Scorpion envenomation causing neuromuscular toxicity (United States, Mexico, Central America, and Southern Africa)

INTRODUCTION — This topic will discuss the clinical manifestations, diagnosis, and management of envenomation by scorpions whose stings cause neuromuscular toxicity (eg, Centruroides species [indigenous to the southwestern United States, Mexico, and Central America] and Parabuthus species [inhabiting Western and Southern Africa]).

The clinical manifestations, diagnosis, and treatment of scorpion envenomations in other parts of the world that cause autonomic storm, myocardial depression, and toxic brain or pulmonary edema are discussed separately [1]. (See "Scorpion envenomation causing autonomic dysfunction (North Africa, Middle East, Asia, South America, and the Republic of Trinidad and Tobago)".)

ENTOMOLOGY — Scorpions, which are grouped in the phylum Arthropoda, have a lobster-like body shape with seven sets of paired appendages: the chelicerae, the pedipalps (claws), four sets of legs, and the pectines (a pair of comb-like structures on the ventral surface) (figure 1). The segmented tail curves upward dorsally, ending in a terminal bulbous segment called the telson, which contains paired venom glands and the stinger. In the United States, a subaculear tooth on a small, slender scorpion is specific to Centruroides sculpturatus, also known as the bark scorpion (picture 1 and picture 2 and picture 3) [2,3].

Envenomation occurs through stinging, not biting. Scorpions clutch prey in their pedipalps (claws) and thrust the tail overhead to sting. Although envenomations are sometimes reported as bites, true scorpion bites have not been documented and would be inconsequential if they did occur. Scorpions can sting multiple times, although the first sting depletes or nearly depletes the telson of venom.

A characteristic physical property of scorpions is that they fluoresce when illuminated by ultraviolet light, as from a black light or a medical Wood's lamp (picture 4) [4]. This property is used in collecting scorpions for breeding or venom harvesting and in providing pest control. The fluorescent pigment in scorpion cuticle is most likely riboflavin.

SCORPION GEOGRAPHY AND APPEARANCE — Scorpions are found on all continents except Antarctica (figure 2). Scorpions characteristically live in desert areas, semiarid grasslands, and the tropics [5].

Estimates vary regarding the number of scorpion species. Researchers report 1400 scorpion species divided into nine families [6]. Buthidae is the largest and the most dangerous family and, with few exceptions, contains the only species capable of producing clinically significant envenomations through their neurotoxic venoms [7]. At least 30 species can inflict potentially fatal stings [6-8].

Dangerous species capable of neuromuscular toxicity after envenomation include Centruroides in the United States, Mexico, and Central America and Parabuthus in Southern Africa (table 1).

Common scorpions associated with neuromuscular toxicity and their characteristic appearance are as follows:

●Centruroides sculpturatus – C. sculpturatus or the bark scorpion, named due to its preference for residing in or near trees, measures 4 to 7 cm long in length and varies in color from yellow to brown or tan (picture 1 and picture 2 and picture 3 and picture 5). The presence of a subaculear tooth, a tubercle at the base of the stinger, is specific to C. exilicauda and is helpful in differentiating this highly neurotoxic scorpion from other species [7,9,10]. C. sculpturatus is found primarily in northern Mexico and the southwestern United States (eg, Arizona, New Mexico, western Texas, southeastern California, and near Lake Mead, Nevada).

●Centruroides vittatus – C. vittatus, the striped scorpion, has a black intraocular triangle and black stripes on the thorax. C. vittatus is found primarily in the Southwest and Texas but also extends into southern Indiana and Illinois [11].

●Centruroides suffusus – C. suffusus, also known as the Mexican scorpion, is yellow to tan with dark longitudinal strips on the abdomen and measures 5 to 7.5 cm. C. suffusus is found throughout Mexico and is considered the most dangerous among the many species found there.

●Parabuthus granulatus – P. granulatus, or the granulated thick-tailed scorpion, is dark yellow to brown and is large; measuring 6 to 15 cm [12,13]. It has thin pinchers and a thick tail versus more harmless scorpions that have thick pinchers and thin tails. It is indigenous to South Africa, western Zimbabwe, Botswana, Namibia, and southern Angola and is found mainly in dry habitats (receiving less than 600 mm of rain per year). P. granulatus burrows under shrubs, grass tufts, and other objects and in sandy soil.

●Parabuthus transvaalicus – The Transvaal thick-tailed scorpion or dark scorpion, P. transvaalicus, is dark brown or black and has a length of 6 to 15 cm. It lives in South Africa, southern Zimbabwe, eastern Mozambique, and eastern Botswana. It has thin pincers and a thick tail. P. transvaalicus can both inject and spray its kurtoxin venom [13,14]. It is predominately nocturnal and burrows in sandy or gritty soil and can be found in thatched roofs.

