Effects of mercury exposure on the visual functions of children from Amazonian riverine villages

Abstract

Purpose: The purpose of the present study was to investigate the color vision of children living in riverine communities located in the Tapajós river basin, in order to evaluate the impacts of mercury exposure earlier in the developing nervous system. We compared the results with those from children living in the Tocantins river basin, considered to be a region without mercury exposure.

Methods: One hundred and seventy eight children, 7-14 years old, were studied. They lived in three riverine communities from two different river basins of the Brazilian Amazon: Barreiras and São Luiz do Tapajós from the Tapajós river, and Limoeiro do Ajuru from the Tocantins river. We quantified their mercury hair concentration using atomic absorption spectrophotometry and evaluated their color vision using the Lanthony D-15 desaturated test (D-15d). We applied the Vingrys and King-Smith (1988) vector analysis to estimate the angle and magnitude of the hue ordering error (C-Index). Kruskal-Wallis test and Dunn post-hoc test was applied to compare the data from each children population.

Results: Children from Tapajós river basin had 10 times higher mercury exposure than children Tocantins river basin (p < 0.05). Most of children from Tocantins river basin showed Lanthony D-15d C-index between 1 and 4 and error angle between +45^o and +900 (normal trichromats), while a few of them had error angle between -750 and -900 (blue-yellow color deficiency). In Barreiras, there was a large cluster of children with C-index between 1 and 3, error angle between +45^o and +900, and two smaller clusters, one with radius between 1 and 4 and angle between +150 and -300 (red-green color deficiency) and another with radius between 1 and 3 and angle between -750 and -90). In São Luiz do Tapajós, there were three well distinct groups with C-index between 1-4: error angle between +450 and +900, +150 and -300, and -750 and -90^o. In São Luiz do Tapajós, the results of color vision defects were associated to mercury hair concentration higher than 1.15 ppm.

Conclusions: We found that in the village with higher mercury exposure, the color vision deficits were associated with the level of mercury exposure. Continuous monitoring of children performance in color vision tests can be important to understand the effects of long term, low levels of mercury exposure in the developing visual nervous system.