Relationship of COMT Gene Polymorphism, Anxiety, and Pain Perception during Labour

Background: The COMT gene polymorphism is associated with neurological and psychiatric disorders and pain perception. This study aimed to determine the relationship between the COMT (catechol-O-methyltransferase) gene polymorphism, pain perception in parturients with inhaled nitrous oxide during labour, and anxiety as a personality trait. Methods: A total number of 181 woman who chose vaginal birth were enrolled in this study. To present the difference in pain perception, the parturients were divided into one group (n=80) that chose labour analgesia with inhaled nitrous oxide (50% nitrous oxide and 50% oxygen) and one group without analgesia. The blood samples were taken during the pregnancy as a part of routine pregnancy controls in the hospital. The COMT gene polymorphism was detected with the PCR technique. Pain perception of labouring women was self-evaluated two times according to the VAS (Visual Analogue Scale) and anxiety as a personality trait was determined with STAI-T (State Trait Anxiety Inventory). Pain perception as well as anxiety were compared according to COMT genotypes. Results: In the 181 pregnant women there were 40 women (22%) of wild homozygotes (GG) of COMT, 95 women (53%) of mutant heterozygotes (GA), and 46 women (25%) of mutant homozygote (AA). A negative association of pain perception with the GA (mutant heterozigote) polymorphism of the COMT gene versus the wild type (GG polymorphism) was observed. The GA polymorphism of the COMT gene was associated with 0.46 units lower pain perception compared to the wild type (GG). The anxiety trait score in group AA was lower than in groups GA and AA. The difference reached statistical significance only when comparing AA versus GA (P>0.042). Analgesic efficacy of nitrous oxide was noticed in 22% of labouring women who reported moderate pain (VAS score 4-7). Conclusions: The COMT gene polymorphism was associated with pain perception and anxiety among labouring women. The COMT gene polymorphism GA was associated with negative pain perception among labouring women. Nitrous oxide showed statistical significance in anxiolytic efficacy during labour in women with mild anxiety as a personality trait. Anxiolytic efficacy of nitrous oxide has shown better efficacy in labouring women with the COMT gene polymorphism AA.

Keywords:

Subject:

Medicine and Pharmacology - Obstetrics and Gynaecology

1. Introduction

Labour is considered to be one of the most painful experiences in women’s life, and 35 to 58 percent described it as severe or intolerable [1,2]. The perceived pain is the result of a number of different interactions involving psychological and physiological mechanisms. Women cannot predict the severity of pain, which can cause fear and anxiety. Pain in childbirth is primarily a subjective feeling, and its threshold varies from woman to woman. The evaluation of pain perception is based on subjective perception of the parturient, previous pain experiences, and on the psychological condition as well as their expectation regarding the pain and duration of childbirth. Despite that, many women still choose “natural childbirth” and try to delay or avoid epidural analgesia in childbirth as well as caesarean section.

Anxiety, fear, and pain during childbirth are a very common occurrence. They can affect the change in the hormone secretion i.e., adrenocorticotropic hormone, cortisol, catecholamines, and endorphin, and a reduction in pain threshold in labouring women. The deterioration of pain affects emotions [3]. The above phenomena result in a fear-tension-pain syndrome and form a vicious circle for pregnant women. Eventually, these adverse factors lead to complications during childbirth, such as increased dystocia and caesarean section rates [4,5]. Anxiety, as a personality trait measured by STAI-T questionnaire, was used to access how anxiety as personality trait affects the pain perception during labour and birth in labouring women with inhaled analgesia and those who did not receive analgesia.

Inhaled nitrous oxide offers an effective pain relief during labour for many women who opted for “natural birth”. The choice of analgesia should be safe for mother and foetus [6,7]. Nitrous oxide has low blood solubility and anaesthetic potency, but it helps women in labour to cope with anxiety. The low blood solubility of inhaled nitrous oxide allows rapid onset and offset of action with peak brain concentrations at 60 s after inhalation without the risk of overdose [8]. With proper patient instruction, the nitrous oxide can be self-administered with a peak effect at the peak of uterine contraction. The mechanism of action of inhaled nitrous oxide is complex, with inhibition of potassium channels in central nervous system (CNS), the release of endogenous opioids with к-receptor activation, and anxiolysis by gamma-aminobutyric acid (GABA) receptor activation. According to the recently introduced theory it is believed that the inhibition of N-methyl-D-aspartate (NMDA) receptor activity by nitrous oxide is mediating primary analgesic effect [9]. Nitrous oxide elicits anxiolysis and sedation, behavioral responses similar to the effects of benzodiazepines [10]. Animal studies have shown that both nitrous oxide and benzodiazepine-mediated anxiolytic behaviors were equally sensitive to flumazenil-induced antagonism. This finding indicates a possible association between the GABA receptor mechanism of benzodiazepine inhibition and the anxiolytic mechanism of nitrous oxide. It remains unclear how nitrous oxide acts at the molecular level to stimulate the benzodiazepine GABA receptor binding site, but it is hypothesized to be related to a release of endogenous benzodiazepine factors stimulating the receptor [10].

Advances in genetics revealed that subtle changes in various genes encoding for receptors could partly explain the variation in individual differences in the individual sensitivity, perception and expression of pain which may influence the way pain could be treated and managed [11]. Catecholamines (nor-adrenaline, adrenaline, and dopamine) are responsible for pain perception and processing which means that they may decrease and/or increase sensitivity to pain [11]. Degradation of catecholamines is controlled by several enzymes among which catechol-O-methyltransferase (COMT) encoded by COMT gene in humans together with the voltage-gated sodium channels play a significant role in human pain perception and sensation [12,13,14]. COMT enzyme has two major isoforms: membrane (MB-COMT) and soluble (S-COMT) bound [15,16]. According to some data, single-nucleotide polymorphism (SNP) is located in the promoter region of MB-COMT and the second SNP is located in the promoter region of S-COMT, while three SNPs are located in the coding regions of both MB- and S-COMT. SNP rs4680 is nonsynonymous and can cause the substitution of valine (Val) with methionine (Met) at codon 158 (Val158Met) which could cause a three- to four-fold decrease in COMT activity, which may lead to enhanced dopaminergic neurotransmission, with lower endogenous levels of enkephalins and thus exaggerated pain sensitivity [17,18]. Obstetricians and midwives are facing anxiety, fear, and pain during childbirth, and with a deeper genetic understanding, they are challenged by gene polymorphism interaction with pain perception and anxiety in parturients [19]. Some studies have shown that COMT polymorphism could regulate psychological and stress factors by changing the activity of catechol-O-methyltransferase (COMT) and affecting the human perception of pain [20]. COMT gene polymorphism is one genetic factor that determines different endurance and response patterns to pain or other stress [21].

