Injection of hypertonic saline into deep tissues of the back (subcutis, muscle, or the surrounding fascia) can create acute low back pain (LBP). Prior to this, no study has analyzed differences in temporal, qualitative, and spatial pain characteristics originating from these tissues. This study aimed to investigate the role of the thoracolumbar fascia as a potential source of LBP. In separate sessions, 12 healthy subjects received ultrasound-guided bolus injections of isotonic saline (0.9%) or hypertonic saline (5.8%) into the erector spinae muscle, the thoracolumbar fascia (posterior layer), and the overlying subcutis. Subjects were asked to rate pain intensity, duration, quality, and spatial extent. Pressure pain thresholds were determined pre and post injection. Injections of hypertonic saline into the fascia resulted in significantly larger area under the curve of pain intensity over time than injections into subcutis (P<0.01) or muscle (P<0.001), primarily based on longer pain durations and, to a lesser extent, on higher peak pain ratings. Pressure hyperalgesia was only induced by injection of hypertonic saline into muscle, but not fascia or subcutis. Pain radiation and pain affect evoked by fascia injection were greater than those of the muscle (P<0.01) and the subcutis significantly (P<0.05). Pain descriptors following fascia injection (burning, throbbing, and stinging) suggested innervation by both A- and C-fiber nociceptors.
The findings of this study reveal that the thoracolumbar fascia is the deep tissue of the back that is most sensitive to chemical stimulation, making it a prime candidate to contribute to nonspecific LBP but not to localized pressure hyperalgesia.