VENOM PROPERTIES — Scorpion venoms are complex mixtures containing mucopolysaccharides, hyaluronidase, phospholipase, acetylcholinesterase, serotonin, histamine, protease inhibitors, histamine releasers, and neurotoxins. Most of these toxins are small peptide toxins that target ion channels found in both mammals and insects. The greatest medical consequences result from scorpion alpha-toxins, which consist of 61 to 76 polypeptides that bind to a specific site on the mammalian voltage-gated sodium channel [15-17]. Once the toxin binds to a site, it inhibits the inactivation of the channel, which results in prolonged depolarization and, hence, neuronal excitation [7,18]. These actions result in excessive neuromuscular activity typical of Centruroides and Parabuthus species and autonomic dysfunction with autonomic storm encountered after stings by Androctonus, Buthus, Leiurus, Hottentotta (Mesobuthus) and Tityus species. (See "Scorpion envenomation causing autonomic dysfunction (North Africa, Middle East, Asia, South America, and the Republic of Trinidad and Tobago)", section on 'Venom properties'.)

Other toxins in scorpion venom act on potassium and calcium channels, which are less important in human envenomation [19-22].

The venom of C. sculpturatus does not cause local tissue destruction or inflammation [6-8,23].

REGIONAL EPIDEMIOLOGY

United States — Centruroides sculpturatus is the culprit scorpion in most reported envenomations. Its sting causes a significant number of systemic reactions, and can be fatal [7,9]. Envenomation from the Centruroides vittatus, the common striped scorpion, has been reported to cause local and allergic reactions [24].

From 1931 to 1940, more than 40 deaths were attributed to C. sculpturatus envenomation, mostly in young children and infants. Mortality is very rare and can be prevented with appropriate supportive care; fatalities are caused by respiratory failure, which may be complicated by metabolic acidosis, hyperthermia, and rhabdomyolysis from excessive muscular activity [25].

Mexico — In Mexico, scorpion deaths previously outnumbered snakebite deaths 10 to 1, but since 2003, the mortality rate has decreased to fewer than 50 deaths annually [26,27]. At least 20 native species are regarded as dangerous. Among the eight members of the Centruroides genus, C. sculpturatus, C. noxius, and C. suffusus (the Mexican scorpion) are usually cited as the most dangerous. Most fatal cases occur in the summer months from April through July [28]. Respiratory failure is typically the proximate cause of death [27].

Southern Africa — Scorpions found in southern Africa include the frequently dangerous Buthidae with characteristic thin pincers and thick tails) and the relatively harmless Scorpionidae, Bothriuridae, and Ischnuridae (with characteristic thick pincers and thin tails) [29]. Approximately 22 Parabuthus species are distributed throughout South Africa, Namibia, Botswana, Zimbabwe, and southern Mozambique. Three other species can produce systemic toxicity after envenomation but are not considered potentially fatal: Parabuthus mossambicensis, Uroplectes planimanus, and Opistophthalmus glabrifrons. Certain Parabuthus scorpions (eg, P. transvaalicus) with large venom vesicles are capable of spraying venom when alarmed [29].

Significant neuromuscular toxicity after envenomation is best described after stings by P. granulatus and P. transvaalicus [13]. In one description of 277 envenomations by P. transvaalicus, 10 percent of patients developed severe symptoms and five patients died (case fatality rate 0.3 percent) [30]. Children younger than 10 years of age and adults over 50 years of age appear to be at greatest risk.

CLINICAL MANIFESTATIONS — Scorpion stings are usually unintentional because scorpions would rather escape humans than attack. As such, stings most commonly occur on an extremity, when a human unintentionally steps on a scorpion or reaches under wood or rocks.

The majority of scorpion envenomations causing neuromuscular toxicity produce local or no pain with minimal to no inflammation. However, approximately 3 percent of patients stung by scorpions of the species Centruroides or Parabuthus develop neuromuscular toxicity that can be life-threatening [2,7,30,31].

After envenomation, symptoms may begin immediately and typically progress to maximum severity within 5 hours. Grading of severity as described in the table and below helps to determine management (table 2). Infants can reach grade IV in as quickly as 15 to 30 minutes after being stung [32,33]. The symptoms abate at a rate that varies with the age of the victim and the grade of envenomation. Symptomatic improvement typically occurs within 9 to 30 hours without antivenom therapy in patients with grade III or IV envenomation [7,31]. Pain and paresthesias are exceptions and have been known to persist for up to two weeks.

