Heavy Metals Overview - Toxic Troublemakers
- Definition: Dense metals/metalloids, toxic at low concentrations. Persist in environment & body.
- Key Examples: Lead (Pb), Mercury (Hg), Arsenic (As), Cadmium (Cd), Chromium (Cr).
- Common Sources:
- Industrial emissions (mining, smelting, manufacturing).
- Environmental contamination (water, soil, air).
- Certain foods (fish for Hg, rice for As).
- Occupational exposure (paints, batteries, welding).
- General Toxicity Mechanisms:
- Enzyme inhibition: Binding to sulfhydryl (-SH) groups.
- Oxidative stress: ↑ Reactive Oxygen Species (ROS) generation.
- Ionic mimicry: Disrupts essential mineral (Ca, Zn, Fe) functions.
- DNA damage & carcinogenicity.
⭐ Arsenic is notorious for Mees' lines (transverse white nail bands) and garlic breath odor.
Major Culprits - Metal Mayhem
| Feature | Lead (Pb) | Mercury (Hg) | Arsenic (As) |
|---|---|---|---|
| Sources | Old paint, batteries, pipes, some cosmetics | Dental amalgam, fish, thermometers, fungicides | Pesticides, groundwater, industrial, herbal meds |
| Features | Burton's line (gums), wrist/foot drop, microcytic anemia (basophilic stippling), encephalopathy, colic. 📌 Pb: Lines, Encephalopathy, Anemia, Drops. | Acrodynia (children), tremors, nephrotic syndrome, erethism (mad hatter). Minamata disease (organic Hg). | Mees' lines (nails), garlic breath, raindrop skin pigmentation, stocking-glove neuropathy. |
| Diagnosis | BLL > 5 µg/dL (action), ↑ urinary ALA, RBC stippling. | ↑ Urine/blood Hg. | ↑ 24hr urine As; hair/nail As (chronic). |
| Chelator | CaNa₂EDTA, Dimercaprol (BAL), Succimer (DMSA), D-Penicillamine. | Dimercaprol (BAL), Succimer (DMSA), Unithiol (DMPS). | Dimercaprol (BAL), Succimer (DMSA), DMPS. |
⭐ Basophilic stippling of RBCs is a classic (though not pathognomonic) sign of lead poisoning, indicating impaired heme synthesis.
Body's Defense & Chelation - Mop-Up Crew
- Natural Defenses:
- Metallothioneins: Cysteine-rich; bind Cd, Zn, Cu, Hg.
- Glutathione (GSH): Antioxidant; conjugates metals (As, Hg).
- Chelation: Ligands bind metals → stable, non-toxic, excretable complex.
⭐ Metallothioneins are inducible, cysteine-rich proteins crucial for intracellular detoxification of heavy metals like cadmium and zinc.
| Chelator | Metals Chelated | Key Notes/Side Effects |
|---|---|---|
| Dimercaprol (BAL) | As, inorganic Hg, Pb (📌 BAL: Be Aware of Lead, Arsenic, Mercury) | IM (painful), nephrotoxic; Not for Fe, Cd, Se poisoning |
| Succimer (DMSA) | Pb, As, Hg (oral) | GI upset, rash; Water-soluble BAL analog |
| Penicillamine | Cu (Wilson's), Pb, Hg | Hypersensitivity, bone marrow suppression, nephrotic |
| Deferoxamine | Fe (acute/chronic) | Rapid IV → hypotension, "vin rosé" urine, ↑Yersinia risk |
| CaNa₂EDTA | Pb (encephalopathy), Zn, Mn | Nephrotoxic, hypocalcemia (give with Ca) |
Diagnosis & Management - Spot & Stop
- Spot (Diagnosis):
- Thorough history: Occupational, environmental, dietary, pica.
- Symptoms: Vary by metal (e.g., Pb: colic, encephalopathy; As: Mees' lines, neuropathy).
- Investigations: Blood/urine metal levels, X-rays (e.g., lead lines), LFTs, RFTs.
- Stop (Management):
- Immediate removal from exposure source.
- Supportive measures: Airway, breathing, circulation; hydration.
- Chelation therapy: Specific agents (e.g., DMSA, CaNa₂EDTA, BAL) based on metal & severity.
- Prevention: Education, workplace safety.
⭐ Dimercaprol (BAL) is administered intramuscularly and can cause hypertension and tachycardia.
High‑Yield Points - ⚡ Biggest Takeaways
- Metallothioneins: Cysteine-rich proteins, bind & detoxify cadmium, zinc, mercury.
- Chelation therapy: Mainstay treatment; agents form stable, excretable complexes with metals.
- Dimercaprol (BAL): Oily IM chelator for arsenic, inorganic mercury, gold, severe lead poisoning.
- Succimer (DMSA): Oral agent for lead, arsenic, mercury; preferred in children, less toxic.
- CaNa2EDTA: IV for severe lead poisoning (encephalopathy); mobilizes lead from bone.
- Penicillamine: Oral chelator for copper (Wilson's disease), also lead, mercury, arsenic.
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