Achieving Initial Control with Antivenom in Snakebite Victims

When someone is bitten by pit viper snakes (examples include rattlesnakes, copperheads, and cottonmouths) in the U.S., they may or may not become envenomed, depending on a number of factors: the size of the snake, whether or not venom was released by the snake into the bite site, and the potency of the venom. When a snake-bitten victim is felt by the treating clinician to have suffered a significant envenomation with symptoms indicating what’s called “envenomation syndrome,” then he or she will likely receive antivenom as part of their treatment.

In an article (Academic Emergency Medicine 2011;18:46-52) entitled “Factors Associated With Difficulty Achieving Initial Control With Crotalidae Polyvalent Immune Antivenom in Snakebite Patients,” Dr. Shan Yin and his colleagues described what they determined to be the risk factors for difficulty achieving initial control of the envenomation syndrome. They looked back at the records (performing a “retrospective study”) of all patients presenting to any one of 17 treatment centers who received the antivenom product CroFab (Crotalidae Polyvalent Immune Fab [Ovine]). CroFab is an antivenom derived from sheep that are immunized to a mixture of snake venoms (western diamondback rattlesnake, eastern diamondback rattlesnake, Mojave rattlesnake, and cottonmouth); the product is purified and sterilized.

Taking into account nine specific venom effects, they used standard criteria to determine whether or not initial control was achieved. The venom effects taken into consideration wer:

• increasing pain
• increasing swelling
• altered blood clotting (determined by laboratory tests)
• decreased platelet count (which can cause easy bruising and bleeding)
• sudden unexplained bleeding anywhere in the body
• difficulty breathing or chest “tightness”
• rapid heart rate (greater than 125 beats per minute) or low blood pressure
• neurologic symptoms (apprehension, headache, weakness, numbness and tingling, confusion, or muscle twitching specifically at the bite site)
• gastrointestinal symptoms (nausea, vomiting, or diarrhea)

Initial control was defined as eliminating or preventing the worsening of any of these venom effects after administration of antivenom (up to 12 vials, with a standard initial dose being four to six vials, repeated as necessary).

Most of the patients were bitten on the upper limb and were young males. This is not surprising, given the tendency for boys and men to handle snakes and to be in a position to become bitten. In most of the patients, initial control of clinical symptoms was obtained. In the patients who remained ill despite an appropriate initial dose of antivenom, the presenting symptoms featured neurologic symptoms, low platelet count, bleeding tendency and what was deemed to be a “severe bite.” Seven patients in whom initial control could not be obtained received more than 24 vials (range 25 to 46 vials). The combination of low platelet count and neurologic symptoms increased the likelihood of not attaining initial control with antivenom by a factor of nearly 14 times.

Knowing about the clinical presentation of victims likely to not be initially responsive to antivenom allows doctors to be certain that there is sufficient antivenom on hand, perhaps be more aggressive with other supportive therapies and anticipate clinical difficulties, and may guide researchers and the developers of antivenom to discover and manufacture more effective future antivenom products.