Pseudocholinesterase Deficiency

Practice Essentials

Pseudocholinesterase deficiency is an inherited enzyme abnormality that results in abnormally slow metabolic degradation of exogenous choline ester drugs such as succinylcholine and mivacurium. If there is a  deficiency in the plasma activity of pseudocholinesterase, prolonged muscular paralysis may occur, resulting in the extended need for mechanical ventilation.A variety of pathologic conditions, physiologic alterations, and medications also can lower plasma pseudocholinesterase activity.^{[1, 2, 3, 4, 5, 6, 7]}

A personal or family history of an adverse drug reaction to one of the choline ester compounds, such as succinylcholine, mivacurium, or cocaine, may be the only clue suggesting pseudocholinesterase deficiency. Most clinically significant causes of pseudocholinesterase deficiency are due to one or more inherited abnormal alleles that code for the synthesis of the enzyme.

This condition is recognized most often when respiratory paralysis unexpectedly persists for a prolonged period of time following administration of standard doses of succinylcholine.^[8] The mainstay of treatment in these cases is ventilatory support until diffusion of succinylcholine from the myoneural junction permits return of neuromuscular function of skeletal muscle. The diagnosis is confirmed by a laboratory assay demonstrating decreased plasma cholinesterase enzyme activity.

Genetic analysis may demonstrate a number of allelic mutations in the pseudocholinesterase gene, including point mutations resulting in abnormal enzyme structure and function and frameshift or stop codon mutations resulting in absent enzyme synthesis. Partial deficiencies in inherited pseudocholinesterase enzyme activity may be clinically insignificant unless accompanied by a concomitant acquired cause of pseudocholinesterase deficiency. Clinically significant effects  are generally not observed until the plasma cholinesterase activity is reduced to less than 75% of normal.^[2]  Pseudocholinesterase deficiency is most common in people of European descent; it is rare in Asians.

Prognosis for recovery following administration of succinylcholine is excellent when medical support includes close monitoring and respiratory support measures. In nonmedical settings in which individuals with pseudocholinesterase deficiency are exposed to cocaine, sudden cardiac death can occur.

The main complication resulting from pseudocholinesterase deficiency is the possibility of respiratory failure secondary to succinylcholine or mivacurium-induced neuromuscular paralysis. Individuals with pseudocholinesterase deficiency may also be at increased risk of toxic reactions, including sudden cardiac death, associated with recreational use of cocaine. Patients with known pseudocholinesterase deficiency may wear a medic-alert bracelet that will notify healthcare workers of increased risk from administration of succinylcholine. These patients also may notify others in their family who may be at risk for carrying one or more abnormal pseudocholinesterase gene alleles.

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Pathophysiology

Pseudocholinesterase is a glycoprotein enzyme, produced by the liver, circulating in the plasma. It specifically hydrolyzes exogenous choline esters; however, it has no known physiologic function.

Pseudocholinesterase deficiency results in delayed metabolism of only a few compounds of clinical significance, including the following: succinylcholine, mivacurium, procaine, and cocaine.^[9] Of these, its most clinically important substrate is the depolarizing neuromuscular blocking agent, succinylcholine, which the pseudocholinesterase enzyme hydrolyzes to succinylmonocholine and then to succinic acid.

In individuals with normal plasma levels of normally functioning pseudocholinesterase enzyme, hydrolysis and inactivation of approximately 90-95% of an intravenous dose of succinylcholine occurs before it reaches the neuromuscular junction. The remaining 5-10% of the succinylcholine dose acts as an acetylcholine receptor agonist at the neuromuscular junction, causing prolonged depolarization of the postsynaptic junction of the motor-end plate. This depolarization initially triggers fasciculation of skeletal muscle. As a result of prolonged depolarization, endogenous acetylcholine released from the presynaptic membrane of the motor neuron does not produce any additional change in membrane potential after binding to its receptor on the myocyte. Flaccid paralysis of skeletal muscles develops within 1 minute.

In normal persons, skeletal muscle function returns to normal approximately 5 minutes after a single bolus injection of succinylcholine as it passively diffuses away from the neuromuscular junction. Pseudocholinesterase deficiency can result in higher levels of intact succinylcholine molecules reaching receptors in the neuromuscular junction, causing the duration of paralytic effect to continue for as long as 8 hours.

