Measuring sensation in the feet of patients with rheumatoid arthritis

(1) To determine the test-retest reliability of monofilaments pressed against the skin as a method of assessing sensation in the feet of patients with rheumatoid arthritis (RA) and in controls using two grades of monofilaments; (2) to determine the stability of findings over 6 weeks; and (3) to calculate initial estimates of frequency of loss of sensation and to investigate its association with disease status.

Clinical examination of the feet was undertaken in 51 patients with RA and 20 normal controls. Six sites on each foot were tested twice with both 10 g and 3 g research grade monofilaments and this was repeated after 6 weeks. Disease status was measured using the Disease Activity Score, the Health Assessment Questionnaire, visual analogue scales of pain, and the acute phase response using erythrocyte sedimentation rate and plasma viscosity.

Reproducibility was high for 3 g (kappa=0.73) and 10 g (kappa=0.75) monofilaments. The best balance between sensitivity (58.8%) and specificity (87.5%) for distinguishing the feet of patients from the feet of controls was using the 3 g filament and defining reduced protective sensation as being sensitive to less than 11 of 12 applications. Using this definition, the prevalence of reduced protective sensation is 59% in the patient group and 12.5% in the feet of controls. There was some variation in sensation over 6 weeks in the patient group, but this was not related to measures of clinical status.

The use of monofilaments in assessing sensation levels in the RA foot is repeatable and reproducible over a six-week period and requires only a short time to perform. The frequency of reduced sensation in the feet of patients with RA was greater than previously reported. Future studies should assess relationships with disease duration and inflammatory status.