Simultaneous recording of late and ultra-late pain evoked potentials in fibromyalgia

Objective: To characterize laser evoked potentials (LEP), pain psychophysics and local tissue response in fibromyalgia patients. Methods: LEP were recorded in 14 women with fibromyalgia in response to bilateral stimulation of tender and control points in upper limbs by 4 blocks of 20 stimuli at each point. Subsequently, heat pain thresholds were measured and supra-threshold magnitude estimations of heat pain stimuli were obtained on a visual analogue scale. Finally, the extent of the local tissue response induced by the previous stimuli was evaluated. Results: Laser stimuli elicited two long latency waves: A late wave (mean latency 368.9 ± 66.9 ms) in most patients (13/14) from stimuli at all points, and an ultra-late wave (mean latency 917.3 ± 91.8 ms) in 78.5% of the patients at the control points and in 71.4% at the tender points. Amplitude of ultra-late waves was higher at the tender points (20.67 ± 11.1 μV) than at the control points (10.47 ± 4.1 μV) (P = 0.016). Pain thresholds were lower in the tender (41.2 ± 2.7°C) than the control points (43.9 ± 3.2°C) (P = 0.008). Local tissue response was significantly more intense at tender than control points (P = 0.004). Conclusions: Ultra-late laser evoked potentials can be recorded simultaneously with late potentials. Our findings are compatible with presence of peripheral C-fiber sensitization, mostly at tender points, probably combined with generalized central sensitization of pain pathways in fibromyalgia.