Postoperative nausea and vomiting (PONV) is the most common complication of general anaesthesia in surgical procedures and occurs in up to 30% of unselected hospitalised patients including patients receiving antiemetic therapy.1,2 Pharmacological prevention and treatment decreases the incidence of PONV by about 30%, but with possible side effects.3,4 In addition, nonpharmacological interventions such as acustimulation lead to a relative risk reduction of up to 30%, especially in high-risk patients.5,6 Conventional risk factors for PONV are a history of PONV or motion sickness, female sex, nonsmoking status, age, the use of volatile anaesthetics and the use of intraoperative or postoperative opioids.7 However, these factors may not fully explain the individual risk potential and it is assumed that PONV, which is caused by the physiological stress of surgery and/or the administration of anaesthetic agents, is modulated by genetic influences.8 Thus, genetic variation may be associated with an increased risk of PONV.
The dopamine type 2 receptor (DRD2) appears to be the cause of PONV because the dopamine receptor antagonists metoclopramide and droperidol are successful in the prevention and treatment of PONV.2 In this context, the DRD2 TaqIA polymorphism has been demonstrated to affect human habits related to dopaminergic neurotransmission.9,10 Moreover, in a Japanese cohort, an association (relative risk of PONV 1.58 for homozygous A2A2 genotypes compared with A1 alleles) with the occurrence of early PONV was shown.11 Until now, however, the potential role of DRD2 TaqIA polymorphism in susceptibility to PONV has not been studied in whites.
Accordingly, we examined the association between DRD2 TaqIA polymorphism and the occurrence of PONV in a white cohort undergoing strabismus repair surgery, which is associated with a higher risk for developing PONV.12,13 We used a standardised anaesthesia regimen and adjusted for conventional risk factors.
Materials and methods
Ethical approval for this study (Ethical Committee No. 08-3686) was provided by the Ethical Committee of the Medical Faculty of the University of Duisburg-Essen, Essen, Germany (Chairperson Prof K.H. Jakobs) on 1 April 2009.
After approval by the local Institutional Review Board and informed written consent from each patient, this single-centre, prospective, double-blind study was performed at the University Hospital Essen. Female and male patients aged 2 to 80 years (quartiles: 2 to 9 years: n = 79; 10 to 21 years: n = 73; 22 to 48 years: n = 77; 49 to 80 years: n = 77) of American Society of Anesthesiologists’ (ASA) physical classes 1 to 3 were eligible for inclusion if scheduled to undergo strabismus surgery requiring general anaesthesia. Patients with neurological or psychiatric disorders were excluded. Demographic and morphometric data as well as risk factors for developing PONV, including PONV and smoking history, were collected preoperatively from the patients’ records and by interviewing patients and/or parents during the consenting procedure.14
Of 510 patients initially screened for eligibility between 2009 and 2012, 185 were ineligible due to meeting exclusion criteria or refusal to participate. Of 325 patients enrolled, 19 could not be assessed due to failed genotyping. Accordingly, data from 306 patients were eventually analysed.
All patients received a standardised anaesthesia regimen. Patients received oral midazolam on the morning of the day of surgery at the discretion of the attending anaesthetist in a dose of 0.2 to 0.5 mg kg−1 if body weight was under 30 kg or 0.1 to 0.5 mg kg−1 if body weight was above 30 kg (maximum dose 15 mg).
General anaesthesia was induced by etomidate 0.3 mg kg−1 intravenously (i.v.), alfentanil 20 to 40 μg kg−1 i.v., mivacurium 0.1 to 0.3 mg kg−1 i.v. was given to facilitate tracheal intubation, and anaesthesia was maintained using an end-tidal concentration of sevoflurane of 1.7 to 2.5% in oxygen/air (30 to 50%) with the end-tidal concentration chosen at the discretion of anaesthetists not involved in the study. Patients younger than 12 years received a paracetamol suppository (125 to 250 mg) 30 min before the end of surgery. For analgesic therapy, rectal paracetamol was repeated in the postanaesthetic care unit (PACU). Rescue medication with piritramide 0.1 mg kg−1 i.v. was administered to any patient who experienced an episode of severe pain in the PACU. In case of PONV in the PACU, rescue medication of granisetron (20 to 40 μg kg−1 i.v.) was administered.
