CHAPTER 2-150: THALLIUM

INTRODUCTION

Thallium is a soft metal that quickly oxidizes upon exposure to air. It is a minor constituent in a variety of ores. Thallium salts are used in the manufacture of jewelry, semiconductors, and optic devices. Thallium no longer is used in the United States as a depilatory or rodenticide because of its high human toxicity.

MECHANISM OF TOXICITY

The mechanism of thallium toxicity is not known. It appears to affect a variety of enzyme systems, resulting in generalized cellular poisoning. Thallium metabolism has some similarities to that of potassium, and it may inhibit potassium flux across biologic membranes by binding to Na⁺/K⁺-ATP transport enzymes.

TOXIC DOSE

The minimum lethal dose of thallium salts is probably 12–15 mg/kg, although toxicity varies widely with the compound, and there have been reports of death after adult ingestions of as little as 200 mg. The more water-soluble salts (eg, thallous acetate and thallic chloride) are slightly more toxic than the less soluble forms (thallic oxide and thallous iodide). Some thallium salts are well absorbed across intact skin.

CLINICAL PRESENTATION

Symptoms do not occur immediately but are typically delayed for 12–14 hours after ingestion.

1. Acute effects include abdominal pain, nausea, vomiting, and diarrhea (sometimes with hemorrhage). Shock may result from massive fluid or blood loss. Within 2–3 days, delirium, seizures, respiratory failure, and death may occur.

2. Chronic effects include painful peripheral neuropathy, myopathy, chorea, stomatitis, and ophthalmoplegia. Hair loss and nail dystrophy (Mees lines) may appear after 2–4 weeks.

DIAGNOSIS

Thallotoxicosis should be considered when gastroenteritis and painful paresthesia are followed by alopecia.

1. Specific levels. Urinary thallium is normally less than 0.8 mcg/L. Concentrations higher than 20 mcg/L provide evidence of excessive exposure and may be associated with subclinical toxicity during workplace exposures. Blood thallium levels are not considered reliable measures of exposure except after large exposures. Hair levels are of limited value, used mainly in documenting past exposure and in forensic cases.

2. Other useful laboratory studies include CBC, electrolytes, glucose, BUN, creatinine, and hepatic aminotransferases. Because thallium is radiopaque, plain abdominal radiographs may be useful after acute ingestion.

TREATMENT

1. Emergency and supportive measures

   1. Maintain an open airway and assist ventilation if necessary.

   2. Treat seizures and coma if they occur.

   3. Treat gastroenteritis with aggressive IV replacement of fluids (and blood if needed). Use pressors only if shock does not respond to fluid therapy.

2. Specific drugs and antidotes. There is currently no recommended specific treatment in the United States.

   1. Prussian blue (ferric ferrocyanide, Radiogardase;) is the mainstay of therapy in Europe and received FDA approval for use in the United States in 2003. This compound has a crystal lattice structure that binds thallium ions and interrupts ...