by Steve Curry, M.D.
Banner – University Medical Center Phoenix
University of Arizona College of Medicine – Phoenix
It’s early March in Phoenix and we are well into the rattlesnake bite season. It is estimated there are about 350 to 400 rattlesnake bites annually in Arizona, which is pretty tiny compared to the estimated 5 million venomous snakebites and 125,000 snakebite deaths annually worldwide. But, unfortunately, Arizona has been honored as being named the #1 state for deaths per capita from venomous snakebites in the U.S (1979 – 2004). Part of this is explained by delays in seeking or reaching medical care (e.g., it can take as long as 2-3 days to get out of the bottom of the Grand Canyon), but it is also because of rapid collapse and shock seen in some patients, the topic of this post. First, let me provide some background.
The number of rattlesnake species changes at the whims of taxonomists. In Arizona, at my last count, we had 13 of the 17 rattlesnake species found in the U.S. (with additional subspecies). Most patients don’t know which species bit them. But in those cases where the snakes are brought in with patients or photographed for our examination, the most common snakebite victims we care for in Phoenix, (keep in mind some of our patients are flown from around the state and surrounding states), result from six snakes:
But we see bites from other species, as well (e.g., Tiger, Massasauga, Midget Faded, Rock, Twin-spotted, Black, etc.) We have neither indigenous copperheads nor cottonmouths.
Rattlesnake venom is incredibly complex. It varies even within species, depending on geographic location, time of year, the age of the snake, and other factors. Within venom, there are thousands of proteins, lipids, metals, and other molecules, each with actions that could result in a Ph.D. thesis (as many have). Yet there are several basic syndromes we commonly see, and these are usually able to be recited by most toxicology fellows.
Soft tissue injury and necrosis
Numerous digestive enzymes, including metalloproteases, produce local necrosis, and through activation of our own TNF-α, in turn, activate endogenous metalloproteases. Local necrosis can be extensive, and swelling and subcutaneous (SQ) hemorrhage extend up the extremity via lymphatics. A bite to a fingertip can produce swelling into the chest and leg, and even pleural effusions/hemothorax can develop. Hypovolemia and anemia from soft tissue edema and bleeding are common.
Thrombin-like enzymes deplete fibrinogen to form fibrin dimers, almost always without producing intravascular coagulation. Defibrination, with unmeasurable fibrinogen levels and non-clotting blood, similar to what might be seen with streptokinase, is enhanced through activation of our own plasmin system. Venom-induced thrombocytopenia in Arizona may be best-explained by phospholipases degrading platelet membranes. In other species (e.g., C. horridus horridus), platelet aggregation also occurs. We rarely see true DIC, though it certainly can occur. When bleeding begins, it can be acute, severe and fatal. The wife of a man who had just been bitten by a rattlesnake was driving him to medical care when she observed he was becoming weaker, vomiting blood, and passing large volumes of grossly bloody stool. She explained to me, though, that “I knew he was really in trouble when he started bleeding from his eyes.” Not a statement one easily forgets. Fortunately, he survived.
Venom phospholipase A2 acts at neuromuscular junctions (NMJs) to cause fusion of ACh-containing vesicles with the synaptic membrane, resulting in exocytosis, but then prevents membrane recycling, with eventual depletion of neurotransmitter and halting of ACh release.1 Initial fasciculations may resolve on their own (commonly with accompanying rhabdomyolysis) or progress to severe weakness and respiratory failure. Like other neuromuscular junction pathology, cranial nerves can be affected early. Some components of venom block presynaptic voltage-gated calcium channels which can also prevent ACh release.
While most fangs do not reach muscle in adults (exceptions include anterior leg compartments and thenar eminences), systemic rhabdomyolysis can result. Some myotoxins collectively increase intracellular Ca2+ concentrations by opening Na+ channels, depolarizing the sarcolemma, preventing Ca2+ loading by the sarcoplasmic reticulum (SR), and promoting Ca2+ efflux from the SR. High cytoplasmic Ca2+ concentrations activate numerous enzymes systems, uncouple oxidative phosphorylation, and produce other effects to cause myocyte dysfunction and death. We also commonly see rhabdomyolysis as the result of muscle fasciculations from Prairie and Midget Faded rattlesnake bites.
