Article, Toxicology

Long-term efficacy of pressure immobilization bandages in a porcine model of coral snake envenomation

a b s t r a c t

Background: Pressure immobilization bandages delay mortality for 8 hours after coral snake envenomation, but long-term efficacy has not been established.

Objective: The objective of this study is to determine the long-term efficacy of pressure immobilization bandages after coral snake envenomation in the absence of Antivenom therapy.

Methods: A randomized, observational pilot study was conducted. Ten pigs (17.3-25.6 kg) were sedated, intubated for 5 hours, and injected subcutaneously with 10 mg of lyophilized Micrurus fulvius venom resuspended in water. Pigs were randomly assigned to a control group (no treatment) or a treatment group (compression bandage and splint) approximately 1 minute after envenomation. Bandage pressure was not controlled. Pigs were monitored daily for 21 days for signs of respiratory depression, decreased oxygen saturations, and paralysis. In case of respiratory depression, pigs were humanely euthanized and time to death recorded. Statistical analysis was performed with Fisher exact test, Mann-Whitney U test, and Kaplan-Meier survival curve as appropriate.

Results: Median survival time of control animals was 307 minutes compared with 1172 minutes in treated animals (P = .10). Sixty percent of pigs in the treatment group survived to 24 hours vs 0% of control pigs (P = .08). Two of the treatment pigs survived to the end point of 21 days but showed necrosis of the distal lower extremity. Conclusions: Long-term survival after coral snake envenomation is possible in the absence of antivenom with the use of pressure immobilization bandages. The applied pressure of the bandage is critical to allowing survival without necrosis. Future studies should be designed to accurately monitor the pressures applied.

Introduction

Snakebites are a leading cause of death worldwide. Of the 5 million Snake bites that occur annually worldwide, 2.5 million result in envenomation causing a significant morbidity and mortality [1,2]. Many countries lack antivenom and other resources to treat Snake bites, demonstrating a need for inexpensive interventions to treat poisonous snake bites in many countries. In the United States, although coral snake (Micrurus species) mortality is rare, the definitive treatment with Wyeth North American coral snake antivenom is no longer available. Since initial production in 1967, there have been no reported deaths from coral snake bites until an untreated victim in 2006 [3]. Before the production of coral snake

? The authors report no declarations of interest.

?? Presentation: Society for Academic Emergency Medicine, May 2013.

? Source of support: Department of Emergency Medicine, Resident Research

Medical Foundation Account (Dr. Smyrnioudis).

* Corresponding author. Division of Toxicology, Department of Emergency Medicine, Brody School of Medicine at East Carolina University, 600 Moye Boulevard, Room 3ED311, Greenville, NC 27834. Tel./fax: +1 252 744 2954.

E-mail address: [email protected] (W.J. Meggs).

antivenom, mortality rate for untreated envenomation victims has been estimated to be 10% [4]. Coral snake antivenom production was discontinued in 2001 leaving a limited supply of antivenom without alternative definitive treatment options [5].

After subcutaneous snakebites, venom reaches the systemic circu- lation via lymphatic transport [6]. In Australia, modalities that impede lymphatic transport have been demonstrated to delay the onset of neurotoxic effects from coral snake envenomation [7-9]. A previous study found that pressure immobilization bandages prevent death from Eastern coral snake (Micrurus fulvius) evenomation in a porcine model up to 8 hours [10]. Pressure immobilization bandages prevented death from severe Western diamondback rattlesnake (Crotalus atrox) bites with a 24-hour delay in treatment with antivenom, with surviving pigs regaining use of infected limb at 1 week [11]. A localizing circumfer- ential compression device has shown efficacy in both Eastern coral snake [12] and Eastern diamondback rattlesnake [13] envenomations in a porcine model. The present study investigates whether extended use of pressure immobilization bandages consisting of an ace wrap and splint can delay morbidity and mortality and prevent death from coral snake bites, even in the absence of antivenom therapy. We examined this hypothesis using a randomized, controlled observational study of

http://dx.doi.org/10.1016/j.ajem.2014.06.002

M.E. Smyrnioudis et al. / American Journal of Emergency Medicine 32 (2014) 1024-1026

morbidity and mortality from coral snake envenomations in an experimental porcine model with a control group that received no treatment and a group treated with pressure immobilization bandages.

