Mercury toxicity

Fortunately mercury poisoning is very rare. The risk of toxicity depends on the type of mercury you have been exposed to. Elemental mercury is found in thermometers, barometers, paints, and pigments, these are benign presentations unless aerosolised. It is the inorganic and organic mercury that can cause toxicity. Inorganic mercury has ‘mercuric’ in its name i.e. mercuric arsenate and it used in fireworks, disinfectants, waterproofing, processing fur or leather. Organic mercury (alkoxyalkyl mercury, alkyl mercury and methyl mercury) are used in embalming fluid, fungicides, pesticides, wood preservatives and found in seafood.

Toxic Mechanism:

Mercury has no cellular function but it binds to multiple intracellular sites, causing inhibition of enzymes and disruption of cellular membranes.

Toxicokinetics: 

• Elemental mercury has minimal absorption from an intact GI tract. However, if inhaled (i.e. when using a vacuum to clean up a spill or heated to a vapour) it is well absorbed in the respiratory tract.
• Inorganic mercury is well absorbed though the skin and about 10% via the GI tract.
• Organic mercury is well absorbed from the GI tract and the respiratory tract.
• Mercury has a large volume of distribution once absorbed and distributes to the kidneys, liver, spleen and CNS.
• Elimination half life varies from 30 – 70 days.
• All types of mercury are excreted in faeces. Mercuric ions are also excreted in the urine.

Resuscitation:

• Usually not required

Risk Assessment

• Benign presentations:
  • Accidental ingestion of elemental mercury (broken thermometer)
  • Concern about dental amalgams
  • Incidental discovery of high mercury levels in a ‘heavy metal screen’ when the patient is asymptomatic.
• Potentially serious presentations:
  • Inhalation of mercury via aerosol (vacuuming a broken thermometer) or vapour (heating mercury) can cause a pneumonitis, acute non-cardiac pulmonary oedema and neurological injury.
  • Ingestion of inorganic mercury salts leading to haemorrhage gastroenteritis, acute renal failure and shock. Potentially lethal at 30 – 50 mg/kg.
  • Exposure to organic mercury by ingestion, inhalation or dermal application, leading to neurological injury.
• Clinical features:
  • Acute exposure elemental mercury – occurs via inhalation, symptoms include a headache, nausea, vomiting, chills, fever, salivation, metallic taste, visual disturbances, dyspnoea and a dry cough all within a few hours. Interstitial pneumonitis may occur over the following days.
  • Acute exposure to inorganic mercury salts – ingestion causes severe haemorrhage gastroenteritis within hours. Severe local oropharyngeal pain, metallic taste, nausea, vomiting and diarrhoea. Grey discolouration of the mucous membranes. Massive fluid loss leading to hypotension, shock and acute tubular necrosis follows.
  • Acute exposure to organic mercury – GI symptoms, respiratory distress, tremor, dermatitis, renal tubular dysfunction and ECG (ST segment) changes. Delayed neurotoxicity develops over weeks or months after initial exposure and usually permanent. Any number of neurological dysfunctions can occur covering sensory loss, motor loss, cerebellar signs and psychological dysfunction.
  • Chronic mercury toxicity – leads to multi-system disorders, predominantly neurological (tremor, neurasthenia, erethism, emotional lability, insomnia, delirium, mixed sensorimotor neuropathy, ataxia and anosmia), gastrointestinal (metallic taste, burning pain in the mouth, loose teeth, gingivostomatitis and hypersalivation), renal dysfunction (proximal tubular atrophy with mercuric deposits within the renal interstitium and macrophages) and acrodynia

Supportive Care

• Monitor fluid resuscitation and general supportive measure for any organ failure.
• Correct electrolyte abnormalities.

Investigations

• Screening: 12 lead ECG, BSL, Paracetamol level
• Specific:
  • Whole body mercury level (levels >200 microgram/L or 1000 nmol/L start producing symptoms). It confirms a recent exposure but does not reflect total body burden.
  • 24 hours urine mercury level: >100 microgram/L or 500 nmol/L is associated with neuropsychiatric disturbance.
  • Xrays – mercury is radio-opaque and where is has been ingested or injected will been shown. IV injections produce pulmonary emboli in a ‘milky way appearance’.
  • Endoscopy maybe required to asses any corrosive injury.

Decontamination:

• Environmental – seek expert help in cleaning up mercury spills. Do not vacuum. Discard contaminated carpets or surfaces.
• Elemental mercury – remove clothing, remove mercury from the skin. Administer oral polyethylene glycol solution. Surgically remove any subcutaneous deposits.
• Organic mercury – administer activated charcoal.

Enhanced Elimination

• Administration of polythiol resin may interrupt enterohepatic circulation of organic mercury compounds.

Antidotes

• Chelation is indicated when there are clinical features of toxicity or high urine or serum mercury levels.
• Option include dimercaprol (only used for inorganic mercury salt exposure otherwise it will increase exposure to the brain if used for elemental or organic mercury), penicillamine or succimer.
• Chelation is only useful once further exposure to mercury has been eliminated i.e. decontamination or removal of submit deposits.

Disposition

• Asymptomatic patients can be discharged if the risk assessment is benign. Those that have been exposed to organic or inorganic mercury require admission for observation and assessment.
• Symptomatic patients require admission and an assessment for aggressive decontamination, chelation or supportive care.

References and Additional Resources:

Additional Resources:

• Tox conundrum 002 – Poisoning by a mercury thermometer
• Little Willie the poisoner
• Top 10 foreign bodies

References:

• Clarkson TW, Magos L, Myers GJ. The toxicology of mercury – current exposures and clinical manifestations. New England Journal of Medicine 2003; 349:1731-1737
• Kales SN, Goldman RH. Mercury exposure: current concepts, controversies, and a clinic’s experience. Journal of Occupational and Environmental Medicine 2002; 44:143-154