a) Outline the clinical features and laboratory abnormalities likely to be found in a patient with envenomation due to an Australian snake-bite. (50% marks)
b) Outline the management of a patient with confirmed snake envenomation. (50% marks)
Local pain, swelling and bruising. This may be absent
Sudden collapse – associated with hypotension and loss of consciousness, rarely cardiac arrest and seizure (5%)
Non –specific systemic symptoms – nausea, vomiting, diarrhoea, headache, sweating.
Neurotoxicity – descending flaccid paralysis – starting with ptosis, diplopia, blurred vision, and then progressing to bulbar weakness, respiratory and limb muscle paralysis.
Myotoxicity – local and generalised myalgia and muscle tenderness. Haemorrhage – rare – intracranial, gastrointestinal or from cannula sites
Venom induced consumptive coagulopathy – characteristic of Australian snake bite – INR >3, APPT >100, fibrinogen < 1, raised D-dimers – can be 100 times assay cut off, Thrombocytopenia <100
CK – 1000 to over 100,000 u/L associated with myotoxicity
Acute renal failure – raised potassium, urea and creatinine.
Fragmented red cells in blood film – microangiopathic haemolytic anaemia.
First aid – Pressure bandage with immobilisation of the limb and the patient, pressure similar to that for a sprained ankle.
Monitor the patient in critical care area with resuscitation facilities – ED, HDU, ICU – neurological state, HR, BP, respiration, bleeding
Resuscitation as appropriate with two large bore cannulas and collect blood for laboratory tests – Coags (INR, APTT, Fibrinogen, D-Dimers), platelets, Urea, creatinine, electrolytes, CK.
Identify the likely snake type; the site of the bite can be swabbed and a venom detection kit (VDK) used or urine but not blood, or consultation with an herpetologist. Administer anti-snake venom (ASV) only if clinical symptoms or signs or lab abnormalities such prolonged INR. Current guidelines are for one vial ASV only and then correct subsequent coagulopathy with FFP
Release pressure bandage only after administration of ASV.
Type of ASV (monovalent or polyvalent) depends on clinical presentation, geography and VDK.
Monitor closely for anaphylactic reaction. Treat with adrenaline. Premedication with adrenaline, steroids or antihistamines not recommended.
Repeat lab investigations at 6, 12 and 24 hours to monitor response such as improvement in coagulopathy (INR).
Supportive treatment such ventilation for muscle paralysis and respiratory failure, dialysis for acute renal failure, inotropes for cardiovascular collapse and FFP for severe coagulopathy and bleeding complications
Specific clinical features
- Local pain, swelling and bruising (eg. brown snake bites)
- Maybe myonecrosis (from black and tiger snakes)
- Fang marks
- Draining lymph nodes may be enlarged and painful
- Systemic effects
- Abdominal pain
- Renal impairment
Laboratory findings and investigations for a snake bite victim:
- CK (rhabdmyolysis)
- Coags (DIC, or "venom-induced consumption coagulpathy)
- FBC (DIC, looking for thrombocytopenia and red cell fragmentation)
- Fibrinogen (DIC)
- EUC (renal failure)
- LFTs (hepatic injury)
- Snake Venom Detection Kit
Indications for polyvalent antidote:
- Unsure which snake species was involved
- SVDK not available
- monovalent antivenom not available
Evidence for premedication for antivenom administration:
- This is no longer recommended in Australia
- polyvalent antidote tends to have a higher rate of anaphylaxis
How do you know your monovalent antivenom is working?
- The short answer is, you dont.
- It takes tme for some of the irreversible features to resolve (eg. it takes time to synthesis the coagulation factors which have been depleted)
- Giving more antivenom will not improve the situation.
Isbister, Geoffrey K., et al. "Snakebite in Australia: A practical approach to diagnosis and treatment." Medical journal of Australia 199.11 (2013): 763-768.