Rattlesnakes are found throughout the Americas and include snakes in the genera Crotalus and Sistrurus. They are also known as pit vipers, lanceheads, and Asian pit vipers (genus Trimeresurus). Their primary defense mechanism is to hide, and then rattle and hiss which are meant to scare predators away. When they are further challenged, they bite and envenomate with potentially deadly effects. Knowing the common snakes in your area is important in managing a snake bite appropriately.
The morbidity and mortality associated with snake bites are usually due to the envenomation. Snake bite wounds usually do not get infected due to the relatively inhibitory effect of the venom on microorganisms. Humans often get bit by stepping on snakes mistakenly or by getting extremely close to them when they are in hiding. Usually, the rattle gives away their presence, but there have been rattlesnakes noted to have a dysfunctional tail, possibly a population selected by humans that kill rattling snakes in highly populated areas. Most deaths related to snake bites are due to immediate anaphylactic reactions or failure to seek medical attention for anti-venom administration. Snakebite is a common complication resulting from improper handling or interaction with snakes for leisure. Unfortunately, it is also common in professional snake handlers. 
The mortality rate is higher with rattlesnake bites compared to all snake bites. In the United States, approximately 9,000 people per year suffer a snake bite, but only five deaths occur. Interestingly, poison center data shows that one in 736 patients who suffer a rattlesnake bite die. Rattlesnakes are found in varying climates in both North and South America.
The symptoms from a snake bite are related to the envenomation. Rattlesnakes have hollow fangs in the anterior mouth that inject venom into subcutaneous tissue. Rarely, intravenous injection occurs, but when it does, it is usually deadly. Crotalid venom causes necrosis due to toxic tissue enzymes. It also causes increased permeability of the cell membrane, which leads to an increased spread of the venom locally. Crotalid venom has a fibrinolytic and protein C activation effect causing coagulopathy in patients. Certain rattlesnakes, such as timbers, have a higher incidence of angioedema. The mechanism by which angioedema associated with timber rattlesnake envenomation occurs is not understood. The incidence of angioedema in patients suffering from timber rattlesnake bites is high and found to be 66% in one study.
There are two neurotoxins found in rattlesnakes. One is an enzyme phospholipase A2 which decreases the LD50 of rattlesnake venom by ten to 80 times, and the other is canebrake toxin. Both of these toxins inhibit neuromuscular transmission by blocking Ca2+ channels on the presynaptic side of the neuromuscular junction. This may lead to paralysis of a muscle group if it is present in high concentrations. This toxin is also found to induce myokymia, which is an involuntary quivering of muscles or muscle fibers in localized areas.
Rattlesnake bite victims may present with a variety of local and systemic symptoms. Local symptoms include pain and swelling around the bite site, bleeding from the bite site, and local tissue necrosis and ecchymosis. Systemic symptoms include angioedema, bleeding from other orifices including hematemesis and hematochezia, nausea, vomiting, diarrhea, dyspnea, and anaphylaxis.
Initial labs are necessary for all snake bites and should include coagulation studies, hemoglobin, platelets, and fibrinogen. While an x-ray of the bite wound is not necessary, it may be prudent if there is a suspicion of a foreign body or a fracture that may have occurred while escaping the snake. Upon initial evaluation, the leading edge of the swelling and redness surrounding the bite site should be marked. Grading the envenomation determines the use of antivenin.
Minimal Envenomation (no antivenin)
Appropriate management should begin in the pre-hospital setting. Facilitate immediate andrapid transport of the patient for evaluation by a qualified medical provider. Immobilize the extremity to reduce the potential dissemination of venom through the lymphatic system, but this should not delay transport. Patients presenting with a snake bite should be stabilized by initially assessing their airway, breathing, and circulation just like any other trauma presentation. Some patients may be able to identify the snake that bit them, but this may not always be accurate. Knowing the common snakes in the locality of the patient may help elucidate the likely culprit. In any case, the treatment algorithm for snake envenomation does not change drastically depending on the snake.
Please refer to the management algorithm attached for a step-by-step approach to crotaline envenomation.
The leading edge of the swelling and redness surrounding the bite site should be marked and tracked every 15 min-30 min. The extremity should be immobilized to reduce motion, and pain should be treated with IV opioids if necessary. Initial labs are necessary for all snake bites and should include coagulation studies, hemoglobin, platelets, and fibrinogen. Tetanus should be updated if necessary, and the local poison center should be notified.
