Medication Summary
Most cases of redback spider envenomation are mild and can be managed symptomatically using oral analgesics. Severe cases may need parenteral opioids and/or benzodiazepines for pain control and sedation. A study by Ryan et al failed to find low-dose ketamine efficacious for redback spider envenomation. Ketamine administration was also responsible for several adverse events. [20]
Redback spider antivenom provides specific treatment in severe cases of envenomation. Indications for use of antivenom include severe headache, vomiting, abdominal pain, hypertension, arthralgia, or myalgia. Severe pain at the envenomation site is not considered an indication for antivenom administration. [1] Increasing concerns exist about the possible ineffectiveness of the intramuscular route of administration of the redback antivenom. [21] A randomized trial showed no significant difference in relief of symptoms or patient outcome between intravenous and intramuscular administration of antivenom. [22] A more recent study did not show significant benefit even via the intravenous route; see the discussion below.
However, a study published in 2022 of 61 pediatric patient encounters at emergency departments in Adelaide, South Australia revealed that, 2 hours after redback antivenom administration, pain resolved in 71%, hypertension resolved in 62%, diaphoresis resolved in 43% and tachycardia resolved in 82%. Adverse events were minimal, with no reported urticaria or anaphylaxis and only a single instance of serum sickness. [23]
Tetanus vaccine should be given to all patients who are not already vaccinated, are due for a booster, or whose vaccination status is unknown.
Antivenom
Class Summary
The antivenom consists of equine immunoglobulin G (IgG) fragments raised against alpha-latrotoxin (aLTX). Each ampule contains 500 units of neutralizing capacity against the Australian redback spider venom, with an average volume of 1-1.5 mL per ampule. The antivenom is typically administered by intramuscular injection, although the intravenous route has become favored. Indications for use of antivenom include severe headache, vomiting, abdominal pain, hypertension, arthralgia, or myalgia. Severe pain at the envenomation site is not considered an indication for antivenom administration.
Increasing concerns exist about the possible ineffectiveness of the intramuscular route of administration of the redback antivenom. A randomized trial showed no significant difference in relief of symptoms or patient outcome between intravenous and intramuscular administration of antivenom. A more recent study did not show significant benefit even via the intravenous route; see the discussion above.
The risk of allergic reaction to the antivenom is 0.5% and is higher in patients with a history of horse allergy or prior exposure to equine immunoglobulin. Before using the antivenom, ensure the ability to manage hypersensitivity reaction and check for availability of the appropriate resuscitation/intubation equipment. The risk of serum sickness after exposure to the antivenom is 1.4%.
Administration of antivenom requires informed consent. If feasible, a discussion of the risks, benefits, and alternatives should be held with the patient prior to administering antivenom.
Patients discharged after receiving antivenom should receive clear return instructions in the case of delayed hypersensitivity reactions.
Redback spider antivenom
Redback spider antivenom is produced by Commonwealth Serum Laboratories Ltd, Australia.
Analgesic Nonsteroidal Anti-inflammatory Drug
Class Summary
Oral acetaminophen or NSAIDs are reasonable as first-line therapy or in those without severe symptoms.
Acetaminophen
Acetaminophen is the drug of choice for pain in patients with documented hypersensitivity to aspirin or NSAIDs or in patients with upper GI disease or who are taking oral anticoagulants. It is effective in relieving mild to moderate acute pain; however, it has no peripheral anti-inflammatory effects. Acetaminophen may be preferred in elderly patients because of fewer GI and renal adverse effects.
Ibuprofen
Ibuprofen is the drug of choice for patients with mild to moderate pain. It inhibits inflammatory reactions and pain by decreasing prostaglandin synthesis. The intravenous form (Caldolor) is available only for hospital use and is indicated for pain or fever.
Analgesic, Narcotic
Class Summary
Those with severe envenomations may need aggressive pain control with parenteral opioids.
Morphine (Arymo ER, Astramorph, Depodur)
Morphine sulfate is the drug of choice for analgesia because of it has reliable and predictable effects, a good safety profile, and is easy to reverse with naloxone. Various intravenous doses are used; it is commonly titrated until the desired effect is obtained.
