Methylmercury (MeHg) easily crosses the blood–brain barrier and accumulates in the brain. Accumulated MeHg will cause neurological symptoms. We report four pediatric cases of neuropsychological findings with high blood MeHg concentrations.

Four children were admitted for follow-up study because their total mercury (THg) concentration in the blood was found to be high during a national survey. Case 1 was a 9-year-old female with a 16.6 μg/ℓ blood THg concentration in the survey. During admission, the blood THg, hair THg, and blood MeHg concentration(mercury indices) were 21.4 μg/ℓ, 7.2 μg/g, and 20.1 μg/ℓ, respectively. In our neuropsychological examination, cognitive impairment and attention deficit were observed. Her diet included fish intake 2–3 times per week, and she had been diagnosed with epilepsy at 3 years of age. Case 2 was a 12-year-old male with blood THg of 15.4 μg/ℓ in the survey and the mercury indices were 12.7 μg/ℓ, 5.7 μg/g, and 11.8 μg/ℓ, respectively, on admission. He was also observed to have attention-deficit/hyperactivity disorder. Case 3 was a 10-year-old male child with blood THg of 17.4 μg/ℓ in the survey, and the mercury indices on admission were 21.6 μg/ℓ, 7.5 μg/g and 21.5 μg/ℓ, respectively. In his case, mild attention deficit was observed. Case 4 was a 9-year-old male with blood THg of 20.6 μg/ℓ in the survey and the mercury indices were 18.9 μg/ℓ, 8.3 μg/g, and 14.4 μg/ℓ, respectively, on admission. Mild attention difficulty was observed.

We suggest that fish consumption may be the main source of MeHg exposure, and that MeHg may have been the cause of the neuropsychological deficits in these cases.