Fetal Methylmercury Syndrome

A rare disorder characterized by a group of symptoms that may be observed in a foetus or newborn when the mother was exposed during pregnancy to excessive amounts of methylmercury.

Epidemiology

Accidents have led to the birth of about 800 affected children worldwide over the last 50 years.

Clinical description

In the late-1950s, methylmercury toxicity became highly publicised after an outbreak of cerebral palsy and microcephaly in newborns from the fishing village of Minimata Bay, Japan. These abnormalities were caused by methylmercury contamination of fish in the bay, and were termed Minimata Disease. Similar intoxications also occurred in Iraq after seeds contaminated with methylmercury were mistakenly used to make bread. In this population, infants exposed in utero displayed psychomotor retardation and cerebral palsy. Similar neurological impairment has been reported in other instances of food contamination, such as in infants of American Indian women who ingested pork contaminated with phenylmercuric acetate.

Etiology

There are two main types of mercury, metallic mercury (a heavy liquid element at room temperature, used in thermometers) and organic mercury (like methylmercury, used as antifungal agent in seed dressings). The selective neurotoxicity of organic mercury does not appear to apply to metallic mercury and its salts. Methylmercury is lipophilic and more readily distributed to the brain tissue than mercuric salts. Metallic mercury does not cross the placenta readily, and it has been estimated that the rate of methylmercury transport into the brain may exceed that of mercury by a factor of 10.

Diagnostic methods

Exposure to methylmercury can be monitored by measuring the concentration in the hair. All fish contain some methylmercury, and frequent consumption of highly contaminated fish can lead to a hair concentration of methylmercury ranging between 10 and 50 ppm. A level of 50 ppm in hair has been established as the lowest toxic level, and the World Health Organization (WHO) has applied a safety factor of 10 to cover risks in the most sensitive populations. Thus, 5 ppm has been generally adopted as the international standard for the upper tolerable level of mercury in hair.

Management and treatment

Since maternal exposure to methylmercury is primarily through fish consumption, it is recommended that women of childbearing age should not consume more than 350g of fish per week. In addition, they should not be occupationally exposed to air concentrations of mercury vapor greater than 0.01 mg. per cubic meter, to inorganic and phenylmercuric compounds at concentrations greater than 0.02 mg per cubic meter, or to any detectable concentration of methylmercury.