Warm and cold sensory thresholds in patients with unilateral sciatica: C fibers are more severely affected than A-delta fibers.

Abstract

In order to evaluate if nerve root compression or inflammation is the most important pathogenetic mechanism in lumbar radicular pain, we investigated unmyelinated C-fiber function (warm sensation) and myelinated A-delta fiber function (cold sensation) in patients with unilateral L5 or S1 sciatica. Forty consecutive patients with clinical and radiological evidence of unilateral L5 (n = 29) or S1 (n = 11) sciatica were studied. The warm and cold sensory thresholds (Somedic thermotest, method of limits) were measured on the anterolateral leg (L5 dermatome) and on the calf (S1 dermatome) on both sides. Warm thresholds were significantly higher on the symptomatic side compared to the non-symptomatic side (8.4+/-3.0 vs 6.2+/-2.5 degrees C, P < 0.0005) in the affected dermatome. In a subgroup with confirmed disk herniation at surgery (32 of the 34 operated), significant differences between the symptomatic and the non-symptomatic side for the affected dermatome, were found for both warm (P < 0.0005) and cold (P = 0.003) thresholds. No threshold difference was seen in patients with disk herniations contained within the outer annulus fibrosis (n = 22) compared to those with non-contained herniations (n = 10). Patients with unilateral sciatica and L5 or S1 nerve root involvement had increased warm thresholds suggesting impaired C-fiber function. Cold thresholds were significantly elevated in a subgroup with operatively confirmed disk herniation. Because myelinated axons are affected more by compression than unmyelinated ones, our results suggest that nerve root inflammation is more important than compression per se in the generation of sciatic pain.