Determinants of WOMAC function, pain & stiffness scores: evidence for the role of low back pain, symptom counts, fatigue & depression in OA, RA & Fibromyalgia (FM)

OBJECTIVES: The Western Ontario MacMaster (WOMAC) is a
validated instrument designed specifically for the assessment
of lower extremity pain and function in osteoarthritis (OA) of
the knee or hip. In the clinic, however, we have noted that OA
patients frequently have other musculoskeletal and
non-musculoskeletal problems that might contribute to the
total level of pain and functional abnormality that is
measured by the WOMAC. In this report, we investigated back
pain and non- articular factors that might explain WOMAC
scores in patients with OA, rheumatoid arthritis (RA) and
fibromyalgia (FM) in order to understand the specificity of
this instrument.

METHODS: RA, OA and FM patients participating
in long-term outcomes studies completed the WOMAC and were
assessed for low back pain, fatigue, depression and rheumatic
disease symptoms by mailed questionnaires.

RESULTS: Regardless
of diagnosis, WOMAC functional and pain scores were very much
higher (abnormal) among those complaining of back pain. On
average, WOMAC scores for back pain (+) patients exceeded
those of back pain (-) patients by approximately 65%,, and 52%
of OA patients reported back pain. In regression analyses,
study symptom variables explained 42, 44 and 38% of the
variance in WOMAC function, pain and stiffness scores,
respectively. In the subset of OA patients, radiographic
scores added little to the explained variance. The strongest
predictor of WOMAC abnormality in bivariate and multivariate
analyses was the fatigue score, with correlations of 0.58,
0.60 and 0.53 with WOMAC function, pain and stiffness,
respectively. The WOMAC performed well in RA and FM, and
correlated strongly with the Health Assessment Questionnaire
(HAQ) disability scale and a visual analogue scale (VAS) pain

CONCLUSION: The WOMAC captures more than just knee or
hip pain and dysfunction, and is clearly influenced by the
presence of fatigue, symptom counts, depression and low back
pain. WOMAC scores also appear to reflect psychological and
constitutional status. These observations suggest the need for
care in interpreting WOMAC scores as just a measure of
function, pain or stiffness, and indicate the considerable
importance of psychological factors in rheumatic disease and
rheumatic disease assessments.

Wolfe F

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