Tell me what to expect: how instructions affect the pain response of patients with chronic myofascial pain with referral.

The aims of the study are to analyze the influence of pain and no pain expectations on the physiological (electromyography (EMG) and pupillometry) and cognitive (Numerical Rating Scale (NRS)) response to pain. Pain expectation and no pain expectation situations were induced by employing instructional videos. The induction of pain was performed by palpating the masseter with an algometer in a sample of 2 groups: 30 healthy participants (control group) and 30 patients (Temporomandibular disorders (TMD) group) with chronic myofascial pain with referral in the masseter muscle (Diagnostic Criteria for Temporomandibular Dissorders (DC/TMD)). Used a mixed design all participants were exposed to pain and no pain conditions in the same session, but the order of the presentation was counterbalanced across participants to control its possible influence. A significantly larger pupillary diameter was observed in the pain expectation relative to the no pain expectation condition in both groups. The TMD group presented larger EMG activity and larger scores in anxiety, somatization, catastrophizing and central sensitization than the control group. In the NRS, the TMD group also showed a significantly higher score than the control group. The TMD group presented similar NRS scores in the expectation condition compared to the no pain expectation condition, while the control group presented higher scores for pain expectation than for no pain expectation. Pain expectation modulated the pain cognitive pain assessment and pupil diameter in controls. The cognitive pain assessment was altered in the TMD group compared to the control group, particularly in the no pain expectation condition, this may be due to a negative reappraisal of pain due to past experiences, as pointed out by the observed level of catastrophizing. Pain expectations did not influence the EMG, significantly higher EMG activity was found in the TMD group compared to the control group regardless of expectation type.