Thallium poisoning

What is thallium?

Thallium is a heavy metal. It has an atomic number of 81, atomic weight of 204.3833, two stable isotopes ²⁰³ Tl and ²⁰⁵ Tl and a specific gravity of 11.9.

It occurs naturally in elemental metallic form but is more frequently found as thallic and thallous compounds. Thallic cations are trivalent or, more commonly, univalent.

Thallium sulfate was once used as an ant and rat poison. The occupational limit for thallium exposure is 0.1 mg/m³ to the skin for eight hours a day¹ . Levels of 15 mg/m³ are considered immediately dangerous to health. The lethal dose is around 15-20 mg per kg body weight and it is rapidly distributed throughout all tissues of the body. Most thallium is excreted by the faecal route but up to 35% may be excreted by the kidneys. Nevertheless, prolonged exposure may lead to build-up and chronic toxicity. Thallium is readily absorbed through the skin as well as during inhalation.

It was discovered in 1861 by Sir William Crookes who named it after its spectroscopic appearance. The name comes from the Greek thallos meaning a green young shoot.

Epidemiology

Thallium poisoning is rare in Western societies. Due to thallium being tasteless, odourless and water-soluble, accidental intoxication has been reported but it has also occasionally been the tool for murder² .

Thallium poisoning presentation

Poisoning can be acute or chronic and the presentation can be highly variable. The doctor may not be aware that the patient has been exposed to thallium and this makes the diagnosis much more difficult. Occupational history is important but the patient may not know that they have been exposed.

Thallium poisoning usually follows oral ingestion but it can be inhaled from contaminated dust from pyrite burners, cadmium manufacturing, and lead and zinc smelting, and in contamination of heroin or cocaine. Its toxic effect is due to its ability to inhibit a number of intracellular potassium-mediated processes. It is used in the manufacture of electronic components, optical lenses, semiconductor materials, alloys, gamma radiation detection equipment, imitation jewellery, artists' paints or tattoo inks, low-temperature thermometers and green fireworks.

Thallium poisoning symptoms

The classical triad of acute poisoning that should suggest the diagnosis of acute thallium toxicity is as follows:

• Nausea and vomiting.

• Followed by painful peripheral neuropathy.

• Alopecia - occurring later.

Acute poisoning tends to produce gastrointestinal (GI) effects, whilst chronic poisoning tends to produce neurological manifestations.

• Abdominal pain, nausea and vomiting occur within a few hours of an acute exposure. There may be blood in the stool.

• Painful sensory neuropathy occurs, especially on the soles and palms. It may take a couple of days to develop. It may be followed by weakness of the lower leg, ataxia, confusion, hallucinations, convulsions and coma.

• Neuropsychiatric complications include anxiety, confusion, delirium, hallucinations and psychosis. Acute agitation and aggression, personality changes, depression, apathy and confabulation have been observed both in adults and in children³ . Psychosis can occur even without a psychiatric history.

• Diplopia, abnormal colour vision and reduced visual acuity are reported.

• Sometimes there is shortness of breath and pleuritic chest pain. The mechanism is unknown.

Chronic poisoning can lead to tiredness, headaches, depression, lack of appetite, leg pains, hair loss and disturbances of vision.

Thallium poisoning examination

In the early stages, abdominal examination shows some tenderness, hyperactive bowel sounds and possible guarding.

Neurology

• A full neurological examination is required including examination of the cranial nerves and assessment for visual field defects. Optic neuropathy and cranial nerve palsies are the most obvious features.

• Signs take 2-4 days to develop with the severity dependent upon the level of poisoning.

• There is peripheral neuropathy with marked leg and foot tenderness and paraesthesia. Weakness may be difficult to elicit because of pain and it tends to be mild.

• With severe neurological involvement, respiratory paralysis can be fatal at the end of a week.

• Examination of the eyes and face may show loss of the lateral half of the eyebrows, drooping eyelids, facial nerve palsy, ophthalmoplegia and nystagmus.

