The Response of the Primary Motor Cortex to Neuromodulation is Altered in Chronic Low Back Pain: A Preliminary Study

Objective. Neuromodulation is increasingly investigated for the treatment of low back pain (LBP). However, the neurophysiological effects of common neuromodulatory techniques (anodal transcranial direct current stimulation [tDCS] and peripheral electrical stimulation [PES]) have not been investigated in people with chronic LBP. Here we aimed to compare the effect of three neuromodulatory protocols (anodal tDCS, high intensity PES, and a priming protocol of combined tDCS/PES) on primary motor cortex (M1) excitability in people with and without chronic LBP. Design. Cross-sectional. Setting. University laboratory. Participants. Ten individuals with chronic LBP and 10 pain-free controls. Methods. Participants received four interventions in random order across separate sessions: 1) anodal tDCS to M1+PES to the back muscles; 2) tDCS+sham PES; 3) sham tDCS+PES; or 4) sham tDCS+sham PES. Motor cortical excitability (map volume, discrete map peaks, and cortical silent period [CSP]) was measured before and after each intervention. Results. Anodal tDCS increased M1 excitability (increased map volume and reduced CSP) in controls but had no effect in the LBP group. PES reduced M1 excitability in both groups. The combined tDCS+PES treatment increased M1 excitability in the LBP group but had no effect in controls. Conclusions. The neurophysiological response to common neuromodulatory treatments differs between people with and without LBP. This has relevance for the design and tailoring of neuromodulation in pain. Further, if the goal of treatment is to increase M1 excitability, a priming protocol (e.g., combined tDCS+PES) may be more effective than tDCS alone.