitative bacterial cultures, the surgeon uses judgement to determine the probable success of closure. If any doubt exists concerning the viability of tissue or amount of infection remaining in the wound, closure should be delayed. Defects created in the thoracic and abdominal wall by debridement of necrotic tissue require reconstruction. Healthy tissue with an adequate blood supply should be used instead of mesh wherever possible. Some defects in the thoracic wall can be managed by advancing the diaphragm. Other muscle flaps available include the latissimus dorsi, external abdominal oblique, and cranial sartorius.
Primary closure in bite wounds requires strict adherence to basic surgical principles. Only healthy tissue should be present after debridement and lavage. Hemostasis must be excellent, as blood clots provide a superior medium for bacterial growth. Tissue layers should be gently apposed to minimize dead space, using as few fine, absorbable sutures as possible. Ideally, the wound should be tension free. One or more Penrose drains may also be required for dead space. They should exit through separate stab incisions in the sterile field, and be covered by a sterile dressing postoperatively.
In many bite wounds, delayed primary closure is the best alternative for wound management. The wound is left open after the initial debridement, and covered with a sterile, permeable dressing, such as vaseline gauze. This is surrounded by a thick absorbent layer of padding, a conforming bandage, and an elastic, adhesive bandage layer. Delayed primary closure allows for ongoing assessment of wound healing. Closure is usually performed 3 to 5 days after the initial surgery, unless further debridement is necessary. In some areas such as the distal limbs, extensive skin loss necessitates the use of a skin graft or flap for definitive repair. A healthy bed of granulation tissue should be allowed to develop before a free skin graft is performed.
Postoperatively, the bite wound patient is monitored carefully. Respiratory function should be assessed using clinical parameters (respiratory rate and effort) and blood gas measurement or pulse oximetry if the chest was damaged by the bite. Cardiovascular status and perfusion are evaluated frequently. These animals are at risk of sepsis in spite of early, aggressive medical management and wound debridement. Tissues which appeared healthy on initial inspection can progress to necrosis in spite of appropriate antibiotic therapy. Any deterioration in the animal's condition should raise suspicion of ongoing tissue necrosis in the wound. The wound(s) are inspected, and if there is any doubt about tissue viability, re-explored. The wound is aggressively lavaged and debrided to healthy, bleeding tissue.