LOW SERUM CREATINE-KINASE ACTIVITY IS ASSOCIATED WITH MUSCLE WEAKNESSIN PATIENTS WITH RHEUMATOID-ARTHRITIS

Objective. In rheumatoid arthritis (RA) serum creatine kinase(CK) is r educed in association with inflammatory response variables. Our object ive was to examine whether low CK is associated with muscle weakness a nd to what extent the hypothesized relationship between low CK and mus cle weakness can be explained by anthropometric and sociodemographic v ariables and/or disease variables. Methods. Cross sectional and longit udinal retrospective analyses of clinical, radiological, and biochemic al data of a prospective cohort of consecutive patients with RA. Isome tric muscle strength was measured with a validated muscle strength ind ex (MSI); CK was measured with an enzymatic assay (N-acetyl-cysteine, 37 degrees C). Results. 65 patients were enrolled in the study and we obtained complete one year followup data from 47. In cross sectional a nalysis, CK was a significant, moderate correlate of the MSI (r = 0.43 , p < 0.01). CK remained a significant explanatory variable of the MSI in multivariate models that controlled for demographic variables and lean body mass, corticosteroid use, and biochemical, clinical; and rad iological disease variables, In longitudinal dichotomous analyses, wor sening in CK was weakly but significantly associated with decreased mu scle strength, whereas in linear analyses the association did not reac h significance. Conclusion In patients with RA, low CK activity is ass ociated with muscle weakness. Demographic, anthropometric, and disease variables related tc, muscle mass or muscle atrophy explain only part of this association. Our findings support the hypothesis that muscle weakness may be partly caused by a disease related reduction of CK act ivity independent of muscle atrophy.