Repeatability of dermatomal warm and cold sensory thresholds in patients with sciatica.

Quantification of thermal thresholds is a useful method to assess and follow up the function of afferent small A-delta and C-fibres in patients with nerve dysfunctions. The object of this study was to estimate thermal test-retest repeatability in 19 patients with unilateral sciatica (14 L5 and 5 S1) in affected and non-affected dermatomes on the symptomatic (S) and non-symptomatic (NS) sides. Detection thresholds were measured at six sites, two within each of the L4, L5 and S1 dermatomes. The test was repeated after 1-2 h and the coefficient of repeatability (CR=2SD of test-retest differences) was calculated. Warm threshold repeatability did not differ between S and NS sides, but cold threshold CR was higher in the affected dermatome on the foot as compared to the contralateral dermatome ( P=0.04). Warm thresholds were more variable (CR=5 degrees C and 4.7 degrees C on S and NS sides) than cold thresholds (CR=2.2 degrees C and 2.1 degrees C on the S and NS sides). The expected range of variation for the second measurement was between 51% and 200% for warm and between 45% and 230% for cold thresholds. The sensitivity was better on the foot than the lateral calf (5 of 14 vs 1 of 14 abnormal thresholds) in the subgroup with L5 sciatica. We conclude that dermatomal thermotesting has acceptable repeatability, particularly at proximal lower extremity sites. The test may be useful in longitudinal investigations of patients with sciatica, e.g. in treatment follow-up studies.