Tenderness of the Skin after Chemical Stimulation of Underlying Fascia Reveals Somatosensory Crosstalk between Superficial and Deep Tissues

Musculoskeletal pain is often associated with pain referred to adjacent areas or skin. Chronic musculoskeletal pain could be due to central sensitization, (i.e., amplified transmission at central nociceptive neurons), which is manifested by pain hypersensitivity that spreads to areas beyond the affected muscle.

Nociceptive free nerve endings densely innervate fascia. Pain originating from deep tissue (i.e., the muscle or fascia) could induce somatosensory changes of the overlying skin. This further can precipitate as increased pain sensitivity to nociceptive (hyperalgesia) or non-nociceptive stimuli (allodynia).

A study aimed to investigate the possible somatosensory crosstalk between deep tissue (muscle or fascia) and superficial tissue (skin).

48 healthy patients with no low back pain were recruited. Pain was induced using a focal high frequency electrical stimulation (HFS) or an injection of hypertonic saline. The tested the pain stimulation at four fascia sites (temporal fascia, IT-band, TLF, trapezius muscle fascia).

The study showed that 100 µL of hypertonic saline injection in the temporal fascia and TLF elicited significant pinprick hyperalgesia in the overlying skin. But that effect was not observed in the trapezius fascia or iliotibial band. High frequency electrical stimulation did not induce any measure of hyperalgesia.

The experiments demonstrated that fascia stimulation at a sufficient stimulus intensity elicited significant across-tissue facilitation to pinprick stimulation (referred hyperalgesia). This hyperalgesia is a sign of nociceptive central sensitization. The balance between pain-inhibiting and pain-facilitating mechanisms is different between fascia and muscle. Fascia a more likely source of hyperalgesia induction.

Reference: https://www.mdpi.com/2075-1729/11/5/370/htm