Interhemispheric Inhibition Is Reduced in Response to Acute Muscle Pain: A Cross-Sectional Study Using Transcranial Magnetic Stimulation.

Bilateral deficits in sensorimotor function have been observed in unilateral musculoskeletal pain conditions. Evidence suggests a reduction in interhemispheric inhibition (IHI) from the "affected" (contralateral to the side of pain) to the "unaffected" primary motor cortex (M1) could contribute. However, the effect of short-lasting acute muscle pain on IHI, and whether any changes are related to early sensorimotor changes in the unaffected limb, is unknown. Using a cross-sectional study design, IHI was investigated in 20 healthy individuals before, immediately after, and 30 minutes after the induction of acute muscle pain in the right first dorsal interosseous muscle via a bolus injection of hypertonic saline. Transcranial magnetic stimulation was used to assess corticomotor excitability and short and long latency IHI. Pain intensity and quality were recorded using an 11-point numerical rating scale and the McGill Pain Questionnaire. Pressure pain thresholds were assessed in the affected and unaffected first dorsal interosseous and both tibialis anterior muscles. Participants reported an average pain intensity of 4.8 points (standard deviation = 1.3 points). Compared with baseline, corticomotor excitability was decreased at all time points in the affected but not the unaffected M1. IHI was decreased at all time points from the affected to the unaffected M1. Pressure pain thresholds were decreased over both first dorsal interosseous muscles at 30 minutes of follow-up. These findings suggest a decrease in IHI from the affected to the unaffected M1 that occurs rapidly after the onset of acute pain and could contribute to the development of bilateral symptoms. PERSPECTIVE: The affected M1 (contralateral to the side of pain) releases inhibition over the unaffected M1 within minutes after the onset of acute muscle pain. This finding could have relevance for the development of bilateral sensorimotor symptoms in unilateral pain conditions.