Musculoskeletal manifestations occur predominantly in patients with later-onset familial Mediterranean fever: Data from a multicenter, prospective national cohort study in Japan.

Background: We showed previously that Japanese individuals with familial Mediterranean fever (FMF) have a more atypical phenotype compared to endemic areas. The clinical differences between young-onset FMF (YOFMF), adult-onset FMF (AOFMF), and late-onset FMF (LOFMF) in Japan are unclear.

Methods: We enrolled 395 consecutive patients. We defined YOFMF, AOFMF, and LOFMF as the onset of FMF at < 20, 20-39, and ≥ 40 years of age, respectively. We compared clinical manifestations and MEFV mutations patterns among these groups.

Results: Median ages at onset were YOFMF 12.5 years (n = 182), AOFMF 28 years (n = 115), and LOFMF 51 years (n = 90). A family history, MEFV mutations in exon 10, and more than two MEFV mutations were significantly more frequent in the earlier-onset groups (p < 0.01, p < 0.0001, and p < 0.001, respectively). In the accompanying manifestations, thoracic and abdominal pain were significantly more frequent in the earlier-onset groups (p < 0.01 and p < 0.0001, respectively), whereas arthritis and myalgia were significantly more frequent in the later-onset groups (p < 0.0001 and p < 0.01, respectively). The multiple logistic regression analysis revealed that the presence of MEFV exon 10 mutations and earlier onset were significantly associated with serositis, whereas the absence of MEFV exon 10 mutations, later onset, and the presence of erysipelas-like erythema were significantly associated with musculoskeletal manifestations. There was no significant between-group difference in the responsiveness to colchicine.

Conclusions: Our results indicate that the later-onset FMF patients had a lower percentage of MEFV mutations in exon 10 and predominantly presented arthritis and myalgia. It is important to distinguish their FMF from other inflammatory diseases.