Envenomation and marine bites — formative SAQs

SAQ 1 — Snake bite first aid, coagulopathy and antivenom (10 marks)

A 6-year-old boy is brought to a rural emergency department thirty minutes after a suspected brown snake bite to the calf while walking in long grass. A pressure-immobilisation bandage was applied at the scene. He is anxious but alert, with a pulse of one hundred and ten, a blood pressure of one hundred and five over sixty, and a tender regional lymph node. His initial coagulation profile shows an unrecordable INR, a D-dimer greater than twenty, and a fibrinogen of zero point six grams per litre. ^{[1] [4]}

Questions

1. Describe your immediate management of the pressure-immobilisation bandage, the blood tests, and the first antivenom dose. (5 marks) ^{[1] [5]}
2. Explain the laboratory pattern, why the INR will remain abnormal for some hours after antivenom, and how you would manage an antivenom hypersensitivity reaction. (5 marks) ^{[4] [8]}

Model answer

Immediate management (5). I confirm that the pressure-immobilisation bandage is correctly applied — firm, from the toes up the whole leg, with a splint, and the child kept still — and I leave it in place until envenomation is excluded, because removing it prematurely can release a lymphatic bolus of venom. I run an airway, breathing and circulation assessment, give oxygen and attach monitoring, establish intravenous access in the unaffected limb, and send a coagulation profile, creatine kinase, electrolytes and urea, full blood count, group and hold, and urinalysis, repeating the coagulation and creatine kinase at set intervals. Where the laboratory is delayed I perform a twenty-minute whole blood clotting test — unclotted blood after twenty minutes confirms a coagulopathy. Because this child has an unrecordable INR with a very high D-dimer and a low fibrinogen, he has venom-induced consumption coagulopathy and I give brown snake antivenom intravenously as an initial one-vial dose, observing him for a hypersensitivity reaction, with resuscitation equipment ready and paediatric retrieval arranged. ^{[1] [5]}

Coagulopathy pattern, recovery and reactions (5). The laboratory pattern is a true defibrination: the brown snake venom is a prothrombin activator that switches on the coagulation cascade, consumes clotting factors and fibrinogen, and generates fibrin degradation products, producing an unrecordable INR with a very high D-dimer and a low fibrinogen. The INR stays abnormal for one to three days after antivenom because the antivenom only neutralises the circulating venom and stops further consumption — the liver must then resynthesise the consumed factors, so a persistently abnormal INR in an improving child is expected and is not a treatment failure. An antivenom hypersensitivity reaction occurs in around five per cent of recipients; I stop the infusion, treat it as anaphylaxis with intramuscular adrenaline at ten micrograms per kilogram of one in one thousand together with oxygen, fluids and an antihistamine, and then make a measured decision about whether to continue the antivenom, because the underlying envenomation still needs neutralising. I warn the family about serum sickness in the week after and follow up the creatinine and the creatine kinase for renal injury. ^{[4] [8]}