The Facial Action Coding System (FACS) is a widely used and validated tool for objectively analyzing facial expressions and decoding emotional and physical states, including pain 21. It provides a systematic framework for identifying and measuring specific facial muscle movements, known as AUs, which correspond to various facial expressions 22. By utilizing FACS, researchers can gain valuable insights into pain experiences and assess pain levels objectively, transcending language and cultural barriers 23.
It is essential to recognize that facial expressions of pain may differ across races and ethnicities 24. Cultural and individual factors can influence the display and interpretation of pain-related facial expressions 25. Therefore, it crucial to consider the specific cultural and social context in which the study was conducted when analyzing and interpreting facial expressions of pain 24.
This study aimed to investigate the association between specific AUs and both genuine and fake pain levels in a sample of Syrian children. Pain assessment and management in children, particularly in dental settings pose significant challenges due to the subjective nature of pain and the limited verbal communication abilities of young children.
The importance of conducting this study within the Syrian community lies in the lack of research available on pain experiences during dental procedures in this population. By focusing on Syrian children, the study contributes to understanding the unique socio-cultural context and potential variations in pain expression and perception within this specific community.
Studying fake AUs provides valuable insights into the complex nature of facial expressions and their role in pain communication 26. The ability to discern between genuine and fake pain expressions can assist in detecting potential instances of malingering or the exaggeration of pain symptoms, leading to more appropriate treatment interventions, resource allocation, and pain assessment 17.
The design of this study employed a cross-sectional approach, aiming to investigate the association between specific AUs and pain in a sample of Syrian children during dental injections. The primary objective was to identify the AUs most significantly associated with pain in this specific population.
The study included a targeted age group of children (6–9 years), recognizing that pain experiences may vary across different developmental stages (motor and social development) 27. By focusing on a specific age range, the study aimed to provide insights into pain responses in a homogenous sample, reducing potential confounding factors related to age differences 28.
To assess pain levels during dental injections, the FLACC (Face, Legs, Activity, Cry, Consolability) pain assessment scale was utilized. This validated tool enabled objective and standardized pain assessment, capturing multiple dimensions of pain expression and behavior in young children 29.
To analyze facial expressions accurately, the Facial Action Coding System (FACS) was employed. FACS allowed for the identification and measurement of specific AUs associated with pain, enabling a precise and detailed assessment of facial responses during dental injections 30. While analyzing the FAUs, lower face AUs that heavily rely on lip and mouth movements were excluded. This aimed to minimize potential confounding factors related to dental interventions involving the lips and mouth, ensuring that the selected AUs specifically captured responses associated with pain experienced during dental injections 31.
The selection of the inferior alveolar nerve block (IANB) as the dental injection technique provided a standardized and consistent pain-inducing procedure for all participants 32. This approach facilitated the comparison of pain responses across individuals and allowed for a reliable assessment of the association between AUs and pain levels.
Furthermore, the insertion point of the needle during the IANB procedure served as the pain reference in this study. Utilizing a consistent pain reference across all participants, it enhanced the accuracy and reliability of the pain assessment and minimized potential variations related to different injection sites 33.
In analyzing fake FAUs, children were asked to mimic pain after the injection rather than before. This offered valuable perspectives on the nature of facial expressions and their relevance to pain assessment. Additionally, this gives us more data about which AUs are the most activated in fake pain.
The results of the study indicated significant associations between specific AUs and pain levels in the study sample. For instance, AU4, AU6, AU41, and AU46 were found to be significantly associated with mild pain. These findings suggest that the activation of these AUs in the upper face may serve as reliable indicators of mild pain experienced during dental injections among Syrian children. Furthermore, the same FAUs were correlated to moderate pain in addition to AU11 (nasolabial deepener). These findings indicate that if AU11 is also activated, moderate pain is suggested.
In severe pain, AU4, AU6, AU7, AU9, AU11 and AU43 were strongly correlated. As a result, lid tighteners, nose wrinkle, and closed eyes are unique indicators of severe pain in Syrian children. Additionally, the presence of AU43 demonstrated a unique negative association with pain absence, suggesting its potential as a protective factor against pain experiences.
