Strange and Schafermeyer's Pediatric Emergency Medicine, Fourth Edition (Strange, Pediatric Emergency Medicine), 4th Ed.

CHAPTER 133. Snake Envenomations

Patrick M. Lank

Timothy B. Erickson

HIGH-YIELD FACTS

• Pit vipers (Crotalids) account for the majority of envenomations in pediatric patients. Because of their small body weight, young children are relatively more vulnerable to severe envenomation.

• Pit viper (Crotalinae) envenomations result in hematotoxicity while coral snakes (Elapidae) cause neurotoxicity.

• Crotaline snakes are responsible for the vast majority of snake envenomations in the United States. Identification of exact species is not essential since treatment is the same for all indigenous American pit vipers.

• Prehospital management of snakebites includes immobilization of the bitten extremity, minimization of physical activity, fluid administration. No “first aid” technique has been demonstrated to improve outcome after envenomation. Rapid transport for administration of antivenom is the most important intervention in prehospital care.

• Antivenom, such as Crotaline Fab antivenom, consisting of highly purified papain-digested antibodies, is the current standard of care for treatment of crotaline snake envenomation.

• Antivenom dosing in pediatric patients is based on potential venom load, not kilogram size of the patient.

EPIDEMIOLOGY

Worldwide, approximately 30,000 fatal snakebites are sustained each year.1 Of the 120 snake species that are indigenous to the United States, approximately 20% are venomous (Table 133-1). Venomous snakes are classified into two families: Viperidae and Elapidae. Crotalinae is a subfamily of Viperidae better known as pit vipers due to the heat-sensing organs on either side of the head. The Crotalinae subfamily includes three genera: Crotalus (rattlesnakes), Agkistrodon (copperheads and cottonmouths), and Sistrurus (massasaugas).2 While copperhead (Agkistrodon contortrix) and cottonmouth (Agkistrodon piscivorus) snakes are primarily found in the southern and eastern United States, several species of rattlesnake are found throughout the continental United States. Although snakebites are typically underreported, it is estimated that 8000 crotaline envenomations occur in the United States annually. Approximately 30% of snakebites called to US poison centers involve patients younger than 20 years, with 12% of all cases being children aged 9 years or younger.3

TABLE 133-1

Indigenous Poisonous Snakes of the United States

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PIT VIPERS

image ANATOMY

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

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FIGURE 133-1. 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.)

image PATHOPHYSIOLOGY

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.8The major toxic effects occur within the surrounding tissue, blood vessels, and blood components.

image CLINICAL PRESENTATION

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.

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FIGURE 133-2. If the pit viper envenomation is severe, extremity swelling, edema, and hemorrhagic vesicles can appear within hours. (ernursey.blogspot.com/2007archive.html)

image PREHOSPITAL MANAGEMENT

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.10,11Cryotherapy 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).

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FIGURE 133-3. Pit viper wound grading system.

DIAGNOSTIC STUDIES

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.

HOSPITAL MANAGEMENT

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

ANTIVENOM THERAPY

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.5,16,2024 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

DISPOSITION

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.2830 Therefore, patients with presumed Mojave envenomation should be admitted and observed for a minimum of 24 hours.

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FIGURE 133-4. Mojave rattlesnake. (www.elmerfudd.us/dp/pictures/animals.htm)

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.

CORAL SNAKES

Three members of the coral snake family (Elapidae) are indigenous to the United States: the Sonoran coral snake (Micruroides euryxanthus) found in Arizona and New Mexico; the more venomous eastern coral snake (Micrurus fulvius fulvius) found in the Carolinas and the Gulf states; and Texas coral snake (Micrurus fulvius tenere). Coral snakes account for only 2% of annual snakebites in the United States.3

Coral snakes have rounded heads and circular pupils similar to many nonvenomous species. The coral snake is often mistaken for certain varieties of the nonvenomous kingsnakes, scarlet snake, and milk snake because they all have red, yellow, and black rings. The old adage “red on yellow, kill a fellow; red on black, venom lack” helps distinguish the venomous coral snake from the nonvenomous snakes (Fig. 133-5). Narrow yellow rings separate larger red and black bands on a coral snake while black rings are adjacent to red bands in the scarlet kingsnake. It is important to note that these distinguishing patterns are only accurate in North America since highly venomous South American coral snakes and other snakes worldwide may have red and black adjacent bands. The smaller, nonretractable fangs of a coral snake may leave little evidence of envenomation; therefore, any suspicion of an elapid bite warrants medical evaluation.

