Toxic compounds – Thallium sulphate and thallium acetate are usually responsible for poisoning.
Uses – As rodenticidal agent, cosmetic (as depilatory), in industries.
Absorption, fate and excretion – Through skin and mucus membrane of G.I. tract and respiratory tract. As a cumulative poison it is deposited in liver, kidneys, muscles and bones. It is deposited in maximum concentration in the epididymis. Excretion is through kidneys mainly. It is also excreted through milk. Thallium can pass through the placental barrier.
Fatal dose – Adult – 200 mgm to 1 gm. Children – about 8 mgm per kg. of body weight.
Fatal period – Variable.
Signs and symptoms – In acute poisoning, signs and symptoms start late, between 12 – 36 hours, though in some cases it may be early. There is G.I. tract irritation with metallic taste in the mouth, nausea, vomiting, anorexia, dryness of mouth, colic, diarrhoea or constipation. There may be respiratory distress, conjunctivitis and in some, running from the nose. Neurological symptoms start with signs of polyneuritis, tingling and pain sensation in hands and feet, muscular weakness with paralysis of some muscles, within a few days. In some cases, there may be tremor and ataxia. There may be insomnia, deafness and scotoma with ultimate blindness. Within some days, there may also be stomatitis. There is loss of scalp hair, eye brows and axillary hair. In subacute cases there are encephalopathy, white stripes across the nails, loss of nails. In chronic exposure, these symptoms appear in milder forms. In fatal cases, death is preceded by delirium, convulsion and coma.
Eosinophilia is a common phenomenon. Symptoms occur at over 40 µg% in blood and 150 µg per litre in urine.
Treatment – Though usually there is vomiting, there may be necessity for induction of emesis or gastric lavage. Stomach wash is performed with 1% sodium or potassium iodide solution. It forms insoluble iodide salts of thallium. Iodide also acts as a systemic antidote. Absorption can also be prevented by administration of activated charcoal, followed by saline purgative. Sodium diethyldithiocarbamate in the dose of 25 mg/kg body weight in 1/2 litre of 5% glucose saline is given by I.V. drip over a period of 5 – 6 hours, repeated for 3 more doses, once daily. Pilocarpine in its usual dose is also considered as a physiological antidote. BAL is also useful. In case of tremor, anti-parkinsonism drug is helpful. Stimulants, dextrose and calcium salts are other drugs which are used according to necessity.
Postmortem findings – Externally, there are loss of hair and anaemia. Internally, there is fatty degeneration of liver and kidneys. Thallium is retained in body for years.
Medicolegal aspects – Poisoning is mostly accidental and chronic due to exposure in industries or use of cosmetic preparation or acute due to consumption of preparations used for rat killing purpose. Suicidal, poisoning occurs, but is comparatively less now-a-days. Homicidal poisoning is not common. One advantage of homicidal use is, late starting of symptoms. But greatest disadvantage is its detectability from body, years after death.