Histopathological studies have revealed that the synovium, capsule, and bursae of the shoulder of patients with PMR have inflammatory changes with infiltration of T lymphocytes and macrophages, and increased vascularity [16, 17]. Various imaging tests including US and MRI have been used to detect inflammation to diagnose PMR [8]. Adding US results increased the sensitivity and specificity (66 and 81%, respectively) over the diagnostic criteria alone; however, this increase is relatively small [15]. No US consensus for PMR diagnosis exists in terms of involved joints, range of sites to be examined, or specific findings [8, 15].
MRI is a useful technique to detect inflammatory changes in joints and adjacent tissues. Reports have described various MRI findings in PMR [8].
Similar to other published results [11] from PMR cases, we found FDG accumulation in all reported PMR-specific sites, and in sternoclavicular joints like the report using bone scintigraphy [8]. Moreover, all PMR patients in the study had PET-positive scores > 2. Three cases in the non-PMR group had FDG accumulation in one site (shoulder, ischial tuberosity, or greater trochanter); however, their PET-positive scores were < 2.
While US, MRI, and FDG–PET/CT may all detect tissue inflammation in patients with PMR, whether these three imaging examinations can discriminate between polymyalgia-like illnesses and PMR is not clear. In the 2012 ACR/EULAR provisional classification criteria for PMR, adding an US examination decreased the specificity for discriminating RA from PMR to 65% [15]. Ochi et al. reported that MRI findings in severe rotator cuff tendinopathy, periarticular soft tissue edema, and large effusions in and around the shoulder and hip joints are useful indicators for diagnosing PMR, and also for discriminating RA from PMR [18].
Takahashi et al. reported the differences in FDG–PET/CT findings between patients with PMR and those with elderly-onset RA: In the shoulders and hips, they observed specific uptake patterns in each group with circular and linear uptake patterns around the humeral head in the case of RA, and focal and non-linear uptake patterns in the case of PMR [19]. Moreover, focal uptake in front of the hip joint, indicating iliopectineal bursitis, tended to be limited to the patients with PMR [19].
We did not include RA patients in our non-PMR group because no RA patients within the study period fulfilled the Bird’s criteria, and imaging examination is an ancillary procedure for diagnosis in the routine clinical practice. When discriminating RA, physicians look for peripheral small joint arthritis, the presence of serum rheumatoid factor and anti-CCP antibody, and the diagnostic RA criteria [20]. We understand that imaging techniques like FDG–PET/CT, US, or MRI, are not the sole basis for distinguishing the different possible etiologies of inflammation in one site.
We found that FDG accumulation in the shoulder joints was a predictor with higher sensitivity and specificity for diagnosing PMR. However, a limitation of this study was that it included a small population. In addition, if we had included RA patients with shoulder joint swelling in the study, the results could have been different.
The pathogenesis of myalgia in a variety of conditions that mimic PMR is still uncertain [1]. Inflammatory changes in polymyalgia-like illnesses have not been confirmed by histopathologic examinations. In our study, patients in the non-PMR group showed significantly less FDG accumulation in PMR-specific sites than those in the PMR group. These results suggest that the PMR pathogenesis may differ from that of non-PMR illnesses, even if the clinical presentations are similar. Inflammatory changes with infiltration of lymphocytes and macrophages, and increased vascularity may not be seen in the synovium and bursae of non-PMR disorders.
In this study, the mean MMP-3 level was significantly higher in the patients in the PMR group than in those in the non-PMR group and untreated RA patients. However, we also found the level of MMP-3 to change over a wide range. Thus, the MMP-3 value may be misleading in individual cases. More data is needed to determine whether a threshold level for serum MMP-3 that can distinguish between these disorders exists or if other blood tests can be combined with MMP-3 values to diagnose PMR.
To diagnose polymyalgia-like illnesses, physical examination findings need to be carefully considered, and blood tests and imaging tests should be planned to address a wide range of diagnoses. An FDG–PET/CT may be helpful, not only for differentiating PMR from polymyalgia-like illnesses, but also for determining the correct underlying diagnosis.