This condition is recognized clinically when paralysis of the respiratory and other skeletal muscles fails to spontaneously resolve after succinylcholine is administered as an adjunctive paralytic agent during anesthesia procedures.

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Causes

A personal or family history of an adverse drug reaction to one of the choline ester compounds, such as succinylcholine, mivacurium, or cocaine, may be the only clue suggesting pseudocholinesterase deficiency. Most clinically significant causes of pseudocholinesterase deficiency are due to one or more inherited abnormal alleles that code for the synthesis of the enzyme. These abnormal alleles may result in a failure to produce normal amounts of the enzyme or in production of abnormal forms of pseudocholinesterase with altered structure and lacking full enzymatic function, as described below.

Patients with only partial deficiencies of inherited pseudocholinesterase enzyme activity often do not manifest clinically significant prolongation of paralysis following administration of succinylcholine unless a concomitant acquired cause of pseudocholinesterase deficiency is present. The acquired causes of pseudocholinesterase deficiency include a variety of physiologic conditions, pathologic states, and medications listed below.

Inherited causes

The gene that codes for the pseudocholinesterase enzyme is located at the E1 locus on the long arm of chromosome 3, and 96% of the population is homozygous for the normal pseudocholinesterase genotype, which is designated as EuEu. The remaining 4% of the population carry one or more of the following atypical gene alleles (See Table 1, below) for the pseudocholinesterase gene in either a heterozygous or homozygous fashion.

Table 1. Atypical Gene Alleles for the Pseudocholinesterase Genotype

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In individuals with an inherited form of pseudocholinesterase deficiency, only a single atypical allele is carried in a heterozygous fashion, resulting in a partial deficiency in enzyme activity, which manifests as a slightly prolonged duration of paralysis, longer than 5 minutes but shorter than 1 hour, following administration of succinylcholine. Less than 0.1% of the general population carries 2 pseudocholinesterase gene allele mutations that will produce clinically significant effects from succinylcholine lasting longer than 1 hour.

One rare variant allele of the pseudocholinesterase gene, designated the C5 variant, actually has higher than normal enzyme activity, resulting in relative resistance to the paralytic effects of succinylcholine.

The dibucaine-resistant genetic variant form of pseudocholinesterase is identified by the percent inhibition of hydrolysis of benzyl choline caused by the addition of dibucaine to the pseudocholinesterase enzymatic assay. The dibucaine number is the percent inhibition of hydrolysis of benzyl choline by dibucaine added to the plasma sample. The normal dibucaine number for the homozygous typical genotype (EuEu) is 80%. Individuals homozygous for the atypical dibucaine resistant genotype (EaEa) have a dibucaine number of 20%, which correlates with a marked prolongation of the paralytic effect of standard doses of succinylcholine to well over 1-hour duration. Heterozygotes (EuEa) have intermediate dibucaine numbers and modest prolongation of muscle paralysis with succinylcholine. The EuEa heterozygous genotype is found in 2.5% of the general population, making it more common than all other abnormal pseudocholinesterase genotypes combined.

The fluoride-resistant pseudocholinesterase enzyme variant is identified by its percent inhibition of benzyl choline hydrolysis when fluoride is added to the assay. The fluoride number (percentage inhibition of enzyme activity in the presence of fluoride) is 60% for the EuEu genotype and is 36% for the EfEf genotype. This homozygous fluoride-resistant genotype exhibits mild to moderate prolongation of succinylcholine-induced paralysis. The heterozygous fluoride-resistant genotype usually is clinically insignificant unless accompanied by a second abnormal allele or by a coexisting acquired cause of pseudocholinesterase deficiency.

The most severe form of inherited pseudocholinesterase deficiency occurs in only 1 in 100,000 individuals who are homozygous for the silent Es genotype, with no detectible pseudocholinesterase enzyme activity. These individuals may exhibit prolonged muscle paralysis for as long as 8 hours following a single dose of succinylcholine. Gene mutations that produce silent alleles are caused by frameshift or stop codon mutations, resulting in no functional pseudocholinesterase enzyme synthesis.