The influence of the COMT polymorphisms (p.158Val/Met, c.472G/A) on inhaled nitrous oxide analgesia during labour and delivery, as well as anxiety as a personality trait, have not been evaluated according to our knowledge. The aim of the study was to investigate the association of the COMT polymorphism and anxiety trait and pain perception in labouring women.

2. Materials and Methods

2.1. Ethical Statement

The Ethics Committee of the General Hospital Zabok and War Veterans Hospital approved this study (No. 97/2). All the women in the study gave written consent.

2.2. Study participants

In the prospective study, 181 healthy pregnant women in the third trimester planning a vaginal birth were enrolled. The women were approached if they met the following criteria: age 18 – 40 years, nulliparous or parous with singleton pregnancy, vertex presentation, 37-41 weeks of gestational age. The exclusion criteria were as follows: multiple pregnancies, cardiovascular or neurological diseases, KOPD, mental or psychiatric disorders, IUGR, oligohydramnios, GDM, hypertension, pathological CTG, and breech presentation. The parturients were divided into two groups according to their personal choice regarding inhaled analgesia with nitrous oxide (n=90) and without it (n=91). Pregnant women were selected according to the above criteria for regular check-ups in the Department of gynaecology in the hospital. This study was conducted in accordance with the Declaration of Helsinki.

2.3. Data Collection

The following parameters were recorded from all parturients: age, parity, gestational age, duration of birth, weight, height, BMI, education as well as the new-born data: gender, birth weight (g), length (cm), Apgar scores in the 1st and 5th minute and umbilical vein pH. To enable maternal and foetal safety, all participants were monitored with the pulse oximetry, heart rate (HR), arterial blood pressure, foetal heart rate (FHR) monitoring, and an obstetrician was present during the study period, from start of applying the analgesia.

2.4. Pain assessment by Visual Analog Scale (VAS)

Visual Analogue Scale (VAS) is used for pain intensity assessment. The range of the pain scale (0-100 mm) is divided into 10 points, from 0 to 10. If the score is higher, pain is more obvious. Numerical values in pain score <3 points represented weak pain, moderate pain to 4-6 points, and severe pain was denoted by 7-10 points. The parturients were explained how to self-evaluate pain strength using VAS questionnaire. Pain intensity was measured in both parturient groups two times during labour using VAS scale. The first time, at the beginning of the active phase of labour defined as: 4-5 cm cervix dilatation and 3 contractions in 10 minutes lasting 1 minute, and the second, referring to the pain during birth which was evaluated after birth. In the group with nitrous oxide, VAS score was measured before applying the analgesia.

2.5. Assessment of the anxiety as a personal trait using State-Trait Anxiety Inventory (STAI-T) questionnaire

STAI – Stait Trait Anxiety Inventory (STAI-T) questionnaire is used for anxiety as a personality trait assessment for adults and consists of 20 claims regarding their general feelings rated from 1 point to 4 points. STAT-T questionnaire filling requirements were explained in detail and the pregnant women filled out a form at the last check-up before labour. If the score is higher, the probability for anxiety is greater. Maximal score is 80 points. If the score range is for 50-59, it indicates mild anxiety, 60-69 is moderate anxiety, and a score higher than 70 represents severe anxiety.

2.6. Assessment of COMT (cathechol-O-methytransferase) gene polymorphism

At the time of study enrolment, venous blood samples (4 mL) for genotyping were taken into Vacuette K3EDTA anticoagulant test tubes (Greiner Bio-One, Austria) and stored at -80 ⁰C. The manufacturer’s protocol for DNA isolation was followed (MagMAXTM DNA Multi-Sample Ultra 2.0 Kit, USA) and performed on King Fisher Flex analyzer, and analyzed by competitive allele-specific PCR (KASP-PCR) on the ABI 7500 analyzer (Applied Biosystems, USA); according to the finished reaction, a signal was read by the end-point method. Specific primers for the COMT polymorphism rs4689 were commercially prepared by LGC Biotechnologies. KASP-PCR method was used for rs4689 COMT genotyping. According to the genotype, the wild type homozygote (Val/Val) was defined as GG, the mutant homozygote (Met/Met) was defined as AA and the mutant heterozygote (Val/Met) was defined as GA.

2.7. Protocol for inhaled nitrous oxide labour analgesia in this investigation

Nitrous oxide (50%/50%) inhaled analgesia was administered at the request of labouring women after regular uterine contractions and when the cervix dilatation was 4-5 cm. Labouring women were educated by the obstetrician and midwives on how to use the inhaled gas mixture by themselves. Analgesia was applied with inhaling the gas mixture with normal breathing through a facial mask. Inhaling opens the demand valve for gas inflow and with exhaling, the demand valve is closed. To achieve successful analgesic, anxiolytic, and relaxing action of nitrous oxide, breathing has to be synchronised with labour dynamics. A labouring woman has to start inhaling 20 to 30 seconds prior to the beginning of contraction. Inhalation has to be stopped when the intensity of contraction declines and start again before the new contraction starts. Nitrous oxide was not used in the expulsion phase of labour.

