Author: Jessica Slim, MD, MPH, PGY3 in Emergency Medicine, Denver Health Medical Center
Editor: Rahul Patwari, MD

Objectives

Upon completing this module, you should be able to:

1. Recognize the clinical presentation of Centruroides excilicauda in the U.S.
2. Understand the key treatment principles of Centruroides exilicauda scorpion stings.
   

Introduction

Centruroides exilicauda (previously known as Centruroides sculpturatus) is the only scorpion in the United States with venom potent enough to produce a life-threatening illness. It is found throughout Arizona and other adjacent areas in the Southwestern United States, including parts of Texas small areas of California as well as part of Northern Mexico. Scorpions can be found outside their typical range of distribution, typically when they crawl into luggage, boxes, or containers and are transported elsewhere.

Centruroides exilicauda is unique in that it is a climbing scorpion and never burrows. The common name “bark scorpion” refers to the species preference to live in or near trees. It measures 4 to 7 cm in length depending on maturity and varies in color from yellow to brown. The bark scorpion has a pair of slender pincers and a segmented tail that has a poisonous spike at the end called a telson which contains the venom apparatus which stings rather than bites. They are not typically aggressive and usually only sting when handled or threatened.

In the United States, Centruroides exilicauda stings are associated with major neurologic toxicity, especially in young children. Supportive care is the key component of management. Antivenom therapy with equine derived Fab fragments reduces the duration of symptoms. Centruroides antivenom is available in Mexico; however, in the United States, its use is restricted to that of an approved investigational drug.

Mechanism of Toxicity

Centruroides exilicauda venom contains neurotoxin that increases sodium channel permeability resulting in sodium channel activation and cell membrane depolarization. This results in over-stimulation of sympathetic and parasympathetic nervous systems, causing excessive acetylcholine and catecholamine release.

Presentation

After envenomation, symptoms may begin immediately, progress, and peak to maximum severity within several hours, and may persist for one to two days.  The patient often presents with severe sensitivity to touch at the site (tap sign).  Numbness, tingling, anxiety, nausea/vomiting, and blurred vision are common findings. Characteristic signs of envenomation include hypersalivation, abnormal roving eye movements (chaotic multidirectional conjugate saccades), fasciculations, and clonus. Hyperthermia, hypertension, tachycardia and excessive respiratory secretions are consistent with a cholinergic syndrome. Mental status is typically preserved. Depending on the severity of envenomation, patients can ultimately develop dysrhythmias, catecholamine-induced myocarditis, myocardial ischemia and cardiopulmonary arrest.  Although adults appear to be envenomed more often, children are more likely to develop severe illness requiring intensive supportive care.

Diagnostic Testing

The diagnosis of Centruroides scorpion sting is based upon clinical findings including recent visit to or living in an endemic region for the scorpion, history of a scorpion sting (although often not present) and characteristic findings of envenomation. As with spider bites, there is no single diagnostic test that is helpful in the diagnosis of scorpion envenomation.

Differential Diagnosis

1. Scorpion envenomation can clinically resemble black widow spider envenomation. However, unlike black widow spider bites, scorpion stings often cause intense local pain at the site of envenomation. Furthermore, black widow spider bites result in localized diaphoresis and possibly lymphangitis.
2. Cardiac manifestations of myocarditis and dysrhythmias can suggest myocardial infarction, infectious myocarditis and other primary cardiac diseases.
3. The neurologic manifestations, including rotatory eye movements, muscle fasciculations and myoclonus may suggest seizures.
4. Botulism is a rare but potentially life-threatening neuroparalytic syndrome resulting from the action of a neurotoxin by the microorganism Clostridium botulinum. It is classically described as the acute onset of bilateral cranial neuropathies associated with symmetric descending weakness. In contrast to a scorpion sting, botulism does not cause hypersalivation, fasciculation, or painful skeletal muscle contractions
5. Additional causes of cholinergic syndrome should be considered.
6. Intoxication with cocaine, amphetamines and other stimulates may be considered.
   

Initial Actions and Primary Survey

As with snakebites, initial treatment of envenomated patients begins with supportive care. Support the airway as necessary, obtain IV access, and administer pain medications.

Treatment

Vital signs need to be monitored for signs of autonomic dysfunction. Most victims of Centruroides excilicauda scorpion bites can be managed with supportive care only, such as local wound care, tetanus prophylaxis, opioids for muscle pain, and benzodiazepines for neuromuscular symptoms. Airway support is important and patients may require ventilation due to hypersalivation and autonomic dysfunction.

Antivenom (Anascorp®, US; Alacramyn®, Mexico) should only be considered when there is severe somatic or cranial nerve dysfunction not controlled by supportive measures. It is widely available in Mexico and Anascorp® is approved for use in the United States.  Antivenom is given intravenously in a dose of three vials dissolved in 20 to 50 mL of normal saline infused over 30 minutes. Subsequent single vial doses of Centruroides antivenom, up to a total of five vials administered at thirty minutes intervals may be given until resolution of symptoms. Prior to the administration of antivenom, medications and equipment for the treatment of anaphylaxis secondary to allergic reaction to antivenom should be immediately available, including IV fluids, epinephrine, and intubation equipment. Further information about administering or obtaining antivenom may be obtained from the Banner Good Samaritan Poison and Drug Information Center at 1-602-253-3334 or from the nearest regional poison control center at 1-800-222-1222.

Pearls and Pitfalls

• Cholinergic Crisis: Patients can present with cholinergic symptoms, especially excessive oral secretions. Atropine has been reported to be helpful in managing these symptoms. However, atropine should not be routinely used and should only be administered to patient who develop severe cholinergic crisis. The benefits of atropine must be weighed against the risk of tachycardia and dysrhythmias.
• Informed consent: It is important for the patient to understand that scorpion envenomation is unlikely to be a fatal disease process. Therefore, antivenom is not lifesaving. However, without antivenom, the patient will likely have a prolonged period of distressing symptoms, and of all available treatments, current evidence indicates antivenom is likely to be effective and may significantly reduce the duration of suffering and hospitalization.
  

Selected References

1. Feng, S and Goto, C. Bites and Stings – Snakes, Spiders, and Scorpions in the United States. Emergency Medicine Practice. May 2007. Volume 4, Number 5. (Accessed from ebmedicine.net August 1, 2014).
2. Jacobstein, C.R. and Baren, J.M.: Bites and Envenomations (Section XXIV Environmental Emergencies) in Wolfson, A.B., Hendey G., et al. (Eds.). Harwood-Nuss’ Clinical Practice of Emergency Medicine, (5th edition), Lippincott, Williams and Wilkins, Philadelphia, 2009.
3. Lovecchio, Frank. Scorpion stings in the United States and Mexico. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed August 8, 2014).
4. “Bbasgen-scorpion-front” by Musides at en.wikipedia. Licensed under CC BY-SA 3.0 via Wikimedia Commons.