• Specialist examination is required with slit-lamp assessment and documentation of colour vision.

• Keratitis and corneal opacities occasionally occur.

Dermatology

• Hair loss, rashes and palmar erythema take 2-4 weeks to appear.

• There is atrophy of the hair follicles. Alopecia is so characteristic that it should prompt suspicion^{4 5} . The hair loss primarily affects the scalp, temporal parts of the eyebrows, the eyelashes and the limbs. Less often, the axillary regions are affected.

• Early signs are not specific and include scaling of the palms and soles and acneform lesions of the face.

• One month after the poisoning, Mees' lines (transverse white lines on the nails) appear in the nail plate. They also occur in arsenic poisoning and renal failure.

• Other findings may include crusted eczematous lesions, hypohidrosis, anhidrosis, palmar erythema, painful glossitis with redness of the tip of the tongue, stomatitis and hair discolouration.

• There is a characteristic band-like dark pigmentation of the scalp that develops within four days.

Investigation

• Few laboratories can perform an assay for thallium. A 24-hour urine thallium concentration is the most accurate way to assess thallium toxicity but a spot urine level is likely to provide a more rapid and reasonably reliable answer. Acute toxicity will produce a thallium concentration many times higher than the reference range of 0-5 ng/mL.

• The urine may appear green.

• FBC and blood chemistry are required, in case of anaemia from GI haemorrhage and to assess renal function.

• Shortly after ingestion, plain abdominal X-ray may show opacities due to the heavy metal.

• Electrophysiology tests of nerves to the feet may show abnormalities.

• Electroretinography (ERG) demonstrates delayed visual evoked response⁶ . Changes may precede clinical symptoms and so it can be used to assess people with known exposure.

• ECG, as there may be cardiac arrhythmias.

Differential diagnosis¹

• Guillain-Barré syndrome.

• Poisoning with other heavy metals, including arsenic and mercury.

• Poisoning from colchicine, isoniazid and hydralazine.

• Poliomyelitis.

• Carbon monoxide poisoning.

• Organophosphate poisoning.

• Thiamine deficiency.

• Diabetic polyneuritis.

• Botulism.

• Vasculitis.

Thallium poisoning treatment^{7 8}

• Remove from the source of exposure if possible. Remove contaminated clothing but be careful not to become exposed too.

• Assess airways, breathing and circulation. With inhalation, possible respiratory failure is a great concern and intravenous (IV) access and administration of oxygen are advised.

• If ingestion has occurred in the preceding 30 minutes, induce vomiting. In an A&E department gastric lavage can be performed if within an hour of ingestion.

• With ingestion, activated charcoal and Prussian blue (potassium ferric hexacyanoferrate) are recommended⁹ . Prussian blue is better than charcoal at keeping thallium out of the enterohepatic circulation and enhancing faecal excretion but it is not often available.

• Prussian blue is better than chelating agents such as penicillamine; the latter may cause redistribution of thallium into the central nervous system¹⁰ .

• Haemoperfusion, haemodialysis or plasma exchange, if employed, should be performed as early as possible but can be effective even after delayed diagnosis^{10 11 12} .

• All patients with significant signs and symptoms of thallium poisoning require hospital admission.

• Measure thallium levels in blood and urine three times per week to confirm a decreasing trend.

• Although not well studied, case reports suggest that Prussian blue treatment can be discontinued when spot urine thallium levels fall to less than 100 mcg/L⁶ .

• Physiotherapy may reduce or prevent muscle contractures.

Thallium poisoning prognosis

• Of those who survive poisoning, about a third to a half have residual neurological or visual disability.

• Delay in receiving treatment is associated with greater likelihood of lasting neurological problems. Patients can have signs and symptoms of peripheral neuropathy up till six years after intoxication¹ . Reports of persistent findings most commonly involve the feet and lower extremities.

• Exposure during pregnancy can lead to fetal abnormalities.

• Chronic cases may lead to dementia, depression and psychosis.