2.8. Statistical Analysis

The difference between two or more groups of parturients was tested for categorical variables using Pearson's chi-square test or Fisher's test if any expected frequency in the 2 x 2 (or r x c) table was less than 5. For continuous variables, the difference between two groups of parturients was tested using the Student's t-test for a variable with a normal distribution (birth weight of the child) or the Mann-Whitney U test for variables with a distribution that deviated from the normal (other continuous variables). Differences between the three groups of parturients were tested for continuous variables using analysis of variance (ANOVA) in the case of normally distributed variables or by Kruskal-Wallis ANOVA for variables with a non-normal distribution. In the case of a significant Kruskal-Wallis ANOVA result, post hoc testing of differences between individual groups in the model was performed using the Mann-Whitney U test with Bonferroni correction for multiple comparisons. Parametric ANOVA models did not reach the appropriate level of statistical significance.

The correlation between two continuous variables was examined using the Spearman test considering that only one variable had a normal distribution.

The potential deviation of COMT rs4680 gene polymorphisms from Hardy-Weinberg equilibrium was tested using the Hardy-Weinberg equilibrium test (https://www.stata.com/users/mcleves/genhw/genhw.hlp).

Before conducting the Student's t-test, the homogeneity of variances was tested by Brown and Forsythe's modification of Levene's test (Levene 1960; Brown and Forsythe 1974), and in ANOVA, the homogeneity of variances was tested by Bartlett's test. The normality of the distribution of variables was tested with the Shapiro–Wilk test (Shapiro and Wilk 1965).

The association of the dependent variable (perception of pain during labour) with relevant predictors was examined in multiple linear regression models. In the model with the perception of pain during childbirth as a dependent variable, the following predictors were included: age, height and parity of the mother, sex and weight of the child, level of anxiety, and polymorphism of the COMT rs4680 gene.

In both models, child length was not included among the predictors due to a strong association with child weight (Spearman's rho = 0.73; P < 0.001).

The level of anxiety was included in the regression model as a categorical variable. Given that anxiety was assessed as moderate in only three parturients (<2% of the total number of parturients), they were added to the group of parturients with mild anxiety for the purposes of the regression analysis.

3. Results

3.1. The influence of pain relief with N₂O during delivery on the perception of pain during delivery

Table 1 shows the characteristics of the examined group of parturient women, both for all test subjects together as well as for the subgroups with regard to analgesia with N₂O during childbirth. Statistically significant differences between the women giving birth with and those without analgesia were found in the following categories: age, parity, and perception of pain after childbirth. The group of mothers who received analgesia were, on average, three years younger, with lower parity (in this group, there was a 1.7 times higher frequency of primiparous women, as well as a three times lower frequency of multiparous women who had their third, fourth or fifth birth), and with a lower perception of pain at the end of childbirth compared to the mothers who didn’t get analgesia.

The two groups of women in labour did not differ with regard to their height, the duration of gestation, the sex, weight and length of the child, the level of anxiety, and the frequency of tested COMT gene polymorphisms.

3.1.1. Pain relief with N₂O and the perception of pain at the beginning of delivery

The perception of pain at the beginning of delivery was connected only with the perception of pain at the end of delivery and with the sex of the baby. A positive correlation between the perception of pain at the beginning of delivery and the perception of pain at the end of delivery was observed both in the women who received no analgesia during childbirth (P = 0.001) and in the women who received analgesia (rho = 0.380, P < 0.001). Among those women giving birth with analgesia, the perception of pain at the beginning of delivery was slightly higher in mothers who gave birth to male children (median 4, interquartile range: 3-5) compared to mothers who gave birth to female children (median 3, interquartile range: 2-4) (P = 0.029). Among those giving birth without analgesia, this difference was not observed.

3.1.2. Pain relief with N₂O and the perception of pain at the end of delivery

Although pain relief did not have any effect on the perception of pain at the beginning of delivery, almost all the women who did not receive analgesia reported very strong pain (98%) at the end of delivery, while among those with pain relief 22% reported only moderate pain, and the rest reported very strong pain (78%). Among the women who did not receive pain relief, the perception of pain at the end of delivery was positively correlated with the length of the child (P = 0.013). In women with analgesia, this association was not observed. Other predictors were not statistically significantly related to the perception of pain at the end of delivery.

The influence of pain relief on the perception of pain at the end of delivery was also examined in a linear regression analysis model. Lower pain perception at the end of delivery remained significantly associated with analgesia, with control of potential influencing factors (age, height, and parity of the mother; sex and weight of the baby, the level of anxiety, COMT polymorphism). Pain relief was associated with a 0.92-unit lower pain perception compared to the women in labour without analgesia (95% confidence interval from -1.27 to -0.57). Other predictors included in the analysis did not have significant association with the perception of pain at the end of delivery, with the exception of the GA polymorphism of the COMT gene.

Table 2. Multiple linear regression analysis of pain perception at the end of delivery.

Predictors:	Regression coefficient	95% confidence interval	P
Pain relief	-0.918	-1.27; -0.568	<0.001
Mother’s age	-0.008	-0.045; 0.029	0.662
Mother’s parity
first childbirth	reference category
second childbirth	-0.011	-0.424; 0.401	0.957
third+ childbirth	0.258	-0.312; 0.828	0.373
Mother’s height	-0.007	-0.037; 0.023	0.659
Baby’s weight	6.5*10^-5	-3.410^-4; 4.710^-4	0.755
Male sex of the baby (vs. female)	-0.127	-0.474; 0.220	0.471
Mild anxiety (vs. no anxiety) *	-0.146	-0.555; 0.263	0.481
COMT gene
GG (wild type)	reference category
GA (mutant heterozygote)	-0.460	-0.885; -0.034	0.034
AA (mutant homozygote)	-0.296	-0.788; 0.196	0.236

The results have been considered statistically significant if P<0,05.

3.2. The perception of pain during delivery with regard to COMT polymorphism

Among the 181 pregnant women, there were 40 women (22%) in the GG group, 95 (53%) in the GA group and 46 (25%) in the AA group. The allele distribution of COMT was found to be in line with the Hardy-Weinberg equilibrium.