Prolonged paralysis due to pseudocholinesterase deficiency has been reported after succinylcholine administration for emergent cesarean section. Abnormal pseudocholinesterase enzyme variants can be present that are undetectable with standard laboratory tests.^[10]

Acquired causes

Neonates, elderly individuals, and pregnant women with certain physiologic conditions may have lower plasma pseudocholinesterase activity.^{[4, 5]}

Pathologic conditions that may lower plasma pseudocholinesterase activity include the following:

• Chronic infections (tuberculosis)
• Extensive burn injuries
• Liver disease
• Malignancy^[11]
• Malnutrition
• Organophosphate pesticide poisoning
• Uremia

One study recommended estimation of the pseudocholinesterase level to classify the severity of organophosphorous poisoning and to estimate prognosis. Pseudocholinesterase levels were reduced in all the cases in this study (N = 70), with the mean level being 3,154.16 ± 2,562.40 IU/L.^[12]

Iatrogenic causes

Iatrogenic causes of lower plasma pseudocholinesterase activity include plasmapheresis and medications such as the following:

• Anticholinesterase inhibitors
• Bambuterol
• Chlorpromazine
• Contraceptives
• Cyclophosphamide
• Echothiophate eye drops
• Esmolol
• Glucocorticoids
• Hexafluorenium
• Metoclopramide
• Monoamine oxidase inhibitors
• Pancuronium
• Phenelzine
• Tetrahydroaminacrine

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Laboratory Studies

Diagnosis of pseudocholinesterase deficiency is made by plasma assays of pseudocholinesterase enzyme activity. A sample of the patient's plasma is incubated with the substrate butyrylthiocholine along with the indicator chemical 5,5'-dithiobis-(2-nitrobenzoic acid), which will produce a colored product that is assayed using spectrophotometry. The resulting amount of spectrophotometric absorption is proportionate to the pseudocholinesterase enzyme activity that is present in the patient's plasma sample.^{[9, 13]}

Because succinylcholine metabolites can interfere with this assay, plasma samples should be collected after muscle paralysis has completely resolved. The dibucaine and fluoride numbers can be determined by repeating this assay in the presence of standard aliquots of either dibucaine (0.03 mmol/L) or fluoride (4 mmol/L) in the reaction mixture to determine the percent inhibition of enzyme activity caused by these agents.

A simplified screening test of pseudocholinesterase enzyme activity can be performed using the Acholest Test Paper (See Table 2, below). When a drop of the patient's plasma is applied to the substrate-impregnated test paper, a colorimetric reaction occurs. The time it takes the exposed Acholest Test Paper to turn from green to yellow is inversely proportional to the pseudocholinesterase enzyme activity in the plasma sample.

Table 2. Reaction Times for Acholest Test Paper

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The complete DNA sequence and amino acid structure of both the normal pseudocholinesterase protein and most of its abnormal variants have now been identified. However, molecular genetic techniques such as polymerase chain reaction (PCR) amplification with allele-specific oligonucleotide probes for identifying abnormal pseudocholinesterase genotypes are currently available only in a limited number of research laboratories and are not in routine clinical use.

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Medical Care

Pseudocholinesterase deficiency is a clinically silent condition in individuals who are not exposed to exogenous sources of choline esters.

Patients with prolonged paralysis following administration of succinylcholine can be treated in the following ways:

• Prophylactic transfusion of fresh frozen plasma can augment the patient's endogenous plasma pseudocholinesterase activity. This practice is not recommended because of the risk of iatrogenic viral infectious complications. However, perioperative transfusion of fresh frozen plasma administered to correct a coagulopathy may mask an underlying pseudocholinesterase deficiency.
• Mechanical ventilatory support is the mainstay of treatment until respiratory muscle paralysis spontaneously resolves. Recovery eventually occurs as a result of passive diffusion of succinylcholine away from the neuromuscular junction.
• Administration of cholinesterase inhibitors, such as neostigmine, is controversial for reversing succinylcholine-related apnea in patients who are pseudocholinesterase deficient. The effects may be transient, possibly followed by intensified neuromuscular blockade.

Consultation with a geneticist may help to identify the specific atypical genotype alleles contributing to pseudocholinesterase deficiency.

Because the DNA sequence of the pseudocholinesterase gene and its amino acid structure is known, atypical alleles now can be identified by PCR amplification studies using DNA extracted from leukocytes in a blood sample.

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