A few anatomic characteristics differentiate venomous pit vipers from nonvenomous snakes. Pit vipers possess a triangular or arrow-shaped head whereas nonvenomous North American snakes have a smooth, tapered body and narrow head. Crotalids have facial pits between the nostril and the eye that serve as heat and vibration sensors, enabling the snake to locate prey (Fig. 133-1). While nonvenomous snakes typically possess round pupils, pit vipers have vertical or elliptical pupils. Members of the genus Crotalus also have characteristic tail rattles and a single row of ventral anal scales.4
Pit vipers classically possess a triangular or arrow-shaped head and facial pits between the nostril and eye that serve as heat and vibration sensors, enabling the snake to locate prey. (Reproduced with permission from Auebach PS. Wilderness Medicine. 5th ed. Philadelphia: Mosby/Elsevier; 2007.)
Since snakes are defensive animals and rarely attack, they will remain immobile or even attempt to retreat if given the opportunity. Bites most commonly occur in small curious children or in individuals who handle and harass the snake. Because of their small body weight, infants and young children are relatively more vulnerable to severe envenomation. The severity of envenomation also depends on the location of the bite. Bites on the head, neck, or trunk can be more severe than extremity bites.5 Bites on the upper extremities are most common and potentially more dangerous than those on the lower extremities although lower extremity bites may result in delayed clinical signs of toxicity. Male children are more likely than female children to suffer Crotaline snakebites that require antivenom therapy. Males are also more likely to be bitten in the upper extremities.6 Direct envenomation into an artery or vein is associated with a much higher mortality rate.
It is important to remember that when envenomation occurs, the smaller pediatric patient is generally exposed to a greater milligram per kilogram venom load, so treating clinicians should anticipate a higher likelihood of systemic symptoms. Intravenous antivenom is always the first-line therapy and dosing should be targeted toward the potential venom load and its clinical sequelae, as opposed to the patient's weight.7
Crotaline venom is a complex mixture of enzymes that primarily function to immobilize, kill, and digest the snake's prey. Proteolytic enzymes cause muscle and subcutaneous necrosis as a result of a trypsin-like action. Hyaluronidase decreases the viscosity of connective tissue, phospholipase provokes histamine release from mast cells, and thrombin-like amino acid esterases act as defibrinating anticoagulants.8 The major toxic effects occur within the surrounding tissue, blood vessels, and blood components.
Local cutaneous changes classically include one or two puncture marks with pain and swelling at the site while nonvenomous snakes may leave a horseshoe-shaped imprint of multiple teeth marks. Approximately 25% of all pit viper bites are considered “dry bites” resulting in no toxicity.3 Children younger than 6 years are more likely than older children and adults to sustain “dry bites,” although when envenomed, these young children are more likely to have major effects from the envenomation.3 If the envenomation is severe, swelling and edema may involve the entire extremity within 1 hour. Ecchymosis, hemorrhagic vesicles, and petechiae may appear within several hours (Fig. 133-2). Systemic signs and symptoms include paresthesias, periorbital fasciculations, weakness, diaphoresis, nausea, dizziness, and a “minty” or metallic taste in the mouth. Severe bites can result in coagulopathies, thrombocytopenia, and disseminated intravascular coagulation (DIC)-like syndrome called venom-induced consumption coagulopathy.9 Rapid hypotension and shock, with pulmonary edema and renal and cardiac dysfunction, can also result, particularly if the victim suffers a direct intravascular envenomation.
The victim's extremity should be immobilized and physical activity minimized, with the primary goal of prehospital management being evacuation to a healthcare facility that can deliver antivenom if needed. Certain first aid measures can be dangerous and exacerbate limb morbidity. Incision and suction of the bite wound with the human mouth is contraindicated as it will result in increased tissue damage and poses a high risk of infection. Mechanical suction devices exist but are not recommended for use. Studies using both animal and human models have found suction devices to be inadequate in venom extraction and possibly contribute to increased local tissue damage.10,11 Cryotherapy can lead to further wound necrosis and is not currently recommended. Electric shock therapy was historically publicized as a first aid treatment for snakebites, but again case reports and animal studies have not documented any improvement with this prehospital technique.12,13 In fact, using the technique suggested by the original case series, in which the authors suggest that “an outboard motor is one commonly available source of such a current,” could directly lead to a great amount of local tissue and systemic injury. It should also be noted that although routine in other areas of the world, pressure immobilization for North American Crotaline snakebite is not recommended because evidence to demonstrate decreased systemic toxicity is lacking, and there is evidence that suggests increased extremity compartment pressures when it is used.14
Optimal therapy instead consists of placing the patient at rest with the affected extremity placed at cardiac level. Emergency evacuation should be arranged as quickly as possible for transport to the closest facility with access to antivenom. During transport, the wound site should be measured and leading edges marked, so that symptom progression can be judged upon hospital arrival. Intravenous access is obtained if possible and analgesics administered as needed. Crotaline snakebite wounds are generally graded as minimal, moderate, and severe based on the degree of envenomation, which can ultimately guide therapy (Fig. 133-3).
Pit viper wound grading system.
Laboratory studies recommended in the assessment of rattlesnake bites include a complete blood count including platelets, prothrombin time or international normalized ratio (INR), partial thromboplastin time, fibrinogen level, and fibrin degradation products. Abnormal hematologic parameters are considered evidence of systemic toxicity and should be incorporated along with clinical examination into the decision to administer antivenom. If initial laboratory testing is normal and minor local tissue swelling and pain are present, it is acceptable to reevaluate these hematologic laboratory values in 6 hours unless there is objective evidence of worsening clinical status. Other laboratory testing such as chemistry panels, creatinine phosphokinase (CPK), and urinalysis should be monitored if evidence of rhabdomyolysis, myoglobinuria, or renal insufficiency is present.
