We are pleased to introduce the collection Frontiers in Pain Research – Highlights in Neuropathic Pain 2021/22. This collection will welcome and showcase a selection of high impact articles authored by leaders in the field. The work presented here highlights the broad diversity of research performed across the Neuropathic Pain section, and aims to put a spotlight on the main areas of interest. This collection aims to further support Frontiers’ strong community by shining a spotlight on our authors' highly impactful research.

Affiliated Nanshan Hospital of Shenzhen University Health Science Center

University of Porto

Exploring exciting new research opportunities in Neurosciences for 2021/22.

This research topic focuses on the diagnosis and prediction of neuropathic pain, the relationship between type 1 and type 2 diabetes, the role of TRPA1 in BCP, and the effects of cerebral palsy on chronic pain in adults. <ul> <li>Corneal confocal microscopy (CCM) is a rapid non-invasive imaging technique that can diagnose and predict neuropathic pain, as well as monitor nerve regeneration and disease progression in multiple sclerosis, Parkinson's disease and dementia.</li> <li>Type 1 diabetes (T1D) was associated with more hypoalgesia than type 2 diabetes (T2D), and T1D was associated with lower vibration detection thresholds than T2D.</li> <li>TRPA1 is involved in BCP, as evidenced by increased TRPA1 expression in the BCP group and reduced expression in the BCP+AS-ODN group, as well as the alleviation of pain behaviors in the BCP group after treatment with a TRPA1 antagonist and AS.</li> <li>Adults with cerebral palsy experience higher levels of chronic pain than neurotypical adults, and this pain is associated with gender, anxiety/depression symptoms, lower household income, better communicative ability, spinothalamic sensory abnormalities, and a greater degree of prematurity.</li> </ul>

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Frontiers in Pain Research

Neuropathic Pain

Highlights in Neuropathic Pain 2021/22

Neuropathic pain has multiple etiologies, but a major feature is small fiber dysfunction or damage. Corneal confocal microscopy (CCM) is a rapid non-invasive ophthalmic imaging technique that can image small nerve fibers in the cornea and has been utilized to show small nerve fiber loss in patients with diabetic and other neuropathies. CCM has comparable diagnostic utility to intraepidermal nerve fiber density for diabetic neuropathy, fibromyalgia and amyloid neuropathy and predicts the development of diabetic neuropathy. Moreover, in clinical intervention trials of patients with diabetic and sarcoid neuropathy, corneal nerve regeneration occurs early and precedes an improvement in symptoms and neurophysiology. Corneal nerve fiber loss also occurs and is associated with disease progression in multiple sclerosis, Parkinson's disease and dementia. We conclude that corneal confocal microscopy has good diagnostic and prognostic capability and fulfills the FDA criteria as a surrogate end point for clinical trials in peripheral and central neurodegenerative diseases.

Neuropathic pain has multiple etiologies, but a major feature is small fiber dysfunction or damage. Corneal confocal microscopy (CCM) is a rapid non-invasive ophthalmic imaging technique that can image small nerve fibers in the cornea and has been utilized to show small nerve fiber loss in patients with diabetic and other neuropathies. CCM has comparable diagnostic utility to intraepidermal nerve fiber density for diabetic neuropathy, fibromyalgia and amyloid neuropathy and predicts the development of diabetic neuropathy. Moreover, in clinical intervention trials of patients with diabetic and sarcoid neuropathy, corneal nerve regeneration occurs early and precedes an improvement in symptoms and neurophysiology. Corneal nerve fiber loss also occurs and is associated with disease progression in multiple sclerosis, Parkinson&apos;s disease and dementia. We conclude that corneal confocal microscopy has good diagnostic and prognostic capability and fulfills the FDA criteria as a surrogate end point for clinical trials in peripheral and central neurodegenerative diseases.

Corneal Confocal Microscopy to Image Small Nerve Fiber Degeneration: Ophthalmology Meets Neurology

Background: Diabetic neuropathy is characterized by the paradoxical co-existence of hypo- and hyperalgesia to sensory stimuli. The literature shows consistently sensory differences between healthy and participants with diabetes. We hypothesized that due to differences in pathophysiology, advanced quantitative sensory testing (QST) might reveal sensory discrepancies between type 1 (T1D) and type 2 diabetes (T2D). Furthermore, we investigated whether vibration detection thresholds (VDT) were associated with sensory response.Method: Fifty-six adults with T1D [43 years (28–58)], 99 adults with T2D [65 years (57–71)], and 122 healthy individuals [51 years (34–64)] were included. VDT, pressure pain detection thresholds (pPDT) and tolerance (pPTT), tonic cold pain (hand-immersion in iced water), and central pain mechanisms (temporal summation and conditioned pain modulation) were tested and compared between T1D and T2D. VDT was categorized into normal (< 18 V), intermediary (18–25 V), or high (> 25 V).Results: In comparison to healthy, analysis adjusted for age, BMI, and gender revealed hypoalgesia to tibial (pPDT): p = 0.01, hyperalgesia to tonic cold pain: p < 0.01, and diminished temporal summation (arm: p < 0.01; abdomen: p < 0.01). In comparison to participants with T2D, participants with T1D were hypoalgesic to tibial pPDT: p < 0.01 and pPTT: p < 0.01, and lower VDT: p = 0.02. VDT was not associated with QST responses.Conclusion: Participants with T1D were more hypoalgesic to bone pPDT and pPTT independent of lower VDT, indicating neuronal health toward normalization. Improved understanding of differentiated sensory profiles in T1D and T2D may identify improved clinical endpoints in future trials.

