The assessment of radiating low back pain by thermal sensory testing

Low back pain radiating into the legs is a common pain syndrome. However, neurological examination, imaging and electromyographic studies are of limited value for prognosis or therapy. The origin of the pain remains unknown. The aim was to evaluate the potential of thermal sensory testing to serve as a diagnostic tool in 24 patients who had low back pain radiating down the S1 dermatome, compared with 26 pain-free controls. The method of limits was used to detect the thresholds of warm sensation, cold sensation, warm pain and cold pain at the L4, L5 and S1 dermatomes of the symptomatic and the non-symptomatic legs. Thresholds on the asymptomatic leg were similar to values obtained in controls. We found a significantly higher threshold for cold sensation in the S1 dermatome of the symptomatic leg of the patients compared with the controls (p< 0.005). In addition, patients who had abnormal neurological examination (50%) had higher thresholds for cold sensation or cold pain in the three dermatomes tested at the symptomatic leg compared with the non-symptomatic leg. No differences in the thresholds of warm sensation or warm pain were detected. We propose that these findings indicate selective damage to the Adelta fibres which are involved in transmission of cold sensation and pain, presumably by root compression. We found no evidence of involvement of C fibres, which transmit warm sensation and pain. Thermal testing should be considered among the testing modalities that are capable of demonstrating objective findings in patients with radiating low back pain.