Snakebites are responsible for a significant degree of morbidity and mortality worldwide, especially in low resource countries. There are over 600 identified species of venomous snakes worldwide, with the majority belonging to the Viperidae and Elapidae families. Common names among the Viperidae family are vipers, pit vipers, and adders. Distinguishing features of the Viperidae family include long, retractable fangs, triangular heads, elliptical pupils, and small scales on their tails. The pit vipers also have a heat-sensing pad adjacent to their nares for infrared vision. Common names among the Elapid family are cobras, coral snakes, mambas, and copperheads. The Elapids can be distinguished from the Viperids as they have fixed, short fangs, less triangular heads, circular pupils, and larger scales on their tails. The remainder of the venomous snake species belongs to the Atractaspididae, Colubridae, and Hydrophidae families.
Snake envenomation worldwide is primarily related to occupational exposure, such as in farmers and hunters, but is also seen among tourists exploring the outdoors. When snake envenomation occurs in humans, the initial damage is local around the site of the bite and may spread to systemic toxicity depending on the species.
Snakes primarily use their venom production for targeting prey, but it is also a form of self-defense. Some species can selectively release their venom leading to the occurrence of “dry bites,” in which there is the delivery of no venom. When venom gets released with their bite, the overall toxicity is dependent on both the volume of the venom released and the median lethal dose (LD50) of the venom.
There are an estimated 1.2 to 5.5 million people affected by snake bites worldwide each year, with as many as 94000 of these leading to death. Three times this many will lead to permanent disability secondary to the snake bite, such as amputation. The regional variation is quite staggering with only five snakebite related deaths occurring in the United States in comparison to India, which has one of the world’s largest number of snake bites annually, with an estimated 50000 snakebite related death. The cause of this variation is twofold. The first being a higher prevalence of venomous snakes in a more densely populated region with poorer living conditions and health literacy. The second factor is that the access to healthcare in these regions is limited with long travel times to reach a suitable treatment center and lack of antivenom availability. Within the United States, the vast majority of snake envenomations occur in the southwest states with envenomations from the Viperidae family. Children only comprised 28% of envenomations, and males were more likely to be victims at 59%.
The symptoms seen from snake envenomations are mainly due to the toxic components in their venom. The exact composition ranges from species to species and can vary significantly from localized tissue destruction to profound coagulopathies. The clinical effect on humans is related to both the potency and the volume of the toxin released during the snake bite. The venom released by the inland Taipan from Australia is the most potent in the world with a lethal dose of only 0.01 mg/kg with an average envenomation of 44 mg. Their envenomations have greater than 80% mortality.
The composition of snake venom from a single species of venomous snake can consist of up to 100 different toxic elements. Phospholipase A2 is a common component of snake venom and causes damage by inhibiting the electron transfer at cytochrome C, leading to damage of mitochondrial-bound enzymes. This compound has both local effects of the surrounding tissues as well as systemic effects on the vascular system and nerve endings. There are a variety of other proteins and polypeptides with toxic effects, such as neurotoxins and hemotoxins.
Most of the neurotoxic effects are secondary to damage at both the presynaptic and postsynaptic terminals of the neuromuscular junction. Presynaptic neurotoxins, such as phospholipase A2, damages the terminal axon, which prevents the release of acetylcholine, causing diffuse paralysis. These toxins typically do not respond well to antivenom or anticholinesterase administration. In contrast, the postsynaptic neurotoxins, such as alpha neurotoxin, responds well to antivenom and anticholinesterase administration as the toxin binds directly to the acetylcholine receptor.
There are a wide variety of hemotoxins with effects on the coagulation cascade, platelet activation, and fibrin clot formation. Most of the toxins lead to an increased bleeding tendency secondary to a consumptive coagulopathy. There are, however, some hemotoxins that promote clotting and thrombosis.
Local tissue destruction becomes increased with the addition of hyaluronidase and proteolytic enzymes to the snake venom, which can lead to local tissue edema, blistering, and tissue necrosis.
A detailed history of a patient suspected of having a snakebite is essential to delineate treatment options moving forward. Information to obtain includes the timing and location of the bite, the onset of any symptoms the patient has been experiencing, and any first aid administered in the field. Gathering a past medical history with detail to which medications they are on, specifically anti-platelet and anti-coagulant medications, and any allergies that would prohibit them from receiving an antivenom, such as horses. If possible, information on the offending snake should be gathered. This information should be compared to the local database of venomous snakes located on the WHO website to see if a local antivenom exists. Factors that contribute to the severity of the bite include size of the victim, with larger patients doing better, part of the body bitten, exertion following the bite, depth of the bite, species of snake causing the bite, time to presentation at the hospital, and initial first aid given at the scene .
