Treatment of venomous snake bites: antidote

General approach to treating venomous snake bites

Immediately after the bite, the victim should move away from the snake to a safe distance or be moved to that distance. The victim should avoid tension, calm down, keep warm, and quickly transport to the nearest medical facility. The bitten limb should be immobilized in a functional position below the level of the heart, remove all rings, watches, and constricting clothing. To prevent the spread of venom during immobilization, the limb should be compressed (for example, with a pressure circular bandage); this can be used for coral snake bites, but is not recommended in the United States, where most bites are from pit vipers. Compression of the limb in this case can cause arterial ischemia and necrosis. First aid providers should maintain patency of the upper airway and breathing, administer O ₂, establish intravenous access on the intact limb, while arranging for the victim to be transported to the nearest medical facility as soon as possible. The benefits of any other prehospital interventions (e.g., tourniquets, oral suction of venom with or without incision, cryotherapy, electric shock) are unproven and may cause harm and delay necessary treatment. However, tourniquets that have already been applied, if there is no risk of limb ischemia, may be left in place while the patient is being transported to the hospital until poisoning has been ruled out or definitive treatment has been initiated.

In the emergency department, attention should focus on airway patency, respiratory status, and cardiovascular status. Limb circumference should be measured on arrival and every 15 to 20 minutes thereafter until enlargement has ceased; it is useful to mark the margins of local swelling with a permanent marker to assess the progression of local manifestations of envenomation. All nontrivial rattlesnake bites require a complete blood count (including platelets), coagulation profile (eg, prothrombin time, partial thromboplastin time, fibrinogen), fibrin degradation products, urinalysis, and serum electrolytes, blood urea nitrogen, and creatinine. For moderate to severe envenomation, patients should have blood typing and cross-matching, an ECG, chest radiograph, and a CPK test, usually every 4 hours for the first 12 hours and then daily or as needed. In case of bites from a coral snake with neurotoxic venom, it is necessary to monitor blood oxygen saturation, determine the initial parameters and dynamics of functional pulmonary tests (for example, peak flow, vital capacity of the lungs).

All victims of rattlesnake bites require close medical observation for at least 8 hours after the bite. Patients without obvious signs of envenomation can be released within 8 hours after appropriate wound care. Victims of coral snake bites should be observed for at least 12 hours, paying particular attention to the possibility of respiratory paralysis. Envenomation, initially assessed as moderate, may become severe within a few hours. Without constant observation and appropriate treatment, the patient may die.

Treatment may include respiratory support, benzodiazepines for agitation, opioid analgesics for pain, fluid replacement, and vasopressors for shock. Most coagulopathies respond to adequate amounts of neutralizing antivenom. Blood transfusions (eg, washed red blood cells, fresh frozen plasma, cryoprecipitate, platelets) may be required but should not be given until the patient has received adequate antivenom. Tracheostomy is indicated if trismus, laryngospasm, or excessive salivation occurs.

Antidote

In moderate and severe poisoning, in addition to aggressive symptomatic therapy, the correct choice of antidote plays an important role.

In the treatment of rattlesnake envenomations, the equine antidote has been replaced by the sheep polyvalent immune FAb antidote to pit viper venom (purified FAb IgG fragments are taken from sheep immunized with rattlesnake venom). The efficacy of the equine antidote is time- and dose-dependent; it is most effective within 4 h after the bite and its efficacy declines after 12 h, although it can prevent coagulopathy when administered after 24 h. According to recent data, the action of the polyvalent immune FAb antidote to pit viper venom is neither time- nor dose-dependent and it can be effective even after 24 h from the bite. It is also safer than the equine antidote. However, it may still cause early reactions (cutaneous or anaphylactic) and late hypersensitivity reactions (serum sickness). Serum sickness develops in 16% of patients within 1-3 weeks after administration of the FAb antidote. Dose - 4-6 vials of reconstituted polyvalent immune FAb antidote to the venom of the pit viper family, dissolved in 250 ml of 0.9% sodium chloride solution, administered slowly at a rate of 20-50 ml/hour during the first 10 minutes. Then, if no unfavorable reactions occur, the remainder is administered during the next hour; the same dose can be repeated if necessary to treat coagulopathy or correct physiological parameters. For children, the dose is not reduced (i.e., the dose is not adjusted for body weight or height). By measuring the circumference of the affected limb at 3 points proximal to the bite site and measuring the expanding border of the edema every 15-30 minutes, a decision can be made on the need to administer additional doses. As soon as the edema stops increasing, the contents of 2 vials dissolved in 250 ml of 0.9% sodium chloride solution are administered after 6, 12 and 18 hours to prevent resumption of limb edema and other effects of the poison.

