Centruroides sculpturatus/exilicauda, better known as the bark scorpion, is a member of the class Arachnida and the family Buthidae and the most dangerous scorpion within the United States.1
The bark scorpion is endemic to the southwestern United States and Mexico, with stings commonly reported in Arizona, New Mexico, Texas, California, and Nevada.1,2Centruroides sculpturatus burrows are commonly found under the bark of trees (hence the name), but also in fallen trees, piles of stones or firewood, lumber, rock walls, and other rocky crevices.2,3 About 10% of all calls received by the Arizona Poison Control Center, based in Phoenix, are related to scorpion stings.3
While mortality is low today for these types of stings, the pediatric population is especially at risk for severe reactions and Centruroides sculpturatus envenomation can be fatal for these patients. Early identification of common signs and symptoms and a better understanding of treatment protocols have resulted in more effective care of patients and a lessening of the adverse reactions of the venom.1,4,5
In 2013, over 18,000 cases of all types of scorpion envenomation were reported to the American Association of Poison Control Centers (AAPCC).6 The number of reported stings greatly outnumbered any other type of bite or envenomation, including from snakes and spiders, and resulted in 691 outcomes labeled as “moderate, major, or death” by the AAPCC.6
While those numbers are alarming, the bark scorpion continues to cause mortality and morbidity within the southwest United States at a much lower rate than in the early 20th century due to a better understanding of the venom components and quick recognition of associated signs and symptoms.4,5 Deaths due to bark scorpion envenomation in Arizona totaled 40 during the 10-year period between 1931 and 1940, but out of 18,270 scorpion stings from all species that occurred in 2013, only one death occurred.1,6 This continues a trend shown in reported mortality data from 1979 to 1990 and 1999 to 2007 that numbered only four and five deaths, respectively.4,5
The venom of the bark scorpion is a combination of mucopolysaccharides, hyaluronidase, phospholipase, acetylcholinesterase, serotonin, histamine, protease inhibitors, histamine releasers, and neurotoxins.1,7 The neurotoxin component of the venom causes the most significant reactions through partial inactivation of depolarized axonal sodium channels that result in increased sodium channel activation and uncontrolled firing of axons.1,2 This repetitive depolarization results in a greater release of acetylcholine at the neuromuscular junction, the release of catecholamines from the postganglionic sympathetic nerves and the adrenal medulla, and results in neuromuscular abnormalities and autonomic dysfunction.1,2,7
Signs and symptoms
Envenomations are graded based upon the severity of clinical findings (see Severity of scorpion envenomation). The most common signs and symptoms of a bark scorpion sting include pain and paresthesias at the sting site and occur immediately after the encounter. Tachycardia, agitation and restlessness, gastrointestinal upset and vomiting, and abnormal eye movement are also common and may be delayed.1,2,8 These manifestations are common in Grade I and II stings, while more severe envenomation (from multiple stings or stings located in highly vascularized areas of the body) results in cranial nerve (CN) abnormalities and skeletal nerve dysfunction. These are indicative of Grade III or IV envenomations.1,2,8
Grade III and Grade IV envenomations include all of the signs and symptoms of Grade I and II plus CN dysfunction and/or skeletal neuromuscular dysfunction. It is important to note that a Grade III envenomation includes either CN or skeletal nerve dysfunction, but not both. A Grade IV classification requires that both of these be present.
The most common CN dysfunctions seen in Grade III or Grade IV envenomation include:
- blurred vision (CN II)
- abnormal eye movements that include nystagmus (involuntary, repetitive eye movements, often resulting in reduced vision) and opsoclonus (involuntary, disorganized, unpredictable, and multidirectional eye movement). Abnormal eye movements are usually involuntary, conjugate, slow, and roving. (CN II, IV, VI)
- slurred speech (CN V, VII, X, XII)
- tongue fasciculations (brief, spontaneous contractions of muscle fibers)(CN XII)
- hypersalivation (CN IX, X).
Bulbar muscle (oral and esophageal muscles) dysfunction may also occur and include dysarthria, stridor, pharyngeal spasm, and dysphagia.1,2,8 Attention to airway patency, ventilation, and oxygenation are priorities in relation to CN and specifically bulbar muscle dysfunction.
The most common skeletal neuromuscular dysfunctions include:
- skeletal muscle fasciculations
- opisthotonos (severe spasm of the spinal muscles causing the patient's head and heels to arch backward)
- emprosthotonos (tetanic forward flexion of the head and feet)
- jerking and flailing of extremities.1,2,7
Care should be taken to prevent further injury to the patient when skeletal neuromuscular nerve dysfunction is present by padding the stretcher or bed.
