Neuropsychological Assessment Tools Used to Evaluate Developmental Neurotoxicity of Methylmercury
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Methylmercury is a well-known developmental neurotoxicant. In an effort to evaluate studies published since the release of the U.S. EPA IRIS Methylmercury Assessment in 2001, which was based on an assessment by the National Academies of Science (NAS) in 2000, a literature search was conducted in March 2017 for studies assessing the effects of methylmercury exposure on developmental neurotoxicity (DNT) in humans. DNT was the focus of the literature search because DNT endpoints were used to derive the Reference Dose in the 2001 Methylmercury IRIS Assessment. PubMed, Web of Science, ToxNet, and Science Direct were searched to find studies published between 1998 and March 2017. This resulted in the identification of 12,558 studies. The software tools, SWIFT© and Distiller©, were then used to screen studies and identified more than 200 that evaluated DNT effects in humans and that also provided quantitative exposure-response data. In addition, studies cited in the NAS assessment were added to the studies identified by the literature search.
Several neurodevelopmental endpoints were evaluated in the identified studies, including: cognition (e.g., verbal ability, nonverbal ability, executive functioning, attention, visual-spatial processing, learning/memory, IQ), motor function, sensory function, mood/affect/personal regulation, and clinical diagnoses of Autism Spectrum Disorder and Attention Deficit/Hyperactivity Disorder. Many different neuropsychological tests, including multiple versions of the same test and numerous subtests for individual ones, were used to assess the neurodevelopmental endpoints. Some of the more commonly used neuropsychological tests were Bayleys Scales of Infant and Toddler Development-II, Brazelton Neonatal Behavioral Assessment Scale, California Verbal Learning Test, McCarthy Scales of Children’s Abilities, Neurobehavioral Evaluation System 2, and Wechsler Intelligence Scale for Children III. However, there were some tests that were designed specifically for the cohort being evaluated and others include commonly used tests that are translated for non-English speaking populations.
Many factors, such as appropriateness of tests for other cultures and conditions under which the tests were given, will need to be considered. Therefore, specific guidelines are being developed to assist in determining the validity and reliability of each test as it is used in individual studies.