Background
Mercury in any form is poisonous, with mercury toxicity most commonly affecting the neurologic, gastrointestinal (GI) and renal organ systems. Poisoning can result from mercury vapor inhalation, mercury ingestion, mercury injection, and absorption of mercury through the skin. (See Etiology and Prognosis.)
Mercury has 3 forms: (1) elemental mercury, (2) inorganic salts, and (3) organic compounds. Perhaps the most deadly form of mercury is methylmercury. 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 methylmercury ingested is absorbed into the bloodstream from the GI tract. (See the images below.)
This is a 1-view, abdominal, upright radiograph in a male patient who intentionally ingested 8 ounces of elemental mercury. Notice how the mercury outlines the large intestine from ascending to descending. Image courtesy of Fred P. Harchelroad, MD, and Ferdinando L. Mirarchi, DO.
Patient with intentional ingestion of mercury from blood pressure instrument. Note how mercury beads can be seen deposited in lung fields. Image courtesy of Shuchi Vyas, MD.
Organic mercury compounds, specifically methylmercury, 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 methylmercury. Fish eat contaminated vegetation, and the mercury becomes biomagnified in the fish. Fish protein binds more than 90% of the consumed methylmercury so tightly that even the most vigorous cooking methods (eg, deep-frying, boiling, baking, pan-frying) cannot remove it. (See Etiology.)
For centuries, mercury was an essential part of many different medicines, such as diuretics, antibacterial agents, antiseptics, and laxatives. In the late 18th century, antisyphilitic agents contained mercury. It was during the 1800s that the phrase “mad as a hatter” was coined, owing to the effects of chronic mercury exposure in the hat-making industry, where the metal was used in the manufacturing process.
In 1889, Charcot, in his Clinical Lectures on Diseases of the Nervous System, attributed some rapid oscillatory tremors to mercury exposure.
In Wilson’s classic textbook of neurology, published in 1940, Wilson concurred with Charcot’s attribution of tremors to mercury poisoning, but also described mercury-induced cognitive impairments, such as inattention, excitement, and hallucinosis.
In 1961, researchers in Japan correlated elevated urinary mercury levels with the features of the previously mysterious Minamata disease. Before the etiology of Minamata disease was discovered, it plagued the residents around Minamata Bay in Japan with tremors, sensory loss, ataxia, and visual field constriction. (See Presentation.)
Minamata disease is an example of organic toxicity. In Minamata Bay, a factory discharged inorganic mercury into the water. The mercury was methylated by bacteria and subsequently ingested by fish. 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 methylmercury in utero were the most severely affected. Furthermore, because mercury was also discovered in the breast milk of the mothers, the babies’ exposure continued after birth.
On January 19, 2013, The Minamata Convention on Mercury was agreed upon at the fifth session of the Intergovernmental Negotiating Committee in Geneva, Switzerland. It is a global treaty to protect human health and the environment from the adverse effects of mercury. The major highlights of the convention included a ban on new mercury mines, the phase-out of existing ones, control measures on air emissions, and the international regulation of the informal sector for artisanal and small-scale gold mining.
Mercury is still found in many industries, including in battery, thermometer, and barometer manufacturing. Mercury can also be found in fungicides used in the agricultural industry. Before 1990, paints contained mercury as an antimildew agent. In medicine, mercury is used in dental amalgams and various antiseptic agents. (See Etiology and Prognosis.)
Mercury may also be contained in some cosmetics, such as skin-lightening products. One study measured international skin-lightening products for their mercury content, focusing on products available to US consumers either online or in stores. The products were screened for mercury content using a portable x-ray fluorescence spectrometer. Of the 549 products tested, 6% contained mercury levels above 1000 ppm, and 45% contained mercury levels that exceeded 10,000 ppm. Of lightening products purchased in the United States, 3.3% were found to contain mercury in excess of 1000 ppm. According to the authors, the Food and Drug Administration limits the amount of mercury in most cosmetic products to 1 ppm.
Complications
Minamata disease has devastating neurologic consequences as a primary outcome of methyl mercury intoxication; unfortunately, these are relatively resistant to treatment. Complications include the following (see Presentation):
Acute perioral and facial paresthesias
Visual-field constriction
Respiratory distress and nonspecific dermatitis
Extremity numbness eventually appears, along with headache, fatigue, and tremor
Ataxia and dysarthria can also be observed
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. (See Presentation.)
Babies exposed in utero are the most severely affected. They are affected by low birth weight, seizure disorders, profound developmental delay, incomplete visual loss (including tunnel vision) or total blindness, and hearing loss.
Neurologic damage in the form of diffuse and widespread neuronal atrophy is most severe in patients exposed in utero. Long-term studies may indicate that even prenatal exposure at low concentrations can cause subtle, but detectable, decrements in the areas of motor function, language, and memory.
Children so affected may have long-term stigmata, including motor impairment, visual loss, hearing loss, developmental delay, and seizure disorders.
Several members of the Plenary Panel on Human Health in the 12th International Conference on Mercury as a Global Pollutant held in Korea in June 2015 wrote an excellent summary article on this meeting.