Scorpion Envenomation Medication

The goals of pharmacotherapy are to reduce morbidity, to prevent complications, and to neutralize the toxin.

Analgesia may be indicated. Exercise caution when using narcotics for a patient with an unsecured airway because respiratory depressive effects may be synergistic with some scorpion venoms. Some recommend against using narcotics to treat scorpion envenomation with signs of systemic toxicity, especially in children. Tetanus prophylaxis is recommended if the patient cannot verify current status. Prophylactic antibiotic therapy is not required. Corticosteroids have not been shown useful in treating venom toxicity. Hypertensive emergencies may require standard antihypertensive therapy. Conversely, hypotension may require fluid resuscitation and/or vasopressors.

Cardiovascular agents can be used to elevate or decrease blood pressure and increase heart rate. Vasopressors and inotropic agents may be necessary in patients who already have been adequately volume resuscitated but remain in shock. Conversely, antihypertensives may be needed in patients with sympathetic-induced hypertension. In particular, the use of the alpha-blocking agent prazosin has been used and recommended. However, most of the published evidence recommending for or against this agent has come from either retrospective observational or prospective cohort studies. Gupta et al compared dobutamine versus prazosin in children and found mortality in both groups to be equal, but the prazosin group had a quicker resolution in their pulmonary edema (28 h vs 72 h).^[15] For prazosin-resistant cardiotoxic cases, a small retrospective observational study that found the addition of dobutamine to the prazosin may be beneficial.^[16]

At this time, no clear evidence exists as to which agent is most beneficial in specific circumstances. Autonomic instability from scorpion envenomation may lead to rapid, dramatic fluctuations in heart rate and blood pressure. Although many agents have rapid onset, they may also have prolonged effects. Should a hypertensive patient receive a longer-acting agent they may still have medication effects if they develop subsequent hypotension. In any case, agents should be chosen with detailed knowledge of their pharmacology and understanding of the pathophysiology of scorpion venom described above. Ideally, the agents are effective, have rapid onset, can be titrated to effect, have a short half-life if discontinued, and have minimal side effects.

A total of 22 types of scorpion antivenom are listed in the American Zoo and Aquarium Association Antivenom Index. They are available for a number of different species and have varied efficacy. Antivenom use remains controversial. Many researchers report decreased morbidity, mortality, and hospital stay with its use. These researchers and clinicians believe that antivenom therapy cannot be matched by supportive care in severe Buthidae scorpion envenomation. Others suggest that adverse effects (eg, anaphylactic reactions, serum sickness) limit or contraindicate antivenom use.

Scorpion toxins are not good antigens because of small size and poor immunogenicity. They do not induce antibodies that cross-react against toxins of other scorpion species unless a 95% amino acid sequence homology exists between the 2 toxins. Thus, no universal antivenin is available.

Furthermore, the neurotoxin component of the scorpion venom tends to be the least immunogenic, resulting in the low efficiency for neurological complications. It usually is prepared from horses because they yield larger quantities. Sheep, goat, or bovine antivenins have been prepared if patient sensitivity to horse serum occurs.

One idea was to mix a batch of different scorpion antivenin together to create a universal antivenin, but this exposes the patient to unnecessary antivenin from scorpion species not from the patient's region.

Skin tests have been performed to test for allergic response with locally produced antivenins. First, dilute 0.1 mL of antivenin in a 1:10 ratio with isotonic sodium chloride solution. Second, administer 0.2 mL intradermally. A positive test result is if a wheal develops within 10 minutes. The skin test has a sensitivity of 96% and a specificity of 68%.

Until mid-2000, the antivenom for stings by the bark scorpion was manufactured in the Antivenin Production Laboratory of Arizona State University. Its use was controversial. It had been shown to produce rapid resolution of systemic symptoms but not to affect pain or paresthesias. Subsequently, many physicians recommended it in grade III and grade IV envenomations. Adverse effects included immediate and delayed hypersensitivity reactions. Initially, these reactions were rare, but they increased in frequency. This leads some physicians to prefer supportive care only, as they felt that the treatment was potentially worse than the disease. As death was rare if existent, they felt supportive care would yield similar outcomes. Currently, it is no longer being produced.

The best result occurs when antivenin is administered as early as possible (preferably within the first 2 h after the sting) and with adequate quantities to neutralize the venom (usually 50-100 times the LD50 amount). A decrease in curative effects occurs with longer sting-serotherapy delay and administration of insufficient amounts of antivenin.

In August 2011, the US Food and Drug Administration approved use of a Mexican Centruroides antivenom (Anascorp, manufactured by Instituto Bioclon for Rare Disease Therapeutics, Inc).^[17]

Class Summary

Composed of venom-specific F(ab’)2 fragments of immunoglobulin G (IgG) that bind and neutralize venom toxins, facilitating redistribution away from target tissues and elimination from the body.

