Stimulating Aδ and C-Fibers in the Lower Limb With a 980 nm Diode Laser

Abstract

Background: Laser evoked potentials are increasingly used to investigate the integrity of the nociceptive system. Laser heat stimuli can activate Aδ fibers, activation of C-fibers remains difficult. This study attempts to stimulate Aδ and C fibers separately with a ‘grid’ to generate respectively late and ultra-late LEPs. A ‘grid’ is a thin aluminum plate used as a spatial filter to stimulate C-fibers. Furthermore, study subjects pressed a button upon detecting a laser stimulus which was used to measure reaction times (RT) following diode laser stimulation.

Methods: Cutaneous heat stimuli were applied at the Th₁₂ and L₅dermatome in seventeen volunteers. Conduction velocities (CV) were calculated by measuring latencies of P2 and reaction times (RT).

Results: Stimulation condition Th₁₂^no-grid showed a P2_late response at 330 ± 47 ms and L₅^no-grid at 413 ± 53 ms. Mean reaction time during Th₁₂^no-grid was 537 ± 146 ms, L₅^no-grid 784 ± 334 ms, Th₁₂^grid 710 ± 195 ms and L₅^grid 1,391 ± 336 ms. During stimulation block Th₁₂^grid and L₅^grid ultra-late LEPS could not reliably be generated. Median conduction velocities (CV) and their corresponding range were calculated. The median CV_RT no grid was 5.8 m/s (range 1.2 - 43.3). The median CV_LEP no grid was 13.8 m/s (range 4.7 - 45.4). The median CV_RT grid was 1.9 m/s (range 0.8 - 3.7). Ultra-late LEPs could not be generated, although subjects mentioned a long lasting burning pain during Th₁₂^grid and L₅^grid.

Conclusions: This study questions the feasibility of the ‘grid’ to reliably generate C-fiber responses. Pressing a button upon laser stimulus detection seems preferable for identifying C-fiber stimulation in the lower limb, whereas for Aδ nociceptive pathways laser evoked potentials might be of use.




J Neurol Res. 2013;3(2):56-61
doi: https://doi.org/10.4021/jnr177w