Diagnosis Of Mercury Poisoning

A history of exposure to mercury, either industrial or environmental, is obviously valuable in making the diagnosis of mercury poisoning. Otherwise, clinical suspicions can be confirmed by laboratory analysis. The upper limit of a nontoxic concentration of mercury in blood generally is considered to be 3-4 mg/dL (0.15-0.20 mM). A concentration of mercury in blood >4 mg/dL (0.20 mM) is unexpected in normal, healthy adults and suggests the need for environmental evaluation and medical examination to assess the possibility of adverse health effects. Because methylmercury is concentrated in erythrocytes and inorganic mercury is not, the distribution of total mercury between red blood cells and plasma may indicate whether the patient has been poisoned with inorganic or organic mercury. Measurement of total mercury in red blood cells gives a better estimate of the body burden of methylmercury than it does for inorganic mercury. Concentrations of mercury in plasma provide a better index of the body burden of inorganic mercury, but the relationship between body burden and the concentration of inorganic mercury in plasma is not well documented. This may relate to the importance of timing of measurement of the blood sample relative to the last exposure to mercury. The relationship between the concentration of inorganic mercury in blood and toxicity also depends on the form of exposure. For example, exposure to vapor results in concentrations in brain approximately 10 times higher than those that follow an equivalent dose of inorganic mercuric salts.

The concentration of mercury in the urine also has been used as a measure of the body burden of the metal. The normal upper limit for excretion of mercury in urine is 5 mg/L. There is a linear relationship between plasma concentration and urinary excretion of mercury after exposure to vapor; in contrast, the excretion of mercury in urine is a poor indicator of the amount of methylmercury in the blood because methylmercury is eliminated mainly in feces.

Table 65-1

Frequency of Symptoms of Methylmercury Poisoning in Relation to Concentration of Mercury in Blood

Cases with Symptoms (%)

mg/mL (mM)

Paresthesias

Ataxia

Visual Defects

Dysarthria

Hearing Defects

Death

0.1-0.5 (0.5-2.5)

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