[1] Hosp Southern Norway Trust, Dept Rheumatol, Kristiansand, Norway
[2] Norwegian Univ Sci & Technol, Fac Med, Dept Neurosci, N-7034 Trondheim, Norway
[3] Norwegian Univ Sci & Technol, Fac Med, Dept Publ Hlth & Gen Practice, N-7034 Trondheim, Norway
[4] Univ Maryland, Sch Med, Dept Med, Baltimore, MD 21201 USA
[5] Univ Maryland, Sch Med, Dept Epidemiol, Baltimore, MD 21201 USA
[6] Univ Maryland, Sch Med, Dept Prevent Med, Baltimore, MD 21201 USA
[7] Univ Agder, Fac Hlth & Sport Sci, Kristiansand, Norway
[8] St Olavs Hosp, Dept Rheumatol, Trondheim, Norway