Mostafa Farghal
Faculty of Veterinary Medicine, University of Calgary, Calgary, Alberta, Canada ![]()
Faculty of Veterinary Medicine, Minia University, Minia, Egypt ![]()
Correspondence to: Mostafa Farghal, mostafa.farhgal@ucalgary.ca

Additional information
- Ethical approval: N/a
- Consent: N/a
- Funding: No industry funding
- Conflicts of interest: N/a
- Author contribution: Mostafa Farghal – Conceptualization, Writing – original draft, review and editing
- Guarantor: Mostafa Farghal
- Provenance and peer-review:
Commissioned and externally peer-reviewed - Data availability statement: N/a
Keywords: automated pain assessment, machine learning, facial expressions, electrodermal activity, electroencephalography
Peer-review
Received: 24 September 2024
Accepted: 26 September 2024
Published: 10 October 2024
Abstract
Pain is a serious health problem in both adults and infants. If left untreated, it results in serious physiological and psychological consequences. Therefore, accurate and quick pain assessment is crucial to avoid these consequences. While self-reports remain the gold standard in verbal humans, other pain assessment tools are used in infants and noncommunicative people, such as facial expressions, visual analog, and numerical rating scales. Manual pain assessment has several limitations, such as its subjective nature, inconsistency, and potential for bias originating from different sources, including observer gender and culture. Automated pain assessment has received much attention in the last few years, combining artificial intelligence (AI) with these manual tools to achieve accurate and objective pain assessment, especially in infants or nonverbal patients. However, a gap between developing AI models for pain assessment and their application exists. This gap needs to be addressed so that clinicians understand the limitations of using AI-powered pain assessment tools. This paper provides an overview of common pain assessment tools powered with AI, which are facial expressions, body and head movements, language analysis, electrodermal activity, and electroencephalography. In addition, it discusses the gap between the AI models developed based on these tools and their applications under clinical conditions.
Introduction
Pain is defined as an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. It is considered a personal experience, which is influenced to varying degrees by psychological, social, and biological factors.1 Pain has many consequences for human health, including sleep disturbance, reduced physical activities, limited daily social activities, mental problems, and reduced life quality.2–4 Pain was reported to increase hospitalization, visits to emergency departments, and financial burden.5,6 Therefore, pain assessment and treatment not only improve patient outcomes but also improve the use of the healthcare sector.6
Pain assessment is a crucial step for effective pain management; however, accurate pain assessment, particularly determining its intensity, could be a challenging task.7 Fast and accurate pain assessment can avoid transformation of acute pain into chronic pain, which results in psychological or physiological suffering.8 The current tools for pain assessment in humans, including self-reports (self-report is considered the gold standard), visual analog scale (VAS), numerical rating scale (NRS), and facial expressions have limitations such as their subjective nature, being inappropriate for noncommunicative patients, dependence on human judgment, and influence by observer bias and gender.9 Therefore, the need for a pain assessment tool that can precisely identify pain and its intensity still exists.10 Artificial intelligence (AI) was proposed to address the challenges in the current pain assessment tools by providing accurate and objective pain assessment. However, a gap between AI development and AI applications exists.11
AI refers to the analysis of large datasets utilizing computational algorithms to organize, predict, and influence future behaviors or outcomes.12 Also, it refers to the notion that computers could be taught pattern recognition with little or no human interference.13 Machine learning (ML), computer vision, natural language processing (NLP), and fuzzy logic are considered subsets of AI, applied in different medical fields.14 Generally, AI was implemented in pain research for three main purposes: pain assessment,15–17 pain prediction and aiding clinical decision making,18–20 and self-management of pain through developing AI-based mobile applications.21–23 In the last few years, AI models were integrated with several pain assessment tools, including facial expressions, body/head movements, language analysis, electrodermal activity, and electroencephalography (EEG), to enhance pain assessment.24 Nevertheless, little is known about the range of applications of these AI-based tools for pain assessment across populations (infants vs. adults) and conditions (research vs. clinical conditions).
We would like to clarify that this paper does not delve into the technical working of AI nor provide in-depth knowledge of AI models, subsets, and classifiers, as these topics are beyond its scope. Therefore, a basic knowledge of AI terminology is required to understand this paper. Comprehensive and simple explanations of AI terms and their subsets can be found elsewhere.25,26 The primary objectives of this paper are: (1) to provide an overview of common applications of AI in the field of pain assessment and (2) to demonstrate the challenges for using AI-powered pain assessment tools by clinicians.
