A Snake in the Grass at St.Emlyn’s

While not fortunate enough to actually attend SMACC 2013, I was pleased to catch the much anticipated SimWars event, particularly as Team GB was one of the successful finalists. As we kind of expected, they were presented with a patient who had sustained a snake bite, something maybe a little outside of a British team’s experience. None the less, thanks to the sound application of resuscitation principles (and some critical pressure bandaging by Dr May….) the Brits won the day.

 But why a snakebite for the UK team? All of the scenarios were (I think) chosen to be a little out of the comfort zone for each team, but why the assumption that a British team would be confounded by a patient bitten by a venomous reptile?

To be fair, our green and pleasant land lacks the exotic fauna seen in the Australian countryside, such as  this cuddly critter here:

We are however, not completely bereft of poisonous creepy crawlies…..

Meet Vipera berus, more commonly referred to as the common European adder, or just the Adder.

As our only venomous snake, seeing patients bitten by an Adder will probably be a UK docs only exposure to a significant envenomation. We have had two cases recently, which provoked much frantic TOXBASE checking…..  St Emlyns to the rescue!

About 100 patients a year present after being bitten by an Adder. Mortality is rare but not unheard of. It does little for my sense of pride in the Great British public to learn that about half of Adder bites occur after some bright spark decides to try and pick the damn thing up…..

Most Adder bites occur in the summer months, when the snakes are most active. They are timid rather than aggressive and most bites occur when they are disturbed or threatened. The bites are extremely painful, but very rarely life threatening. Many (around 70%) are ‘dry’ bites, where no envenomation occurs. Case reports (1) suggest that patients who are well, and asymptomatic at 4 hours after the bite can be considered for discharge.

If adder venom gets into your wound however, significant morbidity can occur, and recovery can take up to a year.

Local envenomation

Bites are extremely painful, and pain is followed by parasesthesia and swelling that spreads proximally. The spreading swelling and erythema looks like cellulitis, complete with swollen lymph nodes and lymphangitic lines, but the spread occurs much more rapidly. This local inflammation can, rarely, progress to compartment syndrome and/or necrosis.

Treatment involves immobilising the limb in a splint or sling (but, alas, no pressure bandaging this time). Tourniquets, ligatures or cutting into or sucking on the wound are (thankfully) not recommended.

Generous analgesia will of course be needed. The patient will need observation to ensure the swelling is not rapidly progressing or becoming severe. Some cases of local envenomation will need antivenom treatment (see below).

Don’t forget good wound care, thoroughly evaluate and clean the wound, look for foreign bodies (tooth fragments have been reported), and consider your patient’s tetanus status.

The vast majority of bites will involve a local reaction only. Rarely, there is a systemic reaction to the bite which is potentially fatal.

Systemic envenomation

Some patients when bitten, experience a rapid onset of anaphylactoid symptoms within minutes. Shock may be profound, and airway threatening angio-oedema can occur. Multi-organ failure can occur, and the potential for coagulopathy means heparin is contraindicated in these patients.

These patients get all the usual anaphylaxis stuff, airway and cardiovascular support as required, and of course, the antivenom.

Antivenom

If you work in the UK, there should be antivenom for Vipera berus in your hospital somewhere (at least that is what CEM recommends (2)). Antivenom is used to reduce morbidity and mortality after bites from venomous creatures. What evidence we have suggests that this is true for Vipera berus. It seems logical, especially in a sick patient, to make the decision and give the antivenom as soon as possible. There is some animal data to support this (3), so if you’re going to give it, don’t muck about…

INDICATIONS FOR ANTIVENOM:

SHOCK

SIGNS OF SYSTEMIC ENVENOMATION E.G ANGIOEDEMA, ACIDOSIS

LOCAL ENVENOMATION WITH RAPIDLY PROGRESSIVE SWELLING E.G SPREADING PAST WRIST OR ANKLE WITHIN 4 HOURS 

LOCAL ENVENOMATION WITH SIGNIFICANT SWELLING E.G >1/2 OF LIMB WITHIN 48 HOURS

The antivenom is given as an IV infusion – one 10ml vial diluted in 500mls saline (or 5ml/kg  salinefor kids), over 30min. The 10ml dose is the same for adults and kids. This dose can be repeated after 1-2 hours if the patient remains unwell. In severe reactions, such as shock or full blown anaphylaxis, the recommendation is to give two 10ml vials straight away*.

Reactions to antivenom are common, like, really common. About 10% of patients will have an anaphylactoid reaction to the antivenom, so have the adrenaline, steroids and antihistamine ready. There doesn’t seem to be any point in giving a ‘test’ dose to predict who’s going to have the nasty reactions.

This paper from the BMJ (4) suggests that giving a small dose of adrenaline before the antivenom may reduce the incidence of reactions, and comes out with a pretty good NNT of 3. This article (5) also supports the idea, suggesting that pre-treating with adrenaline reduces adverse reactions. The snakes in these studies were of a more tropical nature than our Adder, but it sounds like a pretty good idea none the less.

To sum up…..

Even in the UK, significant envenomations from snake bites can occur and we should be prepared to manage them.

The majority are local reactions, and immobilisation and supportive treatment is all that is necessary.

Rarely, significant anaphylactoid reactions can occur, these need managing as any other anaphylaxis.

Antivenom is indicated in any systemic envenomation, and in severe local relations

Antivenom itself has a high chance of causing an anaphylactoid reaction, so be ready!

Cheers y’all,

Gareth

Want more slithering, serpentine FOAMed? Try this PK from the Broome Docs gang, essential viewing for those of you are planning to work down under. (Ed – featuring Laura Smith who is a St.Emlyn’s and Virchester alumnus)

This BMJ article (6) covers Adder bites pretty well, and seems to mirror the UK poisons information advice.

*The doses and method of administration for the antivenom given above were taken from the UK National Poisons Information Service Website (TOXBASE) on the 17/08/2013. Obviously, you should check these facts yourself before using the doses given here. Neither I or anyone else associated with St Emlyns can take any responsibility etc…..

[learn_more caption=”References”]

1. Emergency treatment of adder bites: case reports and literature review. Arch Emerg Med. 1993 September; 10(3): 239–243

2. College of Emergency Medicine Guideline on Antidote Availability for Emergency Departments May 2008. Available on CEM website http://www.collegeemergencymed.ac.uk/asp/document.asp?ID=4684

3. Karlson-Stiber C, Persson H. Antivenom treatment in Vipera berus envenoming–report of 30 cases. J Intern Med. 1994 Jan;235(1):57-61.

4. Premawardhena AP, de Silva CE, Fonseka MM, Gunatilake SB, de Silva HJ. Low dose subcutaneous adrenaline to prevent acute adverse reactions to antivenom serum in people bitten by snakes: randomised, placebo controlled trial. BMJ. 1999 Apr 17;318(7190):1041-3.

5. Williams DJ, Jensen SD, Nimorakiotakis B, Müller R, Winkel KD.  Antivenom use, premedication and early adverse reactions in the management of snake bites in rural Papua New Guinea. Toxicon. 2007 May;49(6):780-92. Epub 2006 Dec 2.

6. Warrell DA. Treatment of bites by adders and exotic venomous snakes. BMJ 2005;331:1244

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