For bites from the water snake, the dose can be reduced. For bites from copperhead snakes and pygmy rattlesnakes, antivenom is usually not required, except for children, the elderly, and patients with certain diseases (e.g., diabetes, coronary heart disease).

For coral snake bites, equine antidote is administered in a dose of 5 vials if poisoning is suspected and an additional 10-15 vials if signs of poisoning develop. The dose is the same for adults and children.

In cases where an equine antidote is required, determination of susceptibility to equine serum by skin testing is questionable. Skin testing has no predictive value for the development of immediate hypersensitivity reactions, and a negative skin test does not completely exclude the possibility of such a reaction. However, if the skin test is positive and the poisoning is limb- or life-threatening, H1- and H2-receptor antagonists are administered in an intensive care unit prepared for the treatment of anaphylactic shock before use of the antivenom. Early pseudoanaphylactic reactions to the antidote are common, usually due to too rapid administration. The infusion is temporarily stopped and epinephrine, H2- and H3-receptor antagonists, and intravenous fluids are given, depending on the severity of the reaction. The antivenom is usually resumed at a lower concentration by dilution and at a slower rate. Serum sickness may occur 7-21 days after treatment and manifests as fever, rash, malaise, urticaria, arthralgia, and enlarged lymph nodes. Serum sickness is treated with H1 receptor blockers and a shortened course of oral glucocorticoids.

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Additional measures

Patients require tetanus prophylaxis based on their immunological history. Infection of snakebite sites is rare, and antibiotics are used only when clinical manifestations occur. If necessary, first-generation cephalosporins (e.g., oral cephalexin, intravenous cefazolin) or broad-spectrum penicillins (e.g., oral amoxicillin + [clavulanic acid], intravenous ampicillin + [sulbactam]) are prescribed. Subsequent antibiotic selection should be based on wound culture results.

Bite wounds should be treated like all other wounds, cleaned and the bite area covered with an aseptic dressing. In case of bites to the limb, it is immobilized in a functional position, splinted and elevated. The wound is examined daily, sanitized, and the dressing is changed. Surgical debridement of blisters, blood bubbles or superficial necrosis is performed on the 3rd-10th day (several stages may be required). Sterile whirlpool baths and other physiotherapeutic procedures can be prescribed for wound debridement. Fasciotomy for compartment syndrome is rarely required, but is used when the pressure in the interfascial space exceeds 30 mm Hg within an hour, causes vascular disorders and does not decrease with a change in the position of the limb, intravenous administration of mannitol at a dose of 1-2 g / kg or when taking an antidote. Motor activity, muscle strength, sensation, and limb diameter should be monitored for 2 days after the bite. To avoid contractures, immobilization is often interrupted by periods of light exercise, moving from passive to active movements.

In the United States, regional poison control centers and zoos are excellent sources of information in cases of human bite by a snake, even if the snake is not endemic to the area. These facilities maintain a list of physicians trained in treating snakebites, and a directory, published and periodically updated by the American Zoo and Aquarium Association and the American Association of Poison Control Centers, catalogs the location and quantity of vials of antivenom for all known and available venomous snakes, as well as the more exotic species.

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