While a detailed health history, clinical findings, and residence in or travel to an area endemic to the bark scorpion is of the utmost importance to diagnosing a bark scorpion sting, patients with Grade III or IV envenomation will require appropriate lab testing to identify potential effects of the envenomation.2
These labs include a complete metabolic panel including serum electrolytes, blood urea nitrogen, and creatinine; a liver panel; serum creatine kinase; and urinalysis.1 These tests help to identify potential end-organ toxicity and rhabdomyolysis.1 If the patient exhibits signs and symptoms of hypoxemia or has a history/family history of congenital cardiac disease, consider a chest X-ray and/or ECG to identify any abnormalities.1
Medical care and antivenom use
For all levels of envenomation, initiation of supportive care should be immediate and focus on maintenance of airway patency and administration of supplemental oxygen.2 The site of the envenomation should be cleaned and dressed per hospital policy for puncture wounds. If the patient has not had a tetanus vaccination within the preceding 5 years, one should be administered.1,2
For Grade I and Grade II envenomations, application of ice to the sting site and oral analgesics are effective in relieving pain.2 Patients who present with signs and symptoms indicative of Grade I or Grade II envenomations should be monitored for a period of between 4 and 5 hours. If signs and symptoms do not progress, no additional intervention is needed.2 Lab studies are not generally indicated for patients identified as having a bark scorpion envenomation unless they have Grade III or IV clinical findings.
In addition to the care indicated for Grade I and Grade II envenomations, patients with Grade III and Grade IV reactions will require additional intensive care. The use of short-acting I.V. analgesics and sedatives, specifically fentanyl or midazolam, will reduce neuromuscular hyperactivity and associated pain.5,6,8 Fentanyl is recommended over morphine as it does not cause the release of histamine (and resulting vasodilation or histamine-induced bronchospasm) and can be used in conjunction with antivenom due to its short half-life without oversedation.1,2 Midazolam is the best sedative choice due to its relatively short duration in comparison with other benzodiazepines, but should be used cautiously to prevent oversedation.2,8
Additionally, consideration should be given to the amount and type of I.V. analgesics administered, specifically if providers are considering the use of an antivenom, as this combination can cause oversedation.1,7 In addition, since antivenom reverses the excitatory effects of the scorpion venom, patients who have received high doses of long-acting benzodiazepines may become oversedated.1
In some cases, atropine has been administered for Grade III and IV envenomations in an attempt to dry secretions for patients with excessive drooling, but should be used cautiously to prevent adrenergic cardiotoxicity.9 Propranolol has been administered I.V. to control tachycardia but has been shown to have no effect on neuromuscular signs and symptoms associated with scorpion stings.1 I.V. scorpion-specific F(ab')2 equine antivenom (Anascorp) uses plasma from horses that have been injected with bark scorpion venom.10 While available for many years in Mexico, Anascorp was approved for use in the United States in 2011 by the FDA and is currently the only antivenom approved by the FDA in the United States.1,9 Recovery time for patients receiving Anascorp is significantly reduced, with most signs and symptoms resolving within 3 hours versus 30 hours for those not receiving the antivenom.1
As with any patient receiving new medications, vaccinations, or blood transfusions, providers should be prepared for anaphylactic reactions and to administer emergency medications as needed.2 The majority of reactions to Anascorp are due to previous exposure to equine-based antivenom or a history of allergies to horse-derived proteins.2 Delayed allergic reactions (serum sickness), though rare, is also a possibility, with 34 incidents occurring among 1,534 patients during clinical trials of the antivenom.11
Continually monitor patients for progression and/or relief of signs and symptoms. In extreme cases, specifically with envenomated infants, endotracheal intubation may be needed to protect the airway and attention should be maintained to prevent hyperthermia-induced rhabdomyolysis.1,2
Ongoing care and interventions
After initial treatment and observation in the ED, patients may return home. Analgesics for continued pain at the sting site may be prescribed.2 For patients receiving antivenom, discharge instructions and teaching must include signs and symptoms of delayed allergic reactions (serum sickness) including delayed rash, fever, myalgia, and arthralgia.11 Education regarding the scorpion's native habitat and burrow locations should also be included to prevent future stings (see Preventing scorpion stings).2 Providers should schedule a follow-up appointment for all patients suffering a bark scorpion envenomation and patients should be encouraged to immediately return to the ED if delayed allergic reactions occur.
Preventing scorpion stings12
Advise patients who are in areas where bark scorpions are found to:
- Wear protective clothing, such as shoes or gloves, when outdoors. Be sure to check shoes, gloves, clothing, and backpacks for scorpions before using.
- Keep yards free from debris, which can serve as a place for scorpions to hide.
- Make sure windows and doors fit tightly to prevent scorpions from entering the house.
- Avoid walking barefoot, especially at night when scorpions are active.
- Use a Wood lamp at night because the cuticle of the Centruroides species is fluorescent under UV light.
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in the United States: morbidity, mortality, and therapeutic innovations. Pediatr Emerg Care
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11. U.S. Food and Drug Administration. Anascorp Centruroides (Scorpion) Immune F(ab)2 (Equine) Injection [prescribing information]. 2011. www.fda.gov/ucm/groups/fdagov-public/@fdagov-bio-gen/documents/document/ucm266725.pdf
12. Cheng D. Scorpion envenomation
follow-up. Medscape. 2015. http://emedicine.medscape.com/article/168230-followup#e5