Low somatosensory cortex excitability in the acute stage of low back pain causes chronic pain

BACKGROUNDDetermining the mechanistic causes of complex biopsychosocial health conditions such as low back pain (LBP) is challenging, and research is scarce. Cross-sectional studies demonstrate altered excitability and organisation of the primary somatosensory and primary motor cortex in people with acute and chronic LBP, however, no study has explored these mechanisms longitudinally or attempted to draw causal inferences. METHODSProspective, longitudinal, cohort study including 120 people with an acute episode of LBP. Sensory evoked potential area measurements were used to assess primary and secondary somatosensory cortex excitability. Transcranial magnetic stimulation derived map volume was used to assess corticomotor excitability. Directed acyclic graphs identified variables potentially confounding the exposure-outcome relationship. The effect of acute-stage sensorimotor cortex excitability on six-month LBP outcome was estimated using multivariable regression modelling, with adjusted and unadjusted estimates reported. Sensitivity analyses were performed to explore the effect of unmeasured confounding and missing data. RESULTSLower primary (OR = 2.08, 95% CI = 1.22 to 3.57) and secondary (OR = 2.56, 95% CI = 1.37 to 4.76) somatosensory cortex excitability in the acute stage of LBP increased the odds of developing chronic pain at six-month follow-up. This finding was robust to confounder adjustment and unmeasured confounding (E-Value = 2.24 & 2.58, respectively). Corticomotor excitability in the acute stage of LBP was associated with higher pain intensity at 6-month follow-up (B = -0.15, 95% CI: -0.28 to -0.02) but this association did not remain after confounder adjustment. CONCLUSIONThese data provide the first evidence that low somatosensory cortex excitability in the acute stage of LBP is a cause of chronic pain. Interventions designed to increase somatosensory cortex excitability in acute LBP may be relevant to the prevention of chronic pain.