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Toxicity, Mercury
Updated: Jul 21, 2008
Introduction
Background
Mercury has various forms and thousands of industrial uses; however, it is probably best known as the silver liquid in thermometers. Mercury is ubiquitous in nature, occurring as mercuric sulfide or cinnabar.
Mercury has 3 forms: (1) elemental mercury, (2) inorganic salts, and (3) organic compounds. Perhaps the most deadly form of mercury is methyl mercury. Only 2-10% of the ingested mercury is absorbed from the gut, and ingested elemental mercury is not absorbed at all; however, 90% of any methyl mercury ingested is absorbed into the bloodstream from the GI tract.
Organic mercury compounds, specifically methyl mercury, are concentrated in the food chain. Fish from contaminated waters are the most common culprits. Industrial mercury pollution is often in the inorganic form, but aquatic organisms and vegetation in waterways such as rivers, lakes, and bays convert it to deadly methyl mercury. Fish eat contaminated vegetation, and the mercury becomes biomagnified in the fish. Fish protein binds more than 90% of the consumed methyl mercury so tightly that even the most vigorous cooking methods (eg, deep-frying, boiling, baking, pan-frying) cannot remove it.
Methyl mercury poisoning, better known as Minamata disease, is one of the most devastating forms of mercury exposure. It is named for Minamata Bay, a body of water in Japan where, in the early 1950s, the fish contained high concentrations of methyl mercury from the polluted waste of a nearby industrial plant.1 Local villagers ate the fish and began to exhibit signs of neurologic damage such as visual loss, extremity numbness, hearing loss, and ataxia. Babies exposed to the methyl mercury in utero were the most severely affected members of the village. Furthermore, because mercury was also discovered in the breast milk of the mothers, the babies' exposure continued after birth.
Pathophysiology
Organic forms, specifically methyl mercury, are the most toxic of the 3 classes of mercurials. The GI tract absorbs more than 90% of the methyl mercury ingested, which then enters the bloodstream. Because mercury binds to the body's ubiquitous sulfhydryl groups, toxicity involves multiple organ systems. Structural proteins, membranes, and enzymes are all disrupted.
Methyl mercury exerts its most devastating effect on the CNS by causing psychiatric disturbances, ataxia, visual loss, hearing loss, and neuropathy. Methyl mercury is lipophilic and readily crosses the blood-brain and placentofetal barriers. Neurologic damage in the form of diffuse and widespread neuronal atrophy is most severe in patients exposed in utero.
Necrosis of the proximal tubules is a common direct renal toxic effect. Unexplained renal abnormalities with neuropsychiatric disturbances should prompt the physician to consider Minamata disease or other forms of mercury poisoning.
Frequency
United States
Mercury poisoning is episodic and usually involves the inhalation of elemental mercury vapor or an exposure to inorganic mercuric salts; methyl mercury intoxication has been reported in the United States.
International
Worldwide, outbreaks of methyl mercury intoxication are sporadic. Minamata Bay in Japan was involved in the first and most famous epidemic but not the largest. In the early 1970s, one of the most severe mass poisonings in history occurred in Iraq when nearly 95,000 tons of seed grains treated with a methyl mercury–based fungicide were accidentally baked into bread for human consumption.2 More than 6000 individuals were hospitalized, and hundreds died. Many were hospitalized for weeks before methyl mercury intoxication was correctly diagnosed.Mercury mining areas in China have also contributed to cases of methyl mercury poisoning through the ingestion of rice grown in contaminated soil.3
Mortality/Morbidity
- Of the original 121 individuals from Minamata Bay who were affected (see Frequency), nearly one third died shortly after their initial presentation.1
- Subsequent investigations over the last 3 decades resulted in the identification of more than 2000 additional patients who were affected by chronic sequelae of Minamata disease.1
- Of the more than 6000 Iraqi patients (see Frequency), 459 died.2
Sex
Both sexes can be affected.
Age
Although Minamata disease can affect children of all ages, babies exposed in utero are most severely affected.
- Autopsy evidence reveals that brain atrophy is significantly worse in children than in adults.
