Low threshold unmyelinated mechanoafferents can modulate pain.
Level 2 - randomized trial
Randomized crossover experimental trial in human clinical and healthy cohorts
PubMed 28915853 · doi:10.1186/s12883-017-0963-6
What was done
Experiment I evaluated 20 patients with small fiber neuropathy (SFN; 12 women, median age 52.0 years) and 20 healthy controls (14 women, median age 43.0 years) using fMRI and psychophysical testing. Heat-pain (HP), C-tactile-targeted slow brushing (CT), and HP combined with CT were applied in randomized order to the left shank, with pain rated on a visual analogue scale. Experiment II tested 10 healthy participants with HP titrated to 50/100 on a numerical rating scale, administered with and without CT stimulation.
What was found
In Experiment I, CT stimulation significantly reduced heat pain in healthy participants (p = 0.016) but not in SFN patients. Baseline HP-induced pain intensity was significantly higher in SFN patients than in controls (52.6 vs 32.2). Pain-related BOLD responses occurred during HP in both groups, but adding CT stimulation elicited no significant difference in BOLD response compared to HP alone. In Experiment II, CT stimulation again significantly reduced HP intensity in healthy volunteers (p = 0.038).
Why it matters
This study shows that gentle C-tactile touch afferents mediate pain reduction in humans, a mechanism that fails when C-fibers are damaged in small fiber neuropathy. The absence of cortical BOLD changes suggests that this sensory gating may occur downstream at the dorsal horn level.
Limits
The sample size was small (40 in Experiment I, 10 in Experiment II). BOLD fMRI failed to detect a central neural correlate corresponding to the psychophysical pain reduction. The abstract omits exact point estimates, standard deviations, and effect sizes for the magnitude of pain relief.