Organomercury Poisoning

Organomercury poisoning results from exposure to alkylmercury compounds, mainly methylmercury and ethylmercury, and also dimethylmercury.

Methylmercury enters food chains: inorganic mercury released by mining (including artisanal gold mining in Kenya, Tanzania and Uganda), coal burning and industrial waste is methylated by aquatic bacteria and accumulates in large predatory fish and shellfish.

Other sources include contaminated grain treated with methylmercury fungicide, laboratory exposure to dimethylmercury (which penetrates latex gloves), and thiomersal containing products.

Methylmercury is absorbed efficiently from the gut (about 95 percent), crosses the blood brain barrier and placenta, binds sulfhydryl groups, and has a blood half life of about 50 days. It destroys neurons in the cerebellar granular layer and the calcarine and precentral cortex. Features are delayed by weeks to months after the dose, so disease may appear after exposure has ended. Prognosis depends on the dose; established neurological injury is largely irreversible.

• Latent period of weeks to months.
• Paraesthesia (perioral, distal limbs), the earliest symptom.
• Cerebellar ataxia of gait and limbs, dysarthria, intention tremor.
• Concentric constriction of the visual fields, hearing loss.
• Headache, fatigue, irritability, depression, memory impairment; in severe cases dementia, coma.
• Prenatal exposure: cerebral palsy like picture, microcephaly, seizures, intellectual disability.
• Dimethylmercury: delayed severe encephalopathy and ataxia after as little as a few drops.
• Red flags: rapidly progressive neurological deficit, visual loss, pregnancy exposure, children.

• Exposure history: fish intake, mining work, contaminated grain, laboratory exposure.
• Whole blood mercury (most useful for methylmercury; normal below 10 micrograms/L; toxicity above 50 to 100 micrograms/L); hair mercury (reflects exposure over months, elevated above 10 micrograms per gram) and segmental hair analysis for timing.
• 24-hour urine mercury reflects inorganic mercury and underestimates organomercury exposure.
• Neurological examination: gait, finger-to-nose, visual fields by confrontation and perimetry, audiometry.
• MRI brain: cerebellar atrophy and calcarine cortex changes in long standing disease.
• Exclude inorganic and elemental mercury poisoning, lead and thallium poisoning, multiple sclerosis, alcoholic cerebellar degeneration, and paraneoplastic cerebellar syndromes.

• Stop exposure: withdraw the contaminated food or workplace source; test household members and pregnant women.
• Chelation after acute poisoning confirmed (blood mercury over 50 micrograms/L or symptomatic): succimer (dimercaptosuccinic acid, DMSA) 10 mg/kg orally every 8 hours for 5 days, then 10 mg/kg every 12 hours for 14 days (a total of 19 days); monitor liver enzymes, full blood count and zinc and copper levels; main adverse effects are rash, nausea, transaminitis and neutropenia. Alternatively sodium dimercaptopropane sulfonate (DMPS) 100 mg orally every 8 hours or 3 to 5 mg/kg intravenously every 4 hours on day 1, then lowered.
• Do not use dimercaprol (BAL): it redistributes organomercury to the brain.
• Penicillamine 250 mg orally four times daily for 5 to 10 days, as second line when DMSA is unavailable; monitor for neutropenia and proteinuria.
• Gastric decontamination after recent ingestion; activated charcoal 50 g orally once, or 1 g/kg in children; polythiol resin where available.
• Blood mercury every 2 weeks during treatment; repeat course after a 2-week gap if still elevated.
• Supportive care: physiotherapy for ataxia, occupational therapy, ophthalmology and audiology, and treatment of spasticity (baclofen 5 mg orally three times daily, increased every 3 days to 20 mg three times daily).
• Pregnancy: avoid fish with high mercury (shark, swordfish, king mackerel, large Nile perch from contaminated waters) and counsel; chelation is individualised.
• Follow up: neurological, visual and audiological review every 3 months for the first year; hair mercury at 3 and 6 months.
• Prevention: ban mercury fungicides, mercury free gold mining, public warnings about contaminated fish, and mercury exposure surveillance.