Ep 163: A 39-Year-Old with a Snakebite

Today’s episode reviews different types of snakes and the clinical consequences of snakebites.

Read more on this topic in Harrison's.

Harrison's Principles of Internal Medicine, 22nd Edition

Transcript

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[Ms. Heidhausen] This is Katarina Heidhausen, executive editor of Harrison's Principles of Internal Medicine. Harrison's Podclass is brought to you by McGraw Hill's AccessMedicine, the online medical resource that delivers the latest content from the best minds in medicine. And now, on to the episode. 

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[Dr. Handy] Hi, everyone. Welcome back to Harrison's Podclass. We're your co-hosts. I'm Dr. Cathy Handy. 

[Dr. Wiener] And I'm Dr. Charlie Wiener, and we're joining you from the Johns Hopkins School of Medicine. 

[Dr. Handy] Hey, everyone. In celebration of the new edition of Harrison's, and to give a shoutout to our global audience, we are covering a topic that we think will be of interest to all of our listeners today, and that is about a 39-year-old with a snake bite. 

[Dr. Wiener] Cathy, we're going overseas for today's case. 

[Dr. Handy] I love to travel. Where are we headed? 

[Dr. Wiener] Today, we're going to India, but obviously, we're going to have a complication. 

[Dr. Handy] Okay. I'm ready. 

[Dr. Wiener] You are with a group visiting friends in Southern India and staying at their suburban house. While taking a late evening walk, one of your friends steps on something that moves, and then she feels a bite on her lower calf. Her colleague shines the phone flashlight and sees a snake slithering away. The bite seems to have two adjacent breaks in the skin, and you are concerned it was a venomous snake 

[Dr. Handy] Oh my. That's a valid concern. Did they see the snake enough to describe it? 

[Dr. Wiener] Yes. In fact, they were able to take a photo on the phone. The snake is cylindrical and about three feet long. It's black with thin white crossbars and a flat head. 

[Dr. Handy] Taking a picture was a better idea than trying to catch the snake. Differentiating between venomous and nonvenomous snake species can be challenging. Identifying venomous snakes by color pattern can be misleading, too, because many nonvenomous snakes have color patterns that closely mimic those of venomous snakes found in the same region. 

[Dr. Wiener] How big a problem are venomous snake bites? 

[Dr. Handy] Death from snake envenomation represents the deadliest, neglected tropical disease in the world. Most snake bites occur in developing countries with temperate and tropical climates. South Asia, including India, along with parts of East Africa are hotspots. And while estimates vary, worldwide data from 2019 suggests there are about 70,000 deaths globally from snake envenomation. But really, we don't know. It's estimated that there are millions of snake bites worldwide each year. Many victims in these areas either do not seek medical attention or have insufficient access to anti-venom, and reporting and record-keeping are generally poor. Also, many of these bites are dry bites where no venom is released. 

[Dr. Wiener] Okay. Well, that gets us to our question today. The question asks, which of the following is the most likely snake given this description? And the option A. is the common krait; B. is the Indian cobra; C. is the Indian saw-scaled viper; or D. is Russell's viper. 

[Dr. Handy] These are the so-called big four venomous snakes in India. The answer is A. This sounds like a common krait. The cobra is distinct because of its large hood. The Indian saw-scaled viper is typically less than two feet long with a pale or reddish color and white spots. The Russell's viper is typically about four feet long with the typical triangular viper head. They're usually tan or brown with dark brown spots. 

[Dr. Wiener] Tell me more about kraits. 

[Dr. Handy] Kraits are in the family of Elapidae, which means they have permanently erect fangs. Their venom is a powerful neurotoxin. They're widely distributed throughout South Asia and live in a variety of environments, including suburban settings and often near water. They're nocturnal, so seldom seen during the day, and accidental human bites are common. 

[Dr. Wiener] You mentioned that the krait's toxin is a neurotoxin. What are the typical symptoms of a bite? 

[Dr. Handy] Early findings may consist of nausea and vomiting, headache, paresthesias or numbness, or altered mental status. Victims may develop cranial nerve abnormalities such as ptosis or difficulty swallowing, and that usually is followed by peripheral motor weakness. Severe envenomation may result in diaphragmatic paralysis and lead to death from respiratory failure and aspiration. 

