Section 30 Toxinology
30.1 Snakebite
Epidemiology
The commonest clinical manifestation is coagulopathy which occurs in about three-quarters of envenomed cases, about half from brown snakes and half from the tiger snake group. Neurotoxicity and myotoxicity are now uncommon and mechanical ventilation is rarely required for treatment. The types of snakes causing major envenoming differ across Australia. Exotic snakebite remains a problem from snakes in zoos and an unknown number of illegally kept snakes, but hospital presentations are rare.
Clinical features and toxinology
Envenoming results when venom is injected subcutaneously and reaches the systemic circulation. Whether or not a snakebite results in envenoming depends on a number of factors including fang length, average venom yield of the snake, effectiveness of the bite and bite site. Recent studies have suggested that only a small amount of the injected venom actually reaches the systemic circulation.1
The medically important Australian snakes and their associated clinical effects are listed in Table 30.1.1.
Coagulopathy
Venom-induced consumption coagulopathy (VICC)
This is the commonest and most important clinical effect in Australian snake envenoming. Venom-induced consumption coagulopathy (VICC) results from a prothrombin activator in the snake venom converting prothrombin (factor II) to thrombin which leads to consumption of fibrinogen, massive increases in fibrinogen degradation products and consumption of factors V and VIII due to thrombin activation. Most dangerous Australian snakes contain such a prothrombin activator including brown snakes, snakes in the tiger snake group and taipans. Venom-induced consumption coagulopathy develops rapidly within 15 to 60 min and the onset may coincide with the initial collapse seen with major envenoming by brown snakes and taipans. Recovery usually takes 12 to 18 h.2
Anticoagulant coagulopathy
This occurs with Mulga and Collett’s snake, and in about half of red-bellied black snake envenomings. It is unlikely to result in haemorrhage and of itself is rarely of clinical importance. However, anticoagulant coagulopathy is a marker of significant envenoming and is rapidly reversed with antivenom.
Treatment
First aid
Australian snake venoms appear to be absorbed via the lymphatic system so absorption is likely to be increased by movement and exercise. The aim of first aid is to minimize movement of venom to the systemic circulation. This is achieved by a pressure bandage (elastic bandage such as ACE®) being applied over the bite site and then covering the whole limb with a similar pressure to that used for a limb sprain. The bitten limb must be immobilized as well as the whole patient, or the first aid is ineffective.3 Immobilization consists of splinting and complete prevention of movement or exercise of the bitten part. It has been shown that movement of all limbs, not just the affected one, needs to be minimized for optimal effect.4 Transport should be brought to the patient and walking must be avoided. Prompt, properly applied first aid probably prevents significant absorption of venom for many hours although there is only anecdotal evidence to support this. Pressure bandaging is clearly impractical for bites that are not on the limbs but direct pressure with a pad and immobilization may be useful. The bite site should not be washed so that it can be swabbed for venom detection.
Initial assessment and treatment
Figure 30.1.1 provides a simple approach to the management of suspected and envenomed snakebite patients. The patient is managed in an area with full resuscitation facilities. Assessment and management proceed simultaneously. The airway, breathing and circulation are assessed and stabilized. The majority of patients are not critically unwell and can have a focused neurological examination for early signs of paralysis (e.g. ptosis, drooling), examination of draining lymph nodes and general examination for signs of bleeding (oozing from the bite site, gum bleeds). Intravenous access should be established and intravenous fluids commenced.
Further management
Two major diagnostic and risk assessment issues exist for snakebite:
If there is no evidence of envenoming after clinical assessment and initial laboratory testing the first-aid bandage can be removed. The patient requires ongoing close observation including repeated investigations one hour after bandage removal, and at 6 and 12 h after the bite (see Figure 30.1.1).
In the majority of cases a combination of these three factors allows determination of the correct monovalent snake antivenom required. If it is unclear which snake is involved then one vial of polyvalent antivenom should be administered. This contains sufficient antivenom for an initial dose for all types of snakes. In some parts of Australia, such as Victoria and Perth, a combination of brown and tiger snake antivenom can be administered in preference to polyvalent. In Tasmania only tiger snake antivenom is required.
