Correlation between skin conductance measurements and subjective pain scales in children after otolaryngological procedures

Surgery is one of the most common causes of acute pain in children [1]. More than 85% of pediatric patients experience postoperative pain [2, 3], which, if inadequately diagnosed and subsequently treated, can lead to chronic pain, even after discharge home from the hospital [4], increased sensitivity to pain, and anxiety about future medical procedures [5]. Postoperative pain management has become one of the major problems in pediatrics [6]. The main reason for insufficient postoperative pain therapy in children is the difficulty in evaluating it [7]. A wide range of pain self-assessment tools have been developed for pediatric patients, of which the most often used in the postoperative period are the Visual Analogue Scale (VAS); the Wong-Baker Faces Pain Rating Scale (WB); and the Face, Legs, Activity, Cry, and Consolability (FLACC) scale [8]. In the last decade, efforts have been directed towards implementing a diagnostic tool that can assess pain intensity as objectively as possible, especially in pediatric patients. These methods include the skin conductance algesimeter (SCA), with which fluctuations in skin amplitude and conductivity frequencies can be measured and used to assess pain [9]. An SCA relies on stimulation of the autonomic nervous system by nociceptive stimuli, which leads to changes in skin conduction caused by the action of acetylcholine – released in the pain response to muscarinic receptors – with the subsequent release of sweat [10]. In the past, skin conductance has also been measured in the postoperative period [11, 12, 13]. However, to our knowledge, it has not been as widely used as the pain self-assessment scales in children. In addition, upon a patient's discharge from the hospital, the burden of pain management is shifted from the medical staff to the parents, making it even more important for parents to be aware of the most appropriate methods to assess pain in children. The purpose of this study was to evaluate whether an SCA is a useful diagnostic tool for pain assessment in the pediatric postoperative care setting and to demonstrate the correlation between SCA measurements and the widely used pain assessment scales.

Approval was obtained from the Ethical Committee of the Medical University of Wrocław to perform the study from July 2019 to January 2021, under the number KB–459/2018. The authors confirm that all procedures comprising this research conformed to the ethical standards for human experimentation and the World Medical Association Declaration of Helsinki.

The clinical trial included a total of 33 children from the Department of Otolaryngology of the University Clinical Hospital in Wrocław. The inclusion criteria were healthy children between the ages of 3 and 17 years who qualified for surgical treatment in the Department of Otolaryngology and the written informed consent of their parents (or legal guardians). The exclusion criteria were intellectual disability, major coexisting diseases, and pain prior to surgery. All children underwent otolaryngological surgeries (adenoidectomy, adenotonsillotomy, or tonsillectomy). Postoperative pain was assessed using the VAS, the WB scale, the FLACC scale, and an SCA. Postoperative pain was measured 1 and 2 hours after the surgery. The most common VAS is a straight, horizontal line 10cm in length. The ends are defined as the extremes of the parameter being measured – pain, in this case – from 1 (no pain) to 10 (very severe pain). The patient is asked to mark a line on the scale to express the intensity of their pain [14]. The WB is a pain scale that represents a series of faces. There are 6 different faces, representing happy at 0, meaning “no hurt,” to crying at 10, meaning “hurts worst.” Based on the faces and written descriptions, the patient selects the face that best describes their pain level [15]. The FLAAC scale [16] is an assessment tool that quantifies pain on a score ranging from 0 to 10 using five categories: facial expression, legs, activity, crying, and consolability. It is an observational scale which can be used by parents or, as it was in this study, by a research team. The observation took 5 minutes; each parameter was evaluated on a scale from 0 to 2. The total score is interpreted as follows: 0 = relaxed and comfortable; 1–3 = mild discomfort; 4–6 = moderate pain; and 7–10 = severe discomfort/pain. A score of more than 3 points suggests the presence of pain. The children were asked to score their pain on the VAS and Wong-Baker scale, while the research team used the FLACC scale to assess it. In addition, an SCA was used, measuring skin conductance via a laptop running MedStorm software and a measuring device connected to the child's hand with a cable and 3 electrodes. The “postoperative” mode was selected in the computer application. After the patient's data were entered, a hand was disinfected with an antiseptic and degreaser, the electrodes were connected, and the signal quality was checked. The results were then recorded. The measurement took 5 minutes each time, and the key parameter considered was the average number of peaks per second, because we investigated the pain reaction to a single pain stimulus over a period of time instead of at a single moment. The threshold of less than 0.13 peaks per second can distinguish between no pain or mild pain and moderate or severe pain in the postoperative period [11].

