Afferent inhibition and cortical silent periods in shoulder primary motor cortex and effect of a suprascapular nerve block in people experiencing chronic shoulder pain

Objective: To characterise short afferent inhibition (SAI) and the cortical silent period (CSP) in the primary motor cortex representations of the infraspinatus muscle in healthy adults and people experiencing chronic shoulder pain, to determine the impact of a suprascapular nerve block (SSNB). Methods: Neurophysiological measures were obtained in 18 controls and 8 patients with chronic shoulder pain, pre and post SSNB and 1 week later. Pain intensity was assessed by a visual analogue scale. Results: SAI was apparent in controls (all P < 0.03) and a CSP was observed which reduced in the presence of SAI (all P < 0.0001). Compared to controls, shoulder pain patients demonstrated higher active motor threshold (P = 0.046), less SAI (P = 0.044), a longer CSP (P = 0.048) and less modulation of the CSP by SAI (P = 0.045). Higher motor thresholds were related to higher pain scores (P = 0.009). The SSNB immediately restored SAI (P = 0.013), with a positive relationship between increased SAI and reduced pain (P = 0.031). The SSNB further reduced modulation of CSP by SAI at 1 week post injection (P = 0.006). Conclusions: SAI and the CSP were present and demonstrated robust interaction in controls, which was aberrant in patients. The SSNB transiently restored SAI but had no effect on the CSP; however CSP modulation by SAI was further attenuated 1 week post injection. Significance: The current findings improve understanding of the neurophysiology of the shoulder motor cortex and its modulation by chronic pain. The effect of SSNB in shoulder pain patients should be interpreted with caution until proven in a larger population. Interventions that target intracortical inhibition might increase efficacy in people with chronic shoulder pain. (C) 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.