3.2.1. The perception of pain during delivery with regard to COMT gene polymorphism

Univariate analysis did not show significant differences in the perception of pain during delivery and in the duration of delivery between the mothers with different polymorphisms of the COMT gene, both among all the mothers and in the subgroups of the mothers with regard to N₂O pain relief.

In the multiple linear regression model, with control for pain relief and other potential influencing factors (age, height, and parity of the mother, sex and weight of the baby, the duration of labour, the level of anxiety), however, a negative association of the perception of pain with the GA polymorphism of the COMT gene compared to the wild type (GG polymorphism) was observed. The GA polymorphism of the COMT gene was associated with a 0.46-unit lower perception of pain compared to the mothers with the wild type (GG).

With the exception of analgesia, the other predictors were not significantly associated with the perception of pain at the end of delivery. Also, no statistically significant interactions were found between the effect of pain relief and COMT gene polymorphism on the perception of pain at the end of delivery.

3.3. The relationship between anxiety and the perception of pain during delivery

3.3.1. Anxiety and the perception of pain during delivery

With regard to the perception of pain during delivery, univariate analysis showed a statistically significant difference between the parturient women without anxiety and those with mild anxiety only for the perception of pain at the end of delivery in the women who did not receive N₂O pain relief (Table 3). The medians of these two groups of women, however, did not differ, and the association between anxiety and the perception of pain at the end of delivery was not observed in the multiple regression analysis model controlled for other predictors (age, height and parity of the mother, sex and weight of the baby, analgesia, polymorphism of COMT) (Table 2). Also, no statistically significant interaction was found between analgesia and the level of anxiety in relation to the perception of pain at the end of delivery.

3.3.2. Association of COMT gene polymorphism with anxiety

The level of anxiety differed with respect to the polymorphism of the COMT gene. Among all the mothers, a discrete but statistically significant difference was found in the Kruskal-Wallis ANOVA for the COMT gene polymorphism, but mutual comparisons between the groups (GG vs. GA; GG vs. AA; GA vs. AA) did not reach statistical significance.

In the women with pain relief, however, it was observed that those with the AA polymorphism (mutant homozygote) had 1.5 points less on the STAI-T (State-Trait Anxiety Inventory) anxiety scale compared to the GG polymorphism (wild type), and 2.5 points less compared to the GA polymorphism (mutant heterozygote). The difference reached statistical significance for the comparison of AA with GA polymorphism (Table 4).

4. Discussion

We investigated the association of the COMT polymorphism with anxiety as a personality trait and pain perception in women with inhaled nitrous oxide.

The analgesic effect of nitrous oxide is difficult to quantify properly and uniformly as pain relief. There are also missing well-defined measures of analgesic effectiveness of nitrous oxide according to the systematic review by Rosen [22] and Likis [23]. The authors were unable to draw conclusion on the degree of pain relief afforded by nitrous oxide. There were a few trials where nitrous oxide efficacy measured by VAS was compared with labouring pain with no analgesia performed. Most studies have been focused on the comparison of different types of analgesia and pain perception among it e.g., nitrous oxide vs. epidural.

We observed the effect of analgesia in the group of women during delivery who received analgesia with nitrous oxide at the end of the active phase of delivery. In the parturients with analgesia, 22% rated the pain at the end of delivery as medium (VAS score 4-7), while the other 78% rated the pain as very strong (VAS 8-10). In contrast, 98% of the women who were not to get analgesia with nitrous oxide, rated the pain at the end of delivery as very strong.

Among those who did not receive analgesia, we observed that the perception of pain at the end of the active phase of delivery was positively correlated with the length of the child (P=0.013). The correlation between the baby's length and stronger pain at the end of delivery, found in this study, can be explained by the fact that taller newborns likely have a greater head circumference, which in turn adds to increased pain. Unfortunately, head circumference was not measured in this study, therefore we can only hypothesise that a larger head circumference may contribute to stronger pain at the end of delivery. While among those who received nitrous oxide, this connection was not observed; We can therefore conclude that the effect of analgesia was to that extent effective that the length of the child in the group with inhaled analgesia did not have a statistically significant effect on pain during delivery.

To get the best possible analgesic effect from the use of inhaled nitrous oxide during labour, appropriate patient education and follow-up is needed as found by Talebi et al. who noted that appropriate nitrous oxide application combined with precise patient instructions and coaching during labour reduced VAS score for 20 mm (2 points) compared to the control group [24].

In our study, the analgesic effect of nitrous oxide was associated with a 0.92 unit (9.2 mm) lower pain perception compared to those who did not receive analgesia, which was a worse result than obtained by Talebi et al., which might be explained by the COVID-19 pandemic and less face-to-face contacts of pregnant women with healthcare providers who had to educate the patients online or by phone calls.

In our study, nitrous oxide was applied intermittently at the concentration of 50% and the results were similar to Westling et al. They examined different concentrations of nitrous oxide, 40% and 70%, and 100% oxygen intermittently and 40% nitrous oxide continuously. In that study, it was shown that the analgesic effect depends on the dose; 40% shows a decrease in VAS score by 10 mm with intermittent use, and with continuous use by 18 mm, and with continuous application of 40% nitrous oxide, VAS score was lower by 27 mm [25]. Intermittent application of nitrous oxide requires increased concentration and attention of the parturient regarding the recognition of the upcoming contraction in order to be able to apply the nitrous oxide and expect the next contraction before the onset of pain, which occurs with a time delay of 15 seconds, which is difficult for some parturients. Many comments recorded in study by Richardson et al. where the women on the first day post-delivery who delivered with nitrous oxide analgesia said that analgesia was incomplete, but that patient expectations were met [25]. Many women stated that the technique enhanced their coping with labour by shifting attention and reducing anxiety. A significant minority said that it was consistent with their birth plan (some women noted that nitrous oxide enhanced their “natural childbirth”) [26]. Women who decided for vaginal, “natural” birth, could get an effective alternative to neuraxial analgesia or caesarean section with nitrous oxide [27]. Nitrous oxide offers a pain relief method which is very safe for mother, foetus, and new-born.

