High-Yield Terms to Learn

Chelating agent A molecule with 2 or more electronegative groups that can form stable coordinate complexes with multivalent cationic metal atoms Erethism Syndrome resulting from mercury poisoning characterized by insomnia, memory loss, excitability, and delirium Pica The ingestion of nonfood substances; in the present context, pica refers to ingestion of lead-based paint fragments by small children Plumbism A range of toxic syndromes due to chronic lead poisoning that may vary as a function of blood or tissue levels and patient age

Toxicology of Heavy Metals

Lead

Lead serves no useful purpose in the body and can damage the hematopoietic tissues, liver, nervous system, kidneys, gastrointestinal tract, and reproductive system (Table 57-1). Lead is a major environmental hazard because it is present in the air and water throughout the world.

TABLE 57-1 Important characteristics of the toxicology of arsenic, iron, lead, and mercury.

Metal Form Entering Body Route of Absorption Target Organs for Toxicity Treatmenta

Lead Inorganic lead oxides and salts Tetraethyl lead Gastrointestinal, respiratory, skin (minor) Skin (major), gastrointestinal Hematopoietic system, CNS, kidneys CNS Dimercaprol, EDTA, succimer, unithiol Seizure control, supportive Arsenic Inorganic arsenic salts All mucous surfaces Capillaries, gastrointestinal tract, hematopoietic system Dimercaprol, unithiol, succimer, penicillamine Arsine gas Inhalation Erythrocytes Supportive Mercury Elemental Inorganic salts Inhalation Gastrointestinal CNS, kidneys Kidneys, gastrointestinal tract Succimer, unithiol Succimer, unithiol, penicillamine, dimercaprol Organic mercurials Gastrointestinal CNS Supportive Iron Ferrous sulfate Gastrointestinal Gastrointestinal, CNS, blood Deferoxamine

aIn all cases, removal of the person from the source of toxicity is the first requirement of management.

Acute Lead Poisoning

Because of the ban on lead over 20 yrs ago in gasoline and because of bans on other industrial products that previously contained lead, acute inorganic lead poisoning is no longer common in the United States. It can occur rarely from industrial exposures (usually via the inhalation of dust) and in children who have ingested large quantities of chips or flakes from surfaces in older houses covered with lead-containing paint. The primary signs of this syndrome are acute abdominal colic and central nervous system (CNS) changes, including, particularly in children, acute encephalopathy. The mortality rate is high in those with lead encephalopathy, and prompt chelation therapy is mandatory.

Chronic Lead Poisoning

Chronic inorganic lead poisoning (plumbism) is much more common than the acute form. Signs include peripheral neuropathy (wrist-drop is characteristic), anorexia, anemia, tremor, weight loss, and gastrointestinal symptoms. Treatment involves removal from the source of exposure, and chelation therapy, usually with oral succimer in outpatients and with parenteral agents (eg, EDTA with or without dimercaprol) in more severe cases. Chronic lead poisoning in children presents as growth retardation, neurocognitive deficits, and developmental delay. Succimer is generally used in such children. In workers exposed to lead, prophylaxis with oral chelating agents is contraindicated because some evidence suggests that lead absorption may be enhanced by the presence of chelators. In contrast, high dietary calcium is indicated because it impedes lead absorption.

Organic Lead Poisoning

Now rare, poisoning by organic lead was usually due to tetraethyl lead or tetramethyl lead contained in “antiknock” gasoline additives, which are no longer used. This form of lead is readily absorbed through the skin and lungs. The primary signs of intoxication include hallucinations, headache, irritability, convulsions, and coma. Treatment consists of decontamination and seizure control.

Arsenic

Arsenic is widely used in industrial processes and is also an environmental pollutant released during the burning of coal. Although the element exists in both trivalent and pentavalent forms, its toxicity is entirely due to the trivalent form.

Acute Arsenic Poisoning

Acute arsenic poisoning results in severe gastrointestinal discomfort, vomiting, “rice-water” stools, and capillary damage with dehydration and shock. A sweet, garlicky odor may be detected in the breath and the stools. Treatment consists of supportive therapy to replace water and electrolytes, and chelation therapy with dimercaprol.

Chronic Arsenic Poisoning

Chronic arsenic intoxication causes skin changes, hair loss, bone marrow depression and anemia, and chronic nausea and gastrointestinal disturbances. Dimercaprol therapy appears to be of value. Arsenic is a known human carcinogen.

Arsine Gas

Arsine gas (AsH 3), an occupational hazard, is formed during the refinement and processing of certain metals and is used in the semiconductor industry. Arsine causes a unique form of toxicity characterized by massive hemolysis. Pigment overload from erythrocyte breakdown can cause renal failure. Treatment is supportive.

Mercury

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