Abstract
Musculoskeletal pain is often associated with pain referred to adjacent areas or skin. So far, no study has analyzed the somatosensory changes of the skin after the stimulation of different underlying fasciae. The current study aimed to investigate heterotopic somatosensory crosstalk between deep tissue (muscle or fascia) and superficial tissue (skin) using two established models of deep tissue pain (namely focal high frequency electrical stimulation (HFS) (100 pulses of constant current electrical stimulation at 10× detection threshold) or the injection of hypertonic saline in stimulus locations as verified using ultrasound). In a methodological pilot experiment in the TLF, different injection volumes of hypertonic saline (50–800 µL) revealed that small injection volumes were most suitable, as they elicited sufficient pain but avoided the complication of the numbing pinprick sensitivity encountered after the injection of a very large volume (800 µL), particularly following muscle injections. The testing of fascia at different body sites revealed that 100 µL of hypertonic saline in the temporal fascia and TLF elicited significant pinprick hyperalgesia in the overlying skin (–26.2% and –23.5% adjusted threshold reduction, p < 0.001 and p < 0.05, respectively), but not the trapezius fascia or iliotibial band. Notably, both estimates of hyperalgesia were significantly correlated (r = 0.61, p < 0.005). Comprehensive somatosensory testing (DFNS standard) revealed that no test parameter was changed significantly following electrical HFS. The experiments demonstrated that fascia stimulation at a sufficient stimulus intensity elicited significant across-tissue facilitation to pinprick stimulation (referred hyperalgesia), a hallmark sign of nociceptive central sensitization.
Highlights
The most prevalent musculoskeletal pain conditions transitioning to chronicity and contributing to the global burden of disease include low back pain, neck pain and temporomandibular disorder pain [1,2]
Such sensitization is possibly initiated by an ongoing musculoskeletal pathology, but it is frequently augmented or evoked by the intense modulation of sensory inputs [11,13,14] Since the fascia is densely innervated by nociceptive free nerve endings [15,16,17] an increased input originating from deep tissue may be associated with somatosensory changes of the overlying skin and precipitate increased pain sensitivity to nociceptive or non-nociceptive stimuli [18]
This was about two- to fourfold lower than that of the pain induced by high-frequency stimulation (HFS) in a common human model of the long-term potentiation of pain in the skin using an epicutaneous array of punctate electrodes, which induced a modality-specific mechanical hyperalgesia to punctate stimuli or pain to light touch [10,20,21,22,23,24,25,26]
Summary
The most prevalent musculoskeletal pain conditions transitioning to chronicity and contributing to the global burden of disease include low back pain, neck pain and temporomandibular disorder pain [1,2]. Recent epidemiological studies revealed that the current global confinement situation due to the COVID-19 pandemic initiated an increased frequency of moderate exercise, lowering the prevalence of musculoskeletal pain complaints [6,7] The development of these chronic musculoskeletal pain states is potentially linked to central sensitization, (i.e., amplified transmission at central nociceptive neurons) [8,9], which is manifested by pain hypersensitivity [10] spreading heterotopically to sites beyond those directly affected by musculoskeletal input manipulation [11,12]. The current study focuses on somatosensory changes of the skin after evoked central and peripheral sensitization of deep soft tissue
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