Fentanyl (Sublimaze)
Fentanyl citrate is a synthetic opioid that is 75-200 times more potent and has a much shorter half-life than morphine sulfate. It has less hypotensive effects and is safer in patients with hyperactive airway disease than morphine because of minimal-to-no associated histamine release. By itself, it causes little cardiovascular compromise, although the addition of benzodiazepines or other sedatives may result in decreased cardiac output and blood pressure.
It is highly lipophilic and protein-bound. Prolonged exposure leads to accumulation in fat and delays the weaning process. Consider continuous infusion because of the short half-life of fentanyl. A parenteral form is the drug of choice for conscious sedation analgesia. It is ideal for analgesic action of short duration during anesthesia and for the immediate postoperative period.
Fentanyl citrate is an excellent choice for pain management and sedation with short duration (30-60 min) and is easy to titrate. Effects are easily and quickly reversed by naloxone.
After the initial parenteral dose, subsequent parenteral doses should not be titrated more frequently than every 3 hours or every 6 hours thereafter.
The transdermal form is used only for chronic pain conditions in opioid-tolerant patients. When using the transdermal dosage form, most patients are controlled with 72-hour dosing intervals; however, some patients require dosing intervals of 48 hours.
Hydromorphone
Hydromorphone is a potent semisynthetic opiate agonist similar in structure to morphine. It is approximately 7-8 times as potent as morphine on mg-to-mg basis, with a shorter or similar duration of action.
Benzodiazepines
Class Summary
Patients may experience significant restlessness and anxiety, which may require the use of sedatives for symptomatic control.
Lorazepam (Ativan)
Lorazepam is a sedative hypnotic in the benzodiazepine class that has a short onset of effect and relatively long half-life.
Diazepam (Valium)
Diazepam modulates the postsynaptic effects of GABA-A transmission, resulting in an increase in presynaptic inhibition. It appears to act on part of the limbic system, the thalamus, and hypothalamus, to induce a calming effect. Diazepam also has been found to be an effective adjunct for the relief of skeletal muscle spasm caused by upper motor neuron disorders.
It rapidly distributes to other body fat stores. Twenty minutes after the initial intravenous infusion, the serum concentration drops to 20% of C MAX . Individualize the dosage and increase cautiously to avoid adverse effects.
Midazolam (Versed)
Because midazolam is water soluble, it takes approximately 3 times longer than diazepam to peak EEG effects. Thus, the clinician must wait 2-3 minutes to fully evaluate sedative effects before initiating a procedure or repeating a dose. Midazolam has twice the affinity for benzodiazepine receptors than diazepam. It may be administered intramuscularly if vascular access cannot be obtained.
Immunizations
Class Summary
Tetanus immunization should be administered following a redback spider bite in all patients who are not already vaccinated, are due for a booster, or whose vaccination status is unknown.
Diphtheria-tetanus toxoid vaccine (Adacel, Boostrix, Decavac)
Diphtheria-tetanus toxoid vaccine is manufactured by first culturing Clostridium tetani and then detoxifying the toxin with formaldehyde. This toxoid commonly is combined with diphtheria toxoid, and both serve to induce production of serum antibodies to toxins produced by the bacteria.
It is used to induce active immunity against tetanus in selected patients. The immunizing agent of choice for most adults and children older than 7 years are tetanus and diphtheria toxoids. It is necessary to administer booster doses to maintain tetanus immunity throughout life.
Pregnant patients should receive only tetanus toxoid, not a product containing the diphtheria antigen.
In children and adults, it may be administered into deltoid or midlateral thigh muscles. In infants, preferred site of administration is the mid thigh laterally.
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Female redback spider showing a distinctive red stripe over the abdomen. Image courtesy of John Paterson.
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Female redback spider with egg sacs. Image courtesy of John Paterson.
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Female redback spider. Image courtesy of John Paterson.
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Female redback spider. Image courtesy of John Paterson.
