Diet- and Mercury-induced Visual Loss

Abstract

Inorganic mercury in our environment is absorbed into our bodies in the form of organic methylmercury. Diet is the main source of methylmercury in people not involved in mercury-related industries. Our knowledge of the toxic effects of mercury poisoning is based on two major industrial accidents, which resulted in widespread illness, ranging from neurological disease such as ataxia and paraplegia up to seizures and death. Visual symptoms reported from the aftermath of these accidents include sudden cortical blindness, constricted visual fields, optic atrophy, and poor night vision. As the average person is exposed to much lower doses of mercury through their diet, the toxic effects on methylmercury are less clearly defined. With many other confounding factors associated with diet and exposure, we describe the different investigations that are used to examine changes in visual function associated with exposure to methylmercury. MRI and electrophysiological testing are described and the abnormal findings associated with methylmercury toxicity are highlighted. Color vision testing can identify dyschromatopsia (color confusion) associated with mercury exposure and in some cases these changes persist for many years after exposure. We describe the recommended guidelines on fish consumption in place to limit exposure to mercury and other heavy metals. We discuss recent studies that look at fish consumption during pregnancy and the evidence on its effect on childhood neurodevelopment. Finally, we emphasize that on balance, the positive benefits of fish consumption outweigh the risks associated with mercury exposure.