A scoping review of the diurnal variation in the intensity of neuropathic pain

Voluntary running-induced activation of ventral hippocampal GABAergic interneurons contributes to exercise-induced hypoalgesia in neuropathic pain model mice

The exact mechanism of exercise-induced hypoalgesia (EIH) in exercise therapy to improve chronic pain has not been fully clarified. Recent studies have suggested the importance of the ventral hippocampus (vHPC) in inducing chronic pain. We investigated the effects of voluntary running (VR) on FosB⁺ cells and GABAergic interneurons (parvalbumin-positive [PV⁺] and somatostatin-positive [SOM⁺]) in the vHPC-CA1 in neuropathic pain (NPP) model mice. VR significantly improved thermal hyperalgesia in the NPP model. The number of the FosB⁺ cells was significantly higher in partial sciatic nerve ligation-sedentary mice than in Sham and Naive mice, whereas VR significantly suppressed the FosB⁺ cells in the vHPC-CA1. Furthermore, VR significantly increased the proportion of activated PV⁺ and SOM⁺ interneurons in the vHPC-CA1, and tracer experiments indicated that approximately 24% of neurons projecting from the vHPC-CA1 to the basolateral nucleus of amygdala were activated in NPP mice. These results indicate that feedforward suppression of the activated neurons via VR-induced activation of GABAergic interneurons in the vHPC-CA1 may be a mechanism to produce EIH effects, and suggested that disappearance of negative emotions such as fear and anxiety by VR may play a critical role in improving chronic pain.

Circadian pain patterns in human pain conditions - A systematic review

BACKGROUND

Chronobiology is the science of how physiological processes in the body follow a pattern of time. Pain has been shown to follow a circadian rhythm, with different types of pain having variable expression along this rhythm.

OBJECTIVE

This article reviews the nature of diurnal variations in pain along with a discussion of the mechanisms of circadian rhythm of pain.

EVIDENCE REVIEW

We conducted a literature search on the PubMed and Google Scholar electronic databases, through April 2022. Publications were screened for English language, full-text availability, and human subjects. Randomized controlled trials and observational trials were included. Data were extracted from studies on patients with acute or chronic pain phenotypes, which provide pain severity data and corresponding diurnal time points.

FINDINGS

The literature search led to the inclusion of 39 studies. A circadian pattern of pain was found to be present in nociceptive, neuropathic, central, and mixed pain states. Postoperative pain, fibromyalgia, trigeminal neuralgia, and migraines were associated with higher pain scores in the morning. Temporomandibular joint pain, neuropathic pain, labor pain, biliary colic, and cluster headaches increased throughout the day to reach a peak in the evening or night. Arthritis and cancer pain were not associated with any circadian rhythmicity. Furthermore, the circadian rhythm of pain was not found to be altered in patients on analgesics.

CONCLUSION

The results of this review suggest that an understanding of diurnal variation may help improve therapeutic strategies in pain management, for instance through analgesic titration.

Pain Fluctuations of Women with Subacute Herpetic Neuralgia During Local Methylcobalamin in Combination with Lidocaine Treatment: A Single-Blinded Randomized Controlled Trial

Purpose

To evaluate the efficacy and pain fluctuations of methylcobalamin in combination with lidocaine local injection treatment for subacute herpetic neuralgia (SHN).

Methods

Seventy-nine women (60.4 ± 2.7 years) with thoracic SHN were enrolled and randomized to receive a combination of methylcobalamin and lidocaine local injection (MI, N=40), or a combination of lidocaine patch 5% and oral methylcobalamin (PO, N=39) for four weeks. Repeated-measures analyses of variance were used to evaluate the effect on pain levels. Generalized estimation equations were used to analyze the cause-effect relationship between pain fluctuations and influencing factors.

Results

At the treatment endpoint, the group, treatment time, and group interacted with treatment time effects of the pain scores and area were statistically significant (P<0.001), The pain scores were 2.9 ±0.9 (MI) and 4.3 ± 1.5 (PO). 80.00% (MI) or 28.21% (PO) of patients had pain scores ≤ 3, the odds ratio was 2.84 (95% CI: 1.68 to 4.79). The incidence of postherpetic neuralgia was 5.0% (2/40) at 3 months. Pain fluctuated repeatedly during treatment. The pain fluctuation increased from 8.75 log folds in the afternoon, to 79.85 log folds at night. With the ADLs level increasing from 1 to 3, the pain fluctuated from 4.28 to 17.70 log folds. Allodynia, itching, sleep quality, and ADLs were the significant influencing factors (P<0.05).

Conclusion

This study validated the efficacy of methylcobalamin combined with lidocaine for SHN, and confirmed that pain levels in patients with SHN had an obvious circadian rhythm. ADLs were an important cause of pain fluctuations.