MRS. B AND HER SON, DB, age 3, presented to the ED in Glendale, Ariz., at 2000 hours. Ms. B said that DB had been playing around a group of landscaping boulders in the yard when he began to cry and complained of being stung on his right lower leg. Mrs. B tells the ED triage nurse that when the incident occurred almost 1 hour ago, she assumed that DB had been stung by a bee. When DB continued to cry and complain of pain that he described as “burning and stinging,” she brought him to the hospital.
Upon examination, DB is agitated, restless, and complaining of pain on the right lower leg that increases when touched. No erythema or edema is seen at the location identified by the patient. In a head-to-toe assessment, the nurse notes that DB exhibits nystagmus, is drooling, and now says, “I hurt all over.”
DB is awake and alert but often closes his eyes. His temperature is 98.6° F temporal (37° C); heart rate, 154 (normal, 60 to 140); respirations, 39 (normal, 24 to 40); and BP, 132/82 (normal, 96/64).1 While waiting for the healthcare provider, DB exhibits occasional fasciculations of his upper and lower extremities bilaterally.
DB is exhibiting the signs and symptoms of a bark scorpion (Centruroides sculpturatus/exilicauda) envenomation, specifically grade III. (See Taking the sting out of bark scorpions.) Early identification and intervention are essential.2-4
Bark scorpion venom is a complex combination containing mucopolysaccharides, hyaluronidase, phospholipase, acetylcholinesterase, serotonin, histamine, protease inhibitors, histamine releasers, and neurotoxins, which cause most of the adverse reactions in patients.5 The neurotoxins partially inactivate depolarized axonal sodium channels, resulting in membrane hyperexcitability and repetitive uncontrolled axonal firing. Ultimately, excessive neuromuscular activity and autonomic dysfunction result.4,5
The most common signs and symptoms of a scorpion sting are immediate pain and paresthesia at the site. Other common signs and symptoms include tachycardia, agitation or restlessness, vomiting or gastrointestinal distress, and abnormal eye movements.4,6
Many patients with multiple stings or stings in highly vascularized areas are diagnosed with grade III or IV. (See Grading the envenomation.) These patients exhibit advanced systemic signs and symptoms that include paresthesias of the face, tongue, and perioral region; dysgeusia (unpleasant or altered taste); chills; sweating; dysphagia; fasciculations; and nausea and vomiting.5
While clinical findings, along with recent travel to or residence in an region endemic for the scorpion, are the key to diagnosing a bark scorpion sting, patients with grade III or IV envenomation need lab testing, which includes the following:
- complete metabolic panel
- liver function panel, including aspartate aminotransferase and alanine aminotransferase
- serum creatine kinase
- serum lipase
These lab tests are meant to identify potential end-organ toxicity and rhabdomyolysis.4 Chest X-ray and ECG should be considered if the patient is hypoxemic or has a history of congenital heart disease.4
Supportive care should be initiated immediately, focusing on maintaining a patent airway, providing supplemental oxygen, and suctioning oral secretions as needed. The sting site should be cleaned. For grade I and II envenomations, applying ice to the sting site and administering oral analgesics such as ibuprofen will effectively manage pain. Tetanus prophylaxis is indicated if the patient hasn't had tetanus vaccination within the last 5 years.4 Patients with grade I or II envenomations should be monitored for 4 to 5 hours; unless signs and symptoms progress, these patients don't need any other interventions.
Patients who've experienced a grade III or IV scorpion sting need more aggressive treatment. Administering short-acting I.V. analgesics and sedatives, specifically fentanyl or midazolam, reduces neuromuscular hyperactivity and associated pain.4-6 Fentanyl is preferred over morphine because it doesn't cause the release of histamines. I.V. benzodiazepines should be avoided if the patient is a candidate for antivenom due to the compounding sedatory effect of the two that can often lead to oversedation. If a benzodiazepine is used, midazolam is preferred due to its short duration of action.5 I.V. propranolol has been administered to control tachycardia, but it has no effect on neuromuscular signs and symptoms.4
Patients with Grade III or IV envenomation should receive I.V. scorpion-specific F(ab')2 equine antivenom (Anascorp). Anascorp uses plasma from horses that have been injected with bark scorpion venom.7 Once the antivenom is administered, the patient must be closely observed for respiratory depression and changes in mental status. Clinicians should avoid administering additional doses of benzodiazepines.
Staff should be prepared for the possibility of an anaphylactic reaction and have emergency medications and equipment readily available. Most patients who have these reactions have previously received an equine-based antivenom or have a history of allergies to horse proteins.8
Serum sickness related to the use of this antivenom, although rare during clinical trials (34 incidents in 1,534 patients), is a possibility.8 The most common manifestations are rash, fever, myalgia, and arthralgia.8
Patients receiving the antivenom recover much more quickly than those who don't receive it, with most signs and symptoms resolving within 3 hours instead of 30 hours.4,5
Patients should be continually monitored for progression or resolution of signs and symptoms. In extreme cases, specifically in infants, endotracheal intubation may be needed to protect the airway.
After initial treatment and observation in the ED, most patients can return home, often with prescription analgesics for pain management.9 If they've received Anascorp, discharge instructions should include education related to the signs and symptoms of serum sickness. Patients and their families will also benefit from education related to the common habitat of the bark scorpion and how to deter future encounters.
End of the tale
DB was immediately treated in the ED. His signs and symptoms indicated a rapid escalation of reaction to the scorpion sting to stage III. The ED staff identified the signs and symptoms related to a bark scorpion sting and provided supportive care (supplemental oxygen, suctioning of excessive saliva, and cleaning of the sting site). A pediatric airway cart was brought to DB's bay as a precaution. I.V. fentanyl was administered to help manage pain. Anascorp, which was available within the hospital, was administered. DB's signs and symptoms resolved completely. He was monitored throughout the night and released the next morning. Mrs. B received education relating to serum sickness and methods to eliminate or reduce the habitat of the bark scorpion.
Taking the sting out of bark scorpions
The bark scorpion, a species endemic to the southwestern United States and Mexico, is particularly common in Arizona, where about 11,000 stings are reported yearly.1 They prefer to make their burrows or nests under the bark of trees, palm trees, and rocky crevices.5 While mortality is low for these stings today, children are especially at risk for severe reactions.4 The bark scorpion continues to cause complications and death in the southwest United States, although at a much lower rate than in the past due to better understanding of the venom components and quick recognition of associated signs and symptoms.2,3
Grading the envenomation
Envenomations are categorized into four clinical grades based on severity, as follows.
- Grade I: pain and/or paresthesias at the sting site without edema or other signs of inflammation. A “tap” test, done by tapping the affected limb, helps identify the sting site and usually causes an increase in pain.4
- Grade II: pain and/or paresthesias at the sting site and other locations.
- Grade III: signs and symptoms of grade II and either cranial nerve or skeletal nerve dysfunction.
- Grade IV: signs and symptoms of grade II and both cranial nerve and skeletal neuromuscular dysfunction.4,5
Cranial nerve dysfunction may be evidenced by blurred vision, abnormal eye movements (nystagmus, opsoclonus), slurred speech, tongue fasciculations, or hypersalivation.
Skeletal neuromuscular nerve dysfunction may be evidenced by restlessness, skeletal muscle fasciculations, opisthotonos (arching of the back), emprosthotonos (tetanic forward flexure of head and feet), ataxia/loss of coordination, or jerking, shaking, or flailing of the extremities.
Hyperthermia may also occur due to excess motor activity along with respiratory failure, pulmonary edema, metabolic acidosis, rhabdomyolysis, coagulopathy, pancreatitis, and multiple organ failure.