Rapid-onset shock and anaphylactoid reactions
The aforementioned major syndromes of tissue destruction, hematotoxicity, neurotoxicity, and myotoxicity combine to produce complicated clinical pictures. Add in underlying factors or diseases such as congestive heart failure, chronic anticoagulation therapy, cirrhosis, or pregnancy, and it is easy to be confronted with a critically ill patient that requires many physician hours at the bedside. But on top of all this, there is yet another major syndrome we recognize in our snakebite patients, bringing us to the main topic of discussion today.
Every year we see patients who experience rapid collapse, hypotension, many times with unconsciousness, and frequently with all or combinations of the following: angioedema of the face and airway, diarrhea, and hemorrhage. Swelling at the site of envenomation can be minimal or absent, especially during the first few hours. GI bleeding is impressive when it occurs. Some patients are found down and not even recognized as being snakebite victims, being mistaken for suffering from septic shock, anaphylaxis, or other disorders. Complications include ischemic bowel, stroke, myocardial infarction, and renal failure from acute tubular necrosis.
There are several reasons rattlesnake bite patients may become hypovolemic over hours to days following envenomation. Third-spacing of fluid and bleeding into extremities can be severe and can represent significantly large portions of blood volume, especially in children. But I am talking about something else here – patients with rapid collapse and hypotension in minutes to a half-hour or so after a bite. Let’s look at a couple of examples of such cases. Then we’ll discuss some of what is known of the pathophysiology.
A middle-aged man was bitten on his hand by a rattlesnake. Within 12 minutes he was unconscious and cyanotic. Paramedics found him profoundly hypotensive, tachycardic, and with lip and tongue edema. They could not establish an airway. SQ epinephrine was given en route to the hospital, where he was immediately endotracheally intubated. No wheezing, high airway resistance or skin rash was found. Swelling was minimal at the bite site. Treatment consisted of large IV fluid challenges, an epinephrine infusion, methylprednisolone, H1 and H2 antagonists, and 10 vials of antivenom. He was sedated as his blood pressure rose. Initial laboratory values showed: hemoglobin 22 g/dL; platelets 51,000/mm3; prothrombin time 19 s; fibrinogen 190 mg/dL; and creatine kinase 1400 IU/L.
While loading him into the helicopter, hematuria and grossly bloody diarrhea began. He arrived at our center actively bleeding from a rectal tube (photo, below) and urinary catheter.
An epinephrine infusion kept mean arterial pressure above 65 mm Hg, and he required large volumes of IV fluids, transfusions, additional antivenom, and was begun on CRRT for acute renal failure. The envenomated arm began to swell, but compartment pressures remained < 20 mm Hg.
Lip and mouth edema had resolved quickly with epinephrine. Over several hours epinephrine was weaned and vital signs normalized. Platelet count reached a nadir of 26,000/mm3, and the peak CK was about 16,000 IU/L. He was discharged 1 week later. He recalled being bitten, the onset of throat swelling, and then profound weakness. He had never been bitten previously. In the end, he received more than 60 vials antivenom. We were able to examine the live snake (below), and it was a Mojave (C. scutulatus).
Here’s another case. This woman, who had never been near snakes, was bitten on the arm. In less than 30 minutes she arrived at a hospital with unrecordable blood pressure, lethargy, and severe lip, tongue, and mouth swelling. Initial treatment comprised endotracheal intubation, 7 L crystalloid IV, IV epinephrine boluses and infusion, methylprednisolone, and H1 and H2 blockers. Antivenom was begun and she was transferred to us.
On arrival extremity swelling was minimal and angioedema had resolved quickly with epinephrine. Hematuria and bloody stools began. After additional antivenom, epinephrine could be weaned over a couple of hours. Her platelet count fell to 15,000/mm3, and her fibrinogen remained > 150 mg/dL. Transient renal failure resolved without dialysis. Additional antivenom was given for thrombocytopenia. She never experienced significant swelling at the bite site. The helicopter crew brought the dead snake with them for us to examine. Another Mojave (C. scutulatus).
Tragically, CT of the head revealed multiple cerebral infarctions as a result of initial hypotension/shock.
Several years ago Ayrn O’Connor from our group looked back on the most recent 15 consecutive rattlesnake bite patients who had collapsed within 30 minutes and found the following:
- 14/15 had rapid onset hypotension, and 11/14 required vasopressors, usually epinephrine.
- 8/15 had airway edema, and 5/15 required endotracheal intubation.
- 7/15 experienced large to massive GI bleeds.
- 5/15 suffered from acute renal failure.
- 5/15 had coma or were obtunded.
- 3/15 had rhabdomyolysis.
- 1/15 experienced ARDS and died.
