Urine mercury levels might be reported in different units of measure (e.g., micrograms per gram creatinine and micrograms per liter) that are not equivalent. This should be considered when interpreting results in children because creatinine levels in children differ by age due to developmental changes. In addition, fluid intake in children differs from fluid intake in adults, which can also affect the volume when converting values into different units. Other factors, such as chronic illness in both children and adults, might need to be considered when interpreting laboratory results. The method used by the laboratory to report values should be known. For example, the National Center for Environmental Health (NCEH) laboratory reports values in micrograms per gram creatinine, which is adjusted per sample by direct urine creatinine measurement. NCEH considers direct creatinine adjustment of urine mercury values imperative for proper interpretation. Age-specific reference ranges for urine volume and urine creatinine excretion are published if a conversion between the two units of measure is necessary (Behrman et al. 1996). However, interpretation should be done in consultation with a specialist who has experience managing cases of childhood mercury poisoning.
For children exposed to mercury, no current standard medical guideline exists for treatment on the basis of mercury urine levels and clinical signs and symptoms. For this reason, the interpretation of laboratory values for urine mercury and choice of treatment regimen, if necessary, should be determined in consultation with a specialist in pediatric environmental medicine who has experience managing patients with mercury poisoning. In situations where a 24-hour urine mercury specimen is not easily attainable for a young child, such as in an outpatient clinic setting, a spot mercury level can be obtained for screening purposes. It is important to remember, however, that for this type of sample, the units of measure used in the reporting of laboratory results and the methods of adjustment for the concentration of the urine (e.g., using specific gravity versus amount of creatinine present) are not standardized across laboratories and, therefore, this should be considered when interpreting reported laboratory values.
Removal from exposure is the first step, followed by an assessment of the child's clinical condition to ensure that the patient is stable. Chelation has been used to reduce body burden of elemental mercury, although whether it reduces toxic effects or speeds recovery in mercury-poisoned children remains unclear (Fullilove 2001). Chelation should only be used for symptomatic patients with known mercury exposure, and only after consideration of the risk and benefits by a specialist experienced in the use of chelators and in consultation with the patient or family. Mercury poisoning should be treated in consultation with experts in the field of environmental toxicology and pediatricians who have experience in management of children with this exposure.
Succimer, which has been used as an oral chelating agent in the treatment of
lead poisoning, also increases urinary mercury excretion. However, its efficacy and long-term benefits are uncertain, thus classifying this treatment mode as experimental. Adverse side effects from succimer include abdominal distress, transient rash, increased liver function, and neutropenia. Other agents available for treatment, but not yet approved in the United States, might be more efficacious at removing mercury (e.g., 2,3-dimercaptopropan-1-sulfonate). Ethylenediamine tetra-acetate and penicillamine are not very effective for mercury. When treating a patient with similar symptoms, consult with a pediatric toxicologist at your local poison control center or at a Pediatric Environmental Health Specialty Unit (Appendix D).
Mercury occurs naturally in the environment and exists in several forms, which can be organized under three headings: metallic mercury (i.e., elemental mercury-Hg 0, quicksilver), inorganic mercury (i.e., Hg +1 [mercurous salts] or Hg +2 [mercuric salts]), and organic mercury (i.e., methyl-, ethyl-, and phenylmercury). Because mercury's absorption and metabolism depend on its chemical and physical form, it is important to determine the form of mercury to which an individual is exposed. Different forms of mercury can have differing health effects (e.g., absorption and metabolism of different forms of mercury vary and, therefore, have different effects on the nervous system). When metallic mercury vapors are inhaled, they readily enter the bloodstream and cross the blood-brain barrier. Inhaling or ingesting large amounts of methylmercury also results in some of the mercury crossing the blood-brain barrier and affecting the nervous system. Inorganic mercury salts, such as mercuric chloride, do not cross the blood-brain barrier like methylmercury or metallic mercury vapor do. Mercury affects other systems in addition to the nervous system (ATSDR 1999, 1992).
The ATSDR Toxicological Profile for Mercury (ATSDR 1999) states that
One way in which people are routinely exposed to extremely small amounts of mercury is through the gradual (but extremely slow) wearing-away process of dental amalgam fillings, which contain approximately 50% mercury. The amount of mercury to which a person might be exposed from dental amalgams would depend on the number of amalgams present as well as other factors. The Centers for Disease Control and Prevention (CDC) has determined that dental amalgam fillings do not pose a health risk, although they do account for some mercury exposure in those having such fillings. People who frequently grind their teeth or often chew gum can add to the small amount of mercury normally released from those fillings over time.... The practice of having all your dental amalgam fillings replaced with nonmercury filling materials just to remove the possibility of mercury exposure is not recommended by ATSDR. In fact, the removal of the mercury amalgam fillings would actually expose the patient to a greater amount of mercury, at least for a while. There are other sources of mercury that may increase your overall exposure, such as the amount of fish consumed per week or an exposure to mercury from a nearby hazardous waste site or incinerator.
Since the 1930s, some, but not all, of the vaccines routinely recommended for children have contained small amounts of thimerosal, a mercury-containing preservative. In July 1999, the U.S. government asked vaccine manufacturers to eliminate or reduce, as expeditiously as possible, the mercury content of their vaccines to avoid any possibility of infants who receive vaccines being exposed to more mercury than is recommended by federal guidelines. For additional information, see Mercury and Vaccines (Thimerosal) (CDC 2001), Thimerosal in Vaccines: An Interim Report to Clinicians (American Academy of Pediatrics1999), and Goldman et al. (2001).
For those populations that rely on local fish for a major portion of their diet, a complete exposure history should include questions exploring subsistence fishing as a possible source of mercury exposure. Bioaccumulation occurs when mercury in lake and stream sediments is converted by bacteria into organic mercury compounds that accumulate in the food chain. Physicians living in active fishing areas with fish advisories related to mercury should ask women and children if their consumption of fish is in accordance with state-issued fish advisories. In general, freshwater fish have higher levels of contaminants than saltwater fish, but not always. Increased methylmercury content has been found in the larger ocean fish (e.g., tuna, swordfish, and shark) because of naturally occurring and man-made sources of mercury pollution. In the 1950s, when pregnant women in Minimata Bay, Japan, ingested fish with high levels of methylmercury, the result was at least 30 cases of infantile cerebral palsy (Klaassen 1996). As listed in the Handbook of Pediatric Environmental Health (Etzel and Balk 1999), to reduce hazards from fish consumption, individuals can be counseled to eat nonpredator fish rather than predator fish (e.g., shark, swordfish, and tuna); to eat small rather than large game fish; and to eat fewer fatty fish (e.g., carp, catfish, and lake trout), which accumulate higher levels of chemical toxicants. Emphasize to women of childbearing age, pregnant women, nursing mothers, and parents of young children the need to follow fish advisory guidelines. Fish advisories can be obtained from state health, environmental, and conservation departments (ATSDR 1999, 1992; EPA and ATSDR 2001).
