Neuropathic pain in humans arises as a consequence of injury or disease of somatosensory nervous system at peripheral or central level. Peripheral neuropathic pain is more common than central neuropathic pain, and is supposed to result from peripheral mechanisms, following nerve injury. The animal models of neuropathic pain show extensive functional and structural changes occurring in neuronal and non-neuronal cells in response to peripheral nerve injury. These pathological changes following damage lead to peripheral sensitization development, and subsequently to central sensitization initiation with spinal and supraspinal mechanism involved. The aim of this narrative review paper is to discuss the mechanisms engaged in peripheral neuropathic pain generation and maintenance, with special focus on the role of glial, immune, and epithelial cells in peripheral nociception. Based on the preclinical and clinical studies, interactions between neuronal and non-neuronal cells have been described, pointing out at the molecular/cellular underlying mechanisms of neuropathic pain, which might be potentially targeted by topical treatments in clinical practice. The modulation of the complex neuro-immuno-cutaneous interactions in the periphery represents a strategy for the development of new topical analgesics and their utilization in clinical settings.

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients’ quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were “topical AND pain”, “topical AND neuropathic”, “topical AND treatment”, “topical AND mechanism”, “peripheral neuropathic”, and “mechanism”. The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Background: The purpose of this study was to investigate the efficacy and safety of the NADA (National Acupuncture Detoxification Association)-standardized ear acupuncture protocol in comparison to medical acupuncture (MA) in the treatment of chronic nonspecific low back pain (LBP) in older adults. Methods: This was a prospective, clinical, single center, open label, comparative study. A total of 60 older patients with chronic nonspecific LBP were enrolled in the study. The patients were divided into two groups. The MA group received treatment with medical acupuncture (MA), while the NADA group received NADA ear acupuncture once a day for 20 min, for a total of 10 sessions. The co-primary outcome measures were the reduction in pain intensity evaluated by the Numeric Rating Scale (NRS) compared to baseline and improvement in patients’ quality of life (QOL) assessed in the SF-36 questionnaire before and after treatment; this was compared between the two groups. Results: After two weeks of treatment, a significant reduction compared to baseline was observed in the NRS scores following treatment with medical acupuncture as well as after the utilization of NADA ear acupuncture protocol: NRS score for average pain experienced by the patients over the previous week (NRSa) MA: p = 0.002; NADA: p < 0.001, maximum NRS score in the past week (NRSm) MA: p < 0.001; NADA: p < 0.001, and NRS score at the time of examination (NRSe) MA: p = 0.001; NADA: p < 0.001. Reduction of the NRSa score compared to baseline was significantly greater in the NADA group (p = 0.034). Significant improvements in the QOL of patients according to the SF-36 questionnaire compared to baseline were observed in the MA group in the following domains: PF (p = 0.003), RP (p = 0.002), SF (p = 0.041), RE (p = 0.005), MH (p = 0.043), HT (p = 0.013), PCS (p = 0.004), and MCS (p = 0.025); and in the NADA group, in the following domains: PF (p = 0.004), RP (p = 0.048), BP (p = 0.001), VT (p = 0.035), RE (p = 0.006), MH (p < 0.001), HT (p = 0.003), PCS (p < 0.001), and MCS (p < 0.001). There were minor complications observed in 35% of patients (total of 20 participants); 31% (9 patients) in the MA group and 39% (11 patients) in the NADA group. These were minor and quickly resolved, including insertion point pain, minor bleeding after needle removal, and one instance of fainting. No patients in either group reported worsening of LBP. These complications occurred in 4.14% of MA sessions (12 times/290 sessions) and in 6.07% of NADA acupuncture sessions (16 times/280 sessions). Conclusion: The outcomes of this study suggest that both MA and NADA ear acupuncture could be a valuable and personalized component of a comprehensive approach to managing chronic nonspecific LBP in older patients. Incorporation of MA and NADA ear acupuncture into the clinical management of chronic nonspecific LBP in elderly patients has the potential to reduce pain intensity and improve the overall quality of life of affected individuals. However, further studies are needed to confirm our findings.

Neuropathic pain remains a clinical challenge due to its complex and not yet fully understood pathomechanism, which result in limited analgesic effectiveness of the management offered, particularly for patients with acute, refractory neuropathic pain states. In addition to the introduction of several modern therapeutic approaches, such as neuromodulation or novel anti-neuropathic drugs, significant efforts have been made in the repurposing of well-known substances such as phenytoin. Although its main mechanism of action occurs at sodium channels in excitable and non-excitable cells and is well documented, how the drug affects the disturbed neuropathic interactions at the spinal cord level and how it influences morphine-induced analgesia have not been clarified, both being crucial from a clinical perspective. We demonstrated that single and repeated systemic administrations of phenytoin decreased tactile and thermal hypersensitivity in an animal model of neuropathic pain. Importantly, we observed an increase in the antinociceptive effect on thermal stimuli with repeated administrations of phenytoin. This is the first study to report that phenytoin improves morphine-induced antinociceptive effects and influences microglia/macrophage activity at the spinal cord and dorsal root ganglion levels in a neuropathic pain model. Our findings support the hypothesis that phenytoin may represent an effective strategy for neuropathic pain management in clinical practice, particularly when combination with opioids is needed.

Chronic pain remains a significant health concern, particularly among older adults. Pain acceptance - engaging in valued activities despite pain without attempting to control or avoid it - is thought to buffer against the negative psychosocial impacts of chronic pain. The present study aimed to adapt to Polish and evaluate the psychometric properties of a widely used measure of pain acceptance, namely the Chronic Pain Acceptance Questionnaire (CPAQ). Two samples of individuals with chronic pain participated: an online sample (N = 418) and a hospital-recruited sample (N = 148). Confirmatory factor analyses assessed one-factor, two-correlated-factor, and two-orthogonal-factor models for both the original 20-item (CPAQ-20) and the short 8-item (CPAQ-8) versions. In both samples, the results consistently supported a two-factor correlated structure for the Polish CPAQ-8: Activity Engagement (AE) and Pain Willingness (PW). The CPAQ-20 did not show adequate fit. The CPAQ-8 demonstrated acceptable-to-good internal consistency (Cronbach's α = 0.63-0.78 total; 0.75-0.82 subscales) and moderate test-retest reliability (ICC = 0.57). The Minimal Detectable Change for the total score was 9.34 points. In the online sample, CPAQ-8 scores negatively correlated with pain anxiety, pain catastrophizing, anxiety, and depression. Across both samples, total CPAQ-8 and PW scores, but not AE, were negatively associated with pain. Participants reporting activity limitations scored significantly lower on CPAQ-8. Additionally, age was positively correlated with AE scores in both samples. Overall, the Polish CPAQ-8 exhibits good psychometric properties and is suitable for both research and clinical applications. Future studies should examine its longitudinal validity.