[Dr. Wiener] Great. Well, that leads us to our next question. This is going to be a two-question day. All the following are recommended for the management of this bite, except? Option A. is anti-venom administration at the first sign of neurotoxicity; B. apply suction to the wound; C. clean the wound with soap and water; D. pressure immobilization; or E. rapid transport to a medical facility. So four are correct, and only one is wrong. 

[Dr. Handy] The most important aspect of pre-hospital care for a person bitten by a venomous snake is rapid transport to a medical facility that's equipped to provide supportive management, and that may include intensive care, and then certainly, specific antivenom therapy. Any jewelry or tight-fitting clothing near the bite should be removed to avoid constriction from anticipated soft tissue swelling. Although wound care should not delay transport, the wound should be cleaned with soap and running water, then covered with a sterile dressing. 

[Dr. Wiener] Okay, so C. and D. are immediately true, wash the bite and transport to the hospital. What about the others? 

[Dr. Handy] Well, I already mentioned specific antivenom. The goal of antivenom administration is to allow antibodies or antibody fragments to bind and deactivate circulating venom components before they can attach to target tissues and cause deleterious effects. Antivenoms may be monospecific, that means that they're directed against a particular snake species, or polyspecific, which covers several species in a geographic region. In India, there's a polyvalent antivenom that targets the so-called big four snakes that we mentioned before. Antivenoms rarely offer cross-protection against snake species other than those used in their production. So antivenom selection must be specific for the offending snake. If the antivenom chosen does not contain antibodies to the snake's venom components, it will provide no benefit and may lead to unnecessary complications. 

[Dr. Wiener] What about for our patient? Option A. says wait for signs of neurotoxicity. 

[Dr. Handy] Antivenom administration after bites by neurotoxic Elapids is indicated at the first sign of neurotoxicity such as cranial nerve dysfunction or peripheral neuropathy. In general, antivenom is effective only in reversing active venom toxicity. It's of little benefit in reversing effects that have already been established. So if a patient already has established paralysis or renal failure, those will improve with time and other supportive therapies. 

[Dr. Wiener] Okay, so option A. is true. That leaves either B. or D. as false. 

[Dr. Handy] So B. is false. You do not apply suction to the wound. Most of the first aid measures recommended in the past are of little benefit and may worsen outcomes actually. Incising or applying suction to the bite should be avoided as these measures exacerbate local tissue damage, they increase the risk of infection, and have not been shown to be effective. 

[Dr. Wiener] Okay, then what about pressure immobilization? That's true? 

[Dr. Handy] In some circumstances. Elapid envenomations, such as our krait that are primarily neurotoxic and have no significant effects on local tissue may be managed with pressure immobilization. So that's when the entire bitten limb is immediately wrapped with a bandage and then immobilized. This technique is meant to restrict lymphatic drainage and has been shown to delay the systemic absorption of venom from predominantly neurotoxic species. For pressure immobilization to be effective, the wrap pressure should not exceed 40 to 70 millimeters of mercury in upper-extremity application and about 55 to 70 millimeters of mercury in lower extremity application. As an estimate, the bandage should be snug enough to apply pressure but loose enough for a finger to slip underneath. 

[Dr. Wiener] But that approach should not be used for viper bites such as cobras or Russell vipers because pressure immobilization may allow greater tissue injury, right? 

[Dr. Handy] That's right. They may restrict the spread of potentially necrotizing venom and may contribute to loss of function or ischemia. 

[Dr. Wiener] Once again, you really need to know what kind of snake bit you. 

[Dr. Handy] That's right. That's the first part. 

[Dr. Wiener] Okay, so today's teaching points are that venomous snake bites remain a substantial public health problem in many parts of the world. Identifying the type of snake and its likely venom is important to guide local care, antivenom therapy, and the anticipation of complications. 

[Dr. Handy] And you can find this question and other questions like it in Harrison's Self-Review. And if you want to read more about this topic, you can check out Harrison's chapter on disorders caused by venomous snake bites and marine animal exposures. Visit the show notes for links to helpful resources, including related chapters and review questions from Harrison's, available exclusively on AccessMedicine. If you enjoyed this episode, please leave us a review, so we can reach more listeners just like you. Thanks so much for listening. 

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