Emergency Medicine · Toxicology
The facts most likely to be tested
Crotalid envenomation is characterized by local tissue necrosis, edema, and hemorrhagic bullae due to the presence of metalloproteinases in the venom.
The primary systemic manifestation of pit viper bites is coagulopathy, specifically fibrinogenolysis and thrombocytopenia leading to spontaneous bleeding.
Crotalidae polyvalent immune fab (CroFab) is the definitive treatment indicated for patients with progressive local swelling, hemodynamic instability, or coagulopathy.
Compartment syndrome is a rare complication of crotalid bites, and fasciotomy is almost never indicated unless there is objective evidence of intracompartmental pressure exceeding 30-40 mmHg.
The initial management of a snakebite includes immobilization of the extremity at the level of the heart and avoidance of tourniquets or suction devices.
Tetanus prophylaxis is mandatory for all patients presenting with a snakebite due to the risk of Clostridium tetani inoculation.
The severity of envenomation is assessed by the progression of edema beyond the site of the bite and the presence of systemic signs such as nausea, vomiting, or hypotension.
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A 34-year-old male presents to the emergency department after being bitten on his left ankle while hiking. On physical examination, there is marked edema extending from the foot to the mid-calf, accompanied by ecchymosis and hemorrhagic bullae at the bite site. The patient is tachycardic, and laboratory studies reveal a platelet count of 85,000/µL and a prolonged prothrombin time. The distal pulses are palpable, and the patient has full range of motion of the toes.
What is the most appropriate next step in management?
Administration of Crotalidae polyvalent immune fab (CroFab)
The patient exhibits signs of systemic envenomation (coagulopathy and thrombocytopenia) and progressive local tissue injury, which are the primary indications for antivenom therapy.
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Etiology / Epidemiology
Caused by pit vipers (rattlesnakes, copperheads, cottonmouths). Outdoor exposure in the southern/western US is the primary risk factor.
Clinical Manifestations
Presence of fang marks and local tissue necrosis. Rapidly progressive edema is the hallmark sign of envenomation.
Diagnosis
Diagnosis is clinical. Serial coagulation studies (PT/INR, fibrinogen) are the gold standard for monitoring systemic toxicity.
Treatment
Crotalidae polyvalent immune fab is the first-line antidote. Avoid fasciotomy unless compartment pressure is definitively elevated.
Prognosis
Most patients recover with antivenom. Anaphylaxis and serum sickness are the primary delayed complications.
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Epidemiology & Etiology
Crotalids are identified by the loreal pit between the eye and nostril. Most bites occur in males aged 18-30 during recreational outdoor activities. Peak incidence occurs in summer months when snakes are most active.
Pertinent Anatomy
The venom apparatus consists of retractable fangs connected to venom glands. The venom is a complex mixture of proteolytic enzymes that cause local tissue destruction and systemic coagulopathy.
Pathophysiology
Venom induces local tissue necrosis via metalloproteinases and hyaluronidase. Systemic effects include fibrinogen depletion and platelet dysfunction, leading to a consumptive coagulopathy. Severe cases result in hypotension and shock due to increased vascular permeability.
Clinical Manifestations
Patients present with fang marks, local pain, and progressive edema spreading from the bite site. Red flags include ecchymosis, bullae, and systemic signs like nausea, vomiting, or hypotension. Metallic taste in the mouth is a classic early systemic symptom.
Diagnosis
Diagnosis is based on clinical presentation and bite history. Serial coagulation panels (PT, PTT, fibrinogen, platelets) are mandatory to assess systemic toxicity. A fibrinogen level < 200 mg/dL is a sensitive indicator of systemic envenomation.
Treatment
Administer Crotalidae polyvalent immune fab (CroFab) for progressive local swelling or systemic coagulopathy. Do not use ice or tourniquets as these worsen local tissue damage. Fasciotomy is rarely indicated and is contraindicated unless compartment syndrome is confirmed by intracompartmental pressure monitoring.
Prognosis
Prognosis is excellent with timely antivenom administration. Monitor for delayed coagulopathy for up to 14 days post-bite. Serum sickness may occur 1-3 weeks after treatment, requiring corticosteroids.
Differential Diagnosis
Elapid envenomation: causes neurotoxicity rather than local tissue necrosis
Dry bite: absence of local edema or systemic symptoms after 8-12 hours
Cellulitis: lacks the rapid progression and systemic coagulopathy of envenomation
Spider bite: usually presents with a central punctum rather than two distinct fangs
Compartment syndrome: usually traumatic in origin; rarely caused by snakebite