The genus Centruroides, also known as the Bark Scorpion, is found throughout the southwestern United States and northern Mexico. Many emergency medicine practitioners in the Southwest are exceptionally familiar with the treatment of envenomation from Centruroides as a quarter million are reported annually1,2. Although typically mild envenomations occur in adults, children and the elderly are at increased risk for severe complications3. The toxic syndrome consists of a sympathetic and parasympathetic storm that can result in myocardial damage, involuntary jerking, wandering eye movements, and most threatening – loss of airway.
Grading of envenomations
Centruroides envenomations are graded by these symptoms, moving from grade I-II (considered mild) to grade III-IV (considered severe)3.
| Table adapted from 3 | ||
| Grade | Signs and Symptoms | Treatment |
| I | At the site of envenomation:
|
1. Support the airway 2. Provide pain control 3. Apply ice to site of sting |
| II | Grade I AND:
|
Same as Grade I |
| III | Any of the following:
|
Same as Grade I/II PLUS consider the following:
|
| IV | Both cranial nerve AND skeletal muscle dysfunction | Same as Grade III |
Who should we treat with antivenom?
In the time before antivenom, morbidity and mortality in severe cases were much higher. Treatment consisted of supportive care, typically involving benzodiazepines, opiates, and intubation, if necessary4. For patients receiving large total doses of benzodiazepines for neuromotor dysfunction, there comes the additional risk of respiratory depression, intubation, and an admission to the intensive care unit. In 1965, the production of whole immunoglobulin antibodies derived from goats, achieved exceptional treatment success rates. This antivenom was not FDA approved and came with relatively high rates of anaphylaxis (3.4%) and delayed serum sickness (61%), which made many practitioners continue to rely on supportive care4,5. In 2001, production of this antivenom was ceased and the stock was depleted by 2004.
Anascorp (Scorpion [Centruroides] Immune F(AB)2) was approved in 2011 by the FDA and is considered standard treatment for grade III-IV envenomations. This product has been available in Mexico (Alacramyn) for many years prior to FDA approval and is dosed more conservatively than Anascorp.
Anascorp Dosing: The Anascorp package insert recommends an initial dose of 3 vials, followed by additional vials as needed with a maximum of five total vials3,6. Anascorp is equine derived and has a low rate of serum sickness (0.5%). No cases of true anaphylaxis have been reported6,7.
The evidence is certainly not robust in determining which patients will benefit most from antivenom treatment and tends to cause variation in standard practices. This has become a point of interest for pharmacy departments as the cost of Anascorp is ~$3,000-4,000 per vial, and we prefer not to break the bank for patients with astronomical medical bills. There are reports of hospitals in the United States charging between $7,950 to $39,652 per vial8. This has caused some institutions in the Southwest to establish protocols administering one vial at a time, with a “watch and wait” methodology before providing additional vials.
Should we deviate from FDA instructed dosing?
This would not be the first drug to be administered “off-label” in the emergency department (i.e. ondansetron in the ED, PO naloxone, IV olanzapine, ketamine takedown, etc.). The trials that granted the approval of Anascorp evolved to using 3 vials initially, but is it really necessary?
Currently, the one study that has citable data attempting to answer this question is in the process of being published; fortunately, the authors presented their results as a poster at the American College of Medical Toxicology conference in 2015. Their study was a retrospective review looking into pediatric patients with at least a grade III-IV envenomation and compared 3 groups: those not receiving Anascorp, those receiving the FDA approved dosing regimen, and those receiving conservative dosing with one vial initially, followed by additional vials if needed based on symptoms9.
| * FDA Dosing vs Conservative Dosing † No Anascorp vs FDA Dosing & Conservative Dosing |
||||
| No Anascorp | FDA Dosing | Conservative Dosing | p value | |
| # Patients (n=156) | 58 | 16 | 82 | |
| Mean Age | 5.7 years | 2.5 years | 4.0 years | 0.001; 0.024* |
| Grade IV | 24 (41%) | 14 (87%) | 76 (93%) | <0.001† |
| Respiratory Distress | 3% | 56% | 38% | <0.041† |
| Mean Vials Used | 0 | 3.25 | 1.83 | <0.001† |
| Mean ED LOS | 261 min | 253 min | 259 min | 0.839 |
| Hospitalization | 3.4% | 0 | 8.5% | 0.167 |
| Intubation | 0 | 0 | 2.4% | 0.607 |
| Aspiration | 0 | 0 | 2.4% | 0.607 |
These results illustrate there were no significant differences between clinically relevant outcomes, regardless of treatment group. It should be noted that there were much higher rates of respiratory distress in the 16 patients from the FDA Dosing group, suggesting a “sicker” population. However, 93% of the patients in the Conservative Dosing group were considered grade IV envenomations. Should respiratory distress be used as a better indicator for treatment? This remains to be answered, but it does question the need for 3 vials from the initial onset in grade III-IV envenomations in pediatric patients.
The original protocol for Anascorp phase 2 and 3 trials started with 1 vial followed by additional vials based on symptoms every 30 minutes. They found increased treatment adverse events with 1-2 vials (42.5%) versus patients receiving 3, 4, or 5 vials (24.5%, 28.3%, and 35.8%, respectively). Thereafter the protocol was changed to start with 3 vials10.
Pharmacology
Anascorp has an exceptionally long half-life of about 160 hours and an area under the curve (AUC) of 706 µg*hr/mL to match6. After administration of 1 vial of antivenom, serum venom levels have been noted to drop from 1,000-4,000 pg/mL to 200 pg/mL within 30 minutes and become undetectable within 2 hours11. In Mexico, antivenom costs about $100 per vial and the dosing regimen is performed 1 vial at a time, followed by observation for 30 to 60 minutes before repeating. Additionally, in Mexico the antivenom is used frequently for lower severity stings (grade I-II), likely due to the low cost12.
Challenges and Final Thoughts
Although the FDA package insert states to give 3 vials initially, the FDA is limited to the studies submitted for approval. It can also be an uphill battle changing the prescribing habits especially of providers who have ordered 3 vials of antivenom to a scorpion envenomation patient and had them practically symptom-free within minutes to a few hours. This is followed by smiles, high fives, grateful patients and families, a discharge home from the ED, and lastly, but certainly not least, a massive bill.
The literature and practice in neighboring countries appears to suggest that using 1 vial and watchfully waiting is a viable strategy with similar outcomes. Next time you have a patient with a grade III-IV Centruroides envenomation who is not on the verge of being intubated, consider starting with 1 vial of Anascorp and observe for 30-60 minutes before administering any more. It may reduce the financial hardship of patients and result in significant cost-savings for the hospital, while also reversing venomous effects.
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