Administration of antivenom
The initial dose of antivenom for children is the same as for adults and is provided in Table 30.1.1. Further doses are usually not required and recovery is determined by the reversibility of effects and the time it takes for recovery once venom is neutralized. The benefits of antivenom are listed in Table 30.1.2, which underlines the importance of waiting for recovery after antivenom administration, particularly with VICC.5 Although there is controversy over the dose of antivenom, recent studies have demonstrated that previously recommended large doses are not required. For brown snake envenoming an initial dose of two vials binds all venom and sufficient time must then be allowed for resynthesis of clotting factors.1,2
Clinical effect | Benefit |
---|---|
Venom-induced consumption coagulopathy (VICC) | Appears to neutralize toxin effect allowing clotting factors to be resynthesized and clotting to recover over 6 to 18 hours |
Anticoagulant coagulopathy | Neutralizes a toxin inhibitor of coagulation with immediate improvement in coagulation studies |
Neurotoxicity | Neutralizes toxin in the intravascular compartment and will prevent further development of neurotoxicity but will not reverse neurotoxic effects already present |
Myolysis/rhabdomyolysis | Neutralizes myotoxins and will prevent further muscle injury but will not reverse effects |
Local effects | Unlikely to reverse any already developed local effects |
Renal damage | Unlikely to have an effect |
Generalized systemic effects: nausea, vomiting, headache, abdominal pain, diarrhoea and diaphoresis.
Rapidly reverses non-specific effects and is a useful indication of antivenom binding venom components.
Premedication for snake antivenom administration has previously been controversial but is no longer recommended in Australia. One randomized controlled trial suggested that adrenaline was an effective premedication for snake antivenom.6 However, the trial was for an antivenom with a much higher reaction rate and was too small to assess the safety of adrenaline.7 Promethazine has been shown to be ineffective as a premedication,8 and often leads to drowsiness that makes neurological assessment difficult. Immediate-type hypersensitivity reactions occur in about a quarter of antivenom administrations in Australia, but are only severe (mainly hypotension) in 5% of administrations.9 Reactions are more common with tiger snake antivenom and polyvalent antivenom compared to brown snake antivenom. Antivenom should always be administered in a critical care area with readily available adrenaline, intravenous fluids and resuscitation equipment.
Clinical investigation
Snake venom detection kit
In non-envenomed patients the SVDK has a high false positive rate, especially in the brown snake well. This is even more problematic for urine testing so a bite site swab is preferred if available. A positive VDK on urine does not indicate systemic toxicity and in asymptomatic patients with normal laboratory studies this is most likely a false positive. The test should not be done on blood. If the snake responsible for the bite accompanies the patient, a swab of the fangs can be tested except that this may require considerable dilution because it will be too concentrated and overwhelms the SVDK with all wells changing colour.
Disposition
Patients with suspected snakebite but no evidence of envenoming one hour after the removal of first aid may be admitted to an observation area. Blood tests including coagulation studies and a creatine kinase (CK) should be repeated at 1 and 6 h after first aid is removed and be observed for 12 h or overnight (Fig. 30.1.1). Envenomed patients requiring ventilatory support should have continued management in ICU, but patients with coagulopathy only are commonly managed in ED observation wards.
1 Isbister GK, O’Leary MA, Schneider JJ, et al. Efficacy of antivenom against the procoagulant effect of Australian brown snake (Pseudonaja sp.) venom: In vivo and in vitro studies. Toxicon. 2007;49:57-67.
2 Isbister GK, Williams V, Brown SG, et al. Clinically applicable laboratory end-points for treating snakebite coagulopathy. Pathology. 2006;38:568-572.
3 Sutherland SK, Coulter AR, Harris RD. Rationalisation of first-aid measures for elapid snake bite. Lancet. 1979;1:183-186.
4 Howarth DM, Southee AE, Whyte IM. Lymphatic flow rates and first-aid in simulated peripheral snake or spider envenomation. Medical Journal. 1994;161:695-700.
5 Isbister GK. Snake bite: a current approach to management. Australian Prescribing. 2006;29:125-129.
6 Premawardhena AP, de Silva CE, Fonseka MMD, et al. Low dose subcutaneous adrenaline to prevent acute adverse reactions to antivenom serum in people bitten by snakes: randomised, placebo controlled trial. British Medical Journal. 1999;318:1041-1043.
7 Lalloo DG, Theakston RD. Snake antivenoms. Journal of Toxicology: Clinical Toxicology. 2003;41:277-290.
8 Fan HW, Marcopito LF, Cardoso JL, et al. Sequential randomised and double blind trial of promethazine prophylaxis against early anaphylactic reactions to antivenom for bothrops snake bites. British Medical Journal. 1999;318:1451-1452.