SCORPIONS

BACKGROUND

Scorpions envenomate humans by stinging them with the telson on their tail. The majority of medically significant envenomations occur in the Middle East, tropics (e.g., Southwest Asia, India, Central and South America), and North Africa. Scorpions are nocturnal, hibernate in the winter, and are active in the warm seasons. Humans are frequently envenomated by scorpions hiding in dark, hidden locations such as inside shoes and small crevices.

Given many scorpion envenomations occur in developing nations lacking significant medical infrastructure, data regarding scorpion envenomations and antivenoms are limited. Studies have estimated over 1.2 million scorpion envenomations per year globally with an overall fatality rate ranging from 0.27% to 0.52%.22,23 The majority of severe and fatal envenomations occur in children under the age of ten due to their decreased body mass.

PATHOPHYSIOLOGY

Scorpion venoms are complex and can include phospholipase, acetylcholinesterase, hyaluronidase, serotonin, and neurotoxins. Scorpion venom increases neuronal release by blocking inactivation of the sodium channel, resulting in an increase in the amplitude and duration of neuron action potential. The overall result is excess stimulation of the central nervous system, the neuromuscular system, the sympathetic nervous system, and the parasympathetic nervous system.24

The components of scorpion venom are species specific and generally fall into the categories of neurotoxic and cardiotoxic; however, this terminology is misleading since the cardiotoxic effects are secondary to an excess release of catecholamines stimulated by the nervous system.25

The venom of one unique species of scorpion, Hemicorpius lepturus, found in Iraq and Iran is predominately cytotoxic, similar to the brown recluse spider.26

CLINICAL MANIFESTATIONS

Scorpion stings produce a painful local reaction often including the sensation of tingling or burning. Erythema at the injection site is common, and discoloration and necrosis sometimes occur.

Symptoms of excess sympathetic nervous system stimulation predominate over symptoms of parasympathetic nervous system stimulation. Sympathetic stimulation via excess catecholamine release produces hypertension, tachycardia, irritability, and agitation. In severe cases, patients may develop seizures and hyperthermia. Excess cardiovascular stimulation may result in myocardial ischemia, myocardial infarction, and cardiac dysrhythmia. In rare cases, severe outcomes occur like bradycardia and hypotension due to excess parasympathetic stimulation, or cardiovascular collapse resulting from catecholamine depletion.27

Clinical manifestations of the parasympathetic stimulation include salivation, nausea, vomiting, abdominal pain, pancreatitis, and priapism.27

Neuromuscular symptoms include tongue fasciculations, muscle spasms, dysphagia, and dysphonia. Roving eye movements is a classic finding of severe centruroides envenomation (the only clinically significant venomous scorpion indigenous to the United States). While infrequent, severe muscle spasm can result in airway compromise and respiratory arrest.24

Pulmonary edema commonly occurs in severe and fatal cases.25 The pulmonary edema is both cardiogenic and non-cardiogenic in nature. The cardiogenic component is due to reduced cardiac output resulting from excessive sympathetic stimulation. The non-cardiogenic component is due to increased vascular permeability and release of vasoactive substances.

Hemiscorpius lepturus envenomations can cause local skin necrosis and in severe cases hemolysis, disseminated intravascular coagulation, and renal failure.28

TREATMENT

The diagnosis of scorpion envenomation is made clinically based on history, symptoms, and signs of envenomation. While older children and adults will almost universally report the initial painful sting, in nonverbal children the diagnosis may be more challenging. Laboratory analysis may reveal an elevated white blood cell count, serum glucose, lactate dehydrogenase, and amylase; however, these tests lack sufficient sensitivity or specificity to be clinically relevant. In those patients with moderate to severe symptoms, an electrocardiogram should be performed to evaluate for evidence of cardiac ischemia or dysrhythmias. A serum troponin may be measured to evaluate for cardiac ischemia.29 If the envenomation occurred in Iraq or Iran and Hemiscorpius lepturus envenomation is suspected, a platelet count, prothrombin time, D-dimer, and fibrinogen level, blood urea nitrogen, and creatinine can be performed to evaluate for evidence of disseminated intravascular coagulation or renal failure.30

The majority of scorpion envenomations can be adequately managed with pain medications (ibuprofen, acetaminophen, and opioids) and routine wound management to include tetanus prophylaxis. Most patients, especially adults, will not develop significant symptoms. Patients should be observed for 4-6 hours after envenomation to ensure no delayed onset of symptoms. Prophylactic antibiotics are not indicated. Application of a tourniquet, cauterization, and incision and drainage are contraindicated in scorpion stings. Benzodiazepines may be used to treat significant agitation (which may cause hypertension) or seizures. Pharmacologic management of hypertension is generally not indicated. Intravenous vasopressors such as nitroprusside, nitroglycerin, labetalol, and phentolamine should be used in patients with severe or symptomatic hypertension refractory to benzodiazepines.31

For clinically significant envenomation, management is supportive and focused on the patient’s symptoms. Benzodiazepines are the first line therapy for sympathomimetic toxicity.31 Note that benzodiazepines under dosing for sympathomimetic toxicity and seizures is not uncommon. Administer Benzodiazepines aggressively to ensure symptom control. Intravenous propofol or phenobarbital in combination with endotracheal intubation may be used in severe cases. Other anticonvulsants such as phenytoin, levetiracetam, and valproic acid are generally not indicated since they do not treat the underlying sympathomimetic or anticholinergic toxidrome causing the seizures. Direct acting vasopressors (epinephrine and norepinephrine) are recommended to treat bradycardia and hypotension.

In patients with significant neuromuscular spasm, oral secretions, sedation, or other threats to the patent airway, perform endotracheal intubation to prevent aspiration and ensure adequate ventilation. Pulmonary edema should be managed with noninvasive or invasive ventilation in combination with optimization of cardiac output.31

The incidence of long-term sequelae resulting from scorpion envenomation is unknown; however, due to the potentially significant risk, priapism should be managed aggressively with antivenom (if available) and other standard treatments of priapism (aspiration and intracavernous injection of phenylephrine) with urology consultation.32

Antivenom is available for some species; data regarding the benefits and risks of many of these antivenoms are significantly limited. In patients with moderate to severe symptoms refractory to analgesics and benzodiazepines, antivenom, if available, may be indicated. Due to the high risk of immediate or delayed allergic reactions to these antivenoms, intravenous histamine antagonists (i.e. diphenhydramine), steroids, and epinephrine should be immediately available at the patient’s bedside prior to antivenom administration.

Appendix B provides scorpion sting clinical grading and treatment guidelines. Appendix C provides a list of scorpion antivenoms available by country and their dosing regimens. Several studies demonstrate improved efficacy of intravenous compared to intramuscular antivenom administration.33 Antivenom dosing does not depend on patient weight or size and is the same for adult and pediatric patients.34