Prolonged Response to Succinylcholine

Abstract

Some of the genetic variants of human plasma cholinesterase (butyrylcholinesterase, pseudo-cholinesterase) with reduced affinities for succinylcholine (the atypical and fluoride-resistant variants) are recognized by their response to differential inhibitors such as dibucaine and fluoride, while others (the silent variants) are detected as a complete or nearly complete absence of plasma cholinesterase activity [1,2]. Because of technical difficulties in assaying succinylcholine hydrolysis, benzoylcholine, butyryl-thiocholine, or other thioesters are usually employed as surrogate substrates in these tests. Reports of prolonged response to succinylcholine in patients whose laboratory results indicate normal plasma cholinesterase activity and type have long suggested the existence of one or more variants of this enzyme that have a decreased capacity to hydrolyze succinylcholine but a normal ability to hydrolyze the conventionally used surrogate substrates and a normal resistance to the conventionally used differential inhibitors [3-5]. Such a variant is reported here. Case Report A 39-yr-old, 65-kg female with a history of incompetent cervix was admitted at the 16th week of pregnancy to our Ambulatory Surgery Unit for a cerclage. She had no significant past medical history and a negative review of systems. Physical examination, vital signs, and laboratory values were unremarkable. She was taking no medications except for prenatal vitamins. The patient insisted on general anesthesia. Because she recalled being told by her anesthesiologist after a prior cerclage that it took her a "little longer to recover" from succinylcholine than normal, the amount of this relaxant given intravenously to facilitate tracheal intubation was reduced to 60 mg, half our customary dose. After the administration of succinylcholine, neuromuscular block, monitored with a peripheral nerve stimulator, was complete for 25 min and required an additional 5-10 min to return to baseline. The patient's plasma was sent to a commercial clinical laboratory (SmithKline Beecham, New York, NY) for cholinesterase testing. The reported dibucaine number, 76, and cholinesterase activity, 2.1 U/mL, were both within the normal range for this laboratory (dibucaine number, 73-90; cholinesterase activity, 1.7-7.4 U/mL). Because of the disparity between the patient's increased sensitivity to succinylcholine and the laboratory results, a sample of her blood was sent to the research laboratory of the Department of Anesthesiology of the University of Michigan Medical School for a more detailed analysis. At this laboratory, the standard phenotyping methods, using benzoylcholine as substrate and dibucaine and fluoride as inhibitors, indicated that the patient was a usual/fluoride-resistant heterozygote Table 1. However, this phenotype is not associated with an increase in duration of response to succinylcholine [6]. The patient's plasma cholinesterase activity was then assayed using succinylcholine as the substrate, by a modification of the method of Artiss et al. [7,8], and was found to be much lower than would be expected in a usual/fluoride-resistant heterozygote Table 2. The ratio of plasma cholinesterase activity determined with succinylcholine to that determined with benzoylcholine was also much lower than would be anticipated in an individual of this phenotype Table 2. These findings suggested that the patient's plasma contained a cholinesterase variant that was not detected by the standard phenotyping methods.Table 1: Dibucaine and Fluoride Numbers and Phenotype of Members of the PedigreeTable 2: Plasma Cholinesterase Activities of Members of the PedigreeBlood was obtained from the patient's father, mother, and daughter. Their dibucaine numbers, fluoride numbers, and phenotypes are shown in Table 1; their plasma cholinesterase activities, determined with benzoylcholine and with succinylcholine as substrates, are shown in Table 2. The patient's father, phenotyped as a usual homozygote, had a plasma cholinesterase activity that was within the normal range when benzoylcholine was used as substrate, but that was much lower than would be expected in an individual of this phenotype when succinylcholine was used as substrate Table 2. In addition, the ratio of plasma cholinesterase activity determined with succinylcholine to that determined with benzoylcholine was much lower than would be expected in an individual of this phenotype Table 2. Again, these findings suggested the presence of a cholinesterase variant with a decreased affinity for succinylcholine that was not detected by standard phenotyping methods. The patient's mother and daughter were both phenotyped as usual/fluoride-resistant heterozygotes and had plasma cholinesterase activities determined with both benzoylcholine and succinylcholine that were consistent with this phenotype Table 1 and Table 2. Desoxyribonucleic acid was isolated from the patient's leukocytes, and the three coding exons of the plasma cholinesterase gene were sequenced [1,2]. This revealed two point mutations: one characteristic of the fluoride 2 variant, nucleotide 1169 GGT right arrow GTT leading to the substitution of valine for glycine in the enzyme [9], and a new mutation, nucleotide 551 GCC right arrow GTC leading to the substitution of valine for alanine. Desoxyribonucleic acid sequencing of the other family members showed that the mutations were carried on different plasma cholinesterase chromosomal strands. The gene allele for the new cholinesterase variant has the formal name BCHE*184V. BCHE designates the butyrylcholinesterase gene. The * separates the name of the gene from information characterizing the allele. The letter V, the single-letter amino acid symbol for valine, combined with the number 184 indicates a mutation resulting in the substitution of valine for the 184th amino acid from the N terminal of the enzyme. We have given the new cholinesterase variant the trivial name SC-variant. The genotypes of the patient, her parents, and her daughter are shown in Figure 1.Figure 1: Genotypes of members of the pedigree: right arrow indicates patient; open square designates male; open circle designates female. U = usual; F2 = fluoride 2-resistant; SC = SC-variant.Discussion The traditional tests that have been used for the past 30 yr to determine plasma cholinesterase phenotype leave many cases of prolonged response to succinyl-choline unexplained [3-5]. Plasma cholinesterase variants with a reduced affinity for succinylcholine can be missed through reliance on surrogate substrates. The new variant described in this report is detected only by using succinylcholine as the substrate for determining plasma cholinesterase activity. For screening plasma for variants of cholinesterase that have a reduced affinity for succinylcholine, this method for phenotyping should be added to those that are commonly used. The techniques of molecular genetics permit precise identification of plasma cholinesterase variants and should now be used in addition to phenotyping methods to characterize plasma cholinesterase from patients who have an abnormal duration of response to succinylcholine or mivacurium. The alteration in the structure of the SC enzyme molecule decreases its ability to hydrolyze succinylcholine to a level comparable to that of the fluoride 2-resistant variant Table 2. We have not yet detected a patient homozygous for the SC-variant, but it is likely that such an individual would have a response to succinylcholine similar to that of the patient described in this report. Note: The Research Laboratory of the Department of Anesthesiology at the University of Michigan will analyze without charge blood samples from individuals who have displayed either resistance or prolonged response to succinylcholine or mivacurium. For further information, contact Bert N. La Du, MD, PhD, Department of Anesthesiology, University of Michigan Medical Center, Ann Arbor, MI 48109-0572; telephone: [phone].