Heavy Metal Poisoning¶
Chapter 469 | Part 14: Poisoning, Drug Overdose, and Envenomation
KEY CLINICAL POINTS¶
- Heavy metals (arsenic, lead, mercury, cadmium) are among the top 10 chemicals of highest health concern globally, with significant environmental and occupational exposure risks.
- Chelation therapy (dimercaprol, DMSA, CaEDTA) is critical for acute poisoning, but chronic low-level exposure contributes to cardiovascular disease, hypertension, and neurodevelopmental delays.
- Arsenic exposure is linked to skin, lung, and bladder cancers, while cadmium causes renal failure, osteomalacia, and cardiovascular mortality.
- Lead toxicity in children causes neurodevelopmental delays, and maternal bone lead levels predict lower birth weight and neurodevelopmental deficits.
- Mercury poisoning (organic and inorganic) leads to CNS dysfunction, with dimethylmercury being 'supertoxic' and requiring immediate chelation.
1. DEFINITION & OVERVIEW¶
Heavy metal poisoning refers to toxic effects from exposure to metals such as arsenic, lead, mercury, and cadmium. These metals are classified as xenobiotics with potential toxicity at any exposure level. Chronic low-level exposure contributes to cardiovascular disease, hypertension, and neurodevelopmental disorders. Acute poisoning requires immediate chelation therapy, while chronic exposure necessitates environmental intervention and nutritional support.
Table 469-1: Heavy Metals - Main Sources, Metabolism, Toxicity, Diagnosis, Treatment¶
| Metal | Main Sources | Metabolism | Toxicity | Diagnosis | Treatment |
|---|---|---|---|---|---|
| Arsenic | Smelting, pesticides, contaminated water, folk remedies | Inorganic arsenic absorbed via GI/air; sequestered in liver/kidneys; biomethylation detoxifies but saturates | Acute: GI necrosis, hemolysis, cardiovascular collapse; Chronic: skin cancer, bladder cancer, peripheral neuropathy, hypertension | Garlicky breath odor, Mees’ lines, 24-h urinary arsenic >67 mmol/d, serum arsenic >0.9 mmol/L | Ipecac, gastric lavage, dimercaprol (3–5 mg/kg IM), oral succimer |
| Metal | Main Sources | Metabolism | Toxicity | Diagnosis | Treatment |
|---|---|---|---|---|---|
| Cadmium | Metal plating, tobacco, incineration, contaminated food | Absorbed via GI/inhalation; bound by metallothionein; reabsorbed via proximal tubules | Acute: pulmonary edema; Chronic: renal failure, osteomalacia, lung cancer, cardiovascular disease | Urinary b-microglobulin >100 nmol/L, urinary cadmium >750 mg/g creatinine | Avoid further exposure, vitamin D, no effective chelation (dimercaprol may worsen nephrotoxicity) |
| Lead | Paint, plumbing, contaminated food/water, gasoline, batteries | 95–99% sequestered in RBCs; 15% in bone; excreted via urine | Acute: encephalopathy, anemia; Chronic: neurodevelopme ntal delays, hypertension, renal failure, infertility | Blood lead >80 mg/dL (children), >120 mg/dL (adults), | Chelation with DMSA (oral), CaEDTA (IV/IM), dimercaprol; calcium supplements (1200 mg/day) |
| Mercury | Industrial exposure, dental amalgams, contaminated fish | Elemental mercury volatilizes; organic mercury (methylmercury) crosses blood-brain barrier | Acute: CNS symptoms, renal failure; Chronic: neurotoxicity, acrodynia, kidney damage | Hair mercury >30 nmol/g, urinary mercury >1 mg/L, neurological exam findings | Chelation with dimercaprol, DMSA, penicillamine; Prussian blue for thallium |
1.1 Metal Toxicity Mechanisms¶
Metals inhibit enzymes, damage DNA, and disrupt metal-dependent proteins. They also induce reactive oxygen species and alter epigenetic processes like DNA methylation. Genetic polymorphisms and nutritional status (iron, calcium, zinc) modulate susceptibility.
1.2 Diagnostic Challenges¶
Blood levels may not reflect long-term exposure; bone lead levels (measured by KXRF) better indicate cumulative exposure. Urinary arsenic and cadmium levels are critical for chronic toxicity assessment.
2. EPIDEMIOLOGY¶
Arsenic, lead, mercury, and cadmium rank among the top 10 chemicals of highest health concern globally. Arsenic exposure is prevalent in Bangladesh, India, and China via contaminated drinking water. Lead poisoning is common in older U.S. housing with lead paint. Cadmium exposure from mining effluents caused the 1946 'itai-itai' disease outbreak in Japan. Mercury contamination from industrial waste affects Arctic populations via bioaccumulation in fish.
2.1 Risk Factors¶
Occupational exposure (mining, battery manufacturing), environmental pollution, contaminated food/water, and poor nutrition. Children are at higher risk for lead poisoning due to hand-to-mouth activity and developing nervous systems.
2.2 Demographics¶
Lead poisoning is most common in children <6 years old. Arsenic exposure disproportionately affects low-income populations in South Asia. Cadmium toxicity is prevalent in industrial workers and smokers.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Metals disrupt cellular processes by inhibiting enzymes (ATPases), damaging DNA, and displacing essential metals in proteins. Organic mercury (methylmercury) crosses the blood-brain barrier, while inorganic mercury causes renal tubular damage. Arsenic induces oxidative stress and DNA methylation changes. Cadmium interferes with calcium homeostasis, leading to renal failure and bone demineralization.