Labour analgesia is used for alleviation of labour pain, but the analgesic effect varies among women. Our study showed that the COMT gene polymorphism GA was correlated with the level of anxiety and pain among parturients. In the multiple linear regression model in this study, a negative association of pain perception with GA (mutant heterozygotes) polymorphism of the COMT gene compared to the wild type (GG polymorphism) was observed. The GA polymorphism of the COMT gene was associated with a 0.46-unit lower pain perception compared to the group of parturients with the wild type (GG) of the COMT gene, while the interaction between the analgesic effect of nitrous oxide and the COMT gene polymorphism was not observed.

Studies have shown that genetic factors are closely related to labour anxiety and pain perception. One study has revealed that the activity of COMT is an important regulator of pain sensitivity [28]. Another study has also indicated that the Val158Met mutation of the COMT gene can affect the sensitivity of the body to pain by altering the activity of COMT and thus changing the hormone levels and the psychological state [29]. Ren et al. showed in their study results that the pain score in groups AA and GA was significantly higher than in the GG group. According the Tang ae al., pain score of women with AA genotype was higher than GG and GA [30]. The findings of our study exhibit discrepancies, and it is plausible that in the studies conducted by Ren and Tang, women received analgesia with fentanyl and dexmedetomidine, in contrast to our study, where nitrous oxide was administered and a limited number of participants were involved.

Psychological factors associated with labour pain experience are, for example, coping strategies and expectations [31]. Numerous studies have evaluated psychological factors associated with labour pain but not many studies have evaluated anxiety as a personality trait as a possible predictor to labour pain perception.

In Dabo Pettersson at al. study, women with increased self-rated pain score according the Spilberger Stait-Trait Anxiety Inventory (STAI-T) reported higher self-rated pain prior to labour analgesia compared to women with low STAI-T [32].

This study showed a statistically significant difference (P=0.029) between non-anxious and mildly anxious parturients in the perception of pain at the end of the active phase of delivery, but only in the parturients who received analgesia with nitrous oxide. Due to the positive analgesic effect of nitrous oxide, the degree of anxiety did not affect the perception of pain among the women with analgesia. The obtained results are in accordance with the research of Vallejo et al., in which also no significant difference was observed in the analgesic effect of nitrous oxide in women with low anxiety [33]. In our study, no statistically significant correlation was found between the degree of anxiety and the perception of pain at the beginning of delivery. However, it should be noted that in the study, we had healthy pregnant women without anxiety or with only mild anxiety. The obtained data are in accordance with the data from the literature that suggest a direct connection between the degree of prenatal anxiety and maximum pain during delivery [34]. In addition, pain scorings were collected retrospectively, i.e., within 24 h of delivery, and recall bias may have influenced the pain severity ratings.

The level of anxiety in parturients differed with regard to the polymorphism of the COMT gene, but only in parturients with analgesia. In these pregnant women, it was observed that those with the AA polymorphism had 1.5 points less on the STAI-T anxiety scale compared to the GG polymorphism, and 2.5 points less compared to the GA polymorphism (mutant heterozygote). The difference reached statistical significance for the comparison of AA with GA polymorphism.

In contrast to this study, Tang et al. have shown that polymorphism of the GA COMT gene affects the degree of anxiety and pain before the administration of analgesia during delivery. In the same study, the author compared the effectiveness of dexmedetomide on the three genotypes, and the results showed that there was no significant difference in the degree of anxiety and pain between the GG, GA, and AA polymorphisms, while the GA polymorphism of the COMT gene did not affect the analgesic effect of dexmedetomide in delivery, which is similar to the results we obtained in our study in which we applied nitrous oxide. Xiaohong et al. found in their study that prenatal anxiety was significantly higher in the AA group of women in contrast to those in the GG group [35].

In this study, we investigated the association of the COMT gene polymorphism with anxiety as a personality type and pain perception in parturients who received pain relief with nitrous oxide and deepened the knowledge about anxiety as a personality state as well as analgesia with nitrous oxide during vaginal births.

This study has limitations due to the small number of participants and the analysis of only one mutation, SNP rs 4860. There wasn't taken into account interference with other SNPs of the COMT gene and other genes which influence pain and analgesic efficacy during delivery, which could explain our study results. An increasing number of studies strongly suggest that genetic predisposition plays an important role in pain and pain-related mechanism [36]. Gender-genotype and ethnicity-dependent genetic interactions have been shown to exist, indicating that cannot generalize findings from different cohorts and extrapolate from one study population to another [37]. Other limitations which could impact results are self-rated pain and VAS score collected within 24 hours after delivery. VAS score results are subjective, affected by different expectations, and driven by strong emotions. Also, cultural, physiological, and social components play important roles in pain perception during childbirth which weren't taken into consideration in this study.

5. Conclusions

The COMT gene polymorphism was associated with pain perception and anxiety among labouring women. In women with mild anxiety, nitrous oxide has been found to be effective in reducing anxiety during labor. Additionally, it may provide mild pain relief for some women. Since the exact effect of genes on analgesia during delivery, additional multicentric research is needed with a large number of subjects, including gene polymorphisms and their combinations. This would allow us in the future to demonstrate, based on genetic research, which analgesic drug would be most effective for a particular group of labouring women. This could allow them to overcome anxiety, fear, and discomfort as efficiently as possible, or make them acceptable, especially for those who desire a vaginal (“natural”) birth and enable a more precise personal approach.

Author Contributions

Conceptualization, J.Š.; methodology, J.Š., K.K., M.Š.G., L.F.D.; Software, A.C., D.N.V., Validation, J.Š., K.K., L.F.D.; Formal Analysis, A.C., D.N.V., Investigation, J.Š., Resources, J.Š., Writing – Original Draft Preparation, J.Š., Writing – Review & Editing, J.Š, M.Š.G.