Our findings were inconsistence with the Nerella et al study (2021), which resulted in the correlation of AU25, 26, and 43 with pain. The difference between the results can be justified for many reasons; they focused on the adult population, different races, and hospital patients who are critically ill 34. While our study aimed to assess pain in children who are receiving IANB.
On the other hand, Facial Action Summary Score was validated in the study of Bringuier et al (2021) and it resulted in the association of AU4, 11, and 43 with pain in children 6. However, they did not state this association regarding pain intensity. Moreover, they found that upper lip raiser and lower lip raiser are also correlated. In contrast, our study did not find those AUs as indicators for pain due to the clinical interventional situation we studied the children (during dental injections).
In the study of Prakachin (1992), AU4, 6, 7, 9, 10, 20, 27and 43 were mentioned as the pain indicators. However, they used different pain indicators on adults and children 35.
Furthermore, our results indicated that specific AUs, such as AU12 and AU38, were predominantly associated with fake pain expressions rather than genuine pain. This finding highlights the importance of distinguishing between genuine and simulated pain when interpreting facial expressions. AU12, which involves the pulling of lip corners, was found in the study of Craig et al (1991) that is more activated in fake pain than in genuine pain 19. In this regard, the study of Lucey et al (2019) also showed that AU12 is activated in fake pain more frequently 36.
AU38, which relates to dilating nostrils, may have particular relevance in the context of fake pain as individuals may consciously manipulate these facial movements to portray a sense of discomfort or distress. The nostrils have been observed to dilate or widen during various emotional and physiological states, including, stress 37.
In this regard, the present study showed that AU4 and AU43 were activated in most of the fake pain expressions of the children. Our findings are in agreement with the study of Ya-Bin Sun et al (2015) in which the authors found that both AU4 and AU12 are involved in mimicking pain expressions 38.
The study's findings have important implications for pain assessment and management in various clinical settings. By recognizing the differences between real and fake pain expressions, healthcare professionals can enhance their ability to accurately evaluate and respond to patients' pain experiences 39.
Our study showed no difference between boys and girls regarding the FAUs associated with pain. In the study of Rocha et al (2003), it was also shown that there is no difference between boys and girls regarding pain facial expressions 40. Additionally, a review (2007) showed that gender does not significantly affect pain experience in children 41.
The findings of this study will have practical implications for dental professionals, enabling them to recognize and respond to pain more effectively during dental procedures involving injections. Ultimately, this research aims to alleviate pain and enhance the dental experience of Syrian children, fostering positive attitudes toward oral health and reducing dental anxiety in this vulnerable population.
While this study contributes valuable insights into the association between specific AUs and pain levels in Syrian children, it is important to acknowledge certain limitations that should be considered when interpreting the findings. Firstly, the study sample consisted of a specific population of Syrian children, which may limit the generalizability of the results to other populations or cultural contexts. Cultural factors, pain perception, and pain expression can vary across different ethnicities and communities. Therefore, caution should be exercised when extrapolating the findings to broader populations.
Secondly, the study focused on a specific age group of Syrian children, and the findings may not apply to other age ranges or developmental stages. Pain perception and expression can differ among infants, toddlers, and adolescents. Therefore, it is crucial to consider age-related factors when interpreting the results and extrapolating them to different age groups.
Moreover, the study solely examined upper-face AUs while excluding lower face AUs that rely on lips and mouth movements. While this approach aimed to isolate pain-related expressions during dental injections, it may have limited the comprehensive assessment of facial expressions associated with pain. Future studies should consider including lower-face AUs to provide a more comprehensive analysis of pain-related facial expressions.
It is important to mention that this research serves as a crucial foundation for our ongoing study aimed at developing an AI algorithm to detect pain levels in children during dental injections. By investigating the association between facial action units and pain experiences, we gain valuable insights into the complex dynamics of pain expression in Syrian children. Understanding the nuances of facial expressions, both genuine and potentially fake, provides us with essential knowledge that can inform the development of an accurate and reliable AI algorithm.