Background Rare diseases (RD) in rheumatology are heterogeneous and thus lack a systemic classification. By definition, prevalences of RD range up to 5/10 000 in Europe. Clinical courses comprise a vast variety of musculoskeletal symptoms such as arthritis, myositis, vasculitis, autoimmune organ involvement and bone diseases. The impact of RD for health care systems is widely unknown and individual patients are prone to unsatisfactory diagnoses and/or treatments.

Objectives The objective of this study was to identify and classify RD in rheumatology and estimate their combined prevalence. The data may give a better understanding for this area of rheumatology, may aid specified centers for RD, and perhaps sharpen the overall perception of health care systems.

Methods Databases (pubmed.org, abstract archives of the European League Against Rheumatism and the American College of Rheumatology, and Orpha.net) were searched for the following terms: rare in combination with arthritis, arteritis, connective tissue disease, rheumatic and vasculitis. Furthermore, terms were used in various combinations including arthralgia, autoimmune, fever, inflammation, joint pain, muscle pain, myalgia and swollen joint. Identified syndromes were then classified according to the following terms and data: differential diagnosis, etiology, genetics, prevalence, principal symptom, prognosis, and therapy. The overall total point prevalence for all RD in rheumatology was estimated by adding up all single point prevalences available.

Results A total of 82 syndromes and diseases were identified. Classification was as follows: arthritis (n=17) and vasculitis (n=18), fever syndromes (n=13), collagen diseases (n=10), myositis (n=9), and overlaps/others (n=15). Fifty-two diseases showed a chronic progressive course. The mortality was variable in many diseases and thus could not be determined precisely. Point prevalence data were available for 49 syndromes and diseases, and added up to a total point prevalence of 49/ 10 000. Forty-nine of these in part pathophysiologically quite distinct RD are treated with corticosteroids.

Conclusions Rare diseases in rheumatology are most often chronic or progressive. Symptoms are variable, they include arthritis, organ and skin involvement. The estimated combined prevalence of all RD in rheumatology is significant with 49/10 000: it is double or more than that of ankylosing spondylitis with 18/10 000 ⁽¹⁾. Treatment options are often restricted to corticosteroids presumably because of the scarcity of randomized controlled trials. Health care systems should assign RD the same importance as any other common disease in rheumatology.

1. Exarchou S, Lindström U, Askling J, Eriksson JK, Forsblad-d'Elia H, Neovius M, Turesson C, Kristensen LE, Jacobsson LT. The prevalence of clinically diagnosed ankylosing spondylitis and its clinical manifestations: a nationwide register study. Arthritis Res Ther 2015; 201; 17(1):118.

Disclosure of Interest None declared