Jeff: Welcome back to Emplify, the podcast corollary to EB Medicine’s Emergency Medicine Practice. I’m Jeff Nusbaum, and I’m back with my co-host, Nachi Gupta and we’ll be taking you through the September 2018 issue of Emergency Medicine Practice - Emergency Department Management of North American Snake envenomations.
Nachi: Although this isn’t something we encountered too frequently – it does seem like I’ve been hearing more about snake bites in the recent months.
Jeff: I actually flew someone just the other day because the local ED ran out of CroFab after an envenomation in Western PA.
Nachi: Yeah, this is definitely more than “just a boards topic,” and it’s really important to know about in those rare circumstances. In terms of incidence, there are actually about 10,000 ED visits in the US for snake bites each year, and 1/3 of these involve venomous species.
Jeff: That’s a good teaser, so let’s start by recognizing this month’s team – the two authors, Dr. Sheikh, a medical toxicologist, and Patrick Leffers, a pharmD, and emergency medicine and clinical toxicology fellow. Both are at the University of Florida Jacksonville, and they reviewed a total of 120 articles from 2006-2017, in addition to reviews from both Cochrane and Dare.
Nachi: And don’t forget our peer reviewers this month, Dr. Daniel Sessions, a medical toxicologist working at the South Texas Poison Center, and our very own editor-in-chief, Dr. Andy Jagoda, who is also Chair of the Department of Emergency Medicine at Mount Sinai in New York City.
Jeff: What a team! But, let’s get back to the snakes. As some background, from 2006-2015 there were almost 66,000 reported snake exposures and 31 deaths from snake envenomation in the US. Of course, this number likely underestimates the true total.
Nachi: And there are two key subfamilies of venomous snakes to be aware of – the Crotalinae – or pit vipers – which includes rattlesnakes, copperheads, and water moccasins; and the Elapidae – of which you really only need to know about the coral snake.
Jeff: And while those are the only two NATIVE snake subfamilies to be acutely aware of, don’t forget that exotic snakes, which are shockingly popular pets -- they can also cause significant morbidity and mortality.
Nachi: Oh, and one other quick note before we get into the epidemiology – most of the recommendations this month come from expert opinion, as high quality RCTs are obviously difficult. In addition, many of the studies were based in other countries, where the snakes, the anti-venoms and their availability, and the general healthcare systems are different from those that most of us work in.
Jeff: Unless we have listeners abroad? Do we have listeners in other countries?
Nachi: Oh we definitely do... but we are going to be a bit biased towards US envenomation today. In any case, venomous snake bites occur most frequently in men aged 18 to 49 during warmer months with provoked bites occurring more frequently in the upper extremities and unprovoked bites in the lower extremities.
Jeff: In one study of poison center data from the last decade, nearly half of all victims of snake bites were victims of unknown type snakes. However, of those that were known, copperheads were the most common, while rattlesnakes caused the most fatalities – 19 of 31 in this dataset.
Nachi: In a separate study of snake bites in the early 2000s, 32% of exposures were from venomous snakes and 59% of those resulted in admission. That’s remarkably high.
Jeff: Snake bite severity depends on several key factors: the amount of venom, the composition of the venom, the body size of the bite victim, the victim's clothing, the size of the bite, comorbid conditions, and the timing and quality of medical care the victim receives.
Nachi: To be a bit more specific - First, the amount of venom will depend on the species of snake, with variations even occurring within the same species. Secondly, while there is a correlation between rattlesnake size and bite severity, there is much more at play. Some snakes can even vary the amount of venom based on threat risk – with defensive bites having different profiles than bites to strike prey.
Jeff: I found it pretty interesting that an estimated 10-25% of pit viper bites are considered dry bites, that is, ones in which no venom is released.
Nachi: Right, this is just one reason why all victims shouldn’t immediately get anti-venom, but we’ll get there.
Jeff: We definitely will. As we already stated – venom composition varies greatly. Pit vipers produce a predominantly hemotoxic venom. Systemic effects include tachycardia, tachypnea, hypotension, nausea, vomiting, weakness, and diaphoresis. Neurotoxicity is rare and is usually due to inter-breeding between species.
Nachi: While rattlesnake bites are associated with higher morbidity and mortality, the more common copperhead bites typically only cause local tissue effects. More serious systemic findings such as coagulopathy and respiratory failure have been reported though.
Jeff: So that’s a solid background to get us started. Let’s talk about the individual snakes. Why don’t you start with the crotalinae family – aka the pit vipers.
Nachi: Sure – the crotalinae includes rattlesnakes, cottonmouths (also known as water moccasins), and copperheads. These make up the vast majority of reports to the poison centers. They can be identified by their heat sensing pits located behind their nostrils (hence pit vipers). As a general rule, you can also identify the venomous snakes by their triangular or spade-like head, elliptical pupils, and hollow retractable fangs.
Jeff: wait, so you want me to walk up to the snake and ask to see if their fangs retract… yea, no thanks.
Nachi: Haha, of course not, I’m just giving you some of the general principles here. In contrast, non-venomous pit vipers have rounded heads, round pupils, a double row of vertical scales, and they lack fangs.
Jeff: In terms of location, rattlesnakes are found in all states but Hawaii, and cottonmouths and copperheads are distributed mostly throughout the southern and southeastern states, with copperheads also extending further north, even into Massachusetts.
Nachi: Moving on to the Elapidae – there are 3 species of coral snakes, only two of which you need to know about, Micrurus fulvius fulvius or the eastern coral snake and Micrurus tener or the Texas coral snake. Of the two, the eastern or Micrurus fulvius fulvius produces more potent venom.
Jeff: As you may have guessed by their names, the eastern coral snake is found in the southeastern united states, specifically, east of the Mississippi -- whereas the Texas coral snake lives west of the Mississippi.
Nachi: Venomous North American coral snakes can be recognized by the red and yellow bands around their bodies whereas their nonvenomous counterparts can be recognized by their characteristic black band between the red and yellow bands. I’m sure you’ve heard the popular mnemonic for this… Red touch yellow kill a fellow, red touch black, venom lack.
Jeff: I have heard that one, and it’s not a bad mnemonic. Just remember that this is more of a guideline than a rule, as it doesn’t always hold true.
Nachi: Coral snakes also tend to chew rather than bite thanks to their short, fixed, hollow fangs. Locally, bites can lead to muscle destruction thanks to a certain myotoxin. Systemic signs of infection include nausea, vomiting, abdominal pain, and dizziness.
Jeff: The venom also contains a neurotoxin which can lead to diplopia, difficulty swallowing and speaking and generalized weakness.
Nachi: Complicating matters even further, the onset of these symptoms may be delayed for many hours.
Jeff: Alright, so I think that about wraps up the background. Let’s move on to the meat and potatoes of this article, starting with the differential.
Nachi: For differential this month, we are really focusing on differentiating a venomous snake from a non-venomous one.
Jeff: Oh yeah, this is where you want us to ask the snake if it can retract its fangs, right?
Nachi: Ha very funny – Although the type of snake may be obvious if the patient...
