Crotalid Snakebite

By Lewis Nelson, MD

One cool October morning in New York City, a 62-year-old man presented to the emergency department complaining of pain and swelling in his hand. He explained to the triage nurse that while playing Frisbee in Central Park, he was bitten on the thumb by an unknown animal. The man said that the bite occurred when he reached down to retrieve the Frisbee and his hand briefly touched the ground.

The patient's entire hand and distal forearm were found to be swollen, tender, tense, and warm. Radial pulse and capillary refill were both adequate. Sensation in the fingers was normal, with pain reported on passive extension. Two "fang" marks were noted at the bite site, both oozing serosanguineous fluid. Two hours later, the swelling had progressed to the shoulder.

The patient's history is both typical and atypical when it comes to venomous snakebites. Snakes are poikilothermic, which means their body temperature conforms to that of their environment. Cool weather reduces a snake's activity, making snakebite quite uncommon in the United States during the nonsummer months. Most bites occur on the foot and ankle after a snake has been startled by the victim; bites on the hand generally occur when someone has attempted to handle the snake. In the case of this patient, the bite was consistent with his story.

Snakebites tend to be more common in men; in fact, according to some studies, 90% of bite victims are men. As with most other types of injury, snakebites are commonly associated with alcohol consumption by the victim. This patient, however, did not appear inebriated.

DIFFERENTIAL DIAGNOSIS
The above description of the patient's wounds leaves little doubt as to their origin. Few animals produce the characteristic fang marks associated with snakebite. Of indigenous United States snakes, only pit vipers (crotalids) have fangs capable of producing such marks. The slow drainage of body fluid from the wound and the extremely rapid progression of the swelling to involve the proximal arm were consistent with envenomation by a domestic crotalid.

However, not all snakebites that occur in the U.S. are caused by indigenous snakes. Importers and collectors of venomous snakes regularly acquire such animals for use as culinary delicacies, medicinal agents, and religious symbols, among other reasons. Since the world's population of venomous snakes varies extensively in type and clinical toxicity, and exotic snakebite wounds are often indistinguishable from those of domestic snakes, the possibility that this patient's story was untrue--and that he was bitten by an exotic snake--could not be discounted.

The clinical presentation of patients bitten by North American pit vipers includes both local and systemic effects. Local tissue necrosis and skin sloughing are the major symptoms. The most pronounced systemic effects of North American crotalid venom are hematologic and include coagulopathy and thrombocytopenia. Although these hematologic effects may be associated with hemorrhage, such responses are uncommon even in the presence of dramatic laboratory abnormalities. The Mojave rattlesnake is unique among domestic crotalids in that its venom is neurotoxic; victims may die from respiratory muscle paralysis.

The tissue effects of crotalid venom are caused by several histotoxic components including hyaluronidase, collagenase, and metalloproteinase. These compounds have a direct impact on body tissue, producing localized swelling by initiating a cascade of proinflammatory mediators. The degree and rate of progression of the swelling are broadly related to the envenoming species of crotalid. For example, copperheads (Agkistrodon contortrix) typically produce only mild tissue effects, whereas the venom of true rattlesnakes (Crotalus adamanteus, for example, known as the eastern diamondback) is generally more aggressive, although exceptions do exist.

TREATMENT
Although tissue necrosis from crotalid envenomation is painful and disfiguring, there is currently no suitable treatment other than impeccable wound care and, occasionally, antivenin. Functional impairment following healing is uncommon, however, and limb loss is even rarer. The development of compartment syndrome, however, substantially increases the risk of disability or loss of limb.

Compartment syndrome results from increased pressure within the fixed spaces confined by the fascia of limbs. The skin surrounding the involved compartment is tense and warm, and patients suffer severe pain in the affected part. Vascular compromise to the distal extremity and its sequelae are the major morbidities associated with this syndrome.

The diagnosis of compartment syndrome is confirmed by direct measurement of the compartment pressures using a percutaneous needle apparatus. Although the clinical findings associated with crotalid snakebite resemble those of compartment syndrome, the compartment pressures of snakebite victims generally remain normal or are only slightly elevated. Since the fangs of most North American crotalids are generally too short to enter these relatively deep spaces, the venom is typically deposited in the subcutaneous tissue. Thus, direct pressure measurement is critical prior to initiating surgical compartment decompression.

Several authors still recommend the routine use of surgical compartment release (fasciotomy) for the majority of snakebite victims who have swelling of the extremity. However, there is little experimental or clinical data to support this practice. Subcutaneous injections of rattlesnake venom into the hind limbs of dogs did not produce elevated compartment pressures, whereas direct injection into the compartment was associated with dramatic elevations. No appropriate controlled clinical study evaluating the need for or benefit of fasciotomy has been performed to date. In fact, most of the literature concerning routine fasciotomy has been published as personal commentaries or uncontrolled case studies.

At the minimum, all patients in whom a fasciotomy is considered should have direct measurement of their compartment pressures. Although it seems reasonable to perform a fasciotomy if the pressures are dramatically elevated, there is both experimental and clinical evidence that the early administration of an appropriate antivenin may be beneficial. Since fasciotomy is disfiguring and perhaps detrimental, it seems reasonable to attempt a trial of antivenin prior to fasciotomy. The consequences of delaying fasciotomy are unknown, but there is certainly a window of several hours before irreversible ischemic damage occurs.

PATIENT OUTCOME
The clinicians doubted the patient's story and questioned him, but he maintained his account of the injury. Elevated compartment pressures (50 mm Hg) were recorded by direct measurement in the patient's hand and arm. Because the envenoming species was believed to be a pit viper and because the swelling was extensive, five vials of polyvalent crotalid antivenin were administered empirically. Since this antivenin has unknown, if any, utility against snakebites other than North American pit vipers, a fasciotomy was performed to relieve the elevated compartment pressures.

Two days after surgery, the patient admitted that he was a purveyor of snakes for their meat. The bite, he confessed, was inflicted by a western diamondback rattlesnake (Crotalus atrox) encountered not in Central Park but at his home. This snake, the largest of the North American crotalid species, is one of the few whose fangs are capable of entering the deep fascial compartments. In retrospect, therefore, it is clear that both antivenin and fasciotomy were appropriate therapeutic measures.

Suggested Reading Parrish HM and Carr CA: Bites by copperheads (Ancistrodon contortix) in the United States. JAMA 201:927, 1967. Mars M, et al.: Direct intracompartmental pressure measurement in the management of snakebites in children. S Afr Med J 80:227, 1991. Glass TG: Early debridement in pit viper bites. JAMA 235:2513, 1976. Russell FE, et al.: Snake venom poisoning in the United States: experience with 550 cases. JAMA 233:341, 1975. Garfin SR, et al.: Rattlesnake bites and surgical decompression: results using a laboratory model. Toxicon 22:177, 1984. Roberts RS, et al.: Upper extremity compartment syndromes following pit viper envenomation. Clin ortho 193:184, 1985. Tunget-Johnson CL, et al.: Resolution of elevated compartment pressures after rattlesnake envenomation with antivenin. J Toxicol Clin Toxicol 36:458, 1998 (abstract). Stewart RM, et al.: Antivenin and fasciotomy/debridement in the treatment of severe rattlesnake bites. Am J Surg 158:543, 1989.