Grading of severity — Findings after Centruroides or Parabuthus scorpion envenomation are categorized into four degrees of severity (table 2):

●Grade I envenomation – Grade I envenomations produce local pain and paresthesias at the sting site. Usually, no local inflammation occurs, and the puncture wound is too small to be observed. If no scorpion is seen, the diagnosis may require historical or epidemiologic clues or other physical signs.

The "tap test" has been empirically recommended to confirm a C. sculpturatus sting, although its reliability has not been rigorously tested. With the patient looking away or otherwise distracted, gently tapping the sting site will greatly exacerbate the pain, a sign that does not occur with other scorpion envenomations [2,3,7].

●Grade II envenomation – Grade II envenomations produce local symptoms as well as remote pain and paresthesias. The more distant symptoms often radiate proximally up the affected extremity but may occur in even more remote sites (eg, contralateral limbs) or as generalized paresthesias. Children may present with unexplained agitation or inconsolable crying.

●Grade III envenomation – Grade III envenomations produce either cranial nerve or somatic skeletal neuromuscular dysfunction, which may be accompanied by autonomic dysfunction:

•Cranial nerve dysfunction – Cranial nerve dysfunction can manifest as dysphagia, drooling, abnormal eye movements with blurred vision, slurred speech, and tongue fasciculations. The combination of bulbar neuromuscular dysfunction and increased oral secretions from parasympathomimetic effects as described below may cause problems with airway maintenance.

Abnormal eye movements most often are involuntary, dysconjugate, multidirectional, slow, and roving. Chaotic multidirectional conjugate saccades resembling opsoclonus and unsustained primary positional nystagmus may also be seen [34]. Many patients with abnormal eye movements prefer keeping their eyes closed.

•Somatic skeletal neuromuscular dysfunction – Somatic skeletal neuromuscular dysfunction typically appears as restlessness, fasciculations, shaking and jerking of the extremities, alternating opisthotonos (arching of the back), and emprosthotonos (tetanic forward flexion of the body). These signs can be mistaken for a seizure. However, the abnormal skeletal muscle activity appears more undulating and writhing than the tonic-clonic movements of generalized seizures. Also, unlike patients with seizures, victims of scorpion stings often remain awake and alert.

•Autonomic dysfunction – The most frequently described autonomic findings after envenomation varies somewhat by species:

-Centruroides – Salivation, vomiting, bronchoconstriction, diaphoresis, and tachycardia

-Parabuthus – Salivation, diaphoresis, and urinary retention

●Grade IV envenomation – Grade IV envenomations manifest both cranial nerve dysfunction and somatic skeletal neuromuscular dysfunction. On close examination, victims with skeletal muscle hyperactivity usually also have cranial nerve dysfunction, meeting criteria for grade IV. In the most severe cases of Centruroides envenomation, stridor and wheezing can occur.

Hyperthermia up to 40°C (104°F) may occur due to excess motor activity. Respiratory failure, pulmonary edema, metabolic acidosis, sterile cerebrospinal fluid pleocytosis, rhabdomyolysis, and multiple organ failure have all been described, especially in children with severe envenomation [35].

Scorpion venoms generally do not produce coagulopathy or other significant hematologic effects. Centruroides or Parabuthus venoms do not directly damage the pancreas, unlike Tityus scorpion envenomation which is commonly associated with pancreatitis [36].

Although adults appear to be envenomed more often, children are more likely to develop severe illness requiring intensive supportive care [2,7]. In an observational study of 673 patients with C. sculpturatus envenomations, over two thirds of stings occurred in adults [2]. However, 26 percent of children less than six years of age had severe envenomations (grade III or IV) versus 6 percent of adults over age 20.

Laboratory evaluation — Laboratory evaluation should be performed according to the severity of envenomation (table 2):

●Grade I to II envenomation – Laboratory studies are not needed.

●Grade III to IV envenomation – We obtain the following studies in patients with severe envenomations (grade III to IV) who ultimately warrant inpatient admission:

•Serum electrolytes

•Liver enzymes (aspartate aminotransferase [AST] and alanine aminotransferase [ALT])

•Blood urea nitrogen and serum creatinine

•Serum creatine kinase

•Urinalysis

Additional testing may be needed depending upon the clinical scenario:

•Complete blood count and coagulation studies in patients with petechiae or purpura (although hematologic toxicity is atypical from Centruroides envenomation) (see "Disseminated intravascular coagulation in infants and children" and "Evaluation and management of disseminated intravascular coagulation (DIC) in adults")