Piritramide administration as well as rescue therapy for PONV was recorded for 24 h in the PACU and then on the ward. Patients were evaluated for the occurrence of nausea, retching, vomiting and pain by an investigator unaware of the patient's genotype, according to recommendations for PONV trials.14–16 Nausea, vomiting and retching were categorised (0, no episode; 1, at least one episode) and incidences were collected at two time points: 6 h for the period 0 to 6 h and 24 h for the period 6 to 24 h after surgery. Nausea or retching/vomiting was defined as at least one episode. Vomiting was defined as expulsion of stomach contents and retching as an involuntary attempt to vomit but not productive of stomach contents. Nausea is a symptom difficult to measure in children and consequently nausea is often ignored or undertreated in children until emesis occurs.17 Therefore, in children younger than 7 years, we used emesis as a common objective outcome for symptoms of either nausea or vomiting.
Buccal mouth swabs were obtained preoperatively from all patients. DNA was extracted using standard DNA extraction techniques (QIAgen, Hilden, Germany). The DRD2 TaqI polymorphism (rs1800497) was genotyped after the period of clinical data collection using the Taqman assay C__7486676_10 (Applied Biosystems, Darmstadt, Germany), as recommended by the manufacturer. The TaqMan SNP Genotyping Assays uses TaqMan 5′ -nuclease chemistry for amplifying and detecting specific polymorphisms in purified genomic DNA samples and takes advantage of minor groove-binding probes for superior allelic discrimination. The SNP Genotyping Assays contain a VIC-dye-labelled probe, FAMT-dye-labelled probe and two target-specific primers. PCR was performed using 20 ng of purified genomic DNA together with a TaqMan Genotyping assay and the genotyping master mix on a 96-well plate on a StepOne Real-Time PCR System (Applied Biosystems, Darmstadt, Germany).
All data are presented as mean ± SD unless otherwise indicated. Parametric variables were compared using an unpaired Student's t-test. Categorical variables were compared using the χ2 test. We used a multivariate binary logistic regression model to calculate odds ratios, 95% confidence intervals (CIs) and P values for the risk of postoperative nausea or retching/vomiting. Logistic regression analysis was performed in a stepwise backward fashion after pooling early and late PONV using age, sex, BMI, anaesthesia duration, postoperative morphine usage, smoking status, history of PONV, history of motion sickness and the DRD2 TaqIA polymorphism as covariates. Differences were regarded as statistically significant with an alpha error P of less than 0.05. All statistical analyses were performed using two-sided tests and the software SPSS, version 21.0 (SPSS, Chicago, Illinois, USA).
The primary endpoint was the incidence of PONV over 24 h. We calculated the required sample size using data from Nakagawa et al.,11 who calculated an odds ratio of 1.58 for homozygous A2 carriers for the development of early PONV in a Japanese cohort. We anticipated a PONV incidence of 30% in our calculation, as predicted.7 Minor allele frequency (MAF) structure analysis in whites revealed an MAF of the A1 allele of 25%.18 Given an alpha error of 0.05, a power of 80% and a MAF of 25%, we calculated a total sample size of 260.
Data from 306 patients were eventually analysed. Genotyping analysis revealed no deviation from the Hardy–Weinberg equilibrium. MAF of the A1 allele was 24%, comparable with data in whites.18 Demographic and morphometric characteristics and factors likely to influence PONV are summarised in Table 1. There was a striking significant association of a history of PONV with TaqIA polymorphism. Although no patient with the A1A1 genotype had a history of PONV, a history of PONV was present in 22.6% of A1A2 carriers and in 10.8% of A2A2 carriers (P = 0.005).
