To the best of our knowledge, this is the first prospective, multicenter, observational study to investigate the frequency of psychosomatic-prone MPS in patients with incurable cancer and its association with the armchair sign.
The first important finding of this study was the high frequency of psychosomatic-prone MPS in patients with incurable cancer. A previous single-center study reported a frequency of 57.1% (6), which is similar to the 61.3% observed in our multicenter study. Participants included patients with incurable cancer, about half of whom were in the anticancer treatment stage and had relatively good PS of 0–2. These results were consistent with those of a previous study (6). Of the enrolled patients, the percentage of patients who met diagnostic criteria for MPS was 43.6%, which is within the range of 31–45% reported in previous studies (4–6).
The findings of this study may be useful to raise awareness of psychosomatic-prone MPS in patients with incurable cancer. MPS in patients with incurable cancer is often overlooked and is a common cause of opioid-induced delirium (16). Furthermore, medical professionals may consider pain that is exacerbated by stress as psychogenic pain. Diagnosis of MPS requires careful manual examination and is considered extremely reliable (17). Physicians' physical involvement has been reported to increase in response to patients' complaints of physical symptoms and decrease in response to psychological complaints (18).
The findings of this study will lead to increased treatment options for MPS in patients with incurable cancer. Behavioral psychosomatic approaches (e.g., biofeedback therapy, hypnosis, cognitive restructuring, and relaxation) have been recommended as psychosocial intervention for pain in cancer patients (19). Indeed, a randomized controlled trial has reported the effectiveness of biofeedback therapy for MPS in cancer patients (20).
A second important finding of this study was the association between armchair sign and psychosomatic-prone MPS in patients with incurable cancer, which demonstrated the clinical utility of the armchair sign for the diagnosis of psychosomatic-prone MPS. Based on the AUC score, the accuracy of the test was close to 1 if the armchair sign false positive (±) was negative (−) (21), which indicated that the test had high accuracy. In this case, specificity and positive predictive value were 100.0%, which suggested that the armchair sign of + was useful for diagnosing psychosomatic-prone MPS. The only clinical characteristic that showed a significant difference between the psychosomatic-prone MPS and control groups was the armchair sign, whereas the sensitivity and negative predictive value of the armchair sign were low, which suggested that the armchair sign of – was not reliable. Thus, our study suggests that the armchair sign is useful as an ancillary test for the diagnosis of psychosomatic-prone MPS in patients with incurable cancer.
We did not find an association between armchair sign and all MPS in patients with incurable cancer. Specifically, there was no association between sustained muscle tension and difficulty in voluntary muscle relaxation. In addition to psychological stress, MPS is associated with physical stress, such as sustained muscle tension due to positional restriction or repetitive movements (22, 23). Despite finding an association between sustained muscle tension and difficulty in voluntary muscle relaxation under psychological stress, we could not ascertain its mechanism. In patients with psychosomatic-prone functional somatic syndrome, a significant negative correlation has been shown between subjective physical tension under psychological stress and objective physiological indices (24). Constant feelings of high physical tension hinders the ability to feel sensations of relaxation (24). This unconscious sustained muscle tension under psychological stress may have resulted in insufficient voluntary muscle relaxation. In addition, alexithymia, a typical characteristic of psychosomatic patients, has been shown to be associated with MPS and is an intensifying factor of cancer pain (25, 26). One possible explanation for the positive correlation between armchair sign and NRS score in this study is that alexithymia was one of the mediating factors.
The study has several limitations. First, this study was a secondary analysis of a prospective observational study. Therefore, sample size was not calculated specifically for the aims of the study.
Second, because psychosomatic disorders are pathological conditions, it was difficult to identify its degree. Given that previous studies have not reported clear criteria (i.e., Rivers’ criteria for MPS) (7), we made a comprehensive judgment based on objective assessments of medical professionals and subjective assessments of patients. In this study, there was no significant difference in diagnoses between psychosomatic physicians who are skilled in diagnosing psychosomatic disorders and oncologists who are less skilled. Third, because there have only been a few studies conducted on the armchair sign, only limited comparisons can be made with other study findings, and discussions regarding the association between the armchair sign and psychosomatic-prone MPS are limited. Finally, our study was a preliminary study.