Question of the Week
August 1, 2011
What do you need to know about venomous snakebites?
It is summer and the snakes are quite active this season and are basking in the sun, just like we humans like to do. In Tennessee there are four types of indigenous venomous snakes: the timber rattlesnake, the pygmy rattlesnake, the cottonmouth (also known as water moccasin, however this causes confusion as some people call any snake on or near water a water moccasin) , and the copperhead.
The venom from these snakes is a complex mixture of proteolytic enzymes. In general, the venom has a local effect on the bite site: local necrosis and swelling. As venom is absorbed and distributed up the limb through the lymphatic system and venous system, swelling and necrosis may progress up the limb as well. Systemic symptoms include nausea and vomiting, headache, metallic taste in mouth, massive edema, hypotension, hemolysis, and coagulopathy similar to DIC, with decreasing fibrinogen and increasing fibrinogen split products. Thrombocytopenia occurs, but is more pronounced in timber rattlesnake envenomations. Approximately 20% of bites are dry; no venom is injected and they are simply a puncture wound. Bites from copperheads tend to be milder compared to other crotalids.
Take off any jewelry or other potentially tight fitting items to avoid a tourniquet effect as swelling progresses. Elevation of the affected extremity should be instituted upon presentation to help reduce extremity edema. Marking the progression of swelling and timing the marks help to discern the rapidity of cytotoxicity. Circumferential measurements provide an objective measure of swelling that can be compared over time. Obtain blood to check hemoglobin and hematocrit, platelets, INR, fibrinogen and fibrinogen split products, BUN and creatinine. A urinalysis may detect presence of hemoglobinuria if there is red cell destruction. A type and screen/match may be necessary if hemoglobin/hematocrit or platelets are low. Administration of antivenin is determined by progression of swelling, bullae, and systemic symptoms. Based on this assessment, and after discussion with the medical toxicologist, Crotaline Fab antivenin (CroFab) is administered until no progression of edema or systemic symptoms occurs, including resolution of coagulopathy. Additional doses of antivenin are given at 6, 12 and 18 hours to prevent recurrence symptoms which may occur because of clearance of the Fab fragments and unbinding from the venom. As treatment progresses, reassessment of the envenomation is important to see if it changes with therapy or if additional antivenin is required. Platelet transfusions are generally not needed, as long as no active bleeding is occurring or invasive procedures planned. Most patients tolerate thrombocytopenia even as low as 10-12,000 platelets/mm3.
During the evaluation and treatment phases, opioids are typically required to provide adequate pain control. Active range of motion (AROM) is encouraged and physical therapy or occupational therapy should be considered to help with maintaining AROM in the hospital and on discharge. Appropriate wound care is determined by the degree of tissue destruction. Prophylactic antibiotics are generally not necessary, but if a wound infection occurs over the ensuing days, then antibiotics should be prescribed.
This question prepared by: John Benitez, MD, MPH Medical Toxicologist