What is thallium?
Discovered in 1861, thallium is a heavy metal chemical. The compound form of thallium sulfate was previously widely used in rodenticide, or rat poison. It was also once used to treat syphilis, gonorrhea and gout but stopped due to side effects. Despite making an effective rodenticide, it is highly toxic and extremely poisonous. The colourless, odourless and tasteless properties cause poisoning to occur unnoticed through ingestion, inhalation or skin absorption. It is so toxic that ingestion of as little as 8 mg/kg of thallium has been reported to cause death in humans. For this reason, it had been banned in the United States and many other countries since the 1970s. Although it is rarely used nowadays, cases of thallium poisonings still occur sometimes, possibly due to accidental contaminated rice or grain, occupational exposure or homicidal intention. Fortunately, there is a cure for thallium.
What does thallium do to the human body?
Thallium disrupts the energy production of the cells by displacing potassium in enzymes and ion channels. As a result, patients suffer from a number of symptoms, including the two major symptoms of hair loss and nerve damage. The effects may vary based on the dosage and duration of exposure. Ingesting high doses kill before some symptoms become apparent, while chronic toxicity to low doses manifests more symptoms.
Within 2 days after exposure, patients begin to have early symptoms of stomach pain, vomiting, constipation and diarrhoea. Patients may also have a rapid heartbeat, high blood pressure and chest pain.
Subsequently, neurological symptoms become prominent in the next 2 weeks, such as:
- Very painful numbness or burning sensation in the upper and lower limbs (stocking-glove paresthesia of extremities)
- Drooping eyelids and involuntary eye movement (ptosis and nystagmus)
- Painful blurry vision due to inflamed eye nerve (optic neuritis)
- Imbalanced gait (ataxia)
- Involuntary fast movements of the hands and feet (choreoathetosis)
- Cognitive and memory impairment
- Delirium, psychosis, lethargy and even coma
Other common symptoms include loss of hair, redness on palms and soles, and skin darkening. If left untreated, thallium poisoning causes permanent blindness, multiorgan failures and death. Diagnosis is difficult but can be confirmed by history of exposure, clinical manifestations, and a 24-hour urine test using atomic absorption spectrometry.
What is the cure for thallium poisoning?
Prussian blue is the cure for thallium poisoning. It is an insoluble chelating agent and absorbent. After administering orally, it works by trapping thallium within its crystal structure, thus keeping thallium from being absorbed into the body. This helps to limit the amount of time the body is exposed to thallium. Usually, Prussian blue is given in a dosage of 250mg/kg daily divided into 2 to 4 doses. While no substance can remove thallium that has already been absorbed, Prussian blue can remove unabsorbed thallium from the digestive tract. Despite having insufficient data on its efficacy, studies have shown that the safety profile of Prussian blue is superior to other therapies. Hence it is accepted as the treatment of choice for thallium poisoning.
Activated charcoal may be beneficial too as thallium is well absorbed to it which can enhance gastrointestinal elimination and faecal excretion of thallium. In the absence of Prussian blue or activated charcoal, whole bowel irrigation may be useful.
Gastric emptying is warranted for patients who presented early to the hospital within 1 hour of ingestion of large doses of thallium. This is because large doses of thallium may not respond to Prussian blue and can cause fatal toxicity in a short time. Gastric lavage is not needed, however, if patients have been vomiting. Other treatments of thallium poisonings are supportive care for the airway, breathing and circulation, as well as analgesic medications for pain relief.