Patients presenting after rattlesnake envenomation should be given tetanus prophylaxis if indicated. Affected extremities should be elevated to the level of the heart and any previously placed constriction bands or wraps removed. Intravenous access in an unaffected extremity should be established for the delivery of antivenom as well as analgesic medications. The liberal use of opioid analgesics is often necessary to control pain. Prophylactic antibiotics are not recommended since rattlesnake venom possesses its own bacteriostatic properties. However, evidence of infection or a history of human mouth suction to the wound may be indications for wound culture and initiation of a first-generation cephalosporin or amoxicillin/clavulanate.15
While prophylactic fasciotomy and digital dermatomy have been advocated as routine crotaline snakebite treatments in the past, these techniques are discouraged and rarely indicated. A true compartment syndrome is unlikely following rattlesnake envenomation.16,17 Rattlesnake strikes generally place venom subcutaneously, not subfascially. The tense edema that is frequently apparent is usually a result of swelling and necrosis of the subcutaneous tissues. Additionally, the myonecrosis seen microscopically in these cases is a direct result of snake venom and not because of increased compartment pressures. As a result, these bitten extremities should not be managed like other potential compartment syndromes. The preferred treatment for significant limb swelling is intravenous antivenom. Surgical therapy should only be considered in children who have had aggressive intravenous antivenom therapy and after consultation with a medical toxicologist, regional poison center, or physician specializing in the medical treatment of envenomation. While surgical debridement of devitalized tissues or amputation of necrotic digits may become necessary after wound stabilization, there is inadequate evidence to support the use of fasciotomies for snakebite-associated elevated compartment pressures, and some evidence to suggest worse outcome if fasciotomies are performed.18,19
Crotalidae polyvalent immune Fab antivenom is an ovine (sheep serum) preparation that is highly purified and consists of only the smaller Fab antibody fragments. Crotaline Fab antivenom is equally effective and safer than the older antivenin crotalidae polyvalent (ACP) product, resulting in a significant reduction in the rates of allergic reaction.5,16,20–24 The older antivenin, ACP, is no longer being produced.
Crotaline Fab antivenom is administered intravenously for patients with moderate to severe envenomation (Fig. 133-3). It is important to remember that dosing is based on venom load as opposed to the kilogram weight of the patient. Patients with envenomation symptoms should initially receive 4–6 vials of Crotaline Fab regardless of the child's size. Lyophilized antivenom must be initially gently reconstituted in 10–25 mL of sterile water before dilution in 250 mL of 0.9% normal saline. Although the use of Crotaline Fab has a much lower rate of anaphylactoid reactions than older antivenoms, it is still recommended that the initial infusion be started slowly and increased as tolerated with a goal of finishing the infusion of 4–6 vials within 60 minutes. In small children, it has been suggested that the reconstitution volume can be reduced and the amount of fluid given as antivenom can be subtracted from any additional maintenance fluid given.21 However, since the vast majority of pediatric envenomations occur in otherwise health children who weigh more than 10 kg, this initial bolus of fluids is generally well tolerated.
After the initial dose of 4–6 vials of antivenom, the child should be assessed for local tissue symptoms and hematologic abnormalities. If control has been achieved, we recommend maintenance antivenom dosing of 2 vials every 6 hours for three doses unless the child's symptoms are minor or if there has been dramatic improvement. If, however, symptoms were not controlled with the initial bolus of antivenom, that same 4–6 vials dose should be repeated and symptoms reassessed as with the initial dose.25
While the severity of acute side effects associated with the new crotaline Fab antivenom appears to be much lower than that of equine-based antivenom, patients should still be observed closely for anaphylactoid reactions. Slowing the infusion rate and administering intravenous diphenhydramine can easily treat most of these reactions. The incidence of serum sickness is low when crotaline Fab is used regardless of the number of vials given in the course of treatment.7
A phenomenon of recurrent hematologic venom effects has been observed following stabilization using crotaline Fab antivenom.26 It is thought that this effect may be due to an imbalance of the physiologic “half-lives” of the venom and the antivenom—that the renal clearance of Fab antivenom is faster than the duration of “depot” venom at the wound site. Therefore, patients should be rechecked for recurrence of local and hematologic venom effects at 48–72 hours after stabilization. The treatment of a patient who has possible recurrence of venom effects should be discussed with a medical toxicologist or other physician expert in envenomations, as decisions to use antivenom or various blood products are complex and controversial.25,27
Asymptomatic patients presenting after a crotaline strike should be observed for a minimum of 8 hours following the injury. If no symptoms or signs of envenomation develop, the patient may be safely discharged with the diagnosis of a “dry” (nonenvenomated) bite. One exception to this rule would include patients with a bite from a Mojave rattlesnake (C. scutulatus scutulatus) (Fig. 133-4). These snakes have been associated with delayed onset of significant neurotoxic symptoms.28–30 Therefore, patients with presumed Mojave envenomation should be admitted and observed for a minimum of 24 hours.
Patients with minor symptoms should be admitted for 24-hour observation. All patients initially treated with antivenom should be admitted to the intensive care unit for further antivenom therapy, monitoring for anaphylactoid reactions, wound care, and analgesia. Wound checks including extremity measurements should be performed hourly during the initial phase of treatment until symptoms have stabilized.