Prenatal methylmercury poisoning and neurocognitive impairment in Minamata

Minamata disease, officially identified in 1956, refers to methylmercury food poisoning that occurred in Minamata City, Japan. Although many children in the affected areas were born with severe neurological signs after birth (known as congenital Minamata disease [CMD]), little investigation has been conducted to elucidate the health effects on those who were born at the same time in the affected areas. We used historical data from a clinical study conducted in 1970 in the city of Minamata (a methylmercury-polluted area) and the island region of Amakusa (a less polluted area) to evaluate the association between prenatal methylmercury exposure and neurocognitive impairment in a total of 416 junior high school students. We divided the participants into three categories: those who lived in a highly exposed area, those who lived in a moderately exposed area (both in Minamata City), and those who lived in a less exposed area (in the Amakusa island region). We then calculated the prevalence of neurocognitive impairment in each category and estimated prevalence ratios with 95 % confidence intervals (CIs) for neurocognitive impairment compared with the less exposed area, adjusting for potential confounders. We then found that the prevalence for both neurocognitive impairment and severe neurocognitive impairment among students in the highly exposed area were 2.08 (95 % CI: 1.26 to 3.44) and 2.84 (95 % CI: 1.18 to 6.81) times higher, respectively, than those in the less exposed area. Prenatal methylmercury exposure affected neurocognitive function in CMD patients and in Minamata residents born at the same time as CMD patients in the affected areas.