Compression at Myofascial Trigger Point on Chronic Neck Pain Provides Pain Relief through the Prefrontal Cortex and Autonomic Nervous System: A Pilot Study

Abstract

Compression at myofascial trigger points (MTrPs), known as "ischemic compression," has been reported to provide immediate relief of musculoskeletal pain and reduce the sympathetic activity that exacerbates chronic pain. We conducted a pilot study to investigate the possible involvement of the prefrontal cortex in pain relief obtained by MTrP compression in the present study, and analyzed the relationships among prefrontal hemodynamic activity, activity of the autonomic nervous system, and subjective pain in patients with chronic neck pain, with and without MTrP compression. Twenty-one female subjects with chronic neck pain were randomly assigned to two groups: MTrP compression (n = 11) or Non-MTrP compression (n = 10). Compression for 30 s was conducted 4 times. During the experiment, prefrontal hemodynamic activity [changes in Oxy-hemoglobin (Hb), Deoxy-Hb, and Total-Hb concentrations] and autonomic activity based on heart rate variability (HRV) were monitored by using near infrared spectroscopy (NIRS) and electrocardiography (ECG), respectively. The results indicated that MTrP compression significantly reduced subjective pain compared with Non-MTrP compression. The spectral frequency-domain analyses of HRV indicated that a low frequency (LF) component of HRV was decreased, and a high frequency (HF) component of HRV was increased during MTrP compression, while LF/HF ratio was decreased during MTrP compression. In addition, prefrontal hemodynamic activity was significantly decreased during MTrP compression compared with Non-MTrP compression. Furthermore, changes in autonomic activity were significantly correlated with changes in subjective pain and prefrontal hemodynamic activity. Along with previous studies indicating a role for sympathetic activity in the exacerbation of chronic pain, the present results suggest that MTrP compression in the neck region alters the activity of the autonomic nervous system via the prefrontal cortex to reduce subjective pain.

Keywords: autonomic nervous system; massage; myofascial pain syndrome; near-infrared spectroscopy; pain; prefrontal cortex.

Figure 1

Participant flow diagram.

Figure 2

Location of NIRS probes and channels. (A) Location of the NIRS head pad on the frontal region. (B) The arrangement of probes (sources and detectors) and channels. (C) Spatial estimation of NIRS channels. The coordinates of each NIRS channel were normalized to MNI (Montreal Neurological Institute) space using virtual registration. The red dots indicate the coordinates of the channels: a, right dorsolateral prefrontal cortex (rDLPFC) (Brodmann Areas 45, 46); b, right dorsomedial prefrontal cortex (rDMPFC) (Brodmann Area 10); c, central dorsomedial prefrontal cortex (cDMPFC) (Area 10); d, left dorsomedial prefrontal cortex (lDMPFC) (Area 10); e, left dorsolateral prefrontal cortex (lDLPFC) (Areas 45, 46).

Figure 3

Comparison of changes in subjective pain scores after compression between the MTrPs and Non-MTrP groups. Ischemic compression at MTrPs significantly decreased subjective pain scores compared with Non-MTrP compression. Error bars indicate SEM. VAS, visual analog scale; ^**P < 0.01.

Figure 4

Comparison of changes in the activity of the autonomic nervous system during ischemic compression between the MTrP and Non-MTrP groups. (A) High frequency (HF) components of HR variability (HF%) were significantly increased during ischemic compression at MTrPs compared with Non-MTrPs. (B) Low frequency (LF) components of HR variability (LF%) was significantly decreased during ischemic compression at MTrPs compared with Non-MTrPs. (C) LF/HF ratio was significantly decreased during ischemic compression at MTrPs compared with Non-MTrPs. Error bars indicate SE; ^**P < 0.01.

Figure 5

Time course and topographical maps of Oxy-Hb changes during Non-MTrP (A) and MTrP (B) compression. Topographical maps indicate Oxy-Hb maps 25 s after starting ischemic compression. NIRS image data were superimposed on individual 3D-MRIs. Yellow dots on the 3D-MRI indicate NIRS channels. Red lines indicate changes in Oxy-Hb concentrations from baseline activity. Note that the Oxy-Hb concentration in the prefrontal cortex was decreased during MTrP compression.

Figure 6

Comparison of hemodynamic responses in the prefrontal regions during ischemic compression between MTrPs and Non-MTrP groups. Hemodynamic responses are shown as effect sizes. Statistical analysis by repeated-measures 2-way ANOVA indicated that MTrP compression significantly decreased the effect size compared with Non-MTrP compression. Error bars indicate SE. rDLPFC, right dorsolateral prefrontal cortex; rDMPFC, right dorsomedial prefrontal cortex; cDMPFC, central dorsomedial prefrontal cortex; lDMPFC, left dorsomedial prefrontal cortex; lDLPFC, left dorsolateral prefrontal cortex.

Figure 7

Correlations between changes in autonomic activity and changes in subjective pain scores (A–C), and between changes in autonomic activity and changes in hemodynamic responses in the DMPFC (D–F). (A) A negative correlation was observed between changes in HF% and changes in subjective pain scores. VAS, visual analog scale. (B) A positive correlation was observed between changes in LF% and changes in subjective pain scores. (C) A positive correlation was observed between changes in LF/HF ratios and changes in subjective pain scores. (D) A negative correlation was observed between changes in HF% and changes in the hemodynamic response in the DMPFC. (E) A positive correlation was observed between changes in LF% and changes in the hemodynamic response in the DMPFC. (F) A positive correlation was observed between changes in LF/HF ratios and changes in the hemodynamic response in the DMPFC.