Background: Diabetic neuropathy is characterized by the paradoxical co-existence of hypo- and hyperalgesia to sensory stimuli. The literature shows consistently sensory differences between healthy and participants with diabetes. We hypothesized that due to differences in pathophysiology, advanced quantitative sensory testing (QST) might reveal sensory discrepancies between type 1 (T1D) and type 2 diabetes (T2D). Furthermore, we investigated whether vibration detection thresholds (VDT) were associated with sensory response.Method: Fifty-six adults with T1D [43 years (28–58)], 99 adults with T2D [65 years (57–71)], and 122 healthy individuals [51 years (34–64)] were included. VDT, pressure pain detection thresholds (pPDT) and tolerance (pPTT), tonic cold pain (hand-immersion in iced water), and central pain mechanisms (temporal summation and conditioned pain modulation) were tested and compared between T1D and T2D. VDT was categorized into normal (&lt; 18 V), intermediary (18–25 V), or high (&gt; 25 V).Results: In comparison to healthy, analysis adjusted for age, BMI, and gender revealed hypoalgesia to tibial (pPDT): p = 0.01, hyperalgesia to tonic cold pain: p &lt; 0.01, and diminished temporal summation (arm: p &lt; 0.01; abdomen: p &lt; 0.01). In comparison to participants with T2D, participants with T1D were hypoalgesic to tibial pPDT: p &lt; 0.01 and pPTT: p &lt; 0.01, and lower VDT: p = 0.02. VDT was not associated with QST responses.Conclusion: Participants with T1D were more hypoalgesic to bone pPDT and pPTT independent of lower VDT, indicating neuronal health toward normalization. Improved understanding of differentiated sensory profiles in T1D and T2D may identify improved clinical endpoints in future trials.

Simple Quantitative Sensory Testing Reveals Paradoxical Co-existence of Hypoesthesia and Hyperalgesia in Diabetes

Background: Bone cancer pain (BCP) significantly affects patient quality of life, results in great bodily and emotional pain, and creates difficulties in follow-up treatment and normal life. Transient receptor potential ankyrin 1 (TRPA1) is an essential transduction ion channel related to neuropathic and inflammatory pain. However, the role of TRPA1 in BCP remains poorly understood. This study aimed to explore the relationship between TRPA1 and BCP.Methods: A BCP model was induced by Walker256 cells to the left tibia. The sham group was induced by normal saline to the left tibia. Thereafter, pain behaviors and TRPA1 expression between the BCP group and the sham group were observed on the 14th day of modeling. The TRPA1 antagonist A967079 (10 mg/kg) was injected via tail vein. TRPA1 antisense oligodeoxynucleotide (AS-ODN, 5 nmol/10 μl) and missense oligodeoxynucleotide (MS-ODN, 5 nmol/10 μl) were intrathecally delivered via a mini-osmotic pump for 5 consecutive days to assess the effect of TRPA1 on BCP. Behavioral tests were assessed preoperatively and postoperatively. Real-time quantitative PCR and western blot analyses were used to measure TRPA1 levels among the different groups.Results: The BCP model was successfully established via X-ray and pathological sections at 14 days. Compared to the sham group, the BCP group was more sensitive to mechanical stimuli, cool stimuli and hot stimuli. Intravenously injected A967079 can relieve paw mechanical withdrawal threshold and paw withdrawal thermal latency in rats with BCP. Moreover, AS-ODN can relieve paw mechanical withdrawal threshold and paw withdrawal thermal latency in rats with BCP. Additionally, relative mRNA and protein expression of TRPA1 in the BCP group were much higher than those in the sham group (14.55 ± 1.97 vs. 1 ± 0.04, P < 0.01). Compared to the BCP group, the relative mRNA and protein expression of TRPA1 in the BCP+AS-ODN group was reduced (14.55 ± 1.97 vs. 2.59 ± 0.34, P < 0.01).Conclusions: The TRPA1 channel mediates mechanical allodynia and thermal hyperalgesia in a rat BCP model.