Funding

This research received no external funding

Institutional Review Board Statement

This study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of General Hospital Zabok (No. 97/2).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The datasets generated and analyzed in the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors would like to express thanks to the patients, physicians, midwives and other employees of the Department of Gynaecology at the General hospital Zabok for providing help with the data acquisition.

Conflicts of Interest

The authors declare no conflict of interest.

References

Agah, J.; Baghani, R.; Tali, S.H.S.; Tabarraei, Y. Effects of Continuous Use of Entonox in Comparison with Intermittent Method on Obstetric Outcomes: A Randomized Clinical Trial. J. Pregnancy 2014, 2014, 1–5. [Google Scholar] [CrossRef] [PubMed]
Attar, A.S.; Feizabadi, A.S.; Jarahi, L.; Feizabadi, L.S.; Sheybani, S. Effect of Entonox on reducing the need for Pethidine and the Relevant Fetal and Maternal Complications for Painless Labor. Electron. Physician 2016, 8, 3325–3332. [Google Scholar] [CrossRef] [PubMed]
Guszkowska, M. The effect of exercise and childbirth classes on fear of childbirth and locus of labor pain control. Anxiety, Stress. Coping 2013, 27, 176–189. [Google Scholar] [CrossRef]
Jokić-Begić, N.; Žigić, L.; Radoš, S.N. Anxiety and anxiety sensitivity as predictors of fear of childbirth: different patterns for nulliparous and parous women. J. Psychosom. Obstet. Gynecol. 2013, 35, 22–28. [Google Scholar] [CrossRef] [PubMed]
Mazzoni, A.; Althabe, F.; Gutierrez, L.; Gibbons, L.; Liu, N.H.; Bonotti, A.M.; Izbizky, G.H.; Ferrary, M.; Viergue, N.; Vigil, S.I.; et al. Women’s preferences and mode of delivery in public and private hospitals: a prospective cohort study. BMC Pregnancy Childbirth 2016, 16, 1–8. [Google Scholar] [CrossRef] [PubMed]
Karlström, A. Women’s self-reported experience of unplanned cesarean section: results of a Swedish study. Midwifery 2017, 50, 253–258. [Google Scholar] [CrossRef]
Pasha, H.; Basirat, Z.; Hajahmadi, M.; Bakhtiari, A.; Faramarzi, M.; Salmalian, H. Maternal Expectations and Experiences of Labor Analgesia With Nitrous Oxide. Iran. Red Crescent Med J. 2012, 14, 792–7. [Google Scholar] [CrossRef]
Waud, B.E.; Waud, D.R. Calculated kinetics of distribution of nitrous oxide and methoxyflurane during intermitent administration in obstetrics. Anesthesiology 1970, 32, 306–316. [Google Scholar] [CrossRef]
Duarte, R.; McNeil, A.; Drummond, G.; Tiplady, B. Comparison of the sedative, cognitive, and analgesic effects of nitrous oxide-induced analgesia. Br. J. Anesth. 2009, 103, 744–749. [Google Scholar]
Emmanouil, D.E.; Quock, R.M. Advances in Understanding the Actions of Nitrous Oxide. Anesthesia Prog. 2007, 54, 9–18. [Google Scholar] [CrossRef]
James, S. Human pain and genetics: some basics. Br. J. Pain 2013, 7, 171–178. [Google Scholar] [CrossRef]
Williams, F.M.K.; Spector, T.D.; MacGregor, A.J. Pain reporting at different body sites is explained by a single underlying genetic factor. Rheumatology 2010, 49, 1753–1755. [Google Scholar] [CrossRef] [PubMed]
Yu, F.H.; Catterwall, W.A. Overview of the voltagegated sodium channel family. Genome Biol. 2003, 4, 207. [Google Scholar]
Wang, W.; Gu, J.; Li, Y.Q.; et al. Are voltage-gated sodium channels on the dorsal root ganglion involved in the development of neuropathic pain? Mol. Pain 2011, 7, 16. [Google Scholar] [CrossRef] [PubMed]
DeMille, M.M.; Kidd, J.R.; Ruggeri, V.; Palmatier, M.A.; Goldman, D.; Odunsi, A.; Okonofua, F.; Grigorenko, E.; Schulz, L.O.; Bonne-Tamir, B.; et al. Population variation in linkage disequilibrium across the COMT gene considering promoter region and coding region variation. Hum. Genet. 2002, 111, 521–537. [Google Scholar] [CrossRef] [PubMed]
Xie, T.; Ho, S.-L.; Ramsden, D. Characterization and Implications of Estrogenic Down-Regulation of Human Catechol-O-Methyltransferase Gene Transcription. Mol. Pharmacol. 1999, 56, 31–38. [Google Scholar] [CrossRef] [PubMed]
Zubieta, J.; Heitzeg, M.; Smith, Y.; Bueller, J.; Xu, K.; Xu, Y.; Кoерре, R.; Stohler, C.; Goldman, D.; J., Z.; et al. Comt val158met genotype affects mu-opioid neurotransmitter responses to a pain stressor (Science. — 2003. —Feb. 21. — 299(5610). — P. 1240—1243: англ.). Neurol. Bull. 2003; XXXV, 78. [CrossRef]