3.1 Metal-Specific Mechanisms¶
Arsenic: Inhibits thioredoxin reductase, induces oxidative stress. Lead: Disrupts calcium channels, causes neurotoxicity. Mercury: Binds sulfhydryl groups, interferes with ion channels. Cadmium: Competes with zinc, induces calciuria.
3.2 Epigenetic Effects¶
Arsenic and cadmium alter DNA methylation patterns, affecting gene expression and microRNA regulation. These changes contribute to chronic diseases like cancer and cardiovascular disease.
4. CLINICAL FEATURES¶
Acute poisoning presents with GI symptoms, CNS dysfunction, and renal failure. Chronic exposure causes neurodevelopmental delays, hypertension, and organ-specific damage. Mercury poisoning may present with 'erethism' (restless legs, irritability). Cadmium toxicity manifests as osteomalacia and renal tubular dysfunction.
4.1 Acute Symptoms¶
Arsenic: Hemorrhagic gastroenteritis, delirium, coma. Lead: Abdominal pain, encephalopathy. Mercury: Pulmonary edema, CNS symptoms. Cadmium: Pulmonary inflammation, renal failure.
4.2 Chronic Manifestations¶
Lead: Neurodevelopmental delays, hypertension, renal failure. Arsenic: Skin cancer, peripheral neuropathy. Cadmium: Osteomalacia, renal tubular acidosis. Mercury: Cognitive deficits, acrodynia.
5. DIFFERENTIAL DIAGNOSIS¶
Arsenic poisoning must be differentiated from other heavy metal toxicities. Lead encephalopathy resembles bacterial meningitis. Mercury toxicity may mimic Parkinsonism. Cadmium nephropathy overlaps with chronic kidney disease. Environmental exposure history is critical for accurate diagnosis.
5.1 Metal-Specific Differentiation¶
Lead: 'Lead lines' on teeth, basophilic stippling. Mercury: Tremors, erethism. Cadmium: Osteomalacia, renal tubular acidosis.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic tests include blood/urine metal levels, urinary β -microglobulin, and bone lead measurement (KXRF). Arsenic is detected via 24-h urinary arsenic >67 µ mol/d. Cadmium is diagnosed with urinary cadmium >750 µ g/g creatinine. Lead levels are measured in whole blood (not serum).
6.1 Laboratory Tests¶
Blood lead (BPb) >80 µ g/dL in children, >120 µ g/dL in adults. Urinary arsenic >67 µ mol/d. Urinary cadmium >750 µ g/g creatinine. Urinary β -microglobulin >100 nmol/L.
7. MANAGEMENT & TREATMENT¶
Acute poisoning requires immediate chelation with dimercaprol, DMSA, or CaEDTA. Chronic exposure management includes environmental intervention, nutritional support (iron, calcium, zinc), and calcium supplements. Thallium poisoning requires Prussian blue and forced diuresis.
7.1 Acute Chelation Therapy¶
Dimercaprol (3–5 mg/kg IM, q4h × 2 days), DMSA (oral 10–20 mg/kg/day), CaEDTA (IV/IM 1–2 g/day). For thallium: Prussian blue (250 mg/kg orally), potassium chloride, peritoneal dialysis.
7.2 Chronic Management¶
Remove exposure sources, calcium supplements (1200 mg/day), vitamin C, and dietary iron/calcium/zinc. Monitor bone lead levels in high-risk populations.
8. PROGNOSIS & COMPLICATIONS¶
Acute poisoning with chelation has good outcomes, but chronic exposure leads to long-term complications. Lead exposure increases cardiovascular mortality. Arsenic and cadmium are linked to cancer and renal failure. Mercury toxicity may cause irreversible CNS damage.
8.1 Long-Term Risks¶
Chronic lead exposure: Hypertension, renal failure, infertility. Arsenic: Skin/lung cancer, cardiovascular disease. Cadmium: Osteomalacia, renal failure, lung cancer. Mercury: Neurological deficits.
8.2 Cardiovascular Risks¶
Arsenic, lead, and cadmium are risk factors for hypertension, atherosclerosis, and coronary artery disease. Chronic exposure increases mortality from cardiovascular events.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Lead crosses the placenta, causing fetal neurodevelopmental delays. Children: Blood lead levels <10 µ g/dL are recommended. Elderly: Increased risk of renal failure from metal toxicity. Occupational exposure requires regular monitoring and removal at BPb >40 µ g/dL.
9.1 Pregnancy & Pediatrics¶
Maternal bone lead predicts lower birth weight. Fetal mercury exposure from fish consumption may cause cognitive deficits. Calcium supplements reduce maternal bone lead mobilization.
9.2 Occupational Safety¶
OSHA mandates regular blood lead testing. Workers with BPb >40 µ g/dL must be removed from exposure. Lead-exposed workers require monitoring for renal and neurological complications.
10. KEY POINTS & CLINICAL PEARLS¶
- Chelation therapy is critical for acute poisoning but controversial for chronic exposure. 2. Bone lead levels (KXRF) better reflect cumulative exposure than blood levels. 3. Mercury poisoning requires immediate chelation due to its neurotoxic potential. 4. Nutritional deficiencies (iron, calcium, zinc) increase metal absorption. 5. Environmental intervention is essential for long-term management of chronic metal toxicity.