Effect of temporomandibular disorder pain duration on facial expressions and verbal report of pain

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Abstract
This study investigated how specific expressive behaviors (verbal report of pain level and the frequency of emitting specific non-verbal facial expressions of pain) may change over the course of a chronic pain condition. Based on the concept of chronic pain behaviors, we hypothesized that both verbal and non-verbal behavior would increase with duration of pain. Thirty-six women with chronic temporomandibular disorder (TMD) pain (duration over 6 months) were compared with 35 recent onset cases (first episode, duration Symbol= 2 months). Subjects completed questionnaires assessing depression, anxiety, somatization, daily hassles and pain coping strategies. They were videotaped during a resting baseline and 2 painful conditions: experimental cold pressor pain and the clinically relevant pain of palpation of the masticatory muscles and temporomandibular joint; tapes were coded for facial expression using the Facial Action Coding System. Visual analog scale (VAS) ratings of the aversiveness and intensity of ongoing TMD pain were collected at baseline, and similar ratings of cold pressor and clinical examination pain were gathered after the painful stimulus. Recent onset and chronic cases did not differ on self-report measures of anxiety, depression, somatization or daily stress. Coping strategies were also similar, although chronic cases showed a greater tendency to catastrophize. Self-report measures of ambient facial pain, as well as the pain of clinical examination and cold pressor stimulation, revealed no significant differences between the 2 groups. In contrast, rates of pain facial expression were significantly higher for chronic cases under all conditions of the experiment, including baseline. The findings suggest that verbal and non-verbal expressions of pain may be differentially reinforced over the course of a chronic pain condition, such that non-verbal pain facial expressions become more frequent with time, while verbal reports of pain remain relatively constant.