Ren, X.; Zhang, L.; Xiao, Q.; Huang, D.; Liu, Q.; Zhang, Y. Association between COMT polymorphism, labor anxiety, and analgesia in pregnant women. J. Pain Res. 2019, ume 12, 779–785. [Google Scholar] [CrossRef]
Hawkins, J.L. Epidural Analgesia for Labor and Delivery. New Engl. J. Med. 2010, 362, 1503–1510. [Google Scholar] [CrossRef]
Lotsch, J.; Skarke, C.; Liefhold, J.; Geisslinger, G. Genetic predictors of the clinical response to opioid analgesics: clinical utility and future perspectives. Clin. Pharmacokinet. 2004, 43, 983–1013. [Google Scholar] [CrossRef]
Abood, M.E. Molecular Biology of Cannabinoid Receptors. 2005; 81–115. [Google Scholar] [CrossRef]
Rosen, M.A. Nitrous oxide for relief of labor pain: a systematic review. Am. J. Obstet. Gynecol. 2002, 186, S110–S126. [Google Scholar] [CrossRef]
Likis, F.E.; Andrews, J.C.; Collins, M.R.; Lewis, R.M.; Serogy, J.J.; Starr, S.A.; et al. Nitrous oxide for the management of labor pain: a systematic review. Anesth; Analg. 2014, 118, 153–167, Most recent meta-analysis summarizing nitrous oxide analgesia effectiveness and side effects. [Google Scholar] [CrossRef] [PubMed]
Talebi, H.; Nourozi, A.; Jamilian, M.; Baharfar, N.; Eghtesadi, P. Entonox for Labor Pain: A Randomized Placebo Controlled Trial. Pak. J. Biol. Sci. 2009, 12, 1217–1221. [Google Scholar] [CrossRef]
Westling, F.; Milsom, I.; Zetterström, H.; Ekström-Jodal, B. Effects of nitrous oxide/oxygen inhalation on the maternal circulation during vaginal delivery. Acta Anaesthesiol. Scand. 1992, 36, 175–181. [Google Scholar] [CrossRef] [PubMed]
Richardson, M.G.; Raymond, B.L.; Baysinger, C.L.; Kook, B.T.; Chestnut, D.H. A qualitative analysis of parturients’ experiences using nitrous oxide for labor analgesia: It is not just about pain relief. Birth 2018, 46, 97–104. [Google Scholar] [CrossRef]
Baysinger, C.L. Inhaled Nitrous Oxide Analgesia for Labor. Curr. Anesthesiol. Rep. 2019, 9, 69–75. [Google Scholar] [CrossRef]
Comasco, E.; Hellgren, C.; Olivier, J.; Skalkidou, A.; Poromaa, I.S. Supraphysiological hormonal status, anxiety disorders, and COMT Val/Val genotype are associated with reduced sensorimotor gating in women. Psychoneuroendocrinology 2015, 60, 217–223. [Google Scholar] [CrossRef]
Qiu, A.; Tuan, T.A.; Ong, M.L.; Li, Y.; Chen, H.; Rifkin-Graboi, A.; Broekman, B.F.; Kwek, K.; Saw, S.-M.; Chong, Y.-S.; et al. COMT Haplotypes Modulate Associations of Antenatal Maternal Anxiety and Neonatal Cortical Morphology. Am. J. Psychiatry 2015, 172, 163–172. [Google Scholar] [CrossRef]
Tang, L.-J.; Han, J.; Feng, Y.-J.; Pu, C.-X.; Zhang, Y. Comparative study of the efficacy of dexmedetomidine and fentanyl on anxiety and pain of parturients with different COMTva1158met genotypes. BMC Anesthesiol. 2022, 22, 1–7. [Google Scholar] [CrossRef]
Curzik, D.; Jokic-Begic, N. Anxiety sensitivity and anxiety as correlates of expected, experienced and recalled labor pain. J. Psychosom. Obstet. Gynecol. 2011, 32, 198–203. [Google Scholar] [CrossRef]
Dabo Pettersson, F.; Hellgren, C.; Nyberg, F.; Akerud, H. Depressed mood, anxiety, and the use of labour analgesia. Arch. Womens Ment. Health 2016, 19, 11–16. [Google Scholar] [CrossRef]
Vallejo, M.C.; Phelps, A.L.; Shepherd, C.J.; Kaul, B.; Mandell, G.L.; Ramanathan, S. Nitrous oxide anxiolysis for elective cesarean section. J. Clin. Anesthesia 2005, 17, 543–548. [Google Scholar] [CrossRef] [PubMed]
Lang, A.J.; Sorrell, J.T.; Rodgers, C.S.; Lebeck, M.M. Anxiety sensitivity as a predictor of labor pain. Eur. J. Pain 2006, 10, 263–263. [Google Scholar] [CrossRef] [PubMed]
Xiaohong, Y.; Xue-ming, H. COMT val1158met Gene Polymorphism Correlation with Prenatal Anxiety and Labour Analgesia. Int. J. Hum. Genet. 2020, 20, 104–109. [Google Scholar]
Max, M.B.; Stewart, W.F. The molecular epidemiology of pain: a new discipline for drug discovery. Nat. Rev. Drug Discov. 2008, 7, 647–658. [Google Scholar] [CrossRef] [PubMed]
Landau, R.; Ortner, C.M.; Vuilleumier, P.H. The Impact of Genetics and Other Factors on Intra- and Post-partum Pain. Curr. Anesthesiol. Rep. 2013, 3, 264–274. [Google Scholar] [CrossRef]

