EXTENDED REPORT Prevalence and clinical characteristics of adult polymyositis and dermatomyositis; data from a large and unselected Norwegian cohort Cecilie Dobloug, 1 Torhild Garen, 1 Helle Bitter, 2 Johan Stjärne, 3 Guri Stenseth, 4 Lars Grøvle, 5 Marthe Sem, 6 Jan Tore Gran, 1 Øyvind Molberg 1,7 Handling editor Tore K Kvien ▸ Additional material is published online only. To view please visit the journal online (http://dx.doi.org/10.1136/ annrheumdis-2013-205127) 1 Department of Rheumatology, Oslo University Hospital, Oslo, Norway 2 Department of Rheumatology, Sørlandets sykehus, Kristiansand, Norway 3 Department of Rheumatology, Betanien Hospital, Skien, Norway 4 Revmatismesykehuset, Lillehammer, Norway 5 Department of Rheumatology, Sykehuset Østfold, Moss, Norway 6 Department of Neurology, Sykehuset Østfold, Fredrikstad, Norway 7 Institute of Clinical Medicine, University of Oslo, Oslo, Norway Correspondence to Dr G Cecilie Dobloug, Department of Rheumatology, Oslo University Hospital, Pb 4950 Nydalen, Oslo N-0424, Norway; gdobloug@ous-hf.no Received 20 December 2013 Revised 11 March 2014 Accepted 16 March 2014 Published Online First 2 April 2014 To cite: Dobloug C, Garen T, Bitter H, et al. Ann Rheum Dis 2015;74: 1551–1556. ABSTRACT Objectives The occurrence of polymyositis (PM) and dermatomyositis (DM) in the general population is largely unknown and unbiased data on clinical and laboratory features in PM/DM are missing. Here, we aim to identify and characterise every PM/DM patient living in southeast Norway (denominator population 2.64 million), 2003–2012. Method Due to the structure of the Norwegian health system, all patients with PM/DM are followed at public hospitals. Hence, all public hospital databases in southeast Norway were screened for patients having ICD-10 codes compatible with myositis. Manual chart review was then performed to identify all cases meeting the Peter & Bohan and/or Targoff classification criteria for PM/DM. Results The ICD-10 search identified 3160 potential myositis patients, but only 208/3160 patients met the Peter & Bohan criteria and 230 the Targoff criteria (100 PM, 130 DM). With 56 deaths during the observation period, point prevalence of PM/DM was calculated to 8.7/100 000. Estimated annual incidences ranged from 6 to 10 /1 000 000, with peak incidences at 50–59 (DM) and 60–69 years (PM). Myositis specific antibodies ( Jo-1, PL-7, PL-12, signal recognition particle (SRP) and Mi-2) were present in 53% (109/204), while 137/163 (87%) had pathological muscle MRI. Frequent clinical features included myalgia (75%), arthritis (41%) dyspnoea (62%) and dysphagia (58%). Positive anti-Jo- 1, present in 39% of DM and 22% of PM cases, was associated with dyspnoea, arthritis and mechanic hands. Conclusions Our data indicate that the population prevalence of PM/DM in Caucasians is quite low, but underscores the complexity and severity of the disorders. INTRODUCTION The idiopathic inflammatory myopathies (IIM) include three clinical syndromes; polymyositis (PM), dermatomyositis (DM) and sporadic inclu- sion body myositis (sIBM). 1 PM and DM are sys- temic, inflammatory disorders, while sIBM appears to be a primary degenerative condition. 2 In the current study, we focus only on PM and DM. PM and DM are characterised by symmetrical weakness of proximal muscle groups. Additionally, patients with DM have a distinct rash. Inflammation in other organ systems are frequent in PM and DM; with interstitial lung disease as a major cause of mor- tality. 3 Cancer is also a major clinical problem, par- ticularly in DM. 4 Autoantibodies are common in PM and DM, and more than 10 different, mutually exclusive myositis-specific antibodies (MSA) have been described. 5 Interestingly, the MSA are asso- ciated with distinct clinical syndromes that often cross the classical distinction between PM and DM. 6–8 The Peter & Bohan diagnostic criteria from 1975 is still the gold standard when it comes to classifying PM and DM cases for research purposes. 9 These cri- teria include key clinical features (muscle weakness and DM rash) and laboratory parameters (serum level of muscle enzymes, electromyography (EMG) and muscle histology), but not MSA or MRI of muscle tissue. Revised classification criteria, building on the Peter & Bohan criteria, with inclusion of MSA and MRI were proposed by Targoff et al in 1997. 10 Other classification criteria have also been suggested. 11–13 More recently, larger-scale consen- sus efforts have been undertaken by the International Myositis Classification Criteria Project (IMCCP). Interestingly, preliminary data from IMCCP, presented at the EULAR-meeting in 2013, indicated that the Targoff criteria show the best sen- sitivity and specificity of established criteria. 14 Data on the epidemiology of PM/DM are limited, probably due to the rarity of the diseases, heterogeneous study populations and difficulties with classification. 1 12 15 To our knowledge, there is only one population-based study on DM, with 29 cases defined solely by clinical features, 16 and some few retrospective studies based on chart reviewing. 17–25 The largest of these chart review studies, performed in the Allegheny County in Pennsylvania from 1963–1982, used predefined clinical criteria for case assignment, and identified 177 PM/DM cases. Interestingly, the study reported that the PM/DM incidence tripled during the study period. 17 Few studies have been undertaken in Europe, but an annual incidence of 7.6 cases/ million was estimated in a Swedish study with 21 PM/DM cases. 18 Prevalence data for PM and DM vary from 5/100 000 to 21.5/100 000 depending on methods used for obtaining data, highest estima- tions done by calculated medical administrative data in the USA and Canada. 26–28 Overall, there appears to be large knowledge gaps on the epidemiology of myositis; little is known about the occurrence of PM/DM in the general population, and unbiased data on the fre- quencies of key clinical and laboratory features are largely missing. Hence, the aims of this study were Clinical and epidemiological research Dobloug C, et al. Ann Rheum Dis 2015;74:1551–1556. doi:10.1136/annrheumdis-2013-205127 1551 group.bmj.com on September 18, 2016 - Published by http://ard.bmj.com/ Downloaded from