The physical exam may or may not reveal fang marks at the injury site. There could be local tissue damage, such as ecchymosis, blistering, or even tissue necrosis. Neurotoxic effects will initially present with generalized weakness, ptosis, and ophthalmoplegia; this may progress to paralysis of the facial muscles, and eventually, respiratory failure secondary to obstruction or paralysis of the diaphragm. Significant bleeding from the puncture site, epistaxis, or evidence of other spontaneous bleeding could indicate a hemotoxic effect. Patients may present with signs of shock secondary to venom-induced vasodilation, hypovolemia, or even anaphylaxis in some patients .
The physical exam could help indicate the species of snake inflicting the bite. In general, Elapid bites are associated with minimal local tissue damage and have a neurotoxic syndrome with systemic toxicity. Viperid bites are associated with profound local tissue damage and have a hemotoxic syndrome with systemic toxicity.
Symptoms that may suggest systemic effects of the envenomation include nausea, vomiting, abdominal pain, lethargy, muscle weakness, muscle fasciculation, and severe headache. It is important to recognize these symptoms early to prompt initiation of antivenom administration.
Ancillary study testing should target the suspected toxin envenomation. In the case of unknown exposure and signs of systemic toxicity, a broad initial workup should take place. Laboratory testing would include a complete blood count, prothrombin time, partial thromboplastin time, fibrinogen in the case of suspected hemotoxin and a creatinine kinase, basic electrolytes, and urinalysis for possible developing rhabdomyolysis. For cases of neurotoxicity, the patient requires end-tidal CO2 monitoring, or serial arterial blood gases to evaluate for developing respiratory acidosis. They should also undergo frequent neurologic checks to monitor the progression of the toxicity. One study commented on the use of electroencephalogram (EEG) in the diagnosis of systemic neurotoxicity, but this did not correlate with envenomation severity. There has been the development of rapid venom testing within some centers, but the data shows that these are only useful to confirm the suspected species of envenomation and that a negative result cannot exclude a particular species.
Additionally, the wound site should be monitored carefully for the spread of edema, development of blisters, or signs of compartment syndrome. If compartment syndrome becomes a concern clinically, the definitive diagnosis is possible by obtaining a direct compartment pressure. If this is within 30 mmHg of the diastolic blood pressure, it is diagnostic of compartment syndrome.
The initial first aid at the scene should be minimal and aim at getting the patient to the nearest treatment center quickly. Varying opinions exist regarding the usefulness of placing the affected extremity in a splint and keeping it at heart level. Therefore, this should only occur if it will not delay transportation. Removal of jewelry and any constrictive clothing on the affected limb is necessary due to the possibility of swelling and circulatory compromise. The patient should be kept calm and encouraged not to exert themselves as this could increase the snake venom absorption. Pressure bandages are another controversial topic. If the identity of the snake species is known to cause neurotoxicity and no local tissue damage, the application of a pressure bandage could slow the spread of the venom. However, if the venom is known to cause local tissue damage, the implementation of the pressure bandage may worsen the damage inflicted to the extremity. Application of a tourniquet proximal to the bite results in higher morbidity without any improvement in outcomes, so this practice has been discouraged. The use of venom extractors has also demonstrated to be ineffective. Local wound manipulation, such as incision or washout, is generally not suggested.
Appropriate identification of the offending snake species will help determine the appropriate treatment modality. If antivenom is available, the administration should take place as soon as systemic toxicity is suspected. Crotalidae polyvalent immune fab is an antivenom approved for use in the United States and has antivenom components for four species of Crotalids. Countries like India and Mumbai, with a more significant variation in venomous snake speciation, carry multiple targeted antivenoms. In high prevalence areas, these antivenoms are stored at a central facility to aid in proper storage and distribution as needed. Antivenom should be administered within four hours from the snakebite for optimal effect. Antivenom administration is intravenous, as a local subcutaneous injection has not proven to result in improved outcomes. There have been reported cases of recurrent neurotoxic envenomation after the initial dosing of a proper antivenom, which may be secondary to manipulation at the bite site, releasing a non-neutralized toxin into the bloodstream. All these cases improved after re-administration of the proper antivenom.
The primary treatment should also include resuscitation of the patient, including intubation for those with respiratory distress or paralysis and IV fluids for those exhibiting signs of shock. Some patients may require the use of vasopressors to counteract the vasodilatory effects of the envenomation. It is essential to recognize the systemic toxicity may progress rapidly, so early recognition and treatment are paramount.
