SN1 SHORT NOTE 5M ★ High-yield

Describe the chelating agents with their uses and precautions.

Forensic Medicine · 2nd Professional — Para-Clinical

Principle and the Agents

A chelating agent is a compound that forms a stable, non-toxic, water-soluble complex with a metal ion, which is then excreted in the urine. The word is from the Greek chele, a claw — the agent grips the metal between two or more binding sites.

Agent	Metals and notes
BAL (dimercaprol, British Anti-Lewisite)	Developed as an antidote to the arsenical war gas lewisite. Used for arsenic, inorganic mercury, gold and (with EDTA) lead. Given by deep intramuscular injection in peanut oil, which is painful. Contraindicated in methylmercury and organic lead, since it redistributes the metal to the brain
Calcium disodium EDTA (edetate)	The agent of choice for lead. It must be the calcium salt — the sodium salt causes fatal hypocalcaemia and tetany, and this substitution has caused deaths. Nephrotoxic; and it may worsen encephalopathy if given alone in severe lead poisoning, so BAL is given first
DMSA (succimer)	An oral analogue of BAL, better tolerated and less toxic; used for lead (especially in children), arsenic and mercury. Now the preferred agent in many settings
Penicillamine	Oral; for copper, and the mainstay of treatment in WILSON disease; also lead, mercury and arsenic. Causes pyridoxine deficiency, nephrotoxicity and marrow suppression, and is contraindicated in penicillin allergy
Desferrioxamine	For iron, and for aluminium in dialysis patients. Turns the urine a characteristic "vin rose" pink, which is used to monitor the response
Prussian blue (ferric ferrocyanide)	For thallium and radioactive caesium; acts within the gut by interrupting enterohepatic circulation

CLINICAL PEARL

The calcium salt of EDTA is the antidote and the sodium salt is a poison. Disodium EDTA chelates the patient’s own calcium, causing acute hypocalcaemia, tetany and cardiac arrest; the calcium disodium salt exchanges its calcium for lead and does not. The two look almost identical on a label, and the substitution has killed children being treated for lead poisoning. Read the ampoule.

Principles of Use

Remove the patient from exposure first. Chelation while exposure continues mobilises the metal, achieves nothing durable, and may increase absorption
Chelation is indicated on the clinical picture and the blood level together, not on the level alone; an asymptomatic slightly raised level is usually managed by removing the source
Chelators are not selective — they also remove essential trace metals: zinc, copper, manganese and iron — so prolonged courses require monitoring and supplementation
They are given in courses with intervals, and the blood level is expected to rebound between courses as the metal redistributes from bone and tissue; a rebound is not a treatment failure
Most are nephrotoxic, and adequate hydration and monitoring of renal function are essential; renal impairment may itself be caused by the metal
In severe lead encephalopathy, BAL is given before EDTA, because EDTA alone may mobilise lead into the brain and worsen the encephalopathy

Applied Aspects

Check that the EDTA is the calcium disodium salt before giving it; the sodium salt causes fatal hypocalcaemia and the error is a recognised cause of death
Do not give BAL in methylmercury or organic lead; it worsens the neurological injury by redistributing the metal to the brain
Remove the source first — the workplace, the remedy, the surma, the water supply — since chelation without removal is futile
Monitor renal function and trace elements during a course, and supplement zinc where the course is prolonged
Expect a rebound between courses and do not interpret it as failure; the metal redistributes from bone, and repeated courses are the norm
Treat the child on a lower threshold; the developing brain is affected at levels that produce no symptoms in an adult, and the damage is irreversible
Chelators are not selective and strip zinc, copper and manganese as well, so prolonged courses need monitoring and supplementation
BAL is given in peanut oil intramuscularly, which is painful, and it is contraindicated in peanut allergy as well as in organic metal poisoning

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