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Chapter 57: Heavy Metals

INTRODUCTION

The heavy metals discussed in this chapter—lead, arsenic, mercury, and iron—frequently cause toxicity in humans. The toxicity profiles of metals differ, but most of their effects appear to result from interaction with essential cations, sulfhydryl groups of enzymes, and regulatory proteins. Chelators are organic compounds with 2 or more electronegative groups that form stable bonds with cationic metal atoms. These stable complexes lack the toxicity of the free metals and often are excreted readily. Chelators, which function as chemical antagonists, are used as antidotes in the treatment of heavy metal poisoning.

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TOXICOLOGY OF HEAVY METALS

A. 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 57–1Important characteristics of the toxicology of lead, arsenic, mercury, and iron.
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TABLE 57–1 Important characteristics of the toxicology of lead, arsenic, mercury, and iron.
Metal Form Entering Body Route of Absorption Target Organs for Toxicity Treatmenta
Lead Inorganic lead oxides and salts Gastrointestinal, respiratory, skin (minor) Hematopoietic system, CNS, kidneys Dimercaprol, EDTA, succimer, unithiol
Tetraethyl lead Skin (major), gastrointestinal CNS Seizure control
Arsenic Inorganic arsenic salts All mucous surfaces Capillaries, gastrointestinal tract, hematopoietic system Dimercaprol, unithiol, succimer, penicillamine
Arsine gas Inhalation Erythrocytes Supportive
Mercury Elemental Inhalation CNS, kidneys Succimer, unithiol
Inorganic salts Gastrointestinal Kidneys, gastrointestinal tract Succimer, unithiol, penicillamine, dimercaprol
Organic mercurials Gastrointestinal CNS Supportive
Iron Ferrous sulfate Gastrointestinal Gastrointestinal, CNS, blood Deferoxamine, deferasirox

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

1. Acute lead poisoning

Because of the ban on lead over 20 years 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.

2. 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 ...

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