Pit Vipers

Pit vipers (crotalids) are a subfamily of venomous vipers that can be found widely across Europe, Asia, and North and South America. Their name comes from the pair of pits on either side of the snake's head that contain extremely-sensitive heat (infrared)-sensing organs. One group of vipers, the rattlesnakes, are only native to the Americas. While more commonly distributed to the American Southwest, several species of pit vipers can also be found in the northeast.

Crotalid venom comprises a blend of peptides and enzymes, and differs in composition depending on the species. Metalloproteinases, serine proteases, and phospholipase A₂ are some of the major contributors to the hemotoxicity and cytotoxicity of crotalid venom. A minority of rattlesnake venoms also have neurotoxic components, such as the Mojave A toxin that inhibits the dihydropyridine receptor.

Presentation:

Envenomation is not subtle, and patients typically present shortly after their unfortunate encounter with a crotalid. Bites are usually to the lower extremities, but upper extremity bites may be seen in provoked encounters. Initial signs are localized pain, edema, ecchymosis, and erythema around the puncture sites; these may progress to localized necrosis. In approximately 20% of cases, bites are "dry" and symptoms will be minimal.

Systemics symptoms are uncommon but not rare, and include nausea and vomiting, bleeding diathesis, fasciculations and myokymia, paresthesia, hypotension, and angioedema (due to bradykinin overactivation). If the degree of coagulopathy is severe, other complications of hemorrhage and thrombosis (such as vessel occlusion) may manifest.

Management:

General puncture wound care includes wound cleaning, pain control, and Tdap booster if not recently received. The border of the edematous/erythematous region around the bite should be marked and regularly reassessed (q1h) for regression or expansion. The envenomed extremity should also be straightened and elevated to prevent pooling of venom.

Coagulopathy should be assessed with a CBC, PT/INR, PTT, and fibrinogen level or D-dimer. Urinalysis and creatine kinase level can assess for myotoxicity.

Blood products are only temporizing, and transfusion of pRBCs, FFP, and platelets should only be done for patients with severe bleeding or who are hemodynamically unstable.

Antivenom treatments are indicated when there are signs of local necrosis or worsening edema/erythema, compartment syndrome, systemic coagulopathies, or other systemic symptoms. Clinical judgment is usually sufficient to assess crotalid envenomation. However, less-experienced providers can consider using the Snakebite Severity Scale (SSS) — a 20-point scale that grades degree of envenomation based on pulmonary, cardiovascular, gastrointestinal, hematological, CNS, and local/soft tissue symptoms.

Two types of antivenom are available:

Crotalidae polyvalent immune Fab (CroFab) is produced in sheep and is derived from Western diamondback, Eastern diamondback, Mojave rattlesnake, and cottonmouth venom. The loading dose is 4-6 vials in 250 mL NS infused over 1 hour, which can be doubled in hemodynamically-unstable patients. If symptoms fail to improve or worsen, repeat the loading dose. If symptoms improve, then a maintenance dose of 2 vials every 6 hours, for 3 doses, should be started. Coagulation labs should be reassessed for 7 days thereafter, with repeat CroFab dosing if coagulopathy persists.

Crotalidae immune F(ab)2 (ANAVIP) is produced in horses and is derived from the venom of the fer-de-lance and the South American rattlesnake. The dose is 10 vials in 250 mL NS infused over 1 hour. Repeat dosing is usually only needed for severe cases, as it remains in the serum for a longer period of time. Limited trials have demonstrated mild superiority of ANAVIP over CroFab in reducing late coagulopathy; both antivenoms have a small chance of causing serum sickness.

Antivenom should be administered before considering wound debridement or fasciotomy. Suction, cryotherapy, NSAID analgesia, and tourniquets have been used in the past but have been shown to increase tissue damage and worsen outcomes.

Infection and anaphylaxis are rare secondary complications of crotalid envenomation. Antibiotic prophylaxis is not indicated; if signs of infection are present, ampicillin or piperacillin/tazobactam may be appropriate. Anaphylaxis may be treated with antihistamines and steroids.