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Thalium


Thallium has been previously used as a treatment for tuberculosis, gonorrhoea, ringworm and syphilis. It is still used in some countries as a rodenticide. There is evidence to suggest that industrial activity (including coal combustion and releases from cement plants) is contributing to environmental contamination with thallium. However there is little evidence to suggest that this poses a substantial risk to human health. Clinically significant exposure is rare, and is more likely to occur through accidental ingestions of rodenticides or via attempted homicide. Thallium can be absorbed systemically via ingestion, inhalation or skin contact. Following absorption, thallium is distributed to soft tissues with the highest concentrations found in scalp hair, kidney and heart. Reports of the half-life vary from 1.7 to 30 days.

Thallium is handled by the body in a similar manner to potassium and accumulates in tissues with a high potassium concentration: the nervous system, muscle and liver. At high concentrations it inhibits potassium dependent enzymes affecting mitochondrial energy generation and muscle depolarisation. Thallium binds to sulphydral groups interfering with the formation of keratin (which may contribute to abnormalities in hair and nail growth) and decreases glutathione production resulting in lipid peroxidation in the brain. Thallium also destroys ribosomes and may affect the utilisation of riboflavin and thiamine. The overall effect is cellular damage, particularly affecting the nervous system.

The principle targets of thallium poisoning are initially the gastrointestinal tract and the central nervous system. Gastrointestinal symptoms, unlike other metal salts, may be minor or absent. The neurological effects typically constitute a painful ascending peripheral neuropathy and CNS depression. Residual neurological effects include impairment of memory and cognition and a motor neuropathy.

Unlike other heavy metals, thallium is bound to activated charcoal and may interrupt its enterohepatic circulation. Prussian blue (potassium ferric hexacyanoferrate) is used as an oral chelator for thallium poisoning. Its use in humans has been restricted to individual cases and there are no controlled trials evaluating its effectiveness. Prussian blue is not available in Australia.

Haemodialysis and charcoal haemoperfusion have been used in cases of thallium poisoning but the amounts removed by these techniques are small, however they should be considered in cases of life-threatening toxicity. Repeat doses of activated charcoal may be more effective.

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