Prevalence of polymyalgia rheumatica in a cohort of patients with idiopathic inflammatory myopathy

Polymyalgia rheumatica (PMR) is a common rheumatological condition occurring in adults aged over 50 years. The association of PMR with other autoimmune diseases such as rheumatoid arthritis is complex. There is a clear relationship with giant cell arteritis. We sought to determine whether there is any association between PMR and idiopathic inflammatory myopathy (IIM). We undertook a database study of adult patients with a diagnosis of IIM under the care of the Rheumatology Department of the Royal Adelaide Hospital, and retrospectively determined the frequency of PMR in this patient cohort. Patients were considered to have PMR if this had been diagnosed by a physician or if they satisfied all five of the following clinical criteria: (1) bilateral shoulder and/or pelvic girdle aching, (2) morning stiffness exceeding 45 min, (3) age greater than 50 years, (4) duration more than 2 weeks, and (5) evidence of an elevated erythrocyte sedimentation rate or C-reactive protein. Amongst 82 patients with IIM, seven (8.5%) were found to have PMR. There was a disproportionate representation of necrotizing autoimmune myopathy (NAM) in patients with IIM/PMR (5/7) compared with patients with IIM alone (19/75), p = 0.02. There was a high prevalence of antibodies to 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) in patients with IIM/PMR (4/7, 57%) compared with those with IIM alone (9/32, 28%), p = 0.19. The prevalence of PMR in our IIM cohort was much higher than the population prevalence 0.91-1.53% and it appears there is a specific association with NAM and anti-HMGCR antibodies. Further studies are required to confirm these findings and explore mechanisms of shared disease susceptibility.