- Exposure to mercury has been suggested to contribute to the development of autism in children.
- Although the mechanism for this disorder has many hypotheses, no evidence has confirmed or disproved a causal relationship between mercury exposure and the development of autism. In fact, in one recent study, the discontinuation of thimerosal-containing vaccines in Denmark seemed to be followed by an increase in incidence of autism.4
- Further studies are needed to elucidate a link, if any, between mercury exposure and autism development.
Clinical
History
- If methyl mercury intoxication is suspected, inquire about the patient's diet, including the following:
- Amount of fish consumed
- Frequency of consumption
- Types of fish consumed
- Source of fish consumed
- Source of water supply
- Also inquire about classic symptoms, including the following:
- Perioral and facial paresthesias
- Extremity numbness
- Dysarthria
- Headache
- Constriction of the visual fields
- Difficulty in hearing
- Memory loss
- Problems with walking
- Methyl mercury exerts its most devastating effect on the CNS by causing the following:
- Psychiatric disturbances
- Ataxia
- Visual loss
- Hearing loss
- Neuropathy
Physical
- Clinical examination typically reveals the following:
- Deficits in the visual field relative to confrontation
- Ataxia
- Tremor
- Psychiatric disturbances such as anxiety and agitation
- Seizures
- Respiratory distress and dermatitis can occur acutely.
- Neurotoxicity is the most damaging syndrome.
- Severe poisoning eventually causes the patient to lie in a mute semirigid posture that is broken only by episodes of crying or primitive reflexive movements.
- Unexplained renal abnormalities with neuropsychiatric disturbances should prompt the physician to consider Minamata disease or other forms of mercury poisoning.
- The most damaging effect of ingested inorganic mercury (eg, mercuric chloride) is caustic gastroenteritis.
- Ingested elemental mercury is considered nontoxic because of its poor absorption in the gut; it is most dangerous as a vapor because it can cause acute lung injury and respiratory failure.
Causes
- Classic methyl mercury intoxication occurs as the result of ingesting contaminated seafood, usually fish.
- Organic mercury compounds, specifically methyl mercury, are concentrated in the food chain.
- Fish from contaminated waters are the most common culprits.
- Larger species, such as swordfish, shark, and large tuna, have higher concentrations of methyl mercury in their tissues.
- Other sources of mercury include batteries and thermometers.
- Mercury-containing disk batteries are a concern because of their ability to cause corrosion and ulceration of the GI mucosa. With a battery ingestion, one would expect signs of inorganic mercury exposure, such as hypersalivation and vomiting, rather than signs of organic mercury poisoning.
- Digital thermometers are electronic and do not use mercury columns to measure temperature; therefore, they are not factors in mercury poisoning.
More on Toxicity, Mercury |
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| Differential Diagnoses & Workup: Toxicity, Mercury |
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References
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Mayo Clinic Health Letter. Mercury in fish: concerns shouldn't dampen your appetite. 1996 Apr.
Morgan JN, Berry MR, Graves RL. Effects of commonly used cooking practices on total mercury concentration in fish and their impact on exposure assessments. J Expo Anal Environ Epidemiol. Jan-Mar 1997;7(1):119-33. [Medline].
Sue, Young-Jin. Mercury. In: Goldfrank LR, Flomenbaum NE, Lewin NA, eds. Goldfrank's Toxicologic Emergencies. 1319-29.
Uchino M, Tanaka Y, Ando Y, Yonehara T, Hara A, Mishima I, et al. Neurologic features of chronic minamata disease (organic mercury poisoning) and incidence of complications with aging. J Environ Sci Health B. Sep 1995;30(5):699-715. [Medline].
Yotsuyanagi T, Yokoi K, Sawada Y. Facial injury by mercury from a broken thermometer. J Trauma. May 1996;40(5):847-9. [Medline].
Further Reading
Keywords
mercury toxicity, mercury intoxication, mercury poisoning, methyl mercury intoxication, methyl mercury toxicity, methyl mercury poisoning, Minamata disease, mercurials, fish protein, autism, hearing loss, anxiety, respiratory distress, dermatitis, gastroenteritis, swordfish, shark, large tuna
