Perception to laser heat stimuli in depressed patients is reduced to Aδ- and selective C-fiber stimulation.

Depression and clinical pain have been shown being strongly associated with each other. However, recent studies have demonstrated that depressed patients are less sensitive to experimental pain than healthy individuals. Reasons for this phenomenon are still elusive. The study investigates whether cutaneous C- and/or Aδ-fibers might contribute to this phenomenon. C- and Aδ-fiber systems were assessed in 12 depressed patients and 12 sex- and age-matched healthy controls using stimulation of tiny skin areas by laser heat stimuli. Detection and pain thresholds as well as proportions of trials associated with C- and Aδ-fiber stimulation as well as of non-perceived trials were compared between groups. Patients showed elevated pain thresholds and significantly less C-fiber responses. They also failed significantly more often to recognize the noxious laser-heat stimuli. Thus, higher pain thresholds to experimental stimuli in depressed patients are not only associated with reduced perception of cutaneous Aδ-, but also with decreased perception of selective C-fiber input. The physiological underpinnings of the phenomenon remain elusive and should be examined in the future to understand whether it is based on changes in the periphery or in central processing or both.