The Application of AI in Medicine
The adoption of AI in various medical fields is rapidly expanding. AI contributes to outcome optimization, cost reductions, new product development, and improved decision-making.27 It has been applied in identifying diseases, predicting prognosis, analyzing health examinations, laboratory results, and imaging, and linking these findings to specific diseases.28 Simply, AI-empowered tools can analyze patient records to identify their patterns and risk factors that may indicate the presence of a specific disease. Additionally, AI-empowered tools can help with selecting treatment options to support favorable outcomes.12
The decision-making process could be significantly enhanced by the AI application through analysis of big datasets. Current clinical decision-making relies on physicians’ knowledge and experience, a process that can be considered slow due to managing a vast amount of information. AI enables fast and precise analysis of big data at both national and international levels, ultimately improving clinical decision-making globally.29 Moreover, AI algorithms can help in opioid monitoring and risk reduction through analyzing patterns in electronic health records and identifying patients at risk of opioid misuse or when opioid use is appropriate.30,31 Another application for AI is tailoring interventions to individual needs. Many chronic pain patients suffer from conditions such as anxiety and depression, making it crucial to address these problems.12 AI could assist in developing personalized psychological treatment protocols through identifying patterns in patients’ behaviors. For example, AI could improve cognitive behavioral therapy for chronic pain by tailoring interventions to individual needs based on their insights.12
Applications of AI in Pain Assessment
AI has become prevalent in the field of pain assessment. AI has the potential to help clinicians in pain assessment through offering enhanced diagnostic capabilities and ultimately get better outcomes.27 By integrating data from various sources, such as patient monitor sensors, electronic health records, and facial expressions, AI can accurately assess pain and its intensities.29,30 The gold standard of pain assessment, self-reports, is not possible in neonates and adult patients unable to communicate due to various reasons, including unconsciousness, installing medical devices, medications, cognitive disabilities, and dementia.32 Therefore, automated pain assessment has been receiving much attention in the last few years.
Several behaviors or tools, including, but not limited to, facial expressions, body and head movements, language, electrodermal activity, and EEG, have been powered by AI to enhance pain assessment.31,33,34 Some of these tools are appropriate for pain assessment in adults, while others are appropriate for pain assessment in infants. These tools vary in terms of accuracy, sensitivity, specificity, and applications under research or clinical conditions.34 The following sections investigate five common pain assessment tools empowered with AI algorithms, which are: analysis of facial expressions, analysis of body and head movements, analysis of language, electrodermal activity, and EEG.
AI-Powered Analysis of Facial Expressions
Facial expressions refer to the changes in the muscles of the face, known as action units, to pain stimulation such as brow lowering, eye closure, orbital tightening, and levator muscle contraction.35,36 The facial action coding system (FACS) is a manual method for describing and analyzing observable changes in facial muscles,14,33 which has been used to study the facial expressions of pain in a variety of populations, including healthy individuals, individuals with psychiatric or neurological disorders, and individuals with chronic pain conditions.37 It is reported that facial expressions of pain are consistent across ages, genders, different types of pain, and mental states and correlate with self-report of pain.38 However, facial expressions of pain have several limitations, such as the impact of observer bias, gender bias, the need for training, and population differences.9,39
The Prkachin and Solomon Pain Intensity scale is a validated facial expression-based pain scale,40 which is commonly used in pain studies.9,17,41 It has a significant correlation with self-reported pain and is considered the gold standard for measuring pain intensity.32,42 It is characterized by being simple, has a total score for facial expressions, and is widely accepted as a pain assessment measure.40 However, it has several limitations, such as pain underestimation on some occasions, it does not reflect pain intensity in all cases, and it is difficult to use when facial muscles are disabled, such as in the case of Parkinson’s patients.43
AI can automatically identify the features in each video frame and changes over time through the training of a classifier to recognize the pain-related facial expressions.44,45 Briefly, ML models are loaded with a huge dataset and then analyze and classify these data to identify pain-related patterns.39 These models have the capability to identify pain and pain intensities.9,46 AI avoids the risk of bias by human observers in pain assessment,47–49 and gender bias, whereas females were reported to overscore pain due to their empathic nature.50,51 Mobile applications were developed using AI to assess pain from a patient’s facial expression, such as the PainCheck app (Fig. 1), which is applied specifically in older nonverbal patients.52

In adult patients, the accuracy of AI-based pain detection through facial expressions varied widely depending on the model; it ranged from 51.7% to 95.57%,53,54 with varying levels of accuracy in between 53%, 83.1%, and 92.26%.9,41,46 Also, AI was more accurate in pain identification than human observers,55,56 but similar to the accuracy of nurses in another study.49 Moreover, the sensitivity and specificity of these models varied. For example, the sensitivity was 77.5% and 89.7%, and the specificity was 45% and 61.5% for two AI models in two different studies.9,46 These variations could be related to many factors, such as AI algorithms, feature extraction tools, or type of painful stimulation used (heat and electrical stimulation, arm movement, shoulder pain, postoperative pain), cross-validation techniques, and video or image quality.57
In infants, manual pain assessment relies on using various scales such as the neonatal infant pain scale, the neonatal facial coding system, the neonatal pain, agitation, and sedation scale, as well as the Faces Pain Scale-Revised, which have proven to be reliable, valid, and accurate.34,58 These scales have several limitations, including a lack of continuous pain assessment, observer bias, being time-consuming, and requiring trained personnel.34,59,60 Several AI-based models were developed for pain assessment in infants. These models demonstrated varying levels of accuracy from 80% to 96% across several studies.49,61–63 The model built by Sikka et al.49 demonstrated good-to-excellent accuracy (0.84–0.94) in both ongoing and transient pain conditions. The model was equivalent to nurses in pain detection and to parents in estimating pain severity. Heiderich et al.64 reviewed 15 articles about AI-based pain assessment using facial expressions in neonates; the models varied in their accuracy, sensitivity, and specificity, and the author concluded that AI was not effective enough to replace the human eye for pain assessment in neonates. AI-based pain detection through facial expressions in infants is still a promising area and needs more advancements.
