Most people with mercury levels above the EPA Reference Dose (RfD) or 'safe level', are asymptomatic or have subtle, nonspecific symptoms (Hightower 2003). In women of reproductive age, it is important to diagnose overexposure to mercury even if the patient herself is asymptomatic due to the serious developmental neurotoxicity of this metal (Steuerwald 2000). There may be a reason to look for mercury overexposures even in men and older women. Recent studies have found an association between mercury exposures over time and coronary artery disease, including myocardial infarction (Guallar 2002). The only way to diagnose mercury poisoning is to be clinically alert to the diagnosis, and to take an occupational history and a fish-consumption history in patients with suggestive symptoms, and in all women of reproductive age.

Metallic mercury poisoning is almost exclusively confined to the occupational setting. However, some cases have been reported among people who use mercury for hobbies, such as gold mining, where mercury is added to the pan to amalgamate the gold and is subsequently boiled off. Metallic mercury is also used in a Caribbean religious practice known as Santaria. In these rituals mercury can be sprinkled inside a home and can result in serious over-exposures (Wendroff 1997).

Signs and symptoms of metallic mercury poisoning include:

• Pneumonitis -- dyspnea, chest pain, cough, interstitial infiltrates (after inhalation)
• Salivation, gingivitis
• Erethism -- memory loss, excitability, delirium, personality changes, shyness
• Tremor, peripheral neuropathy
• Nephrotic syndrome

Inorganic mercury poisoning is unusual in the United States. In the past, cases of Acrodynia occurred in infants due to inorganic mercurials used as disinfectants in laundry detergents and baby powders (Weiss 1999). Since these uses have been discontinued, this syndrome has essentially disappeared.

Inorganic mercury can cause:

• Ingestion -- ulceration and necrosis of GI tract
• Renal failure -- necrosis of proximal tubule
• Acrodynia -- pink, peeling skin, salivation, hypotonia, swelling of hands and feet

Organic mercury poisoning is the most common and most worrisome form of mercury poisoning today (Clarkson 2003). Except in very rare cases (eg. exposure to laboratory reagents) this disorder is exclusively associated with ingestion of fish.

Classic symptoms of methyl mercury poisoning in the adult include:

• Visual field constriction
• Behavioral changes, memory loss, headaches
• Ataxia, tremor, loss of fine motor control, spasticity
• Paresthesias of extremities and lips
• Hair loss

Prenatal exposure to methyl mercury at high levels can cause overt Minamata Disease, with symptoms which include:

• Mental retardation
• Seizures
• Cerebral palsy
• Blindness and deafness
• Disturbances of swallowing, sucking, and speech
• Hypertonia, rigidity, hyperreflexia

At the lower levels more likely to be encountered today, signs and symptoms of prenatal exposure to methyl mercury may include (Mahaffey 2000):

• Delayed learning
• Shortened attention span
• Memory deficits
• Delayed language acquisition
• Poorer motor control or coordination

Testing patients for mercury exposure

• Urine (when exposure to inorganic or metallic mercury is suspected) Toxic range >50 mcg/g creatinine (occupational exposure levels)
  
  
• Whole Blood (when exposure to organic mercury is suspected) "Safe" range (below EPA reference dose equivalent level) <5 mcg/L Toxic range >60 mcg/L (Rice 2003, Schober 2003)
  
  
• Hair (less accurate, but provides information over a longer time line) "Safe" range (below EPA reference dose equivalent level) <1 mcg/g

References

For an annotated bibliography, click here.

Clarkson TW, Magos L, Myers GJ. "The toxicology of mercury -- current exposures and clinical manifestations," in New England Journal of Medicine, 349 (18), 2003, pp. 1731-7.

Guallar E, Sanz-Gallardo MI, van't Veer P, Bode P, Aro A, Gomez-Aracena J, Kark JD, Riemersma RA, Martin-Moreno JM, Kok FJ. Heavy Metals and Myocardial Infarction Study Group. "Mercury, fish oils, and the risk of myocardial infarction," in New England Journal of Medicine, 347 (22), 2002, pp. 1747-54.

Hightower JM, Moore D. "Mercury levels in high-end consumers of fish," in Environmental Health Perspectives, 111 (4), 2003, pp. 604-608.

Mahaffey KR. "Recent advances in recognition of low-level methyl mercury poisoning," in Curr Opin Neurol, 13(6), 2000, pp. 699-707.

Rice DC, Schoeny R, Mahaffey K. "Methods and rationale for derivation of a reference dose for methyl mercury by the U.S. EPA," in Risk Anal, 23(1), 2003, pp. 107-15.

Schober SE, Sinks TH, Jones RL, Bolger PM, McDowell M, Osterloh J, Garrett ES, Canady RA, Dillon CF, Sun Y, Joseph CB, Mahaffey KR. "Blood mercury levels in US children and women of childbearing age, 1999-2000," in JAMA, 289(13), 2003, pp. 1667-74.

Steuerwald U, Weihe P, Jorgensen PJ, Bjerve K, Brock J, Heinzow B, Budtz-Jorgensen E, Grandjean P. "Maternal seafood diet, methyl mercury exposure, and neonatal neurologic function," in J Pediatr, 136(5), 2000, pp. 599-605.

Weiss J, Trip L, Mahaffey KR. "Human exposures to inorganic mercury," in Public Health Rep, 114(5), 1999, pp. 400-1.

Wendroff AP. "Magico-religious mercury exposure," in Environmental Health Perspectives, 105(3), 1997, 266.

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