•Electrocardiogram and cardiac biomarkers if signs of exacerbation of underlying ischemic heart disease (see "Initial evaluation and management of suspected acute coronary syndrome (myocardial infarction, unstable angina) in the emergency department")

•Serum lipase and amylase and pancreatic imaging in patients with findings suggesting pancreatitis such as abdominal pain or vomiting (see "Clinical manifestations and diagnosis of acute pancreatitis")

DIAGNOSIS — The diagnosis of Centruroides or Parabuthus scorpion envenomation is based upon the following findings:

●Recent visit to or residing in an endemic region for the scorpion (see 'Regional epidemiology' above)

●History of a scorpion sting (often not present)

●Characteristic signs of envenomation (table 2) (see 'Clinical manifestations' above)

Laboratory evidence of rhabdomyolysis supports the diagnosis of severe (grade III or IV) envenomation (table 2).

Clinicians should have a high suspicion for scorpion envenomation when caring for young children from endemic areas presenting with unusual neurologic symptoms (such as agitation, choreiform movement, or abnormal eye movements) even when a history of a sting or presence of a scorpion is absent. However, the clinician should always consider other possible diagnoses when evaluating a patient suspected of scorpion envenomation. (See 'Differential diagnosis' below.)

In older patients, a history of scorpion sting is typically present. However, scorpion stings may be confused with a wide array of other diagnoses as described below. Although geographically confined in the United States, scorpions have been known to "hitchhike" on passenger baggage or freight and cause stings in regions far removed from the Southwest.

DIFFERENTIAL DIAGNOSIS — Conditions with symptoms that may overlap with those of scorpion envenomation include:

●Spider bite – Bites or stings from other arthropods may present with symptoms similar to scorpion envenomation. Specifically, black widow envenomation may produce hypertension, tachycardia, sweating, and other signs of adrenergic excess. However, it does not produce the abnormal eye movements, fasciculations, or paresthesias, or induce a positive tap test as found with C. sculpturatus scorpion stings. In addition, black widow spider bites frequently produce a characteristic halo lesion at the site (picture 6), whereas C. sculpturatus stings produce no such lesion. (See "Widow spider bites: Management".)

●Tetanus – Generalized tetanus infection may present up to one month after sustaining a tetanus-prone wound. Signs include autonomic symptoms of irritability, restlessness, diaphoresis, and tachycardia. These findings are accompanied by trismus, dysphagia, opisthotonus, and severe muscle spasms with an intact mental status. Although similar in many respects to grade III or IV scorpion envenomation (table 2), the characteristic dysconjugate multidirectional ocular movements seen  in victims of a scorpion sting help to differentiate these diagnoses. (See "Tetanus", section on 'Clinical features'.)

●Botulism – Botulism is a rare but potentially life-threatening neuroparalytic syndrome resulting from the action of a neurotoxin elaborated by the microorganism Clostridium botulinum. Botulism is classically described as the acute onset of bilateral cranial neuropathies associated with symmetric descending weakness. The Centers for Disease Control (CDC) has also suggested that the following be considered as key features of the botulism syndrome:

•Absence of fever

•Symmetric neurologic deficits

•The patient remains responsive

•Normal or slow heart rate and normal blood pressure

•No sensory deficits with the exception of blurred vision

Thus, in contrast to a scorpion sting, botulism does not cause hypersalivation, dysconjugate multidirectional ocular movements, fasciculation, or painful skeletal muscle contractions. (See "Botulism", section on 'Clinical manifestations'.)

●Neuroblastoma – Opsoclonus-myoclonus syndrome (abnormal eye movements) is a well-known paraneoplastic syndrome that occurs in 1 to 3 percent of cases of neuroblastoma. Conversely, as many as 50 percent of children with opsoclonus-myoclonus have an underlying neuroblastoma. The characteristic symptoms of OMA are rapid, dancing eye movements, rhythmic jerking, and/or ataxia. Unlike children with severe scorpion envenomation, those with neuroblastoma do not typically demonstrate hypersalivation, acute onset of cranial nerve deficits, or skeletal muscle effects. (See "Clinical presentation, diagnosis, and staging evaluation of neuroblastoma", section on 'Opsoclonus myoclonus'.)

●Toxic exposures – Several poisons have physical findings similar to those of scorpion envenomation:

•Organophosphates – Organophosphate compound and nicotine ingestion can cause excessive oral secretions and muscle fasciculations. However, organophosphate and nicotine overdose frequently cause paralysis and true seizures with loss of consciousness, which is not seen with scorpion envenomation. (See "Organophosphate and carbamate poisoning".)