AI-Powered Analysis of Head and Body Movements
While body and head movements have been used to identify emotional states in humans,65 they also have proven to be effective in pain assessment, albeit not receiving much attention compared to other indicators such as facial expressions.66,67 Behaviors such as head aversion, downward gaze, and leaning the body forward have been identified as indicative of pain in adult patients.68 Head orientation and posture leaning downward or toward the pain area are considered reliable signs of pain in both infants and adults.69 Zamzmi et al.34 have considered head shaking, arms and leg extension, and splaying fingers to be pain-related behaviors in infants, despite the fact that they were primarily used to diagnose diseases.70–72
ML models based on body and head movements were developed to assess pain, achieving an accuracy of 87.5% in infants and 92% in adults.73,74 While ML models of body and head movements showed promising results in pain assessment, their application could be limited to nonverbal population, such as infants or adults who cannot communicate verbally,69 as self-reported pain remains the gold standard for verbal adults. However, in certain conditions where body movements are restricted, alternative pain assessment tools such as other manual pain scales or AI-based facial expression models may be prioritized. Werner et al.69 used AI to assess pain in human adults, achieving good reliability and high accuracy, but the study findings were not fully applicable under clinical conditions due to several limitations, including confounding factors, the absence of a control group, and the use of an artificial rather than natural pain source.
AI-Powered Language Analysis
AI has been applied to analyze patients’ language as an indicator of pain, focusing on language feature extraction and classification.14 NLP, a subset of AI, represents an advanced step in automatic pain assessment through language analysis. NLP uses linguistics and computer science to extract, interpret, and retrieve data from unstructured spoken and written words.75 The integration of AI, linguistics, and computer science represents the foundation for NLP.14 NLP is capable of performing tasks such as language translation, text summarization, and sentiment analysis with a wide range of applications, including chatbots, language-enabled applications, and virtual assistants.14 In the medical field, NLP has been applied to enhance computerized clinical decision support systems and improve healthcare management through analysis of patient feedback.68,76
In pain research, NLP has been employed to extract and analyze key information from text data such as electronic medical records and patient-reported outcomes, identifying location, intensity, and duration of pain.77 This is beneficial to understand the patient’s experience of pain and identify patterns in pain management. Chaturvedi et al.78 validated a lexicon comprising 382 terms, which facilitates the extraction of pain-related information from electronic databases. Similarly, Naseri et al.79 proposed an AI method to automatically identify and categorize pain reported by physicians in clinical notes, even when the pain is not documented through structured data entry.
