Case # 28 – Envenomation
Author: Jason Murphy, MD
Peer Reviewer: Jenny Lu, MD
A mother brings her 15 year old son to your ED. She states that he was out in the national park near their house when he was bitten by what he claims was a rattlesnake. The mother confirms that she also saw the rattlesnake. He was bitten on both his right arm and left leg, and he is writhing in pain. His past medical history is unremarkable.
Vitals: Temp: 98.2, HR: 115, RR: 25, BP: 120/75, O2 Sat: 100% on RA.
On exam, your worst fears are confirmed when you note his right arm and left leg are swollen, covered in black necrotic eschar, and there is serosanguanous drainage from both sites. You also note blood in his oral and nasal mucosa.
What are the two major categories of venomous snakes in the United States? How can you differentiate them?
There are five families of venomous snakes worldwide. Two of these families, Viperidae (subfamily Crotalinae) and Elipadae, cause the majority of envenomations in the United States.
| |
Viperidae |
Elapidae |
| Appearance |
Elliptical eyes, central pit between the eyes, triangular heads. |
Round eyes, no central pit, double row of plates at distal tail, brightly colored.
Coral snakes: Red/yellow/black bands, with the red and yellow bands touching each other.
“Red on yellow, kill a fellow; Red on black, venom lack.” |
| Mechanism of Toxicity |
Necrotoxins cause local tissue necrosis. Hemotoxins cause coagulopathy. |
Irreversibly binds acetylcholine receptors causing systemic toxicity |
| Symptoms |
Severe pain and swelling at the initial site, ecchymoses, hemorrhagic bullae, rhabdomyolysis, metallic taste, fasciculations, coagulopathy |
Minimal local reaction, weakness, numbness, cranial neuropathies, respiratory paralysis. |
| Treatment |
Immobilize the extremity, antivenin |
Supportive care |
| Common species |
Rattlesnakes, Copperheads, and Water Moccasin |
Coral snakes, Cobras, Mambas, and Kraits. |
Does every bite by a venomous snake result in an envenomation? What findings would you expect on physical exam to suggest envenomation?
Every species of snake is capable of controlling how much venom they inject. Because venom consumes energy to make and only so much can be stored at one time, it is important for them to be able to control how much they use. If someone who is bitten develops no findings of envenomation, it is called a ‘dry bite’. Approximately 25% of snake bites by poisonous snakes are ‘dry bites’. With Viperidae, envenomation will often produce significant tissue injury near the bite. Alternatively, Elapidae envenomations may not cause a significant local reaction and symptoms may be delayed, but they can cause significant systemic symptoms.
What labs do you want? What are you looking for?
Hemotoxin: This venom acts like endogenous thrombin, thereby producing a state of Disseminated Intravascular Coagulation (DIC). When this venom is suspected, you should obtain a
CBC,
PT/INR/PTT,
fibrinogen, and
fibrin split products (FSP). In cases of mild envenomation, abnormalities in these can suggest the need for antivenom.
Necrotoxin: This venom causes severe tissue and muscle necrosis can lead to rhabdomyolysis. When this venom is suspected, you should obtain a BMP, Phosphorus level, CK, and Urinalysis. Hydration is the key to preventing rhabdomyolysis in severe envenomation.
Your patient’s labs are remarkable for platelets of 125, INR 2.2, elevated FSP, low fibrinogen and normal electrolytes and CK.
What is the recommended treatment for this envenomation?
In the United States, all envenomations by Viperidae (except the Mojave rattlesnake) are covered by CroFab™ (polyvalent Crotalidae ovine immune Fab). The dosing is based upon symptomatology, in conjunction with the help of a trained toxicologist. This has replaced horse serum-derived antivenom, which was associated with a significant risk of anaphylaxis and serum sickness.
As he is being wheeled up to the ICU, his mother asks you if there is anything she could have done differently before they went to the hospital (tourniquets, suction devices, immobilizations) that could have helped her son. What does the evidence say?
Tourniquets are not indicated for envenomations of any kind. Mouth to wound suctioning has little effect on removing the venom and carries a significant risk of infecting the wound. Commercial suction kits are able to remove very little (if any) venom.
However, there is evidence that light lymphatic compression dressings and immobilization of the extremities may decrease the systemic absorption of venom without sacrificing local tissue.
Is there any role for antibiotics?
Evidence suggests that the rate of bacterial infection following dry bites and mild envenomations not requiring hospitalization is very low and prophylactic antibiotics are not recommended. Everyone bitten by a reptile should have their tetanus updated.
References:
Corbett SW, Anderson B, Nelson B, et al. Most lay people can correctly identify indigenous venomous snakes. Am J Emerg Med. 2005 Oct;23(6):759-62.
Hall EL. Role of surgical intervention in the management of crotaline snake envenomation. Ann Emerg Med. 2001 Feb;37(2):175-80.
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 Feb 3;11:2.
LoVecchio F, Klemens J, Welch S, et al. Antibiotics after rattlesnake envenomation. J Emerg Med. 2002 Nov;23(4):327-8.
McKinney PE. Out-of-hospital and interhospital management of crotaline snakebite. Ann Emerg Med. 2001 Feb;37(2):168-74.
Shaw BA, Hosalkar HS. Rattlesnake bites in children: antivenin treatment and surgical indications. J Bone Joint Surg Am. 2002 Sep;84-A(9):1624-9.
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