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Cholinesterases are enzymes that hydrolyze the acetylcholine released at central and peripheral sites. Two types of cholinesterase are present; acetylcholinesterase, or true cholinesterase, and pseudocholinesterase. Acetylcholinesterase is found in large amounts in red cells and neurons. Pseudocholinesterase is present in serum, plasma, liver, pancreas, heart, and other tissues. Both plasma and tissue cholinesterase activity are markedly inhibited by organophosphate pesticides and herbicides, such as diazinon and malathion. Acetylcholinesterase is theoretically the best enzyme to follow because its presence in red cells more accurately reflects enzyme concentration in the central nervous system. However, serum pseudocholinesterase can be measured more easily and is suppressed enough to be useful in diagnosis of acute organophosphate toxicity. Pseudochoinesterase levels correlate fairly well with cholinergic signs. Headache and fatigue are common in mild poisoning in which pseudocholinesterase activity is inhibited 20 to 50%. Weakness, muscle fasciculation, and difficulty talking are symptomatic of moderate poisoning, in which pseudocholinesterase activity is reduced 80 to 90%. In severe poisoning, when enzyme activity is inhibited more than 90%, patients present with coma, flaccid paralysis, and cyanosis. Both plasma and red cell enzyme levels should be measured when monitoring chronic exposure to low level organophosphate chemicals, especially in high-risk workers. Pregnancy and liver disease can also cause a decrease in cholinesterase activity. Caution must be exercised when diagnosing pesticide or herbicide intoxication in the presence of these conditions. The major use of serum cholinesterase activity is to evaluate the effect of succinylcholine, a muscle relaxant used in surgery. Normally, cholinesterase rapidly hydrolyzes succinylcholine. However, some patients have an "atypical" form of serum cholinesterase that does not hydrolyze succinylcholine efficiently. Patients that are homozygous for the "atypical" cholinesterase gene have extremely low levels of cholinesterase activity. These patients may develop prolonged muscle relaxation and apnea after the administration of succinylcholine. Reference ranges are:
One lavender top (EDTA) tube of blood is required. |
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