Statistical analysis was performed using the STATISTICA v.13.3 software (TIBCO, Software Inc., USA). For quantitative variables, mean values, median standard deviations, quartile ranges, and extreme values were calculated. The Shapiro-Wilk test was used to assess the normality of the distribution. As the empirical distributions deviated from the normal distribution, nonparametric tests were used in further analysis. The Mann-Whitney U test was used to estimate the statistical significance of the difference between independent groups. In the case of related groups (results after 1 hour and after 2 hours), the Wilcoxon test was used. The relationship between the parameters was checked by calculating the Rho Spearman correlation coefficient.

Data were collected on 33 children (17 girls and 16 boys). Eleven children (33.3%) had undergone adenoidectomy, 15 (45.5%) adenotonsillotomy, and 7 (21.2%) tonsillectomy. The mean age was 6.1 years (SD=3.0; range: 3–17). The groups were properly matched and there were no statistically significant differences between the two research samples: the gender (p=0.869) and age (p=0.186) ratios were similar (Table 1).

The median pain measurements by gender are shown in Table 2. The boys had significantly higher skin conductance measurements 2 hours after surgery. There was no significant correlation between self-reported pain scores and gender.

There was no statistically significant correlation between self-reported pain level and the number of skin conductance fluctuations per second in either the entire study group or in subgroups differentiated by gender (p>0.05) (Tables 3, 4, 5). There was a statistically significant correlation between all subjective self-reported pain scales in the entire study group (p<0.05) (Table 6).

Assessing postoperative pain in children, especially in the youngest patients, can be challenging. The available pain self-assessment scales, in use for several decades, have been continuously questioned because the child indicates their pain level on their own. A young child does not have a reference for the full spectrum of pain sensations and their experience of pain, often their first, may be influenced by a tendency to mark the highest possible value on the scale. Younger children generally rate their pain experience as higher, even with the same procedure being performed [17, 18]. Therefore, the prospect of creating an objective pain measurement tool, and its subsequent implementation, would facilitate accurate pain management. To date, most clinical studies using an SCA have been based on measuring the acute pain that occurs at the moment of measurement [19, 20, 21]. However, assessing pain in children who do not experience a single pain incident, such as in the postoperative period, is more difficult; previous studies have also failed to conclusively confirm the usefulness of the SCA after surgical procedures [11, 13, 22].

The issue of pain perception by gender has also been the subject of many studies, though scientific reports continue to collate conflicting data between men and women [23, 24]. The current study is the first to compare three subjective pain rating scales with skin conductance measurements in the postoperative period in pediatric patients. Our clinical trial found no statistically significant correlation between self-reported pain scores and the skin conductance fluctuations in the children under study, regardless of gender or age. However, a statistically significant correlation was found between existing subjective pain scales in children. In the study group of 33 children, the number of skin conductance fluctuations per second increased between the first and second hour (0.08 vs. 0.11 Hz; p = 0.008) and the subjective pain ratings decreased. Perhaps the children had become accustomed to the pain. This finding suggests that these commonly used scales that are easy to use and – above all – easy for young patients to understand, have been validated and proven reliable. The skin conductance measurements do not provide an additional reliable tool for assessing pain in patients after otolaryngological procedures. The existing self-reported pain scales are sufficient for assessing postoperative pain in children.

The limitations of the present study rest on the fact that it is a single-center study with a small number of patients. It is therefore uncertain whether the results can be replicated in other larger centers, with variable experience and standardization in their practices.