Objective: In rheumatoid arthritis (RA) serum creatine kinase (CK) is reduced in association with inflammatory response variables. Our objective was to examine whether low CK is associated with muscle weakness and to what extent the hypothesized relationship between CK and muscle weakness can be explained by anthropometric and sociodemographic variables and/or disease variables.
Methods: Cross sectional and longitudinal retrospective analyses of clinical, radiological, and biochemical data of a prospective cohort of consecutive patients with RA. Isometric muscle strength was measured with a validated muscle strength index (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 analysis, 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 radiological disease variables. In longitudinal dichotomous analyses, worsening in CK was weakly but significantly associated with decreased muscle strength, whereas in linear analyses the association did not reach significance.
Conclusion: In patients with RA, low CK activity is associated with muscle weakness. Demographic, anthropometric, and disease variables related to 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 activity independent of muscle atrophy.