•Drugs of abuse and selected pharmaceutical agents – Sympathomimetic agents (eg, methamphetamines, cocaine), dissociative agents (eg, phencyclidine [PCP]), and anticholinergic agents (eg, antihistamines, Jimson weed) often cause motor excitability and hyperthermia that is similar to the skeletal muscle findings seen after a severe scorpion sting. (See "Anticholinergic poisoning" and "Phencyclidine (PCP) intoxication in children and adolescents" and "Phencyclidine (PCP) intoxication in adults" and "Methamphetamine: Acute intoxication".)

A case series of 18 inadvertent methamphetamine poisonings among children in central Arizona included three victims initially misdiagnosed with a scorpion sting and inappropriately treated with antivenom; one patient had an anaphylactic reaction [37]. However, poisoning with these agents does not typically cause cranial nerve findings and is usually associated with delirium.

•Strychnine – Strychnine intoxication, tetanus, or dystonic reactions from medicines that antagonize dopamine receptors (eg, antipsychotic agents) may all present with painful muscle contraction (eg, opisthotonus), tonic-clonic movements, and a preserved mental status, similar to the clinical picture of severe scorpion envenomation. A history of an infected wound (tetanus), exposure to a precipitating agent (dystonic reaction), or strychnine ingestion can facilitate making these alternative diagnoses. In addition, patients with tetanus and strychnine exposure do not typically have abnormal eye movements, and patients with dystonia usually respond rapidly to administration of diphenhydramine or benztropine. (See "Tetanus" and "Strychnine poisoning" and "Treatment of dystonia in children and adults".)

●Seizure – Patients with seizures may have muscle movements similar to victims of scorpion envenomation but usually do not have an intact mental status or the characteristic dysconjugate, roving eye movements that are seen in victims after a scorpion sting.

●Meningitis – Fever, neck stiffness, hypertonia, and cerebrospinal fluid pleocytosis are seen in children with meningitis and, rarely, in those with severe scorpion envenomation [35]. As with many other diagnoses, abnormal eye movements are characteristically not seen with meningitis. (See "Bacterial meningitis in children older than one month: Clinical features and diagnosis", section on 'Clinical features'.)

●Esophageal or airway foreign body – Excessive oral secretions and respiratory distress from a scorpion sting may mimic the presentation of an upper airway or esophageal foreign body. However, paresthesias, skeletal muscle abnormalities, and cranial nerve abnormalities are not seen in patients with foreign bodies.

●Status asthmaticus – Occasional wheezing may occur following a Centruroides sting and be mistaken for an asthma exacerbation until systemic symptoms (eg, paresthesias, neurologic abnormalities) become evident.

MANAGEMENT — The management of scorpion stings is determined by the severity of envenomation (table 2).

Pain without neuromuscular toxicity — Most scorpion stings result in grade I or II envenomation (table 2) [2]. Treatment consists of [15]:

●Pain management:

•Mild to moderate local pain and/or paresthesias (grade I envenomation): Pain control such as oral ibuprofen, other nonsteroidal antiinflammatory drug (NSAID), or, if NSAID therapy is inadequate, oral opioid medications

•Severe local or remote pain and/or paresthesias (grade II envenomation): Short-acting opioids (eg, intravenous or intranasal fentanyl)

●Wound management:

•Cleansing of the sting site

•Tetanus prophylaxis, as needed (table 3)

These patients should be observed for 4 hours to ensure no further progression of symptoms. Progression of symptoms in young children may occur rapidly [7,30]. (See 'Clinical manifestations' above.)

Prior to discharge, patients should tolerate oral intake, have no progression of symptoms, and their pain should be adequately controlled with oral medications.

Neuromuscular toxicity — When managing a suspected scorpion envenomation with grade III or IV severity (table 2), we encourage consultation with a regional poison control center or physician with experience managing scorpion stings. To obtain emergency consultation with a medical toxicologist, in the United States, call 1-800-222-1222, or the nearest international regional poison center. Contact information for regional poison centers around the world is available separately. (See 'Additional resources' below.)

Antivenom — We suggest that patients with grade III or IV envenomation (table 2) after Centruroides scorpion envenomation (skeletal muscle and/or cranial nerve dysfunction) receive intravenous scorpion-specific antivenom [38,39]. In a patient with grade III envenomation or a patient older than five years of age with grade IV envenomation without respiratory distress or uncontrolled pain/neuromuscular toxicity, supportive care alone without antivenom administration is an acceptable option if antivenom is unavailable or the patient refuses (eg, due to cost) [40]. We advise consultation with a medical toxicologist or other physician with expertise in managing scorpion envenomations if not administering antivenom to a patient with grade III or IV envenomation.

