143 SPONTANEOUS ACTIVITY IN HUMAN C-FIBERS ASSESSED BY MICRONEUROGRAPHY: OBSERVATIONS FROM NEUROPATHIC PAIN PATIENTS AND BASIC MECHANISMS

Background Offset analgesia, in which a disproportionally large amount of analgesia becomes apparent upon a slight decrease in noxious heat stimulation, has not been described previously in patients with chronic pain. Methods Offset analgesia responses in 10 patients with neuropathic pain (in both legs) were compared with 10 matched healthy controls and volunteers from a convenience sample (n = 110) with an age range of 6-80 yr. Offset analgesia was defined by the reduction in electronic pain score upon the 1°C decrease in noxious heat stimulus relative to the peak pain score where pain was administered at the volar side of the arm. Results Offset analgesia was present in healthy volunteers irrespective of age and sex (pain score decrease = 97 ± 1% [mean ± SEM]). In contrast, a reduced or absent offset analgesia response was observed in patients with neuropathic pain (pain score decrease = 56 ± 9% vs. controls 98 ± 1%, P < 0.001). Intravenous treatment with ketamine, morphine, and placebo had no effect on offset analgesia in patients, despite sharp reductions in spontaneous pain scores. Conclusions These data indicate that offset analgesia is fully developed at the age of 6 yr and does not undergo additional maturation. The reduced or absent responses observed in patients with chronic neuropathic pain indicate the inability to modulate changes in pain stimulation, with perseverance of pain perception in situations in which healthy subjects display signs of strong analgesia. Both central and peripheral sites may be involved in the altered offset analgesia responses in these patients.

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Electroencephalography During Nociceptive Stimulation in Chronic Pain Patients: A Systematic Review

Pain Medicine ◽

10.1093/pm/pnaa131 ◽

2020 ◽

Vol 21 (12) ◽

pp. 3413-3427

Author(s):

Dorine Lenoir ◽

Ward Willaert ◽

Iris Coppieters ◽

Anneleen Malfliet ◽

Kelly Ickmans ◽

...

Keyword(s):

Systematic Review ◽

Chronic Pain ◽

Neuropathic Pain ◽

Subjective Experience ◽

Event Related Potential ◽

High Temporal Resolution ◽

Healthy Controls ◽

Somatosensory Processing ◽

Chronic Pain Patients ◽

Pain Patients

Abstract Background With its high temporal resolution, electroencephalography (EEG), a technique that records electrical activity of cortical neuronal cells, is a potentially suitable technique to investigate human somatosensory processing. By using EEG, the processing of (nociceptive) stimuli can be investigated, along with the functionality of the nociceptive pathway. Therefore, it can be applied in chronic pain patients to objectify whether changes have occurred in nociceptive processing. Typically, so-called event-related potential (ERP) recordings are used, where EEG signals are recorded in response to specific stimuli and characterized by latency and amplitude. Objective To summarize whether differences in somatosensory processing occur between chronic pain patients and healthy controls, measured with ERPs, and determine whether this response is related to the subjective pain intensity. Design Systematic review. Setting and Methods PubMed, Web of Science, and Embase were consulted, and 18 case–control studies were finally included. Subjects The chronic pain patients suffered from tension-type headache, back pain, migraine, fibromyalgia, carpal tunnel syndrome, prostatitis, or complex regional pain syndrome. Results Chronic neuropathic pain patients showed increased latencies of the N2 and P2 components, along with a decreased amplitude of the N2-P2 complex, which was also obtained in FM patients with small fiber dysfunction. The latter also showed a decreased amplitude of the N2-P3 and N1-P1 complex. For the other chronic pain patients, the latencies and the amplitudes of the ERP components did not seem to differ from healthy controls. One paper indicated that the N2-P3 peak-to-peak amplitude correlates with the subjective experience of the stimulus. Conclusions Differences in ERPs with healthy controls can mostly be found in chronic pain populations that suffer from neuropathic pain or where fiber dysfunction is present. In chronic pain populations with other etiological mechanisms, limited differences were found or agreed upon across studies.

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