This study has examined motor-evoked potentials (MEPs) and inhibition of voluntary contraction induced by transcranial magnetic stimulation (TMS) of the motor cortex in the paraspinal muscles of the lumbar region in patients suffering from chronic low back pain (LBP).

With local ethical approval and informed consent eighteen patients with LBP were recruited from the orthopaedic and fracture clinic at Charing Cross Hospital and compared with ten control subjects. The patients were on the waiting list for surgery or caudal epidural drug injections to alleviate the symptoms. Electromyographic (EMG) recordings were made bilaterally from the erector spinae (ES) muscles at the L4 spinal level using self-adhesive surface electrodes. TMS was delivered using a MagStim 200 stimulator connected to an angled double-cone coil, which was positioned with its cross-over above the vertex, and the induced current in the brain flowing in a posterior to anterior direction. The threshold (expressed as percentage of maximum stimulator output, %MSO) for inducing MEPs in muscles contracted to 20 % of their maximum voluntary contraction (MVC) was assessed. TMS was delivered in steps equivalent to 0.1 X threshold starting at a minimum intensity of 1.2 X threshold and working downwards. Sets of a minimum of six stimuli were delivered during voluntary contraction to 20 % MVC of ES muscles. The duration and latency of the MEPs were measured and the duration and extent (percentage of voluntary EMG remaining) of the subsequent inhibition were measured even at stimulus intensities subthreshold for inducing MEPs. Statistical comparisons were made using Student’s t test within patients between the left and right muscles (paired) and between patients and control subjects (unpaired). Statistical significance was taken when P < 0.05.

At 1.2 X threshold TMS there was no difference in latency or duration of the MEPs between the left and right back muscles in the patients or controls; the data for both sides were therefore pooled and there were no differences between the patients and the control subjects. The duration and extent of inhibition over a range of stimulus strengths were not different between left and right muscles in the patients or between the patients and the control subjects. There were no significant differences in the thresholds for MEPs or inhibition between left and right muscles in the patients or in the controls. However, the mean (± S.E.M.) thresholds for eliciting MEPs and inhibition in the ES muscles were higher in the patients (MEPs: 42.85 ± 1.63 %MSO; inhibition: 37.12 ± 1.78 %MSO) than in the control subjects (MEPs: 35.80 ± 2.32 %MSO; inhibition: 29.60 ± 1.70 %MSO).

The changed excitatory and inhibitory thresholds seen in LBP patients indicates altered corticospinal excitability in the ES muscles, possibly related to protective strategies. The raised inhibition threshold, in particular, suggests altered neurophysiological function at a cortical level. We will investigate whether the changed excitability persists after remission of pain, as this could help to explain why patients are vulnerable to further episodes of LBP.

This work was supported by the Arthritis Research Campaign.