Methods

1025

A randomized, observational study of the effects of pressure immobilization bandages on M. fulvius (Eastern coral snake) enveno- mations was performed using a porcine model. A porcine model was used given availability, affordability, and similarity of porcine and human subcutaneous tissue distribution. The Institutional Animal Care and Use Committee approved the protocol for this study. Subjects were 10 female pigs ranging from 17.3 to 25.6 kg. Pigs were sedated with tiletamine/zolazepam 5 mg/kg and xylazine 1 mg/kg and intubated using standard procedure. Isoflurane 1% to 3% was titrated to maintain sedation for approximately 5 hours. Each pig was positioned in right lateral decubitus and injected subcutaneously in the left distal foreleg with 10 mg of lyophilized M. fulvius fulvius venom resuspended in water at a concentration of 10 mg/mL. This dose was used because it had 100% fatality rate in a previous study [10]. Each pig was injected witha 27-gauge needle at a depth of 3 mm in the mid-distal phalanx, lateral to the midline. To avoid intravascular injection, negative pressure was applied before injecting venom. Venom was obtained from The Natural Toxins Research Center in Kingsville, Texas.

Pigs were randomly assigned to either a control group (no compression bandage and splint) or a treatment group (compression bandage and cast applied) approximately 1 minute after envenomation by blindly choosing cards labeled “treat” or “no treat.” Pigs in the treatment group had 2-in wide compression bandages applied then splinted with circumferential fiberglass casts to provide immobilization and prevent the pig from removing the bandage. Compression bandages were applied from distal foreleg to proximal using typical fashion to provide compression without vascular compromise. For standardiza- tion, the same individual applied each bandage and ensured a finger breath could be inserted under the bandage. The pigs were monitored every 15 minutes for approximately 5 hours for signs of respiratory depression, decreased oxygen saturations, and change in blood pressure, heart rate, or temperature. For signs of respiratory depression or respiratory arrest, pigs were humanely killed with pentabarbitol 150 mg/kg intravenously and time to death recorded. Bandages and splints were evaluated for correct placement and excessive compression at 24 hours and daily throughout course of study and adjusted if needed. Surviving pigs were monitored daily for signs of respiratory depression and Oxygen desaturation with intermittent pulse oximetry checks and for paralysis until the end point of 21 days. They were accessed daily for any other sign of distress, including abnormal behavior, poor dietary intake, and lack of ambulation. Necropsies were performed on 3 pigs at the discretion of the veterinary staff. Fisher exact test was used to compare rates of survival up to 21 days. Mann-Whitney U test was used to compare median survival times, and a Kaplan-Meier survival curve was constructed.

Results

Pigs in the treatment group were more likely to survive to the 24-hour period than pigs in the control group (60% vs 0%; P = .08). Two of the treatment pigs survived to the end point of 21 days. The median time to death for the remaining pigs was 307 minutes for controls and 1172 minutes for treated pigs (P = .10). survival curves are given in Figure.

One treated pig died immediately when anesthesia was discon- tinued at 5 hours. A necropsy was performed to determine cause of death and demonstrated areas of hemorrhage distal to the limb injection site with concern for venom access to vascular circulation. Two of the treatment pigs required reapplication of loose bandages and casting, which was performed under anesthesia without adverse effects. The

Figure. Survival curves comparing no treatment with treatment with pressure immobilization bandages.

time to death for these 2 pigs was 1480 minutes (24.67 hours) and 1172 minutes (19.53 hours), and necropsies indicated no gross abnormalities. The 2 treatment pigs that survived to the end point of 21 days showed necrosis of the injected distal lower extremity. Both had redness and swelling above the bandage and demonstrated minimal weight bearing. Animals were evaluated twice a day by veterinary staff for signs of pain and distress, including vocalization, immobility, lack of interest in food, and lack of socialization. Analgesia was administered as needed.