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FIGURE 133-5. “Red on yellow, kill a fellow; red on black, venom lack” helps distinguish the venomous North American coral snake from the nonpoisonous scarlet king snake. (http://www.tpwd.state.tx.us/learning/junior naturalists/compare.phtml)

image CLINICAL PRESENTATION

As with other Elapidae, the venom of the coral snake is primarily neurotoxic. The bite site will initially exhibit local cutaneous edema and tenderness. However, given their fangs anatomic differences when compared with Crotalidae, there have been reports of envenomation without evidence of actual tooth marks. Within several hours of significant coral snake envenomation, the patient may experience paresthesias, vomiting, weakness, diplopia, fasciculations, confusion, and occasionally respiratory depression. Convulsions have been observed in smaller children. Prior to the introduction of coral snake antivenom, the fatality rate from eastern coral snake bites was said to be as high as 10%,3 but with the use of supportive respiratory care and antivenom, fatalities from coral snake envenomations are incredibly rare.

image MANAGEMENT

Coral snake bites are treated conservatively since clinical manifestations may be delayed up to 12 hours but can lead to neurologic symptoms and respiratory depression within 24 hours.31 Antivenin (Micrurus fulvius) (Equine Origin) had previously been routinely administered in patients suspected to have true envenomation from an eastern coral snake. In recent years the pharmaceutical company that previously made the antivenom has since discontinued its manufacture. However, the pharmaceutical company has annually extended previous lots’ expiration dates and encouraged hospitals in areas where the eastern and Texas coral snakes are indigenous to keep their existing inventory of antivenom.32 Given the extremely limited supply of North American Coral Snake Antivenin, anyone considering using the hospital’s supply of antivenom is strongly encouraged to contact their regional poison center.

image DISPOSITION

Any child who has sustained a documented bite from a coral snake should be admitted to the intensive care unit for observation, airway management, and potential antivenom administration.

EXOTIC SNAKES

Several bites occur each year from nonindigenous snakes.33 Many of these snakes are illegally imported into the United States as exotic pets or purchased over the Internet from international distributors. Physicians encountering victims of exotic snake envenomation may receive assistance in treatment by calling the regional poison control center who may, in turn, contact local zoo’s herpetologists. The general approach is local wound care, supportive treatment, and specific antivenom therapy, if available.

REFERENCES

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2. Sing KA, Erickson TB, Aks SE, et al. Eastern massasauga rattlesnake envenomations in an urban wilderness. J Wild Med. 1994;5:77–87.

3. Seifert SA, Boyer LV, Benson BE, Rogers JJ. AAPCC database characterization of native U.S. venomous snake exposures, 2001-2005. Clin Toxicol. 2009;47:327–335.

4. Norris RL, Bush SP. North American venomous reptile bites. In: Auerbach PS, ed. Wilderness Medicine. 5th ed. St. Louis, MO: Mosby; 2007:1051–1085.

5. Richardson WH 3rd, Barry JD, Tong TC, Williams SR, Clark RF. Rattlesnake envenomation to the face of an infant. Pediatr Emerg Care. 2005;21:173–176.

6. Matteucci MJ, Hannum JE, Riffenburgh RH, Clark RF. Pediatric sex group differences in location of snakebite injuries requiring antivenom therapy. J Med Toxicol. 2007;3:103–106.

7. Richardson W, Offerman S, Clark R. Snake envenomations. In: Erickson T, Ahrens W, Aks S, et al. Pediatric Toxicology: Diagnosis & Management of the Poisoned Child. New York, NY: McGraw-Hill; 2004:548–555.

8. Iyaniwura TT. Snake venom constituents: biochemistry and toxicology (part 1). Vet Human Toxicol. 1991;33:468–474.

9. Gulati A, Isbister GK, Duffull SB. Effect of Australian elapid venoms on blood coagulation: Australian Snakebite Project (ASP-17). Toxicon. 2012;11:94–104.

10. Bush SP, Hegewald KG, Green SM, Cardwell MD, Hayes WK. Effects of a negative pressure venom extraction device (extractor) on local tissue injury after artificial rattlesnake envenomation in a porcine model. Wilderness Environ Med. 2000;11:180–188.

11. Alberts MB, Shalit M, LoGalbo F. Suction for venomous snakebite: a study of “mock venom” extraction in a human model. Ann Emerg Med. 2004;43:181–186.