Background: Bone cancer pain (BCP) significantly affects patient quality of life, results in great bodily and emotional pain, and creates difficulties in follow-up treatment and normal life. Transient receptor potential ankyrin 1 (TRPA1) is an essential transduction ion channel related to neuropathic and inflammatory pain. However, the role of TRPA1 in BCP remains poorly understood. This study aimed to explore the relationship between TRPA1 and BCP.Methods: A BCP model was induced by Walker256 cells to the left tibia. The sham group was induced by normal saline to the left tibia. Thereafter, pain behaviors and TRPA1 expression between the BCP group and the sham group were observed on the 14th day of modeling. The TRPA1 antagonist A967079 (10 mg/kg) was injected via tail vein. TRPA1 antisense oligodeoxynucleotide (AS-ODN, 5 nmol/10 μl) and missense oligodeoxynucleotide (MS-ODN, 5 nmol/10 μl) were intrathecally delivered via a mini-osmotic pump for 5 consecutive days to assess the effect of TRPA1 on BCP. Behavioral tests were assessed preoperatively and postoperatively. Real-time quantitative PCR and western blot analyses were used to measure TRPA1 levels among the different groups.Results: The BCP model was successfully established via X-ray and pathological sections at 14 days. Compared to the sham group, the BCP group was more sensitive to mechanical stimuli, cool stimuli and hot stimuli. Intravenously injected A967079 can relieve paw mechanical withdrawal threshold and paw withdrawal thermal latency in rats with BCP. Moreover, AS-ODN can relieve paw mechanical withdrawal threshold and paw withdrawal thermal latency in rats with BCP. Additionally, relative mRNA and protein expression of TRPA1 in the BCP group were much higher than those in the sham group (14.55 ± 1.97 vs. 1 ± 0.04, P &lt; 0.01). Compared to the BCP group, the relative mRNA and protein expression of TRPA1 in the BCP+AS-ODN group was reduced (14.55 ± 1.97 vs. 2.59 ± 0.34, P &lt; 0.01).Conclusions: The TRPA1 channel mediates mechanical allodynia and thermal hyperalgesia in a rat BCP model.

The TRPA1 Channel Mediates Mechanical Allodynia and Thermal Hyperalgesia in a Rat Bone Cancer Pain Model

Impact of SARS-CoV-2 Infection on the Epidemiology of Chronic Pain and Long-Term Disability: Prepare for the Next Perfect Storm

Chronic pain is prevalent in adults with cerebral palsy. We aimed to explore associations between chronic pain and somatosensory, motor, cognitive, etiologic, and environmental factors in adults with cerebral palsy. This cross-sectional study enrolled 17 adult participants with cerebral palsy (mean age 31 years; 8 female; Gross Motor Functional Classification Status levels I-V) able to self-report and 10 neurotypical adult volunteers (mean age 34 years; 9 female). Participants reported pain characteristics, demographics, and affective factors. Physical examination included somatosensory and motor evaluation. Between-group comparisons used a ranksum test, and correlation analyses estimated effect size in terms of shared variance (ρ2). Individuals with cerebral palsy reported greater pain intensity, neuropathic qualities, and nociceptive qualities than control participants. Higher pain intensity was associated with female gender (ρ2 = 16%), anxiety/depression symptoms (ρ2 = 10%), and lower household income (ρ2 = 19%). It was also associated with better communicative ability (ρ2 = 21%), spinothalamic (sharp/temperature) sensory abnormalities (ρ2 = 33%), and a greater degree of prematurity (ρ2 = 17%). This study highlights similarity of chronic pain associations in people with cerebral palsy with patterns seen in other populations with chronic pain. Spinothalamic sensory abnormalities suggest central pain mechanisms.

Chronic pain is prevalent in adults with cerebral palsy. We aimed to explore associations between chronic pain and somatosensory, motor, cognitive, etiologic, and environmental factors in adults with cerebral palsy. This cross-sectional study enrolled 17 adult participants with cerebral palsy (mean age 31 years; 8 female; Gross Motor Functional Classification Status levels I-V) able to self-report and 10 neurotypical adult volunteers (mean age 34 years; 9 female). Participants reported pain characteristics, demographics, and affective factors. Physical examination included somatosensory and motor evaluation. Between-group comparisons used a ranksum test, and correlation analyses estimated effect size in terms of shared variance (ρ2). Individuals with cerebral palsy reported greater pain intensity, neuropathic qualities, and nociceptive qualities than control participants. Higher pain intensity was associated with female gender (ρ2 = 16%), anxiety/depression symptoms (ρ2 = 10%), and lower household income (ρ2 = 19%). It was also associated with better communicative ability (ρ2 = 21%), spinothalamic (sharp/temperature) sensory abnormalities (ρ2 = 33%), and a greater degree of prematurity (ρ2 = 17%). This study highlights similarity of chronic pain associations in people with cerebral palsy with patterns seen in other populations with chronic pain. Spinothalamic sensory abnormalities suggest central pain mechanisms.