Snakebite versus Stickbite

aka Toxicology Conundrum 005

A 20 year-old male was walking in rural New South Wales (Australia!) when he noticed a brown-coloured snake. He was startled and stepped backwards onto a tree branch which snapped under his weight. He then saw the snake slither away. Relieved that the danger had passed, he walked on. A few minutes later he glanced down at his shin and saw two bleeding scratch marks. With growing concern he walked 4 km to the nearest ambulance station. A volunteer ambulance officer applied a compression bandage to his leg and immobilized it with a splint prior to transferring the anxious young man to the nearest hospital.

Four hours after the bite, he had blood tests taken that were all normal. The doctor assessing him calls you to find out if the patient can be discharged now.

Questions

Q1. Can the patient be discharged? Why/ why not?

NO! This man MUST NOT be discharged yet…

Although this presentation may well be a “stick bite”, all suspected snakebites must be admitted to hospital for a minimum of 12 hours for investigation and observation. They should never be discharged at night.

If a patient or bystander so much as mentions the word “snake” – or if the thought of a snake crosses your mind – then the patient should be treated as a case of potentially life-threatening snakebite envenoming until proven otherwise.

All suspected Australian snakebites need assessment and management as follows:

  • First aid – pressure-immobilisation bandaging (PIB)
  • Transport to a hospital that meets these 3 criteria:
    • Doctor present who is willing and able to manage a snakebite
    • Laboratory able to operate “24 hours” a day
    • Sufficient and appropriate stocks of antivenom for definitive treatment
  • Determine if the patient is envenomed – based on serial assessment over 12 hours, which includes:
    • History
    • Clinical examination
    • Laboratory investigations
  • If the patient is envenomed, determine the type of antivenom required according to:
    • Geographic area (which snakes are found in the area?)
    • Clinical and laboratory features of envenoming
  • CSL Snake Venom Detection Kit (SVDK) results

Aspects of this approach will be expanded upon in later problems.

Q2. What type of snake might this have been?

The colour and appearance of a snake CANNOT be trusted as a guide to identifying an Australian snake (e.g. not all brown snakes are Brown snakes…). Only a small number of recognized snake experts can be relied upon for accurate snake identification. Snake identification is not necessary for the management of snakebite in Australia – never try to catch a snake after being bitten!

In inland New South Wales the following life-threatening venomous snakes are found:

  • Brown snakes (Pseudonaja spp.)
  • Black snakes (Pseudechis spp.)
  • Tiger snakes (e.g. Notechis scutatus) – in southern and eastern regions
  • Death adder (Acanthophis spp.)

Also, the Inland Taipan (Oxyuranus microlepidotus) might be encountered in the north-western corner of New South Wales.

Q3. What blood tests should be performed on a suspected snakebite victim?

In conjunction with history and clinical examination, laboratory tests are essential to determine if a suspected snakebite victim is envenomed. A snakebite victim may have minimal symptoms or signs yet still have a potentially life-threatening condition.

The following blood tests are useful:

  • Coagulation profile(INR, aPTT), fibrinogen and D-dimer or fibrinogen-degradation products (FDPs)
    • May show a venom-induced consumptive coagulopathy (VICC) characterized by:
      ↑INR and aPTT (may be immeasurably high!), ↓or absent fibrinogen, and ↑FDPs/ D-dimers – this occurs with envenoming by:

      • Brown snakes
      • Tiger snakes
      • Taipans
    • May show an anticoagulant coagulopathy characterized by:
      mildly ↑INR and aPTT, NORMAL fibrinogen, and NORMAL FDPs/ D-dimers (may be minimally elevated) – this occurs with envenoming by:

      • Black snakes
    • May be normal – this occurs if:
      • Not envenomed
      • Envenomed by a death adder
      • On rare occasions this might occur if the patient presents very early (e.g. prior to removal of PIB) or very late (e.g. after resolution of the envenoming syndrome).
  • Full blood count (FBC)
    • Anaemia may occur with microangiopathic haemolytic anaemia (MAHA). MAHA is an uncommon occurrence in envenoming by snakes that cause VICC:
      • Brown snakes
      • Tiger snakes
      • Taipans
    • Thrombocytopaenia may variably occur in the context of MAHA or as part of VICC.
    • Leukocytosis may occur non-specifically after snakebite, regardless of whether or not the patient is envenomed.
  • Creatine kinase (CK)
    • Rhabdomyolsis may occur due to snake venom myotoxins – this can occur with envenoming by:
      • Black snakes
      • Tiger snakes
      • Taipans
      • Sea snakes
  • Renal function (UEC) and urinalysis
    • Myoglobinuria suggests rhabdomyolysis, haemoglobinuria suggests haemolysis, and the presence of red blood cells may indicate the presence of a coagulopathy.
    • Elevated creatinine suggesting renal impairment may occur as a result of:
      • Rhabdomyolysis (see above)
      • Uncommonly due to nephrotoxins or MAHA – or some other mechanism we don’t understand yet – with envenoming by:
        • Brown snakes
        • Tiger snakes
        • Taipans
  • Lactate dehydrogenase (LDH)
    • Elevated in the presence of haemolysis/ MAHA (see above).
  • Whole blood clotting test (WBCT)
    • Requires the use of GLASS (not standard plastic) tubes.
      • The sample is inverted gently once and the left standing. The sample is re-inverted at 10 minutes – if the blood has clotted then the test is normal. If the blood is not clotted then the sample should be inverted once every minute until clotted… However, in a severe envenoming the blood may never clot!
    • A clotting time >10 min is suspicious for the presence of coagulopathy (either VICC or anticoagulant envenoming – see above).
  • WBCT is most useful in remote settings where coagulation profiles may not be readily available.

Note that in some settings not all of these laboratory tests will be available; indeed, they are not all essential for the management of suspected snakebites in every situation. Furthermore, all blood tests may be normal yet life-threatening envenoming may still be present – for instance, in the case of death adder-induced neurotoxicity that may cause respiratory paralysis and death.

Q4. What is the role of the snake venom detection kit (SVDK)?

The role of the SVDK is often misunderstood.

The SVDK does NOT help determine whether or not a patient is envenomed!

  • False positives and false negative can occur
    (anecdotal clinical experience suggests up to 20% of urine samples from non-envenomed patients will test positive for brown snake).
  • Always treat the patient, not the SVDK result, if the two do not match.

The SVDK helps to indicate the correct monovalent antivenom to use in an envenomed patient – i.e. what to use ONCE the decision to give antivenom has ALREADY been made.

I’ll talk some more about SVDKs later…

Q5. What is ASP?

ASP is the Australian Snakebite Project. All suspected snakebites in Australia should be enrolled in the ASP study to help improve our knowledge and understanding of the clinical presentations and natural history of Australian snakebites, as well as the effectiveness and adverse effect profile of snake antivenoms. ASP is also investigating the role of fresh frozen plasma (FFP) in the treatment of VICC – exciting stuff!

To enroll patients in ASP, call the Poisons Information Centre (13 11 26) (Australia only!…) for advice on the management of a suspected snakebite victim. The on-call clinical toxicologist will facilitate inclusion of the patient into the ASP study.

References

To err is human… If you disagree with the recommended answers or have other suggestions please leave a comment below!

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Comments

    • says

      Hi Daniel,
      To check the whole blood clotting time in the assessment of australian snake bites you need to use a plain glass blood tube -- no heparin!
      These are not that widely available unfortunately.

      Chris

Comments