Medical causality is imputed when the association between a medical condition and a given exposure (physical, biologic, or chemical) is such as to lead one to believe that the condition would not have occurred in the absence of the exposure. The temporal relationship between the exposure or injury and the medical condition (or symptoms suggestive of the condition) is the first factor that must be assessed. The illness or disease should occur after the exposure (referred to as “temporal ordering”) and within a time period that is reasonable given the nature of the exposure (temporalcontiguity). In certain situations (such as asbestos, lead, and benzene exposure) there is a long latency between the time of exposure and the appearance of disease. Hence, regardless of whether a temporal relationship appears to be present, determining causality also requires one to assess whether a causal relationship is biologically plausible.
A causal relationship is biologically plausible when:
1. The relationship between the medical condition and the exposure or injury can be explained anatomically or physiologically.
2. The duration, intensity, or mechanism of exposure or injury was sufficient to cause the illness or injury in question.
3. There is evidence suggesting that the exposure is consistently or reliably associated with the process under investigation in the population under investigation or in peer-reviewed literature.
4. Cause and effect are contiguous--ie, there is a readily understandable relationship between the two, in which an increase in the magnitude of the exposure reliably leads to an increase in the severity of its alleged effect upon the injured or exposed person, and vice versa.
5. There is literature providing biologic or statistical evidence indicating that the symptoms or disorder could develop as a result of the exposure (coherence).
6. There is specificity of the association for the injury (ie, the absence of other factors, especially pre-existing disease, that could have caused or contributed to the problem).
The independent examiner is obligated to evaluate the validity and strength of all postulated causal mechanism. Mechanisms that appear weak, or are clearly flawed, must be identified as such and accepted as likely only when at least two other criteria for biologic plausibility have been met. Optimally one would wish to satisfy all criteria. There are, however, circumstances when contiguity cannot be demonstrated, as some exposures lead to disease in a noncontiguous fashion. Specificity of association is also difficult to illustrate definitively given the multifactor nature of many disease processes. Literature supportive of causality is generally available, but must be closely scrutinized before relying upon it as it is often poor quality.