Fetal methylmercury syndrome (ORPHA:1917)
A toxic embryofetopathy characterized by a group of symptoms with unspecific neurologic involvement that may be observed in a foetus or newborn when the mother was exposed during pregnancy to excessive amounts of methylmercury.
Epidemiology
Severe methylmercury poisoning cases were reported in the context of two large epidemics: in the Minamata area, in Japan, and in Iraq, both affecting hundreds of people. The Japanese cases resulted from local seafood consumption and sea mercury pollution by a local industrial installation. Iraqi cases resulted from contaminated flour consumption. Large cohort studies in New-Zealand, the Faroe Islands and the Seychelles have documented a risk of less severe prenatal (and/or postnatal) methylmercury poisoning associated with frequent fish consumption (more than 2 or 3 times a week) in thousands of individuals.
Clinical description
In severe cases, the clinical picture of prenatal methylmercury poisoning is one of unspecific infantile cerebral palsy, with ataxic motor disturbances, psychomotor retardation and seizures. In populations with high fish consumption, the only manifestations of methylmercury encephalopathy are impaired psychomotor and cognitive performances. They are associated with increased risks of preterm delivery, lower birth weight, and impaired postnatal growth.
Etiology
Methylmercury is absorbed from the intestinal tract, through the skin and by inhalation. Most of the human exposure is through food ingestion. In blood, methylmercury being lipophilic is mostly in red cells. It is transported through the placenta to the fetus and accumulated in fetal brain. In cells, methylmercury is partly demethylated into inorganic mercury which is mostly distributed in the liver and kidneys. Elimination is mainly through the fecal route; about 10 % of the amount excreted is in the urine, in the form of inorganic mercury. Methylmercury is also excreted in milk (where its concentration is about 5 % of the concentration in the maternal blood).
Diagnostic methods
The best biomarkers of methylmercury exposure and internal dose are mercury concentrations in hair and whole-blood. In most individuals from the general population, they are under 2 µg/g and 8 µg/L, respectively.
Differential diagnosis
The clinical picture in prenatal or postnatal severe methylmercury poisoning is one of unspecific encephalopathy. The responsibility of methylmercury should be considered when a high fish consumption is characterized.
Management and treatment
Since exposure to methylmercury is primarily through fish consumption, it is recommended to consume not more than 2 portions of fish per week, especially for children and women of childbearing age. Fish consumption should be reduced in individuals with hair or whole-blood mercury levels of more than 2.5 µg/g or 10 µg/L, and without delay, when these levels exceed 10 µg/g or 40 µg/L, respectively. Biomonitoring and search for neurodevelopmental disorders are recommended for children with hair or whole-blood mercury levels above 10 µg/g or 40 µg/L, respectively or born from mothers who exceeded these levels during pregnancy. Mercury chelation should be considered only in those cases where severe neurological complications are possible.
Prognosis
Methylmercury-induced cognitive and psychomotor impairments are not always completely corrected by the decrease of internal methylmercury dose. However, a correct diet, a careful surveillance of neurodevelopment with attentive learning stimulations, can counterbalance the effects of this neurotoxic substance.

                            Last update: 
                            April 2023


                            - Expert reviewer(s): 
                            Dr Robert GARNIER  - Dr Magali LABADIE  - Dr Christine TOURNOUD