Signs of envenomation may vary between presentations but should be assessed in all snake bite victims. Systemic signs include hypotension, bleeding or oozing from IV sites, vomiting, diarrhea, angioedema and neurotoxicity. Assessment for facial edema including tongue swelling and respiratory distress should be recognized, and a definitive airway should promptly be obtained if there are concerns for airway compromise.
A patient with minimal signs of envenomation should be monitored for at least eight hours and should get a repeat coagulation panel before discharge.
Crotalidae polyvalent immune Fab (CroFab) Dosing
Initial bolus four to six vials mixed in 250 mL normal saline (NS) administered over one hour (same number of vials in less fluid for children). Initiate treatment with a bolus of 10 mL/hr observing for adverse effects. If none, then increase every few minutes to achieve complete administration in one hour. Observe patient for local swelling and systemic symptoms. If signs of progression then repeat with four to six vials over one hour. Do not administer to normalize abnormal coagulation markers. Repeat until initial control is achieved (local swelling improves or stops, resolution of systemic signs, and resolution of clinically relevant bleeding. After achieving control, maintenance doses of two vials every six hours for 18 hours are recommended for rattlesnakes, patients with a coagulopathy, and those with severe clinical envenomations. This is not usually required for moderate copperhead envenomations. Remember that if recurrent swelling or coagulopathy occurs during maintenance doses, reduce with bolus protocol.
Recheck coagulation labs prothrombin time (PT), fibrinogen, platelets, and hemoglobin at day two to three and days five to seven. Recurrent coagulopathy without clinically significant bleeding has been observed. Some retreat and others follow parameters to normalization.
Consider contacting a Poison Control Center to discuss with a medical toxicologist.
Progression of angioedema is unpredictable as it may progress beyond presentation or may start regressing soon after presentation. Elevated protime and/or decreased platelets and fibrinogen levels are also signs of envenomation. Coagulopathy responds to antivenin treatment, but the thrombocytopenia may persist. Life-threatening bleeding is rare despite severe coagulopathy and thrombocytopenia. Platelet or fresh frozen plasma (FFP) transfusion should be avoided except when there is life-threatening bleeding because antivenin is the definitive treatment.
If no signs of envenomation are present, the patient should be observed for at least six hours, and labs should be repeated before discharge. If the patient develops signs during the observation period or presents with signs of envenomation, erythema and swelling should be marked, measured, and tracked for progression. Prophylactic antibiotics are not indicated for snake bites as they have an extremely low likelihood of infection owing to the proteolytic properties of snake venom.
Overall Indications for Crotalidae polyvalent immune Fab (CroFab)
CroFab will help local symptoms and coagulopathy, and local symptoms are used to monitor response to therapy. Cessation in the progression of swelling and erythema is reassuring.
Worsening angioedema is not a sign of the failure of treatment with CroFab. CroFab is not expected to reduce or stop the progression of angioedema. Typically, the angioedema resolves with supportive care.
Coagulation studies will improve with CroFab treatment, but persistent thrombocytopenia has been noted despite CroFab treatment and is not considered the failure of treatment. Other systemic signs are expected to improve with treatment as well. Local swelling and pain may persist for weeks to months despite CroFab therapy. CroFab is contraindicated in a patient with known allergy to sheep protein. CroFab use is associated with an 8% incidence of immediate hypersensitivity and a 13% incidence of serum sickness (which is rare and almost always clinically insignificant).
The diagnosis and management of rattlesnake bite is best done with an interprofessional team that includes the emergency department physician, toxicologist, poison control, surgeon, internist and an intensivist. Appropriate management should begin in the pre-hospital setting. Facilitate immediate and rapid transport of the patient for evaluation by a qualified medical provider. Immobilize the extremity to reduce the potential dissemination of venom through the lymphatic system, but this should not delay transport. Patients presenting with a snake bite should be stabilized by initially assessing their airway, breathing, and circulation just like any other trauma presentation. All patients must be monitored by the nurse until stable. Those who are asymptomatic may be discharged after a period of observation. Those with signs of envenomation need admission to the ICU for close clinical and biochemical monitoring.
The prognosis for patients treated promptly is good.
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