We also suggest antivenom administration for grade III or IV envenomation caused by Parabuthus species. However, as discussed below, evidence is less clear that Parabuthus antivenom is effective.

Centruroides antivenom (Anascorp, United States; Alacramyn, Mexico) is widely available in Mexico and Central America. Anascorp is also approved for use in the United States [41]. In the United States, further information about administering or obtaining scorpion-specific F(ab')2 equine antivenom may be obtained from the nearest regional poison control center at 1-800-222-1222.

●Dosing and administration – The approved dosing regimen for Centruroides scorpion-specific F(ab')2 equine antivenom consists of three vials dissolved in 20 to 50 mL of normal saline and infused over 30 minutes [41]. Subsequent single-vial doses of Centruroides antivenom at 30-minute intervals after the initial three vials, up to a total dose of five vials administered may be given until resolution of symptoms.

However, in order to decrease the total amount of antivenom required for treatment, some experts use an off-label approach that consists of a single-vial of Centruroides scorpion-specific F(ab')2 equine antivenom as the initial dose that is repeated at 30- to 60-minute intervals as needed for persistent signs of envenomation [40,42]. As discussed below, this regimen may delay time to complete resolution of symptoms but appears to provide similar control of envenomation at four hours without increasing the need for supportive care interventions (eg, analgesia or sedation). This approach should only be performed under the guidance of a medical toxicologist or other physician with expertise in managing scorpion envenomation. In the United States, call 1-800-222-1222, or the nearest international regional poison center.

For Parabuthus scorpion-specific F(ab')2 equine antivenom dosing and administration, consult the manufacturer’s directions (SAIMR Scorpion Venom Antiserum SAVP, South Africa).

It is important for the patient and/or guardian to understand that the vast majority of Centruroides scorpion envenomations are unlikely to be life-threatening; the exception is severe envenomations in very small children. However, without antivenom, the patient will likely have a prolonged period of distressing symptoms. Of all available treatments, current evidence indicates antivenom is likely to be effective and may significantly reduce the duration of suffering and need for hospitalization.

Prior to the administration of antivenom, medications and equipment for the treatment of anaphylaxis should be immediately available, including IV fluids, epinephrine, and intubation equipment. Whenever possible, antivenom should be administered in settings capable of emergency or intensive care.

Allergic reactions should be managed by immediately stopping intravenous infusion of the antivenom (if applicable) and treating symptoms appropriately (table 4 and table 5). (See "Anaphylaxis: Emergency treatment".)

All patients receiving antivenom should be informed of the possibility of serum sickness and the symptoms suggestive of serum sickness (eg, fever, rash, arthralgias, and arthritis) and advised to seek medical care if such symptoms occur. However, limited evidence suggests that the risk of serum sickness after scorpion-specific F(ab')2 equine antivenom is low [28,38]. (See "Serum sickness and serum sickness-like reactions".)

●Efficacy – The evidence for efficacy of scorpion antivenom varies by species and region as follows:

•United States – Centruroides scorpion-specific F(ab')2 equine antivenom has been distributed to rural hospitals in Arizona by legislative mandate. Previously used Centruroides goat-derived antivenom was associated with a 3 percent frequency of immediate hypersensitivity reactions and up to a 60 percent risk of delayed serum sickness and is no longer used [9,38].

Preliminary evidence indicates that intravenous scorpion-specific F(ab')2 equine antivenom markedly reduces the duration of clinical signs and symptoms in patients with grade III or IV and appears to have a lower risk of anaphylaxis [38,39]. In a blinded trial of scorpion-specific F(ab')2 antivenom (Anascorp) in 15 children, ages 6 months to 18 years, who were admitted to a pediatric intensive care unit, total resolution of clinical symptoms occurred within 4 hours of treatment in all eight antivenom recipients versus one of seven placebo recipients [38]. Plasma venom concentrations were undetectable in all eight antivenom recipients but in only one placebo recipient 1 hour after treatment. No immediate hypersensitivity or symptoms of serum sickness occurred in patients receiving the antivenom.

Although Centruroides F(ab)2 antivenom is expensive, its use is probably cost-effective when given to patients with severe envenomation because the length of intensive care and hospitalization is significantly shortened.

As previously mentioned, based on evidence from retrospective observational studies, some experts have adopted an off-label antivenom dosing approach or supportive care alone as alternatives to US Food and Drug Administration (FDA)-recommended dosing in patients with grade III or IV scorpion envenomation:

-In a retrospective chart review of 156 children with grade III or IV scorpion envenomation, outcomes (emergency department length of stay, hospitalization, intubation, or aspiration) did not differ significantly regardless of treatment (supportive care, three vials initial antivenom dose, or one to two vials initial antivenom dose followed by additional doses as needed) [42]. However, there were important baseline differences in the groups: patients who received antivenom had more severe envenomation than patients receiving supportive care alone and patients who received a three-vial initial antivenom dose were younger and were more likely to have respiratory distress.