An age-stratified analysis in 154 patients aged 2 to 20 years revealed a history of PONV in 14.3% of A1A2 carriers and 4.3% in A2A2 carriers (P = 0.08). Analysis of 154 patients aged 21 to 80 years revealed a history of PONV in 30.5% of A1A2 carriers and 18.1% of A2A2 carriers (P = 0.035). No further significant genotype-related associations were detected.
The incidence of early (0 to 6 h) nausea was 39.5% and late (6 to 24 h) nausea occurred in 8.5% of patients (Table 2). However, it should be noted that almost all patients who suffered from late nausea had at least one episode in the early postoperative period, resulting in 40.1% of patients suffering from at least one episode of nausea in the total observation time (0 to 24 h). Results stratified by DRD2 TaqIA genotypes are displayed in Table 2. Univariate analysis showed that postoperative nausea was not associated with DRD2 TaqIA genotypes. However, retching/vomiting showed a significant association with genotypes, both for the early postoperative period and for the total observation time. Although no patient with the A1A1 genotype suffered from retching/vomiting, the incidence was 34.8% in A1A2 genotype patients and 34.1% in A2A2 patients, respectively (P = 0.022). Retching/vomiting occurred mainly in the early postoperative period (0 to 6 h), and although 17 patients (5.6%) suffered from late retching/vomiting, there was no new episode of retching/vomiting in the late postoperative period, thus resulting in an overall incidence of 32.7% for retching/vomiting in the early postoperative period as well as for the total postoperative period (0 to 24 h).
Finally, we investigated covariables such as age, sex, body weight, anaesthesia duration, postoperative morphine usage, smoking status, history of PONV, history of motion sickness and the DRD1 TaqIA polymorphisms in a multivariate model, to determine which risk factor or method is most capable of predicting PONV. Using a logistic regression analysis for the occurrence of nausea and retching/vomiting, we found that age, sex, smoking status and past history of PONV were all independent predictors for nausea as well as for retching/vomiting. In contrast, DRD2 TaqIA polymorphism in this multivariate model had no independent significant influence on PONV beyond its striking association with a history of PONV (Table 3).
In patients undergoing ocular muscle surgery, we found that the specific DRD2 TaqIA A2 allele is associated with an increased incidence of PONV, as shown by a striking increase in the past history of PONV and the occurrence of postoperative retching and vomiting, especially in the early postoperative period. Although 22.6% of patients with the A1A2 genotype and 10.8% with the A2A2 genotype had a history of PONV, no patient with the A1A1 genotype presented with such a history. Moreover, no A1A1 patient suffered from postoperative retching and vomiting while its overall incidence was 32.7%. Finally, multivariate analysis confirmed younger age, female sex, no-smoking status and history of PONV as risk factors for developing PONV. However, although the DRD2 TaqIA polymorphism predisposes to PONV, it confers no extra risk beyond the increased incidence of a past medical history of PONV.
The gene cluster ANKK1/DRD2 of chromosome 11q23.2 harbours the TaqIA polymorphism (rs1800497). Its location is within the 3′ untranslated region; therefore, the functional significance is still unclear. However, it may reflect linkage disequilibrium with other functional but yet unidentified polymorphisms that alter DRD2 density.19 In this regard, it has been reported that the A1 allele is associated with decreased dopamine D2 receptor binding and decreased DRD2 density in human brain.20,21
The TaqIA polymorphism lies within the ankyrin repeat and kinase domain containing 1 protein (ANKK1) and was shown to replace an amino acid within an ankyrin repeat of ANKK1, finally potentially affecting substrate-binding specificity.22 In this regard, it has to be determined how the ANKK1 gene harbouring the TaqIA polymorphism influences the DRD2 gene. However, TaqIA polymorphism might affect the DRD2 gene through other polymorphisms. Moreover, a linkage disequilibrium with other functional polymorphisms in DRD2 could play a role. Here, the TaqIA polymorphism was shown to be in linkage disequilibrium with C957T, a synonymous polymorphism in the DRD2 gene.23 The C957T polymorphism was reported to alter mRNA folding and results in decreased mRNA translation and stability. These mRNA changes have been shown to diminish dopamine-induced upregulation of DRD2 expression23 and this mechanism could therefore explain the finding of decreased brain DRD2 density in A1 allele carriers.20,21,24 These data are supported by results from PET indicating altered DRD2 availability affected by TaqIA polymorphism.25 Moreover, different homovanillic acid concentrations in urine, which are dopamine metabolites associated with the TaqIA polymorphism, have been demonstrated,26 suggesting either an indirect or a direct role of TaqIA polymorphism in regulating DRD2 expression.