Hemoconcentration with hematocrits exceeding 58% was common, including before and during the initial stages of bleeding.
In the few instances in which we’ve been able to identify the snake, we have always found a Mojave (C. scutulatus), except in one instance in which it was a Sidewinder (C. cerastes). In contrast, the most common snakebite we see is from C. atrox. However, this syndrome has been reported throughout the Americas and with multiple snake species.2–6
So, what’s going on? Could at least some of these cases represent IgE-mediated anaphylaxis? This has been documented in the literature,7 and I’ve seen clear-cut cases of such in patients with previous snakebites, including professionals with known histories of snake venom allergy. But the patients we are discussing with the syndrome of rapid collapse and shock, including young children, have had no previous history of contact with snakes. And in those rare, previous exposure anaphylaxis cases, patients experienced various degrees of urticaria, lacrimation, conjunctival injection, and wheezing. The patients we are discussing today do not, though an occasional patient may display some mild flushing.
Could it be an intravascular injection of venom? This might be a rare possibility. For example, below is a histologic section of an envenomation site from a patient who was unconscious in less than two minutes of a snakebite to the leg, and who experienced true DIC with multiple organ infarctions, diffuse bleeding, and profound shock. The red arrow points to eschar at the envenomation site. The black arrow shows the disruption of collagen directly towards a vessel lumen. Interestingly, in animals, venom injection into a vessel wall produces shock about as quickly as venom injected into the vessel lumen. Suicide attempt with IV venom injection has even been reported, and the patient looked similar to our patients with rapid collapse.8
That most or all of our patients with rapid collapse might suffer intravascular venom injections, however, is implausible, especially with bites to fingers. Furthermore, we see snakebites to highly vascular areas without rapid collapse and hypotension, such as bites to the tongue, lips, and face. Swelling in these cases, of course, is the counterpart of that seen in extremities and results from the digestive/inflammatory actions of venom.
What we are left with, then, is a syndrome that develops over about 5 to 30 minutes comprising hypotension, tachycardia, and shock along with various combinations of diarrhea, angioedema, rapid third-spacing of fluid with hemoconcentration. There is often minimal to no swelling at the bite site and, about half the time, serious GI bleeding develops. The GI bleeding certainly may be from ischemia in the face of shock and venom-induced hematotoxicity. But most of this sounds like actions of either bradykinin, as in ACEI-induced angioedema, or degranulation of mast cells and basophils, as in anaphylaxis, or both. Investigators have established mechanisms of action for snake venom that include these possibilities. Let’s take a look.
Bradykinin and snake venom
The following simplified figure outlines the formation, actions, and degradation of bradykinin.
Precursors of bradykinin are collectively called kininogens, and they reside in tissue and in blood. Enzymes capable of converting kininogen to bradykinin or kallidin (a direct precursor to bradykinin) are named kallikrein. Thus, kallikrein(s) produces bradykinin, and there is strong evidence that plasmin, also activated in rattlesnake envenomations, may form bradykinin from HMW kininogen.9,10 (Remember that IV tPA can cause angioedema.) Bradykinin, in turn, activates G protein-coupled B1 and B2 receptors to produce effects as shown in the figure. Bradykinin is degraded directly or secondarily by angiotensin-converting enzyme (ACE; Kinase II).
Bradykinin is a short, nine-amino acid peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg)
Snake venom contains 5-14 amino acid peptides, structurally similar to bradykinin, that have been named bradykinin-potentiating peptides (BPPs).11–1415 These peptides were first described in Bothrops species from South America, but have since been identified in a broad variety of snake venoms, including those from pit vipers and old world vipers. BPPs display two main actions. First, some mimic the effect of bradykinin and activate bradykinin receptors.16,17 Second, they competitively inhibit ACE to prevent the degradation of bradykinin. In fact, the original 1949 paper which led to the identification of BPPs in Bothrops jararaca venom led to the development of captopril, the first ACE inhibitor.18 Captopril’s structure is based on the common C-terminal end of selected BPPs.11 During development of ACE inhibitors, a nonopeptide BPP from B. jararaca was synthesized and actually administered IV to hypertensive human volunteers. Blood pressure fell within minutes of infusion, with complete effect within 1-2 hours. One patient experienced transient erythema of the neck without itching or other signs of allergic reaction.19
If venom BPPs were not enough, kallikrein-like enzymes have been isolated from multiple pit vipers and old world vipers.20–22 These enzymes convert kininogen to bradykinin, and also commonly display thrombin-like activity, contributing to defibrination.
