As an expert in clinical and forensic Neuropsychology, I am frequently asked to opine on alleged neurotoxic exposure through my evaluations of individuals in a forensic context. These evaluations are requested by attorneys, insurance companies, and courts in a number of legal contexts, including occupational injury claims, environmental contamination disputes, and product liability litigation. The primary and often contested question is whether an individual's cognitive complaints can be validly identified and, even more importantly, whether an individual's cognitive complaints can be attributed to a specific neurotoxic agent.

As a forensic neuropsychologist, I am acting in my capacity as an expert witness to the legal profession, and I have a responsibility to provide data-based opinions regarding the neurotoxic exposure to individuals at the intersection of science and the law, converting complex neurocognitive data into opinions that are admissible and relevant to the law (Sweet, 1999; Heilbronner, 2008; Leonard, 2015). These requests have also reaffirmed that the civil litigations that include brain-related claims are seeking neuropsychological expertise to support their claims.

As noted in the forensic literature, neuropsychologists have a unique capacity to identify and evaluate these cognitive changes when there are subtle cognitive changes that are not readily identified by routine medical examinations or imaging, and where there may be diffuse or low-level neurotoxic exposure (Boone, 2012; Sweet & Kaufmann, 2017; Hartman, 2012). However, with these requests come a substantial amount of responsibility. Unlike clinical practice, where my primary responsibility is to provide care for the individuals I evaluate, my key responsibility is to provide evaluations that are objective, methodologically valid, transparent, and compliant with the evidentiary standards associated with the legal system. The opinions that I provide must withstand scrutiny from the opposing experts, as well as the evidentiary standards that apply to all of the expert testifying (Kaufmann, 2013; Richards et al., 2015).

The domain of forensic neuropsychology as it relates to toxic exposure litigation is highly technical and a complex combination of neuroscience, clinical evaluation, and legal standards. While traditional clinical practice addresses the diagnosis and treatment of cognitive impairment, forensic neuropsychology is concerned primarily with the objective evaluation of cognitive impairment and the translation of scientific findings into a legally meaningful conclusion. In toxic exposure cases, neuropsychologists will be expected not only to determine if the claimant is experiencing cognitive impairment but also to establish whether there is a causal link between the cognitive impairment and an identified hazardous environmental, occupational, or chemical exposure. This dual function creates a higher level of analytical and ethical rigor than can be found in therapeutic practice.

Causation is an essential principle in the field of toxic exposure litigation. Establishing a claim of causation in toxic exposure litigation requires a clearly defined, structured, and methodologically sound protocol that differentiates between general and specific causation. General causation refers to the establishment of the ability of the substance to produce a dangerous situation, which is based on toxicological evidence, epidemiological evidence, and mechanistic evidence. Specific causation requires a thorough analysis of the claimant's personal experience with respect to the hazardous substance to determine if the claimant had an adequate dose, duration of exposure, and time of exposure necessary to cause the cognitive impairment.

It is critical to note that courts will not accept under any circumstances a general epidemiological association between a toxin and a population as proof of injury to a particular claimant. To establish specific causation, the neuropsychologist must evaluate and integrate exposure documentation, biologic plausibility, and temporal relationships while systematically excluding other potential explanations for the presence of the claimant's cognitive impairment. In addition to the challenges described above, the neuropsychologist must also carefully select and interpret the assessment tools used to evaluate neuropsychological evidence in litigation.

Cognitive deficits due to toxic substances often appear as very slight or diffuse and will be shown in several domains, such as working capacity and skills. Therefore, these areas of performance must be sensitive to these changes, as well as possess strong psychometric characteristics. In addition to test results, courts will require that all test findings be linked to measurable productive outcomes, such as job performance and activities of daily living. As such, all tests must be validated through third-party collateral data, such as, but not limited to: employment, education, and/or vocational rehabilitation. The strength of neuropsychological data lies in both the degree of deviation from the normative data and the degree of social or occupational function impaired due to the neurocognitive impairment.

Validating the Assessment of Neuropsychological Evidence

A major goal of forensic neuropsychology is the emphasis placed upon the validity of cognitive and symptom reports. In the realm of forensic settings, which include legal and financial responsibilities that will impose motivation to produce deceitful cognitive and symptom evidence, the credibility of all cognitive/symptom reports must be carefully evaluated. Therefore, performance validity tests (PVT) and symptom validity tests (SVT) are integral parts of the complete forensic evaluation process. These tests will help the forensic neuropsychologist differentiate between complete effort and intentional exaggeration of performance, as well as between true and false symptom validity.

The presence of a forensic case does not imply that an individual is being deceitful or has made an intentional act of malinger; thus, most individuals will make a valid effort; nonetheless, a proper, balanced, data-driven approach by the evaluator is critical to account for both genuine and non-credibly performed, evaluated cognitive processes that exist. Factors that are to be considered when interpreting the validity of evaluations include, but are not limited to: fatigue, numerous psychiatric and medical comorbidities.

