Make the Diagnosis: Did This Snakebite Cause Severe Envenomation?
Prior Probability of Severe Snakebite Envenomation
In North and South America, the probability of severe tissue injury from a snakebite is 14% (95% CI, 12%-16%). The incidence of severe hematologic venom effects is 18% (95% CI, 8%-27%). The probability of severe systemic toxicity is 14% (95% CI, 9%-21%).
Population Among Whom Severe Envenomation Should Be Considered
Most patients who present for emergency care after snakebite will have pain, erythema, ecchymosis, and edema spreading from the venom injection site. Severe envenomation should be considered in any patient who seeks medical care following a snakebite.
Assessing the Likelihood That a Patient Following Snakebite Has Severe Envenomation
Severe snakebite envenomation can be categorized as systemic toxicity, tissue injury, and/or hematologic venom effects. Historical factors associated with increased likelihood of severe envenomation include when the time from bite to treatment ≥ 6 h, younger patient age (≤ 12 y), a bite from a snake that is large for its species, and location of the bite on a finger or toe (see Table 106-1). Snakebites from copperhead or cottonmouth snakes are less likely to cause severe systemic envenomation than rattlesnake bites (LR, 0.28; 95% CI, 0.10-0.78).
Table 106-1.Useful Findings for Identifying Severe Envenomation (Quality Level 3 Studies) ||Download (.pdf) Table 106-1. Useful Findings for Identifying Severe Envenomation (Quality Level 3 Studies)
| ||LR+ (95% CI) ||LR– (95% CI) |
|Time from bite to treatment ≥ 6h ||3.4 (1.1-6.4) ||0.69 (0.34-0.98) |
|Patient age ≤ 12 y ||3.2 (1.5-7.1) ||0.70 (0.52-0.94) |
|Large snake for given species ||3.1 (1.5-5.7) ||0.45 (0.23-0.74) |
|Bite on a digit ||3.1 (2.4-4.0) ||0.33 (0.16-0.68) |
Measurement of serum fibrinogen and the platelet count are the most useful laboratory findings for hematologic venom effects (see Table 106-2). While most studies describe the results as either abnormal or normal, experts recognize that the risk of severe envenomation is increasingly likely with greater abnormalities in these test results. Because initial laboratory tests do not rule out severe envenomation, many patients are observed and have repeat tests.
Table 106-2.Useful Clotting Tests for Identifying Severe Envenomation ||Download (.pdf) Table 106-2. Useful Clotting Tests for Identifying Severe Envenomation
| ||LR+ (95% CI) or Range ||LR– (95% CI) or Range |
|Initial hypofibrinogenemia (< lower limit normal) ||5.1 (1.7-15) ||0.60 (0.35-1.0) |
|Initial thrombocytopenia (< 150000/mm3) ||3.7 (1.9-7.3) ||0.36 (0.15-0.88) |
Severe envenomation following snakebite may create shock, internal hemorrhage from coagulopathy, tissue necrosis, compartment syndrome, renal failure, respiratory failure, or death. While these are all clinical diagnoses, severe systemic toxicity is quantified by commonly used comprehensive grading scales.1,2,3