Although muscle pain is common in primary Sjögren's syndrome (SS), the underlying mechanisms are mainly unknown. We studied all patients with SS at our rheumatology unit with respect to muscle pain in general and to fibromyalgia (FM), and correlated clinical data to muscle biopsy findings.
We investigated 48 patients with SS according to the modified European diagnostic criteria. The ACR criteria for FM were used to subgroup the patients. Muscle biopsy was performed in 36 patients. Light microscope morphology and immunohistochemical expression of MHC class I, MHC class II, and membrane attack complex (MAC) were studied.
We found 44% of patients complained of muscle pain; 27% fulfilled the ACR criteria for FM, whereas 17% had other forms of myalgia. Muscle pain could not be related to histopathological findings. Signs of inflammation were found in 26 of 36 biopsies (72%), and inflammation combined with degeneration/regeneration (i.e., histological signs of polymyositis) in 17 biopsies (47%). However, only 5 patients (14%) had clinical as well as histological signs of polymyositis. Eight muscle biopsies (22%) showed histological features of inclusion body myositis (IBM). However, no patient had clinical symptoms suggestive of this disease. Abnormal expression of MHC class I, MHC class II, and MAC was found in 18 (50%), 16 (44%), and 27 (75%) patients, respectively.
Muscle pain, especially FM, is common in SS. Histopathological signs of myositis are very common in SS. However, muscle symptoms are not related to histological signs of muscle inflammation. IBM-like findings may represent vacuolar myopathic degeneration due to previous subclinical muscle inflammation rather than a specific clinical entity.