Thus far we have discovered mechanisms for increased bradykinin formation from kallikrein-like enzymes and endogenous plasmin, and impaired bradykinin degradation and direct bradykinin receptor activation by BPPs. But snakes are not the only animals with venom that contains kallikrein-like enzymes. Anyone recognize this skull?
This is Heloderma suspectum, the Gila monster, a venomous lizard whose habitat we also share in Arizona.
The venom glands are modified salivary glands in the mandible. The Gila monster must bite, hang on, and commonly chew for quite some time so that venom flows into the wound through grooved teeth.
The literature contains various articles on how best to remove a Gila monster from a hand, arm or leg, including setting the lizard on fire with gasoline (not recommended). But after a few minutes, by the time a person does get the Gila monster disengaged, which frequently involves it becoming attached to the other hand, enough venom has been absorbed to produced hypotension, tachycardia, and profound angioedema, such as in the gentleman, below.
Gila monster venom contains enzymes that exhibit kallikrein activity, raising bradykinin levels.23,24 Interestingly, small peptides that act as bradykinin receptor antagonists have been isolated both from rattlesnake (and other vipers) and Gila monster venom. But clinically, an activated kallikrein/bradykinin system appears to prevail in many patients.
There is an observation that should make at least some of you question whether most of what we are seeing with early collapse and hypotension, sometimes with angioedema, mainly or entirely results from BPPs and kallikrein-like enzymes. In our experience angioedema resolves rather quickly, over minutes to 3-4 hours, with epinephrine infusions, whether from a rattlesnake or Gila monster envenomation. This rapid response of angioedema to epinephrine contrasts with the virtual complete lack of response to epinephrine infusions in patients with other forms of relatively pure bradykinin receptor-mediated angioedema, such as from ACE inhibitors or C1 esterase inhibitor (C1-INH) deficiency. BPPs are of too-low a molecular weight to think that antivenom necessarily contains high titers of antibodies against them, though antivenom certainly may inactivate kallikrein-like enzymes (antivenom definitely inactivates other thrombin-like enzymes). Perhaps BPPs are rapidly cleared from the circulation and tissue, just as is bradykinin. In animals, about 50% of administered BPPs are found in urine within about 3 hours of dosing.25 Most BPPs are generally resistant to enzymatic degradation because of proline(s) at the C-terminal end, and an N-terminal modified amino acid, though some appear to be able to undergo some enzymatic hydrolysis.26 Regardless, we must keep an open mind and look for other factors that may contribute to the formation of angioedema, vasodilation, and vascular leak, and maybe even contribute to the formation of bradykinin – something that might respond to an epinephrine infusion.
Natriuretic peptides and phospholipase A2
Degranulation of mast cells and basophils, such as during anaphylaxis, causes release and synthesis of various mediators that can cause angioedema. Furthermore, significant degranulation also appears to activate the kallikrein/bradykinin system. As an example, basophils release a kallikrein-like enzyme.27,28 The fact that mast cell and basophil degranulation can be rapidly halted by producing sustained elevations of cAMP concentrations through beta receptor activation by epinephrine causes us to examine other components of venom that might trigger degranulation. First to mind are natriuretic peptides.
Venom natriuretic peptides (NPs) bind to NP receptors to produce various actions, including intense vasodilation. Interestingly, NPs also bind to mast cells to cause degranulation.29–31 What is particularly fascinating about crotalids (and other snakes) is that a single gene codes for a single mRNA transcript that includes both NPs and BPPs.32 Take a look at the very simplified schematic illustration of a single gene product, below.
Both BPPs and a C-type NP are transcribed at the same time (more than one BPP may be in the transcript), and the BPPs and NPs are separated in post-translational modifications. While BPPs are inhibiting ACE and activating bradykinin receptors, NPs are producing vasodilation and degranulating mast cells, possibly adding to the severity of the syndrome. Toss in some additional kallikrein and plasmin activation to further increase bradykinin levels, and things are really hopping. But there is more.
