Module 3: Toxicology - Section 12: Heavy Metals

OHM12.X: Mercury - Part 12

E. Clinical toxicity

It is believed that metallic mercury is oxidized to mercuric ion (Hg++) and that toxicity results from the action of this ion in precipitating protein and in inhibiting enzymes that contain sulfhydryl groups. This explains why mercury vapor and other mercury compounds show similar patterns of toxicity. The preferential CNS toxicity of metallic mercury may reflect that it is taken up by the brain before it is metabolised to mercuric ion, whereas mercury salts do not cross the blood brain barrier. Mercurous (Hg++) compounds are considerably less toxic than mercuric (Hg+++) compounds. The inhibition of enzymes by mercury is reversible upon removal of the mercury and this forms the basis of chelation therapy.

The classical syndrome of subacute and chronic mercury poisoning in industry has three features: Inflammation of the mouth, muscle tremors, and psychic irritability. Any combination of these features may be present. The oral symptoms may include gingivitis, loosening of the teeth, swelling of the salivary glands, excessive salivation, and stomatitis. In individuals who have poor oral hygiene, pyorrhoea may develop and a blue-gray line of mercury sulfide may be visible along the gum margins. Muscle tremors may develop slowly, affecting the eyelids, tongue, and fingers first. It may worsen to include the arms and legs, making walking difficult. The tremors in the fingers interfere with writing, giving it a characteristic jerky appearance. The tremor increases with efforts to control it, decreases with relaxation or rest, and is absent during sleep. In this respect it is similar to Parkinson’s disease. Removal from exposure usually results in recovery from the tremor.

The peculiar pattern of psychic irritability (called erethism) is the most intriguing characteristic of mercury poisoning. Workers become timid and anxious, shrink from observation, become discouraged, lose self-confidence, are easily embarrassed, and become fearful of criticism or of losing their jobs. They also become irritable and may have extreme outburst of anger if they are given orders or if their orders are not followed. Insomnia, nightmares, and excessive drowsiness during waking hours are also common.

Skin lesions may be caused by contact with mercury compounds but are more commonly associated with organic mercurials (especially mercury fulminate) than inorganic compounds. Pruritic rashes with erythema, edema, papules, pustules and vesicles have been described. Deep ulcers may result from penetration of skin abrasions.

Renal effects have not been well characterized, although proteinuria is common among heavily exposed workers. There is evidence that the kidney can eliminate low levels of mercury for long periods without damage. Individual cases of nephrosis, anuria, and renal failure have been reported after ingestion of mercury bichloride (HgCl₂), but this is not the form of poisoning seen in industry.

Workers exposed to mercury have also demonstrated abnormal patterns of lactate dehydrogenase (LDH), a sulfhydryl dependent enzyme, in which isoenzymes 12 and 5 were elevated, consistent with liver and kidney dysfunction.

Postgraduate Diploma in Occupational Health (DOH) - Modules 3: Occupational Medicine & Toxicology (Basic) by Profs Mohamed Jeebhay and Rodney Ehrlich, Health Sciences UCT is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License. Major contributors: Mohamed Jeebhay, Rodney Ehrlich, Jonny Myers, Leslie London, Sophie Kisting, Rajen Naidoo, Saloshni Naidoo. Source available from here. For any updates to the material, or more permissions beyond the scope of this license, please email healthoer@uct.ac.za or visit www.healthedu.uct.ac.za. Last updated Jan 2007.
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