Abstract
Acute low back pain can be experimentally induced by injections of hypertonic saline into deep tissues of the back, such as fascia and muscle. The current study investigated the dose-dependency of peak-pain and spatial extent of concomitant radiating pain following 50, 200 and 800 μL bolus injections of hypertonic saline (5.8%) into the thoracolumbar fascia and multifidus muscle, since data on dose-dependency is lacking in humans. Sixteen healthy subjects rated (11 female, 5 male; 23.3 ± 3.1 years, mean ± SD) intensity and spatial extent of pain. Injections into the fascia resulted in significantly higher peak-pain (+86%, p < 0.001), longer pain durations (p < 0.05), and larger pain areas (+65%, p < 0.02) and were less variable than intramuscular injections. Peak-pain ratings and pain areas were 2–3-fold higher/larger for 200 μL vs. 50 μL. In contrast, peak pain increased only marginally at 800 μL by additional 20%, while pain areas did not increase further at all in both, fascia and muscle. Thus, higher injection volumes did also not compensate the lower sensitivity of muscle. Peak-pain ratings and pain areas correlated between fascia and muscle (r = 0.530, p < 0.001 and r = 0.337, p < 0.02, respectively). Peak-pain ratings and pain areas correlated overall (r = 0.490, p < 0.0001), but a weak correlation remained when the impact of between-tissue differences and different injection volumes were singled out (partial r = 0.261, p < 0.01). This study shows dose-dependent pain responses of deep tissues where an injection volume of 200 μL of hypertonic saline is deemed an adequate stimulus for tissue differentiation. We suggest that pain radiation is not simply an effect of increased peripheral input but may afford an individual disposition for the pain radiation response. Substantially higher pain-sensitivity and wider pain areas support fascia as an important contributor to non-specific low back pain.
Highlights
IntroductionFascia tissue has been identified to play an important role in the sensitivity to pain stimuli, that fascia is more sensitive to both chemical and electrical stimulation than the underlying muscle leading to higher pain intensities, pain duration, and larger pain distribution [4,5]
It has been shown that the human thoracolumbar fascia is more sensitive to chemical stimulations by 400 μL hypertonic saline than the underlying erector spinae to chemical stimulations by 400 μL hypertonic saline than the underlying erector spinae muscle according to peak pain, pain duration and, pain radiation [4]
We demonstrate that hypertonic saline injections elicited a graded dose-dependent pain we demonstrate hypertonic saline injections elicited a graded dose-dependent pain lasting for severalthat minutes with the thoracolumbar fascia being generally more prone to lasting for several minutes with than the thoracolumbar beingmuscle generally moretoprone to respond to chemical stimulation the underlyingfascia multifidus leading higher respond to chemical stimulation than the underlying multifidus muscle leading to higher pain scores, longer pain durations and larger pain distribution patterns
Summary
Fascia tissue has been identified to play an important role in the sensitivity to pain stimuli, that fascia is more sensitive to both chemical and electrical stimulation than the underlying muscle leading to higher pain intensities, pain duration, and larger pain distribution [4,5]. The stimulated fascia nerve endings lead to a distinctly higher affective and sharper mechanical pain character compared to other deep tissues [4,6]. They are able to induce longer-lasting pain amplification [5] and we recently revealed a somatosensory crosstalk between deep fascia tissue and superficial cutaneous tissue [7]
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