Prevalence and clinical characteristics of adult polymyositis and dermatomyositis; data from a large and unselected Norwegian cohort.

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.