The SNARE proteins responsible for exocytosis of ACh-containing vesicles at the NMJ (and at other synapses) are also involved with exocytosis of mast cell and basophil granules during degranulation. In fact, experiments have demonstrated that botulism toxins, which are metalloproteases that inactivate various SNARE proteins, prevent mast cell degranulation, just as they prevent ACh exocytosis at the NMJ.33 Given that degranulation is similar to neurotransmission at the NMJ, think back to the phospholipase A2 in rattlesnake venom that acts as a β-neurotoxin (acts presynaptically). I mentioned that phospholipase A2 causes ACh-containing vesicles to fuse with the presynaptic membrane (exocytosis), explaining initial fasciculations. We shouldn’t be surprised, then, that venom phospholipase A2 causes degranulation of mast cells and, presumably, basophils.34,35 Venom phospholipase A2 also causes the release of membrane-bound arachidonic acid from phospholipids, beginning synthesis of leukotrienes through the lipoxygenase pathway during and following degranulation. I’ve tried to put this together in an abbreviated figure, below. With everything potentially going on, it’s surprising all patients with rattlesnake envenomation don’t become hypotensive and develop profound third-spacing, hemoconcentration, and angioedema.
It is fortunate that most of our patients with rapid collapse usually respond over minutes to a few hours to fluids, epinephrine and antivenom. Though antihistamines and corticosteroids are also given, I don’t know that they actually have much of an effect in saving lives, just as in anaphylaxis, but single doses are not harmful and might be beneficial. Rapid resolution of angioedema in our rattlesnake bite patients with epinephrine and antivenom is presumed to be from some combination of halting degranulation and antivenom’s inactivation of kallikrein-like enzymes, thrombin-like enzymes (resulting in plasmin activation), NPs and phospholipase A2. The rapid clearance of BPPs might contribute if this is the case. The rapid resolution of angioedema in our patients envenomated by Gila monsters, for whom there is no antivenom, speaks more to the likelihood of halting degranulation. In fact, Gila monster venom also contains NPs and phospholipase A2.36 Here is our Gila monster victim shown earlier, now about 3-4 hours after epinephrine.
Unexpected benefits of mast cell degranulation
But there is always another twist to the plot. It turns out studies have demonstrated a protective effect of mast cell degranulation. Proteolytic enzymes found in mast cells degrade various venom proteins. This protective action extends beyond snake venom to even include Hymenoptera.37 Much of this protective effect results from the release of mast cell carboxypeptidase A.
Sarafotoxins are short peptides structurally similar to endothelins and are found in many venoms, including those from rattlesnakes. Like endothelins, sarafotoxins display various actions on vascular and non-vascular smooth muscle. As one example, by activating endothelin B (ETB) receptors, sarafotoxins cause nitric oxide formation and vasodilation. Furthermore, mast cells contain endothelin receptors that may trigger degranulation in response to sarafotoxins.
Carboxypeptidase A and, possibly, other mast cell proteases degrade sarafotoxins. And wouldn’t you know it, carboxypeptidase A and ACE share a very similar structure, and carboxypeptidase A degrades bradykinin.38 Perhaps proteolysis after degranulation is part of the explanation as to why we don’t see anaphylactoid responses as frequently or for as long a duration as we might expect.
Our discussion of various components of venom certainly does not and cannot include all components that may contribute to hypotension. I chose to focus on those that appear most likely to be associated in some way with the angioedema we encounter. As we finish up, I want to present a case from last week to illustrate that rapid-onset hypotension cases are not always so severe, and some patients get well quickly. A very pleasant woman was bitten on the leg. The snake was shot and killed, and once again, you can see it was a Mojave (C. scutulatus).
She was quickly taken to an emergency department where BP was found to be 60 mm Hg systolic. She had experienced diarrhea, but without blood, and had no angioedema. She was promptly treated with IV fluids challenges, epinephrine, methylprednisolone, H1 and H2 blockers, and antivenom was begun. She responded very quickly with normalization of vital signs; her leg had begun to swell.
On arrival, she displayed significant leg edema that was moving proximally, and additional antivenom was given.
Hematology studies remained satisfactory, and she looked much better within two days and wanted to say hello to everyone.
Medical toxicology fellows, what do you think might be the implications of a patient normally taking an ACE inhibitor on the severity of illness following a rattlesnake envenomation?
And what might be implications for treatment of rapid hypotension and collapse if the victim normally takes a beta blocker? What might glucagon offer? And icatibant?
Finally, head over to YouTube and search for “Jackie Bibby”. Jackie holds additional world records that include lying in a bathtub and in a sleeping bag with the greatest number of rattlesnakes (195 and 109 rattlesnakes, respectively). You might imagine some world records you want to set with rattlesnakes, but I don’t recommend it. Leave that up to Jackie. But if you do set a new record, be sure to leave a comment here for everyone, and have your colleagues be ready with IV fluids, epinephrine, good airway equipment, and antivenom.
And if you need to resuscitate a rattlesnake…