Some patients will develop severe coagulopathies from their snake envenomations. Treat life-threatening bleeding with direct pressure, and if needed, temporary use of a tourniquet to bridge to definitive repair is an option. There is no evidence on the empiric use of blood products, such as platelets or fresh frozen plasma. Transfuse packed red blood cells in cases of severe blood loss.
The cytotoxic effects of the venom, unfortunately, are not improved with the antivenom. These can lead to significant tissue necrosis and compartment syndrome around the wound, so consultation with the general surgery team for local tissue debridement and fasciotomy as needed are important. These procedures should delay until other systemic toxicities, such as coagulopathies, are corrected.
Additional treatment targets the toxin side effects. There have been some studies that demonstrate the benefit of edrophonium and other long-acting anticholinesterase medications to counter the effects of the neurotoxic components of the venom. Additionally, if life-threatening bleeding is present, the administration of blood products may be considered.
In the absence of an observed snake bite, alternative envenomation, such as scorpion, tick, or spider bite, should be considered based on the region. For patients with a neurotoxic syndrome, one must consider Guillain-Barre or botulism poisoning as an alternative. Tick paralysis can also produce similar symptoms. Those with coagulopathies should undergo evaluation for an underlying hereditary abnormality or an acquired disease, such as disseminated intravascular coagulation or thrombotic thrombocytopenic purpura. Local tissue destruction can be related to trauma to that area or a soft tissue infection, such as cellulitis, abscess formation, or necrotizing fasciitis.
The majority of morbidity and mortality from snake bites are secondary to the toxin production associated with the bite. Patients who seek proper medical attention within the first 6 hours after the bite have significantly lower morbidity and mortality. The half-life of various toxins ranges from a few hours and up to two days. Patients who are monitored and given supportive treatment during this period typically do not have any long-term side effects. Those with significant local tissue injury secondary to the snake envenomation may develop longstanding paresthesia, muscle damage, or even amputation in severe cases.
The primary complications from snake envenomations are due to the direct toxic effects. The localized tissue damage may require debridement or even amputation in severe cases. Reports exist of massive coagulopathies leading to profound blood loss. These coagulopathies usually resolve within 48 hours of the snake envenomation. Profound neuromuscular blockade can also occur, leading to pulmonary collapse if the diaphragm is involved. These symptoms also typically resolve within 72 hours of envenomation.
The administration of snake antivenom requires monitoring for signs of adverse reactions. These include an anaphylactic reaction that may occur within the first few minutes of administration and up to two hours. If a severe anaphylactic response is suspected, the infusion should stop, and the administration of epinephrine and an anti-histamine should follow. Patients may also develop a hypersensitivity reaction leading to pruritus, hives, nausea, and mild hypotension. This reaction may occur at any time during the antivenom administration and will dissipate once the infusion is complete. There are also documented cases of serum sickness as a side effect that may occur up to two weeks after administration. This condition responds well to a short course of antihistamines and, in severe cases, corticosteroids.
When a snake bite is suspected, it is essential to notify the local healthcare authorities and present to the nearest emergency center for prompt evaluation. There should be no attempt at local wound exploration or irrigation, and a tourniquet should not be applied. It is important to remain calm following the snake bite and to keep the affected extremity still. Although most snake bites, even when from a venomous species, do not lead to systemic toxicity, the systemic effects have significant morbidity and mortality. The emergency medicine team will monitor the snake bite and any progression to systemic toxicity. The decision to administer an antivenom depends on the development of systemic toxicity as there are some side effects of the antivenom itself. Expect to remain in the hospital for up to 48 hours to monitor symptom progression. Those that do well throughout the observation period typically do not have any long-term effects relating to the snake envenomation.
The proper evaluation and management of a snake envenomation depend on an interprofessional team approach. This type of management starts from the emergency medical service team gathering information regarding the snake species and rapidly transporting the patient to the hospital. The emergency medicine physician will begin the initial resuscitation, preferably while being in contact with a toxicologist and poison center. The nursing staff will continuously monitor the patient for any progression of systemic toxicity. The pharmacist will be coordinating the delivery and administration of antivenom, if available, as well as preparing to assist clinical staff in the event of anaphylaxis. Early consultation with a general surgeon and an intensivist should be considered for optimal monitoring and may need to assist in the treatment of the patient. These interprofessional strategies are crucial in managing snake envenomation. [Level 5]
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