A role for the autonomic nervous system in low back pain with nerve root compression?

Jacqueline A. Dean & Muftah S. Eljamel

Pain Research Institute, Rice Lane, Liverpool L9 1AE, UK

Introduction

It is an established fact that patients with nerve root compression in the lumbar region (L4/5/S1) often display temperature changes in the foot on the painful side (Lindholm et al, 1981; Uematsu et al. 1988). This study has examined, by means of laser-Doppler flowmetry, the skin blood flow and the responses to autonomic challenge in both feet in patients with low back and leg pain, with and without nerve root entrapment.

Materials and Methods

22 recently diagnosed (<12 months) patients were included in the study of whom 10 had L4/L5 or L5/S1 disc displacement, 9 had lumbar spinal stenosis and 4 had pain from causes other than root compression. Diagnoses were confirmed by myelograms and CT scans. Patients were stabilised in a temperature-controlled environment for 20 minutes prior to the tests. During this time skin blood flow was monitored from the dorsal surface of the large toe by a dual channel laser-Doppler flowmeter (MBF3D, Moor Instruments UK). When readings had stabilised autonomic challenge was delivered in the form of a maintained hand grip to approximately one third maximum for 1.5 minutes, followed 10 minutes later by immersion of one hand in a bucket of ice cold water for 1 minute. Skin temperature was monitored continually from the soles of the feet during the whole procedure by skin surface thermistors (YSI). Liquid crystal contact thermographic pictures were taken of the soles of the feet.

Results

A typical normal response to autonomic challenge is shown in Fig Of the 15 patients with root compression and unilateral symptoms, 12 responded in the manner shown in Fig. ie the unaffected side showed greater blood flow fluctuations in response to autonomic challenge than did the affected side (U > A). One responded in the opposite manner (A > U) and 2 gave a parallel response. 2 patients with bilateral symptoms also gave parallel responses. Two cases who had had root compression diagnosed but who were pain free at the time of the tests showed parallel responses. Of the 4 patients with no root compression, none responded in the manner shown.

Conclusion

It is suggested that the disturbances in blood flow seen in patients with nerve root compression could reflect bilateral central or autonomic changes to a unilateral injury in the dorsal horn of the spinal cord. Transsynaptic degenerative changes in response to chronic constriction of the sciatic nerve in rats have been demonstrated in the dorsal horn of the spinal segments innervated by the damaged nerve (Sugimoto et al., 1990). Several lines of evidence indicate that high levels of ectopic discharge in damaged primary afferents may be a critical factor in the production of transsynaptic degeneration.

[...]