Sentiment analysis is another significant application for NLP in pain research, which combines ML algorithms with NLP processes to classify text as positive, neutral, or negative.80 This method has been used to analyze language in patient-reported outcomes, online patient forums, and patient surveys, providing insights regarding patients’ emotions and opinions about their pain and treatment.14 Additionally, facial expression analysis was integrated with language analysis for pain assessment, resulting in the development of software named “the ELAN tool” (Fig. 2).81

Interestingly, infants’ cries have been used to assess various emotions, including pain, hunger, and discomfort.34 AI models have been developed to analyze infant cries and identify pain, achieving varying levels of accuracy. These models have demonstrated accuracies ranging from 30.56% and 83.33%82 to 92%83 and as high as 97.83%.84 While AI-based language analysis offers a valuable pain assessment tool under research settings; its application could be limited to analyzing big data on a large scale and providing general assessments and recommendations. Also, other common methods of pain assessment, such visual analog or numerical rating scales, are easier to use and not time-consuming under clinical settings compared to language analysis.85
AI-Powered Analysis of Electrodermal Activity
Electrodermal activity (EDA) or galvanic skin response has been used to identify and quantify pain intensity, especially for poorly communicating individuals.32 EDA refers to the changes in skin conductivity or skin resistance due to moisture secretion. When humans are exposed to stimuli, sweat glands are induced to secrete moisture, termed emotional sweating, which changes the skin conductivity.86 EDA was integrated into ML devices such as the BITalino® multichannel platform (Fig. 3). It is an open-source biosignal platform compatible with easy-to-use software such as OpenSignals that can be used for obtaining data from EDA, electrocardiography, electromyography, and EEG.14

ML models using EDA were built to predict pain intensity in postoperative adult patients,87 with an accuracy of >80%, but the accuracy decreased to <60%, when the pain level increased. AI models based on EDA were built and validated for pain assessment in infants.88,89 Moreover, the ML model was able to differentiate between nonpainful and moderate-to-severe painful states under clinical conditions in infants.90 Employing AI-empowered EDA has proven applicable for pain assessment in both infants and adults unable to communicate.87 The use of EDA for pain assessment faces some challenges, such as that it requires specific equipment, such as a BITalino® device, needs a specialist to operate this device, and is time-consuming. These limitations could restrict the use of AI-empowered EDA for pain assessment.
AI-Powered Electroencephalography
Electroencephalography, a non-invasive neuroimaging method capable of measuring the electrical voltage changes on the patient’s scalp caused by the activity in the cortex, has the potential to monitor brain activities under different conditions.91 Pain causes changes in EEG patterns; therefore, EEG can be used as a useful tool for pain assessment.91 EEG can detect the functional and structural changes associated with chronic pain.92 In particular, the neuronal activities in the sensorimotor cortex, including increased gamma and theta oscillations, have been linked to painful states.93 In addition, gamma-band oscillations have been identified as a predictor of acute thermal pain.94
AI has been applied for EEG-based pain detection by Chen et al.,95 who validated an ML model capable of distinguishing between nonpainful and painful states, achieving an accuracy of 81%. Other ML models for EEG pain detection in adults achieved even higher accuracy, such as 97.07% and 96.89%.96,97 However, these studies acknowledged that the application of these models under clinical conditions has not been investigated yet. In another study by Elsayed et al.,98 ML models were used to analyze pain-related brain signals and classify them into four pain intensity levels (no pain, low pain, moderate pain, and high pain), achieving an accuracy of 94.83%. This analysis referred to a direct correlation between the pain intensity and the alpha frequency band power.
In addition, EEG can play a vital role in advancing the understanding of pain. In a study by Tayeb et al.,99 EEG source localization was combined with ML techniques to decode pain perception for a real-time reflex system, demonstrated in a single-subject case study. Moreover, EEG has proven to be accurate, non-invasive, and effective for pain assessment in newborn infants.100 However, its application for pain assessment under clinical settings could be limited due to the need for specialized equipment, trained personnel, and the inconsistencies in cortical activation patterns.101–103 As a result, EEG may be most useful for objective pain assessment in nonverbal patients who are unable to self-report or within a research setting.104
We would like to note that heart rate variability, despite its common use in pain assessment, has not been discussed in this paper due to the presence of conflicting evidence about its capability for pain assessment. Bandeira et al.105 screened 2,873 papers about the use of heart rate variability for assessing pain in low back pain patients and revealed limited evidence about the association of heart rate variability with chronic pain. Furthermore, Rampazo et al.106 screened 4,893 papers about pain assessment using heart rate variability in musculoskeletal pain patients and reported heterogeneous and moderate quality evidence about this association. We acknowledge the limitations of this review. First, it does not provide a comprehensive overview of AI applications in pain assessment. Instead, it focuses only on commonly used tools enhanced by AI for pain assessment. Second, the performance of most AI models discussed here has been evaluated under research settings, with little evidence regarding their performance under clinical practice.
Conclusion
AI has a broad range of applications in the medical sector, particularly in the field of pain assessment. Recently, traditional tools for evaluating pain, such as analyzing facial expressions, body and head movements, language, electrodermal activity, and EEG, are powered with AI to offer more objective and accurate pain assessment. Numerous AI models have been developed based on these tools with varying levels of accuracy, specificity, and sensitivity, but their applications are often limited to specific populations, such as infants or people unable to self-report or under research settings. The implementation of AI-empowered pain assessment tools in clinical conditions remains a developing area with the potential for greater integration. References
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