12. Ben Welch E, Gales BJ. Use of stun guns for venomous bites and stings: a review. Wilderness Environ Med. 2001;12:111–117.

13. Guderian RH, Mackenzie CD, Williams JF. High voltage shock treatment for snake bite. Lancet. 1986;26:229.

14. American College of Medical Toxicology, American Academy of Clinical Toxicology, American Association of Poison Control Centers, et al. Pressure immobilization after North American Crotalinae snake envenomation. Clin Toxicol. 2011;49:881–882.

15. Clark RF, Selden BS, Furbee B. The incidence of wound infection following crotalid envenomation. J Emerg Med. 1993;11:583–586.

16. Tanen DA, Danish DC, Clark RF. Crotalidae polyvalent immune Fab antivenom limits the decrease in perfusion pressure of the anterior leg compartment in a porcine crotaline envenomation model. Ann Emerg Med. 2003;41:384–390.

17. Tanen DA, Danish DC, Grice GA, Riffenburgh RH, Clark RF. Fasciotomy worsens the amount of myonecrosis in a porcine model of crotaline envenomation. Ann Emerg Med. 2004;44:99–104.

18. Corneille MG, Larson S, Stewet RM, et al. A large single-center experience with treatment of patients with crotalid envenomations: outcomes with and evolution of antivenin therapy. Am J Surg. 2006;192:848–852.

19. Cumpston KL. Is there a role for fasciotomy in Crotalinae envenomations in North America? Clin Toxicol. 2011;49:351–365.

20. Clark RF, Williams SR, Nordt SP, Boyer-Hassen LV. Successful treatment of crotalid-induced neurotoxicity with new poly-specific crotalid Fab antivenom. Ann Emerg Med. 1997;30:54–57.

21. Pizon AF, Riley BD, LoVecchio F, Gill R. Safety and efficacy of Crotalidae Polyvalent Immune Fab in pediatric crotaline envenomations. Acad Emerg Med. 2007;14:373–376.

22. Trinh HH, Hack JB. Use of CroFab antivenin in the management of a very young pediatric copperhead envenomation. J Emerg Med. 2005;29:159–162.

23. Dart RC, McNally J. Efficacy, safety and use of snake antivenoms in the United States. Ann Emerg Med. 2001;37:181–188.

24. Offerman SR, Bush SP, Moynihan JA, Clark RF. Crotaline Fab antivenom for the treatment of children with rattlesnake envenomation. Pediatrics. 2002;110:968–971.

25. Lavonas EJ, Ruha AM, Banner W, et al. Unified treatment algorithm for the management of crotaline snakebite in the United States: results of an evidence-informed consensus workshop. BMC Emerg Med. 2011;11:2.

26. Seifert SA, Boyer LV. Recurrence phenomena after immunoglobulin therapy for snake envenomations: Part I. Pharmacokinetics and pharmacodynamics of immunoglobulin antivenoms and related antibodies. Ann Emerg Med. 2001;37:189–195.

27. Miller AD, Young MC, DeMott MC, et al. Recurrent coagulopathy and thrombocytopenia in children treated with crotalidae polyvalent immune fab: a case series. Pediatr Emerg Care. 26:576-582, 2010.

28. Farstad D, Thomas T, Chow T, Bush S, Stiegler P. Mojave rattlesnake envenomation in southern California: a review of suspected cases. Wilderness Environ Med. 1997;8:89–93.

29. Jansen PW, Perkin RM, Van Stralen D. Mojave rattlesnake envenomation: prolonged neurotoxicity and rhabdomyolysis. Ann Emerg Med. 1992;21:322–325.

30. Bush SP, Siedenburg E. Neurotoxicity associated with suspected Southern Pacific rattlesnake (Crotalus viridis helleri) envenomation. Wilderness Environ Med. 1999;10:247–249.

31. Kitchens CS, Van Mierop LH. Envenomation by the Eastern coral snake (Micrurus fulvius fulvius). A study of 39 victims. JAMA. 1987;258:1615–1618.

32. Pfizer, Inc. Important product supply information: antivenin (Micrurus fulvius) (Equine origin) North American coral snake antivenin. http://www.pfizer.com/files/products/hcp_antivenin.pdf. New York, NY: Pfizer, Inc; 2012.

33. Chippaux JP. Snake bites: appraisal of the global situation. Bull World Health Organ. 1998;76:515–524.