Facial muscle movements encoding pain—a systematic review

Abstract

Facial expressions of pain are not undefined grimaces but they convey specific information about the internal state of the individual in pain. With this systematic review we aim to answer the question of which facial movements are displayed most consistently during pain. We searched for studies that used the Facial Action Coding System (FACS) to analyze facial activity during pain in adults, and that report on distinct facial responses (Action Units, AUs). Twenty-seven studies using experimental pain and 10 clinical pain studies were included. We synthesized the data by taking into consideration (i) criteria used to define whether an AU is pain-related; (ii) types of pain; and (iii) the cognitive status of the individuals. When AUs were selected as being pain-related based on a "pain>baseline" increase, a consistent subset of pain-related AUs emerged across studies: lowering the brows (AU4), cheek raise/lid tightening (AUs6_7), nose wrinkling/raising the upper lip (AUs9_10) and opening of the mouth (AUs25_26_27). This subset was found independently of the cognitive status of the individuals and was stable across clinical and experimental pain with only one variation, namely that eye closure (AU43) occurred more frequently during clinical pain. This subset of pain-related facial responses seems to encode the essential information about pain available in the face. However, given that these pain-related AUs are most often not displayed all at once, but are differently combined, healthcare-professionals should use a more individualized approach, determining which pain-related facial responses an individual combines and aggregates to express pain, instead of erroneously searching for an uniform expression of pain.

The Pain Assessment in Impaired Cognition scale (PAIC15): A multidisciplinary and international approach to develop and test a meta‐tool for pain assessment in impaired cognition, especially dementia

“…The five selected items on facial expressions are in agreement with previous findings on facial expressions of pain (Prkachin, ; Kunz, Meixner, & Lautenbacher, ). In a recent review article (Kunz et al, ), the findings of 37 studies on facial activity elicited during clinical and/or experimental pain were analysed, and it was found that a consistent subset of pain‐related facial responses emerged across studies: frowning, narrowing eyes, raising upper lip and opening mouth. These are the same facial responses that emerged as the best facial items in our PAIC15 item selection process.…”

Pain assessment for cognitively impaired older adults: Do items of available observer tools reflect pain‐specific responses?

“…The item “pained expression”, for example, is originally defined as “facial expression of pain”, thus being subject to interpretation. We decided to consider a “pained expression” as present whenever at least two empirically derived pain‐specific facial movements occurred (Kunz & Lautenbacher, ; Kunz, Meixner, & Lautenbacher, ; Lautenbacher, Walz, & Kunz, ; Prkachin, ; Williams, ). Due to this operationalization the item ‘pained expression’ turned out to be particularly relevant in our further analyses.…”

“…The disadvantage is the higher chance of certain distress behaviours occurring, making it more difficult for certain items to be confirmed as pain specific. This may in particular be true for “frowning” and “narrowing eyes”, facial movements that empirically were shown to be pain specific (Kunz & Lautenbacher, ; Kunz et al, ; Prkachin, ; Williams, ), but also occur if individuals are cognitively engaged.…”

Equine Facial Action Coding System for determination of pain-related facial responses in videos of horses

“…However, data driven methods do not exist for the interpretation of such data in horses or other animals. We therefore explored statistical methods used in human FACS [17] and also developed novel statistical approaches to analyze the FACS data. This study describes for the first time the facial activities of horse in pain by use of EquiFACS.…”

“…While studies applying EquiFACS methods are few, research on human facial expressions of pain is mature and extensive. Kunz et al [17] presents a systematic review of studies on human facial expressions of pain and describe current approaches for the identification of action units associated with pain. The first criterion for defining an AU to be pain related is if it occurs frequently -forming more than 5% of total pain AU occurrences [18].…”