Tuesday, March 22, 2011

The Cobra Bite

Motivation: Yesterday, my travels took a turn for danger.  I thought I saw a king cobra.  I was walking through some lush green bushes when something black and hooded caught my sight five feet to my left.  I froze in mid-step with my heart bumping.  Then, I ran.  I looked back every other step to make sure I was in the clear.   Nothing followed me.  When I slowed down, I remembered videos of Steve Erwin stroking the heads of hooded cobras.  Of course, he probably had king cobra anti-venom handy in his pocket while I was in the dry.  I wondered what happens if you get bitten by a king cobra and don't have anti-venom on you. 

I couldn't find very many evidence-based reports but did find a great case-report that typifies many of the therapies and complications. 

Paper: Veto, T. et. al.  Treatment of the first known case of king cobra envenomation in the United Kingdom, complicated by severe anaphylaxis. Anaesthesia (2007) 62: 75-78. (http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2044.2006.04866.x/full)

Summary of Report:
A 22 year old man presented to the ED 20 minutes after getting bitten on the left index finger by a king cobra he was trying to feed.  He was alert and oriented except for feeling of dizziness.  His heart rate was 124 bpm at presentation.  Within 10 minutes of arrival, bilateral ptosis was noted.  At 30 minutes, he developed shortness of breath and had to be intubated.  He was transferred to the ICU.  Initial blood tests were all normal except for neutrophilia.  To prevent further lymphatic drainage of venom into circulation, pressure bandage was applied to the entire limb. 

On transfer to the ICU, the patient developed an erratic pulse (45-85 bpm) and systolic hypertension (200-220 mmHg).  At six hours after being bitten, the patient received king cobra specific equine anti-venom (which are antibodies to the venom).  The patient developed anaphylaxis in reaction to the infusion of the anti-venom.  However, after resuscitation with adrenaline, the infusion was continued along with a hydrocortisone infusion.  At 17 hours after the bite, some motor function was noted.  Sedation was stopped, and the patient was extubated.  He was discharged home 24 hours after the bite!

Discussion: The first lesson is that don't try to feed a king cobra.  When bitten in the extremities, patients probably have about half an hour to an hour to seek help in a hospital.  Interestingly, the king cobra venom is heterogenous and contains neurotoxins, myotoxins, cardiotoxins, haemolysins, anticoagulants, and many other proteases.  Even though there is no single mechanism of action, the most lethal component in the cocktail is a post-synaptic neurotoxin that results in paralysis.  Consequently, one of the first signs of venom action is cranial nerve dysfunction (ptosis in the patient).  To me, the most interesting part of the case was the first six hours when no anti-venom was available.  The best approach in a patient with a dangerous snake bite appears be to follow the regular ABCs and resuscitate symptomatically almost like paralysis in a Guillain-Barre patient.  The only difference is that pressure dressing of the entire limb is necessary.  Another important aspect of the case was the anaphylaxis following anti-venom infusion.  Unfortunately, this is a common side-effect of injecting heterogenous equine antibodies.  The anaphylaxis should be treated, but the infusion of the anti-venom continued after treatment with corticosteroids.  For me, if I had gotten bitten, the best course would have been to apply pressure dressing of my entire limb and hop into a taxi to take me to a hospital!