Facial muscle movements encoding pain-a systematic review

Area of Science:

• Pain research
• Nonverbal communication
• Facial action coding system

Background:

• Facial expressions of pain provide crucial information about an individual's internal state.
• Understanding these expressions is vital for accurate pain assessment.
• Previous research has identified various facial movements associated with pain.

Purpose of the Study:

• To systematically review and identify the most consistently displayed facial movements (action units [AUs]) during pain.
• To determine if these AUs vary based on pain type or cognitive status.

Main Methods:

• Systematic review of 37 studies (27 experimental, 10 clinical pain).
• Analysis of facial activity during pain using the Facial Action Coding System (FACS).
• Synthesis of data considering AU pain-related criteria, pain types, and cognitive status.

Main Results:

• A consistent subset of pain-related AUs emerged: brow lowering (AU4), cheek raise/lid tightening (AUs6_7), nose wrinkling/upper lip raise (AUs9_10), and mouth opening (AUs25_26_27).
• This subset was consistent across experimental and clinical pain, irrespective of cognitive status.
• Eye closure (AU43) was more frequent in clinical pain settings.

Conclusions:

• A core set of facial action units reliably signals pain.
• Pain expressions are individualized, with varying combinations of AUs.
• Healthcare professionals should adopt an individualized approach to interpreting pain expressions rather than seeking a uniform pattern.