-In a separate retrospective study of 141 children with grade III or IV envenomation who received antivenom, all patients had complete resolution of signs and symptoms within four hours [40]. Compared with patients who initially received three vials of antivenom, those who initially received one vial of antivenom took longer to become asymptomatic (90 versus 60 minutes) and had a longer emergency department length of stay (236 versus 192 minutes). A minority of children required symptomatic medical therapy (eg, anticholinergic agents, benzodiazepines, opioids, or antiemetics) or rapid sequence intubation; frequency of supportive care was similar in both groups.

-A retrospective Poison Control Center review of 252 patients (166 patients <10 years old) with scorpion envenomation in Arizona between 2017 and 2021 found that symptoms resolved in all patients within four hours of receiving antivenom [43]. However, the severity of envenomation was not reported, and the course of patients who received supportive care alone was not discussed.

•Mexico and Central America – In Mexico, scorpion-specific equine antivenom with activity against several Centruroides species has long been used with low risk of medical complications [27]. In a large observational study of 38,068 adults and children from Mexico, 20,000 received horse serum antivenom with no immediate hypersensitivity reactions and no deaths [28].

•Southern Africa – Although recommended by regional experts, evidence is lacking to show efficacy of Parabuthus antivenom [13,29,30]. Victims of scorpion stings in Africa often have delayed presentation for medical care and, in many cases, postpone medical care while receiving traditional treatments. This delay makes assessment of the effectiveness of antivenom difficult because it is less likely to be effective once severe toxicity is established in the patient.

Supportive care — All patients with grade III or IV scorpion envenomations require intensive supportive care. Key interventions for these patients include:

●Frequent suctioning of oral secretions.

●For patients with wheezing, inhaled beta₂ adrenergic agonists (table 6 and table 7) and systemic or inhaled corticosteroids as for status asthmaticus (table 8).

●Endotracheal intubation in patients with significant difficulties maintaining their airway or with pulmonary edema accompanied by hypoxemia.

●Close monitoring for and treatment of myocardial ischemia and/or acute decompensated heart failure in patients at risk.

●Intravenous fentanyl for pain. Fentanyl is preferred if antivenom administration is planned because, unlike morphine, fentanyl does not cause histamine release.

●If antivenom is not used, intravenous benzodiazepines (lorazepam or continuous midazolam infusion), titrated to effect, for sedation and to treat muscle spasticity.

Benzodiazepines should be used carefully or avoided if antivenom administration is planned. Antivenom reverses the excitatory effects of the scorpion venom and children who have received high doses of long-acting benzodiazepines (eg, lorazepam) may become oversedated, occasionally requiring intubation. Thus, a short-acting benzodiazepine (eg, midazolam) is preferred for management of excess muscle activity and anxiety in pediatric patients. Once antivenom is administered, the clinician should closely monitor respiratory and mental status for signs of oversedation and further dosing with benzodiazepines should be avoided.

PREVENTION — Reducing small cracks and crevices in homes decreases the risk of human-scorpion interactions. In scorpion-infested areas, clothing, shoes, packages, and camping gear should be shaken out and checked for scorpions. Footwear is recommended. Unnecessary ground cover and debris should be removed to reduce potential nesting places.

Certain insecticides, including organophosphates, pyrethrins, and several chlorinated hydrocarbons, are known to kill scorpions. Spraying insecticides around the home can work indirectly by killing other insects in the area and reducing the scorpions' food supply. A village-wide scorpion eradication program with pyrethroid insecticides in the state of Morelos, Mexico reduced the incidence of scorpion stings by 17 percent [44].

ADDITIONAL RESOURCES

Regional poison control centers — Regional poison control centers in the United States are available at all times for consultation on patients with known or suspected poisoning, and who may be critically ill, require admission, or have clinical pictures that are unclear (1-800-222-1222). In addition, some hospitals have medical toxicologists available for bedside consultation. Whenever available, these are invaluable resources to help in the diagnosis and management of ingestions or overdoses. Contact information for poison centers around the world is provided separately. (See "Society guideline links: Regional poison control centers".)

Consultation with medical toxicologists experienced in treating Centruroides and other scorpion envenomations in the United States can be obtained by calling the Banner Poison Control Center in Phoenix at 1-602-253-3334 or the Arizona Poison and Drug Information Center in Tucson at 1-520-626-6016.

