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CLINICAL REVIEW: David A Warrell Treatment of bites by adders and exotic venomous snakes BMJ 2005; 331: 1244-1247 [Full text]

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Treatment of bites by adders and exotic venemous snakes

Alan W Fowler   (6 December 2005)

Treatment of bites by adders and exotic venomous snakes

David A Warrell   (6 January 2006)

David Warrell's clinical review: query

IMRE J. P. LOEFLER   (16 February 2006)

Treatment of bites by adders and exotic venemous snakes 6 December 2005
Alan W Fowler, Retired orthopaedic surgeon Home Send response to journal: Re: Treatment of bites by adders and exotic venemous snakes	In his review of the treatment of snake bites, it is not clear why David Warrell deals separately with the management of bites by British adders and bites by exotic snakes ¹ In particular why does he advocate pressure bandaging for bites by exotic species but forbids it for British adder bites? In his references, Warrell quotes the ground-breaking research by Sutherland et al² in Melbourne Australia which showed that pressure bandaging plus immobilisation was effective in delaying for long periods the absorption of venom injected subcutaneously. Venom is absorbed via the lymphatic vessels which, like veins, have one way valves and depend on movement to drive the lymph to the heart. The lymph vessels are thin walled and only need low pressure to obliterate them. So, as Sutherland showed, a lymphatic tourniquet only needs to exert a pressure of 55mm Hg to be effective. Surely the principles that apply to the absorption of venom apply to all types of venom. As with cardio-pulmonary resuscitation, it is highly desirable that we have a global consensus on the most effective first aid management of snake bite. In seeking this it is appropriate that we should follow the advice of experts in countries such as Australia which has far more dangerous snakes and only one or two deaths annually from about 3000 snake bites. 1. Warrell AW. Treatment of bites by adders and exotic venomous snakes. BMJ 2005;331:1244-47 2. Sutherland SK, Coulter AR, Harris RD. Rationalisation of first-aid measures for elapid snakebite Competing interests: None declared
Treatment of bites by adders and exotic venomous snakes 6 January 2006
David A Warrell, Professor of Tropical Medicine and Infetious Diseases John Radcliffe Hospital, Oxford OX3 9DU Send response to journal: Re: Treatment of bites by adders and exotic venomous snakes	Alan Fowler is incorrect in assuming that “the principles that apply to the absorption of venom apply to all types of venom”. Snake venoms vary enormously in their composition and in the physicochemical properties and pathophysiological actions of their constituent toxins in human snake bite victims. The theoretic basis for Sutherland’s pressure-immobilisation (PI) method is clearly explained in Fowler’s reference 2 and in papers by Barnes and Trueta 2 and Hamilton Fairley.3 In snake bite victims at risk of developing life-threatening respiratory paralysis before they can reach medical care, the systemic absorption of phospholipase A2 neurotoxins of the kind found in Australian and other elapid venoms can be delayed by compressing lymphatic vessels through which these large molecules are transported from the tissues to the blood stream. There are a few species of medically important neurotoxic vipers but their bites do not cause early respiratory paralysis. Early anaphylactoid life-threatening effects of adder (Vipera berus) venom may be caused by oligopeptides (autacoids) which inhibit endogenous angiotensin converting enzyme and potentiate bradykinin leading to shock. Direct absorption of these small molecules into the blood stream will not be prevented by lymphatic occlusion. Venoms of most Viperidae and some cobras (notably African spitting cobras) contain cytotoxic hydrolases and polypeptides whose tissue necrotic action might be potentiated if restrained locally by PI. Compression also increases the pressure in tight fascial compartments (often the anterior tibial compartment in those bitten on the feet and ankles) increasing the risk of ischaemic necrosis of their contents. Local envenoming is usually negligible after bites by Australian elapids and so this concern has not limited the promotion of PI for all snake bites in that country. These are the reasons why I recommend PI for bites by exotic neurotoxic elapid snakes but not for British adder bites. My late friend Struan Sutherland published a series of elegant laboratory studies in support of the use of PI, but prospective clinical studies of this method have not been attempted, even in Australia.3 There are, however, some clinical reports, supported in some cases by measurement of venom antigenaemia before and after release of PI, indicating that it can be effective in delaying systemic envenoming.4 Practical difficulties, such as supplying the necessary materials (long wide stretchy bandages and splints) to those at risk, the problem of training these people to apply the pressure bandage correctly (not too loosely, not too tightly) 5 and uncertainties about the optimal method, seem to pose insuperable barriers to the widespread use of PI in those tropical developing countries where snake bites kill and maim tens of thousands of people each year. 1 Barnes JM, Trueta J. Absorption of bacteria, toxins and snake venoms from the tissues. Importance of the lymphatic circulation. Lancet 1941; 1: 623-6. 2 Hamilton Fairley N. Criteria for determining the efficacy of ligature in snake bite. (The subcutaneous-intravenous index). Medical Journal of Australia 1929; 1: 377-94. 3 Cheng AC, Currie BJ. Venomous snakebites worldwide with a focus on the Australia-Pacific region: current management and controversies. J Intensive Care Med. 2004;19(5):259-69. 4 Sutherland SK, Tibballs J. Australian Animal Toxins. The creatures, their toxins and care of the poisoned patient. Melbourne, Oxford University Press 2001, 2nd edition. 5 Howarth DM, Southee AE, Whyte IM. Lymphatic flow rates and first-aid in simulated peripheral snake or spider envenomation. Medical J Australia 1994; 161: 695-700. Competing interests: None declared
David Warrell's clinical review: query 16 February 2006
IMRE J. P. LOEFLER, EDITOR THE NAIROBI HOSPITAL PROCEEDINGS Send response to journal: Re: David Warrell's clinical review: query	David Warrell in his instructive review “Treatment of bites by adders and exotic venomous snakes ”(BMJ 2005,331:1244-1247)” suggests that the pressure immobilization method may be beneficial under certain circumstances. In Figure 5, the bandaging is illustrated. In caption (1) it reads, “do not remove trousers as the movement of doing so will assist venom to enter the bloodstream”. Caption (2) suggests that, “the bandage should be as tight as you would apply to a sprained ankle.” I submit that the bandaging would press more venom into the bloodstream than the removal of the trousers. I understand that there is benefit in immobilization - it may prevent the spreading of venom, but I have seen no explanation, neither in this review or nor elsewhere for the rational of pressure. Could David Warrell, please enlighten us? Imre Loefler, MD FRCS Competing interests: None declared