Table 1. The characteristics of the examined group of parturient women with regard to pain relief during delivery.

	All mothers	Without pain relief	With pain relief	Differences between groups
	All mothers	Without pain relief	With pain relief	χ²/ t-test/ z value	P
Number of mothers	181	90	91
Age [years]	30 (26-34) [21-44]	31 (27-36) [22-44]	28 (26-32) [21-42]	2.83^d	0.005
Mother’s height [cm]	165 (162-170) [153-183]	165 (163-169) [156-183]	165 (162-170) [153-182]	0.458^d	0.648
Mother’s parity
first childbirth	93 (51)	34 (38)	59 (65)	15.8^a	< 0.001
second childbirth	59 (33)	34 (38)	35 (27)
third+ childbirth	29 (16)	22 (24)	7 (8)
Weeks of gestation
38 weeks	22 (12)	11 (12)	11 (12)	5.16^b	0.277
39 weeks	52 (29)	32 (36)	20 (22)
40 weeks	36 (20)	17 (19)	19 (21)
41 weeks	64 (35)	28 (31)	36 (40)
42 weeks	7 (4)	2 (2)	5 (5)
Male sex of the baby	91 (50)	42 (47)	49 (54)	0.933^a	0.334
Baby’s weight [g]	3530 ± 445 (2200-4750)	3547 ± 481 (2200-4650)	3514 ± 408 (2345-4750)	0.500^c	0.618
Baby’s length [cm]	51 (50-51) [45-56]	51 (50-52) [45-56]	50 (50-51) [45-53]	0.746^d	0.457
Perception of pain at the beginning of delivery	3 (3-5) [1-7]	3 (3-5) [1-7]	4 (2-5) [2-7]	1.58^d	0.115
low pain	94 (52)	53 (59)	41 (45)	3.47^a	0.063
medium pain	87 (48)	37 (41)	50 (55)	3.47^a	0.063
very strong pain	0	0	0
Perception of pain at the end of delivery	9 (8-10) [4-10]	9 (9-10) [6-10]	8 (8-9) [4-10]	5.22^d	< 0.001
low pain	0	0	0
medium pain	22 (12)	2 (2)	20 (22)	16.5^a	< 0.001
very strong pain	159 (88)	88 (98)	71 (78)
Level of anxiety	46 (44-49) [33-67]	46 (44-49) [38-60]	46 (44-49) [33-67]	0.013^d	0.990
no anxiety	142 (78)	72 (80)	70 (77)	0.467^b	0.836
mild anxiety	36 (20)	17 (19)	19 (21)
moderate anxiety	3 (2)	1 (1)	2 (2)
COMT gene
GG (wild type)	40 (22)	19 (21)	21 (23)	1.14^a	0.565
GA (mutant heterozygote)	95 (53)	45 (50)	50 (55)
AA (mutant homozygote)	46 (25)	26 (29)	20 (22)

The results are presented as frequencies (percentage) for categorical variables; as arithmetic mean +/- standard deviation (range) for a normally distributed variable; or as the median (interquartile range) for variables with a distribution which deviates from normal. The difference between the two groups of women was tested using ^aPearson’s chi-test; ^bFisher test; ^cStudent’s test (for variables with a normal distribution); or Mann-Whitney U test (for variables with distribution that deviates from normal). The results have been considered statistically significant if P<0,05.

Table 3. The perception of pain during delivery and the duration of delivery with regard to the level of anxiety and pain relief during delivery.

	Level of anxiety		Differences between groups
	No anxiety	Mild anxiety	Pearson’s or K-W χ²	P
ALL MOTHERS
Number of mothers	142	39
Perception of pain at the beginning of delivery	4 (3-5) [1-7]	4 (3-5) [2-6]	-0.50^c	0.611
mild pain	75 (53)	19 (49)	0.21^a	0.650
medium pain	67 (47)	20 (51)	0.21^a	0.650
very strong pain	0	0
Perception of pain at the end of delivery	9 (8-10) [4-10]	9 (8-10) [5-10]	1.25^c	0.215
mild pain	0	0
medium pain	16 (11)	6 (15)	0.49^b	0.326
very strong pain	126 (89)	33 (85)
MOTHERS WITHOUT PAIN RELIEF
Number of mothers	72	18
Perception of pain at the beginning of delivery	3 (3-5) [1-7]	3 (3-5) [2-6]	-0.09^c	0.939
mild pain	43 (60)	10 (56)	0.10^a	0.748
medium pain	29 (40)	8 (44)	0.10^a	0.748
very strong pain	0	0
Perception of pain at the end of delivery	9 (9-10) [6-10]	9 (8-10) [7-10]	2.21^c	0.029
mild pain	0	0
medium pain	1 (1)	1 (6)	1.15^b	0.362
very strong pain	71 (99)	17 (94)
MOTHERS WITH PAIN RELIEF
Number of mothers	70	21
Perception of pain at the beginning of delivery	4 (2-5) [2-7]	4 (3-4) [2-6]	-0.37^c	0.721
mild pain	32 (46)	9 (43)	0.05^a	0.817
medium pain	38 (54)	12 (57)	0.05^a	0.817
very strong pain	0	0
Perception of pain at the end of delivery	8 (8-9) [4-10]	8 (8-9) [4-10]	-0.26^c	0.800
mild pain	0	0
medium pain	15 (21)	5 (24)	0.05^b	0.515
very strong pain	55 (79)	16 (76)

The difference between the two groups of women was tested using ^aPearson’s chi-test; ^bFisher test; ^cStudent’s test or Mann-Whitney U test. The results have been considered statistically significant if P<0,05.

Table 4. The level of anxiety with regard to COMT rs4680 gene polymorphism and pain relief during delivery.

	COMT rs4680 gene polymorphism			Differences between groups
	GG	GA	AA	Pearson’s or K-W χ²	P
ALL MOTHERS
Number of mothers	40	95	46
Level of anxiety	46 (43.5-48.5) [40-67]	47 (45-50) [33-63]	45 (43-49) [39-56]	6.36^c	0.042
no anxiety	33 (83)	70 (74)	39 (85)	2.76^a	0.252
mild anxiety	7 (18)	25 (26)	7 (15)	2.76^a	0.252
MOTHERS WITHOUT PAIN RELIEF
Number of mothers	19	45	26
Level of anxiety	46 (42-49) [40-53]	46 (45-49) [38-60]	47 (43-49) [39-56]	0.52^c	0.772
no anxiety	16 (84)	34 (76)	22 (85)	1.11^b	0.651
mild anxiety	3 (16)	11 (24)	4 (15)	1.11^b	0.651
MOTHERS WITH PAIN RELIEF
Number of mothers	21	50	20
Level of anxiety	46^ab (44-48) [40-67]	47^a (46-50) [33-63]	44,5^b (43-46) [40-53]	8.58^c	0.014
no anxiety	17 (81)	36 (72)	17 (85)	1.61^b	0.531
mild anxiety	4 (19)	14 (28)	3 (15)	1.61^b	0.531

The difference between the three groups of women was tested using ^aPearson’s chi-square test; ^bFisher test; or Kruskal-Wallis ANOVA test. The results have been considered statistically significant if P<0,05. The letters in superscript indicate statistically significant differences between the groups, with Bonferroni correction for multiple comparisons (0.017).

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