Antivenin, Centruroides (scorpion) (Anascorp)

Centruroides (scorpion) immune F(ab)2 (equine) injection; antivenom indicated for treatment of clinical signs of scorpion envenomation. Initiate treatment as soon as possible in patients who develop clinically important signs of scorpion envenomation, including but not limited to, loss of muscle control, roving or abnormal eye movements, slurred speech, respiratory distress, excessive salivation, frothing at the mouth, and vomiting.

Class Summary

Prevent the histamine response in sensory nerve endings and blood vessels. They are more effective in preventing histamine response than in reversing it.

Cimetidine (Tagamet)

An H2 antagonist that, when combined with an H1 type, may be useful in treating itching and flushing in anaphylaxis, pruritus, urticaria, and contact dermatitis that do not respond to H1-receptor antagonists alone. Use in addition to H1 antihistamines. Other H2 antagonists are also available.

Diphenhydramine (Benadryl, Benylin, Bydramine)

Used for the symptomatic relief of allergic symptoms caused by histamine released in response to allergens.

Class Summary

Wounds resulting from scorpion sting are at risk of Clostridium tetani infection. A booster injection in previously immunized individuals is recommended to prevent this potentially lethal syndrome. Administer tetanus immune globulin (Hyper-Tet) to patients not immunized against C tetani products (eg, persons who have immigrated, elderly individuals).

Diphtheria-tetanus toxoid (dT)

Used to induce active immunity against tetanus in selected patients. Tetanus and diphtheria toxoids are the immunizing agents of choice for most adults and children >7 y. Booster doses are necessary to maintain tetanus immunity throughout life because tetanus spores are ubiquitous.

In children and adults, administer into the deltoid or midlateral thigh muscles. In infants, preferred site of administration is the mid thigh laterally

Class Summary

These agents induce passive immunity. Administer to patients not immunized against C tetani products (eg, persons who have immigrated, elderly individuals).

Tetanus immune globulin (TIG)

Used for passive immunization of any person with a wound that might be contaminated with tetanus spores.

Class Summary

By increasing the action of GABA (inhibitory neurotransmitter), counteract scorpion-induced excessive motor activity and nervous system excitation.

Lorazepam (Ativan)

Sedative hypnotic with short onset of effects and relatively long half-life.

By increasing action of GABA, which is a major inhibitory neurotransmitter in the brain, may depress all levels of CNS, including limbic and reticular formation.

Midazolam (Versed)

Short-acting benzodiazepine that can be administered in continuous infusion for severe nervous system excitation.

Class Summary

Used to counteract scorpion-induced hyperactivity.

Pentobarbital (Nembutal)

Short-acting barbiturate with sedative and anticonvulsant properties used to produce barbiturate coma for severe CNS hyperexcitation. Requires patient intubation prior to use.

Class Summary

Tend to be more effective than opiates to control paresthesia and pain at the sting site.

Bupivacaine (Marcaine)

May reduce pain by slowing nerve impulse propagation and reducing action potential, which, in turn, prevents initiation and conduction of nerve impulses.

Class Summary

Used to counteract the scorpion-induced adrenergic cardiovascular effect.

Labetalol (Normodyne, Trandate)

Blocks beta1-adrenergic, alpha-adrenergic, and beta2-adrenergic receptor sites, decreasing blood pressure.

Prazosin (Minipress)

Counteracts scorpion-induced adrenergic cardiovascular effects. May improve pulmonary edema through vasodilatory effects.

Hydralazine (Apresoline)

Decreases systemic resistance through direct vasodilation of arterioles

Class Summary

Used to counteract scorpion-induced cholinergic symptoms. Current recommendations are for use in treating symptomatic bradycardias. Traditionally, its use to dry venom-induced, excess, respiratory secretions has been warned against because of its potential adverse cardiopulmonary effects. It may exacerbate pulmonary edema and hypertension and may lead to a subsequent tachycardia. A recent case series has suggested relative efficacy and safety with its use in 5 pediatric patients treated for C sculpturatus sting. However, this should be considered an area in need of further study rather than a change in recommendations.

Atropine IV/IM (Atropair)

Used to increase heart rate through vagolytic effects, causing an increase in cardiac output. Also treats bronchorrhea associated with scorpion envenomations. Atropine causes a reversible blockade of muscarinic receptors with subsequent anticholinergic effects. Has been used to reverse vagally induced symptomatic bradycardias, which may be associated with scorpion envenomation. Its use for dry secretions is debated. Will not reverse the somatic or other cranial nerve symptoms.

Class Summary

Used to combat hypotension refractory to IV fluid therapy.

Norepinephrine (Levophed)

Indicated for persistent hypotension not responsive to judicious fluid loading and sodium bicarbonate.

Dobutamine (Dobutrex)

Sympathomimetic amine with stronger beta than alpha effects. Increases inotropic state with afterload reduction.

Milrinone (Primacor)

Positive inotropic agent and vasodilator with little chronotropic activity.