At 12 days from injection dates, the 2 surviving animals were sedated and a tourniquet placed above the bandage before removal for limb evaluations. Both subjects’ limbs had swelling and redness proximal to the compression bandage and distal to the injection site and hoof. Limbs were atrophied with dark necrotic ulcers at the site of venom injection. Wound care was performed with betadine solution flush, triple antibiotic ointment (neomycin 3.5 mg, polymixon B sulfate 5000 U, bacitracin 400 U), and sterile gauze. Compression bandages and casting were reapplied within 5 to 8 minutes. Pigs resumed normal behavior, feeding and ambulating without signs of distress or discomfort. At 2 weeks, both pigs had temperatures of 106.0?F or greater. Both pigs were given meloxicam (0.4 mg/kg), and antibiotic therapy was initiated with clavomox (22 mg/kg orally, twice a day) and ciprofloxacin (20 mg/kg orally, twice a day). On Day 16, after envenomation, both pigs had bandages removed under anesthesia. Distal limbs had mild improvement of redness and swelling with persistent necrotic wounds and distal sloughing of skin. Wound care was performed using betadine, triple antibiotic ointment, and gauze before replacement of compression bandages and splints. Blood was drawn while under anesthesia for complete blood count, comprehensive metabolic analysis, and Creatinine kinase. Both pigs had a leukocytosis (21.4 and 28.7) and elevated creatinin kinase (179 and 351). After initiation of antibiotics, both pigs had resumption of healthy behavior and temperatures resolved, with survival to study end point of 21 days. When bandages were removed, both pigs had distal limb necrosis and nearly autoamputated limbs at time of death. Necropsies of the 2 pigs surviving to the 21-day study end point were normal except for the gross findings on the envenomated extremity.

Limitations

The pressure of the compression bandage was not standardized in an attempt to reproduce field conditions. The variability in survival in treatment pigs is most likely related to variability in the tightness of the bandage. There were also differences in amount of time taken for pigs to recover from anesthesia with some becoming immediately

1026 M.E. Smyrnioudis et al. / American Journal of Emergency Medicine 32 (2014) 1024-1026

mobile and weight bearing, whereas others took hours to slowly return to baseline activity. Because increased muscle activity is a factor in lymphatic flow of venom to the central circulation, early mobility may have played a role in the variable responses to treatment. The study is limited by the small sample size, which was appropriate for a preliminary observational study of this nature.

Discussion

The significance of this study is that it is the first to demonstrate that long-term survival after coral snake envenomation is possible in the absence of antivenom with the use of pressure immobilization bandages in a porcine model. Unfortunately, there was significant damage to the limb, which is attributed to the treatment and not the venom, as coral snake venom generally does not cause extensive necrosis. The applied pressure of the bandage is therefore critical to allowing survival without secondary consequences of local limb necrosis with envenomation. Future studies must be designed to accurately monitor the pressures applied and determine if there is an optimal pressure that can prevent mortality without damage to the extremity.

The pathophysiology of coral snake venom is neurotoxicity leading to muscle paralyzation and respiratory failure. Local edema and necrosis as well as coagulopathy do not occur. For injections that avoid vascular compartments, the venom must reach the systemic circulation via lymphatic transport. Unlike crotalid bites, local edema and necrosis and coagulopathy do not occur. Hence, coral snake venom is the ideal venom for explorations of this type.

The treatment pig that died immediately when anesthesia was terminated had a necropsy that indicated distal hemorrhage to the injection site, suggesting a vascular injection. It is possible that the venom easily gained circulatory access, thus, causing toxic systemic effects despite treatment with compression bandage. The 2 treatment pigs that died at approximately 24 hours were ambulating and exhibiting baseline behavior soon after anesthesia was discontinued at 5 hours. These pigs appeared to bear full weight on the injected limb with minimal change in gait. It is possible that these pigs did not survive to the end point of 21 days due to the lymphatic mobilization

of venom with increased early ambulation and weight bearing. Future studies should ensure injected limbs have minimal weight bearing.

There is insufficient data at this time to recommend long-term use of pressure immobilization bandages for Eastern coral snake bites. This preliminary work has established that long-term survival is possible and motivates the direction for further research to see if survival can be achieved without the unwanted consequence of necrosis.

References

AJEM