Conditioned pain modulation and nociception measured by pain-related evoked potentials in patients with polyneuropathy

Objective: Aim of the study was to assess alterations in nociception and conditioned pain modulation (CPM) in polyneuropathy (PNP) patients depending on the presence of pain and the pattern of affected nerve fibers. Methods: Pain-related evoked potentials (PREP) after painful cutaneous electrical stimulation (PCES) were recorded in 44 PNP patients and 19 healthy controls (HC) with stimulation via concentric surface electrodes. Using the same electrodes for electrical stimulation as test stimulus (TS) and cold water as the conditioned stimulus (CS), we additionally assessed the CPM effects based on changes in pain intensities and amplitudes of evoked potentials elicited by PCES. Subgroup analysis included painless, painful, small (SFN), mixed (MFN), and large (LFN) fiber neuropathies. Results: During the PREP procedure, patients required higher stimulus intensities compared to HC and reported stronger pain. SFN patients had the highest pain scores despite similar stimulus intensities as in HC. N1-latencies of PREP were longer in LFN, N1P1-amplitudes of PREP did not differ. During the CPM testing, the CS induced a significant decrease in pain ratings in HC and painful PNP. The CPM-effects were similar between groups. N1P1-amplitudes for PCES were lower in patients throughout the CPM assessment. Patients with painful MFN and SFN required lower TS-intensities. Discussion: The CPM effects in painful PNP and HC were similar, despite hints for sensitization in CPM and PREP-parameters, especially in small fiber involvement. Amplitude changes during CPM-procedure were independent of pain. Deafferentation influences both PREP and CPM parameters, such as N1-latencies or N1P1-amplitudes, but not the CPM-effect itself.