Selective Laser Stimulation of Aδ- or C-Fibers Through Application of a Spatial Filter: A Study in Healthy Volunteers

Abstract

Background: Pain is perceived through different pathways involving thinly myelinated Aδ-fibers and unmyelinated C-fibers. Aδ-fibers are responsible for a quick, sharp pain, whereas C-fibers relate to a late-onset, burning sensation. Several studies suggest that it is essential to investigate nociceptive fibers separately and in relation to each other. The aim of this study was to selectively stimulate Aδ- and C-fibers using a 980-nm diode laser by varying the laser settings and the stimulated surface area in healthy subjects. Methods: Selective activation of Aδ- and C-fibers is possible using their distinctive physiological characteristics. We used the differences in heat activation threshold and surface density to selectively activate Aδ- and C-fibers. Stimuli from a 980-nm diode laser were applied to 44 healthy volunteers. Two different laser settings were applied for selective stimulation of Aδ-fibers (20 ms at 2.7 W) and C-fibers (50 ms at 0.8 W). A spatial titanium filter, containing 40 holes with varying diameters (0.4, 0.6, 1, and 2 mm), was used to apply the stimuli with varying surface areas. The test subjects received 80 stimuli in total and were asked to press a button when the stimulation was felt. Reaction times between 300 and 650 ms indicate Aδ-fiber activation, whereas reaction times between 650 and 2,000 ms indicate C-fiber activation. Results: First, the usage of response time to discriminate between Aδ- and C-fiber activation was validated. Then, the combined use of the two different stimulation protocols and a spatial filter turned out to be effective to achieve different probabilities of stimulating Aδ- or C-fibers. With the Aδ-protocol and a grid diameter of 2 mm, an Aδ:C response ratio of 1.17:1 was reached, and with the C-protocol and a grid diameter of 0.4 mm, the Aδ:C ratio was 0.05:1. Conclusions: Our results indicate that cutaneous heat stimuli applied with a 980-nm diode laser, using a specific stimulation paradigm and a spatial filter, allow us to selectively activate Aδ- and C-fibers. These findings could serve as a basis for clinical investigation of different involvements of Aδ- and C-fibers in patients suffering from small fiber neuropathies. J Neurol Res. 2016;6(1):1-7 doi: http://dx.doi.org/10.14740/jnr361w