Snake Envenomations: Crotalinae & Elapidae
Comprehensive emergency evaluation and protocolized management of North American venomous snakebites: Crotalinae (rattlesnakes, copperheads, cottonmouths/water moccasins) vs. Elapidae (coral snakes); hemotoxins, myotoxins, and neurotoxins; distinguishing dry bites from progressive envenomation; serial limb circumference measurements; antivenom selection and dosing (CroFab [Crotalidae polyvalent immune Fab] vs. Anavip [Crotalidae immune F(ab')2]); management of snakebite coagulopathy; and critical avoidance of prophylactic fasciotomy.
Resuscitation Quick Actions • First 2 Minutes
Leading Edge Tracking
Mark the leading edge of erythema/swelling with a permanent pen every 15–30 minutes; measure limb circumference at reproducible points
CroFab Initial Dosing
4 to 6 vials IV in 250 mL NSS over 60 min (start slow at 25–50 mL/hr to watch for anaphylaxis); repeat 4–6 vials until initial control
Anavip Dosing
10 vials IV over 60 min; repeat 10 vials if swelling/coagulopathy continues to progress (F(ab')2 fragments prevent late rebound)
Avoid Fasciotomy
DO NOT perform fasciotomy! Tissue swelling is primarily subcutaneous, not subfascial. High compartment pressures resolve with adequate antivenom
Dangerous Interventions
NO tourniquets, NO arterial bands, NO ice immersion, NO wound incisions, and NO electrical shock or suction devices
Coral Snake Rule
'Red on yellow, kill a fellow; red on black, venom lack' (valid in North America only); respiratory arrest can develop silently after 12h
Bottom-Line Clinical Pearl
Crotalinae (pit viper) envenomations cause progressive tissue damage (swelling, bullae, ecchymosis) and systemic hemotoxicity (defibrinogenation, thrombocytopenia, coagulopathy). Indications for antivenom (CroFab or Anavip) are: progressive local swelling crossing a major joint, severe thrombocytopenia (platelets < 100,000), hypofibrinogenemia, or systemic shock/neurotoxicity. Anavip provides longer circulating half-life with less late hematologic rebound than CroFab. NEVER perform prophylactic fasciotomy for elevated compartment pressures in snakebites; the swelling is subcutaneous and tissue-toxic, and pressures normalize rapidly following adequate antivenom dosing. For Elapidae (coral snakes), neurotoxicity (respiratory paralysis) may be delayed 12 hours; admit for prolonged observation.
| Family & Genera | Physical Characteristics & Species | Primary Venom Pathophysiology |
|---|---|---|
| Crotalinae (Pit Vipers) | Triangular head, vertical slit-like pupils, heat-sensing facial pits between eye and nostril, subcaudal single row of scales. Crotalus (rattlesnakes), Agkistrodon contortrix (copperhead), Agkistrodon piscivorus (cottonmouth). | Complex digestive enzyme mixture: zinc metalloproteinases (destroy vascular endothelial basement membranes), phospholipase A2 (myonecrosis), serine proteases (cleave fibrinogen causing consumptive coagulopathy/defibrinogenation), and platelet aggregation inhibitors. |
| Elapidae (Coral Snakes) | Round head, round pupils, brightly colored alternating rings ('Red on yellow, kill a fellow; red on black, venom lack'). Micrurus fulvius (Eastern coral snake). | Potent neurotoxins that bind postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction, producing descending motor paralysis and respiratory failure with minimal or no local tissue reaction. |
Approximately 20–25% of all pit viper strikes are 'dry bites' with zero venom injection. Antivenom is indicated whenever there is evidence of progressive envenomation:
| Antivenom Parameter | CroFab (Crotalidae Polyvalent Immune Fab) | Anavip (Crotalidae Immune F(ab')2) |
|---|---|---|
| Antigen-Binding Structure | Monovalent Fab fragments derived from ovine (sheep) serum | Bivalent $F(ab')_2$ fragments derived from equine (horse) serum |
| Circulating Half-Life | Short elimination half-life (~12–15 hours) | Long elimination half-life (~5.5 days) |
| Initial Control Dose | 4 to 6 vials IV reconstituted in 250 mL NSS over 60 minutes | 10 vials IV reconstituted in 250 mL NSS over 60 minutes |
| Repeat Dosing for Control | Repeat 4 to 6 vials every 1–2 hours until progression halts | Repeat 10 vials if swelling or coagulopathy continues to progress |
| Maintenance Dosing | Mandatory 2 vials IV every 6 hours for 3 doses (total 6 maintenance vials) to prevent rebound | No routine maintenance dosing required due to long persistence of $F(ab')_2$ molecules |
| Late Hematologic Rebound | Common: late recurrence of thrombocytopenia or hypofibrinogenemia occurs in up to 30% at 3–7 days | Significantly reduced incidence of late hematologic rebound |
Critical Pitfall / Contraindication
FASCIOTOMY IS CONTRAINDICATED IN CROTALINE SNAKEBITES: Massive extremity edema, tense skin, and elevated compartment pressures measured via manometer are common after rattlesnake bites. However, this reflects severe cytotoxic subcutaneous tissue edema and myonecrosis from venom metalloproteinases, NOT a mechanical closed-compartment syndrome. Performing fasciotomy in a patient with active venom-induced consumptive coagulopathy precipitates catastrophic, uncontrollable hemorrhage and limb loss. Antivenom alone normalizes compartment pressures within hours. Reserve fasciotomy only for objective compartment pressures > 30–40 mmHg persisting DESPITE full antivenom administration.
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