Society guideline links — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Envenomation by snakes, arthropods (spiders and scorpions), and marine animals".)

SUMMARY AND RECOMMENDATIONS

●Medically important scorpions – Dangerous species capable of neuromuscular toxicity after envenomation include Centruroides in the United States, Mexico and Central America (picture 5) and Parabuthus in Southern Africa (table 1). (See 'Scorpion geography and appearance' above.)

Scorpion stings are usually accidental because scorpions would rather escape humans than attack. Stings most commonly occur when a human unintentionally steps on a scorpion or reaches under wood or rocks. (See 'Clinical manifestations' above.)

●Clinical manifestations – Findings after Centruroides or Parabuthus scorpion envenomation are categorized into four degrees of severity (table 2). The majority of scorpion envenomations cause local or no pain with minimal to no inflammation. When present, symptoms may begin immediately and typically progress to maximum severity within 5 hours. Young children (<5 years old) are most likely to develop life-threatening envenomation requiring intensive supportive care. (See 'Clinical manifestations' above and 'Grading of severity' above.)

●Laboratory evaluation – Laboratory studies are not needed in patients with mild (grade I to II) envenomation. We obtain the following studies in patients with severe effects (grade III to IV) who ultimately warrant hospital admission (see 'Laboratory evaluation' above):

•Serum electrolytes

•Liver enzymes (aspartate aminotransferase [AST] and alanine aminotransferase [ALT])

•Blood urea nitrogen and serum creatinine

•Serum creatine kinase

•Urinalysis

Additional studies are necessary for patients with findings of:

•Acute coronary syndrome (see "Initial evaluation and management of suspected acute coronary syndrome (myocardial infarction, unstable angina) in the emergency department")

•Disseminated intravascular coagulopathy (see "Evaluation and management of disseminated intravascular coagulation (DIC) in adults")

•Pancreatitis (see "Clinical manifestations and diagnosis of acute pancreatitis")

●Diagnosis – The diagnosis of Centruroides or Parabuthus scorpion envenomation is based upon the following findings (see 'Diagnosis' above):

•Recent visit to or residing in an endemic region for the scorpion (see 'Regional epidemiology' above)

•History of a scorpion sting (often not present)

•Characteristic signs of envenomation (table 2) (see 'Clinical manifestations' above):

●Management – The management of scorpion stings is determined by the severity of envenomation:

•Grade I or II envenomation – Treatment of grade I or II scorpion envenomation consists of pain control, wound management with tetanus prophylaxis (table 3) as needed, and observation for 4 hours to ensure no progression of symptoms. Prior to discharge, patients should tolerate oral intake and have adequate pain control with oral medications. (See 'Pain without neuromuscular toxicity' above.)

•Grade III or IV envenomation – When managing a suspected scorpion envenomation with grade III or IV severity (table 2), we encourage consultation with a regional poison control center or physician with experience managing scorpion stings. (See 'Neuromuscular toxicity' above and 'Additional resources' above.)

For patients with grade III or IV envenomation (table 2) caused by Centruroides sculpturatus scorpions in the United States, we suggest scorpion-specific antivenom (Grade 2B). Supportive care alone as directed by a medical toxicologist or other physician with similar expertise is an acceptable option in a patient with grade III envenomation or a patient older than five years of age with grade IV envenomation without respiratory distress or uncontrolled pain/neuromuscular toxicity if antivenom is unavailable or the patient refuses (eg, due to cost).

For patients with grade III or IV envenomation caused by Centruroides scorpions in Mexico and Central America, we recommend scorpion specific antivenom (Grade 1C). (See 'Antivenom' above.)

For patients with grade III or IV envenomation caused by Parabuthus species, we suggest antivenom administration (Grade 2C). (See 'Antivenom' above.)

•Supportive care – All patients with grade III or IV scorpion envenomations require intensive supportive care. Key supportive interventions for these patients include (see 'Supportive care' above):

-Frequent suctioning of oral secretions.

-For patients with wheezing, inhaled beta₂ adrenergic agonists (table 6 and table 7) and corticosteroids as for asthma exacerbations (algorithm 1 and algorithm 2).

-Endotracheal intubation in patients unable to maintain their airway or who develop pulmonary edema.

-Close monitoring for and treatment of myocardial ischemia and/or acute decompensated heart failure in patients at risk.

-Intravenous fentanyl for pain. Fentanyl is preferred if antivenom administration is planned because it does not cause histamine release.

-If antivenom is not used, intravenous benzodiazepines (eg, continuous midazolam infusion), titrated to effect, for sedation and to treat muscle spasticity. Benzodiazepines should be used carefully or avoided if antivenom administration is planned.