Ruling Out Alternative Causes of Cognitive Impairment

Differential diagnosis is one of the foundation elements of defensible forensic analyses. Symptoms caused by toxicity are sometimes indistinguishable from those arising from mental illness, neurological condition, or lifestyle factors. Cognitive impairment due to such factors as depression, anxiety, disturbed sleep, vascular disease, and substance abuse can create the same degree of cognitive deficit as seen with toxic injury. A neuropsychologist must conduct a comprehensive evaluation of each possible explanation for the cognitive impairment in question, and the temporal proximity between the injury and the cognitive impairment must also be evaluated against the pattern of test results before formulating an opinion as to the cause of the cognitive impairment.

If the differential diagnostic work-up is not completely done, the expert could jeopardize not only the credibility of their testimony but also its admissibility in court. Courts require experts to exhibit that they based their conclusions on a thorough examination of all competing theories, rather than a violation of the principles of good science regarding evidence-based reasoning. When presenting evidence for forensic cases involving toxic exposure, longitudinal data is critical, especially when determining the impact of toxic exposure on an individual. A single evaluation will only show a "snapshot" of an individual's functioning, while serial evaluations provide a longitudinal database that allows one to determine how the individual has changed over time.

In addition, the longitudinal database will allow acquisitions of trends of stable, improving, or declining functioning to be used to determine possible underlying causes, such as neurotoxic-related injuries or degenerative-related injuries. Likewise, longitudinal data will also assist in making judgments regarding functional capacity, future impairments, and compensation to be awarded to the injured party. However, it is important that the longitudinal evaluation incorporates consideration for possible measurement error, due to practice effects, to prevent inaccurate conclusions from being drawn. Morality and ethics exist at the foundation of all forensic neuropsychology assessments.

An evaluator's duty is to the court and not to the party that hired them. Therefore, they must be completely objective, transparent, and professional. There can be substantial risks due to prejudiced opinions, conscious or otherwise, especially in an adversarial situation, as both sides want to obtain information that will help them win. Neuropsychologists should ensure they use structured reasoning, review alternative explanations, and document their methodology and conclusions clearly to limit that risk. They must also avoid dual roles, as there is a potential to lose their objectivity and credibility if there are two competing relationships (that is, if you are both a therapist and a forensic evaluator).

In addition, ethical practices require that evaluators acknowledge the issue of uncertainty, particularly when using unrevised or evolving scientific data. When neuropsychologists are called upon to testify in court, they must communicate complex scientific concepts in an understandable way without losing the accuracy of the content. They need to be able to clearly explain the difference between general and specific causes and the importance of testing procedures used to identify valid results. They should maintain an objective and professional attitude while providing a complete and balanced overview of the evidence and should not be perceived as advocates for either party. It is acceptable for attorneys to assist in structuring testimony; however, attorneys should never be allowed to compromise an evaluator's scientific independence.

Toxic exposure litigation is important to society and to public policy beyond simply the immediate outcome of an individual case. Through the course of litigation, exposure to the environment may be made, regulations may be imposed, and/or the public may become more aware. Neuropsychological evidence is valuable to this process for its ability to document the cognitive health effects associated with exposure. Nevertheless, the legal system is incapable of resolving scientific uncertainty; experts must not overreach their conclusions in pursuit of larger objectives. The efficacy of forensic neuropsychology depends upon a clear distinction between scientific analysis and policy advocacy.

To summarize, forensic neuropsychologists assist the courts in making informed decisions regarding toxic exposure through the integration of science, clinical expertise, and the legal system. As a result of thorough methodology, logical reasoning, and ethical commitment, they systematically analyze causation, ensure the causal relation between toxic exposure and impairment, conduct the differential diagnosis, and communicate their findings to assist the courts in their decision-making processes. The assistance provided by forensic neuropsychologists has far-reaching implications, including improving understanding by all stakeholders of the impact(s) of toxic exposure on cognitive function. As the interface between neuropsychology, toxicology, and the legal system continues to evolve, there is an increased focus on this issue in both clinical practice and academic research. ◆

In light of the increased need to integrate scientific disciplines, I will be publishing a textbook later this year through Springer Nature that will provide a comprehensive framework for understanding the negative cognitive effects associated with exposure to toxins in clinical, forensic, and public health contexts. My textbook will assist clinicians to utilize mechanistic science, neuropsychological assessment, and apply the findings in litigation and to inform public policy; thereby assisting to bridge the gap between research and forensic practice.

As the courts continue to struggle with complex issues regarding causation, impairment, and liability issues, neuropsychologists have an increasingly important role in assisting courts to resolve individual case disputes, and in assisting society in understanding the cognitive risks associated with environmental and occupational exposure (Sweet, 1999; Boone, 2012; Leonard, 2015; Hartman, 2012).

References

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