DRD2 receptors are located in various tissues in the body, including the nuclei tractus solitarii, stomach and the chemotrigger zone. These receptors are believed to be involved in the development of PONV. Although an association between DRD2 TaqIA polymorphism and DRD2 density in those tissues has not been directly demonstrated, carriers of the Taq IA A2 allele may also exhibit a greater DRD2 density in these tissues and therefore may be more sensitive to dopaminergic stimulation, finally resulting in a higher incidence of PONV, as supported by our study. However, when we compare our results with data from Nakagawa et al.,4 there are differences in the phenotype when heterozygous patients are taken into account. Nakagawa et al.4 showed an association between homozygous A2A2 genotypes and the occurrence of PONV (early PONV rate 14.4%), while A1A1 homozygous and A1A2 heterozygous patients were associated with similar early PONV rates of 9.0 and 9.3%, respectively. In our study, A1A1 homozygous patients had less retching/vomiting (0%), whereas A1A2 heterozygous and A2A2 homozygous were associated with similar retching/vomiting rates of 34.8 and 34.1%, respectively. The reason for this discrepancy in heterozygous patients may be caused by different effects of inheritance modalities wherein phenotypes of heterozygous patients may be influenced by other, yet unidentified ethnic-specific mechanisms. In genetic association studies, it is critical to present all genotypes and not only alleles to compare the association effects of each genotype on the observed phenotype.
Logistic regression analysis revealed that TaqIA polymorphism was not a significant independent risk factor for developing PONV on a multivariate level when the past medical history of PONV was accounted for, although the latter was strongly associated with TaqIA polymorphism. Another reason for the discrepancy on univariate analysis may be the low frequency of A1A1 genotypes in whites in comparison to other ethnicities. The frequency of the A1 allele in whites (25%) is much lower than that in a Japanese cohort (38%), wherein a positive association of the A2 allele with PONV was detected on a multivariate level.11 However, considering the onset time of PONV, our data are in line with results by Nakagawa et al.,11 showing a significant association of TaqI polymorphism with PONV in the early postoperative period (0 to 6 h), while late PONV (6 to 24 h) was not associated with TaqI polymorphism.
Our study does have some limitations. First, the age range of our study cohort is very large. However, the aim of the study was to reflect all potential patients without any selection due to age. Second, assessment of nausea in children may be underestimated because nausea may often be ignored in younger children until emesis occurs.27 However, we detected a significant association of the TaqIA polymorphism with vomiting in our study cohort, a condition that could be monitored objectively. Third, due to our study protocol, all children younger than 12 years received a nonsteroidal analgesic intraoperatively as well as in the PACU, while older patients received opioids only as a rescue therapy with the risk of nausea also being a side effect of opioid therapy. Although postoperative morphine usage is associated with a higher incidence of PONV,5 we did not observe a significantly higher incidence of PONV as assessed by multivariate analysis.
In conclusion, in a white cohort, the TaqIA A2 allele is significantly associated with a history of PONV, which may explain the increased incidence of PONV but has no further independent influence. Therefore, our study may help to generate more information about genetic effects of the incidence of PONV and underscores the importance of taking possible confounders into account when performing genetic association studies.
Acknowledgements relating to this article
Assistance with the study: none.
Financial support and sponsorship: none.
Conflict of interest: none.
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