The evolving landscape of publishing in the field of pain: An automated bibliometric analysis from 1975 to 2020

BACKGROUND

The aims of this bibliometric analysis were (1) a longitudinal analysis of the publication landscape in the field of pain (1975-2020) and (2) to characterize the overall publication profiles for two selected journals: European Journal of Pain and PAIN® utilizing an automated approach.

METHODS

Database searches in Scopus extracted all journals with 'pain' in their title. For the two specific journals, papers were manually/automatically profiled into preclinical, human and translational studies.

RESULTS

A gross list of 64 journals in the field of pain consisting of both active and ceased journals in Scopus were included in this analysis which identified 62,565 papers with approximately 4000 papers published/year. These papers include 2759 and 9156 papers in Eur. J. Pain and PAIN®, respectively. Currently, there are 24 active 'pain' journals. Authors/paper increase from 2 to 7 indicating a development from mono-disciplinary to multi-disciplinary studies. The overall publication profiles assessing preclinical, human (experimental/clinical) and translational papers in Eur. J. Pain and PAIN® were almost similar (14%, 75% and 10% versus 26%, 63% and 10%). Papers have changed over the years from mono-disciplinary studies (e.g. behavioural studies) to multi-disciplinary studies (e.g. combined behavioural and cell studies). After optimization, the search model matched the manual screening by 100%, 98% and 96% for the preclinical, clinical and healthy volunteer categories.

CONCLUSIONS

Over the last 45 years, more than 60,000 pain-related papers have been published. Papers develop over the years from mono-disciplinary to multi-disciplinary studies. The overall publication profile including preclinical, human (experimental/clinical) and translational papers was almost similar in Eur. J. Pain and PAIN®.

SIGNIFICANCE

The bibliometric analysis of a pain journal provides information on which specific areas of research are published, how this may have changed over the years and how a journal is positioned compared with other journals in the field.

Pain trajectories and neuropathic pain symptoms following lung cancer surgery: A prospective cohort study

BACKGROUND

Persistent postsurgical pain (PPSP) after lung cancer surgery is common and current definitions are based on evaluations at a single time point after surgery. Pain intensity and symptoms may however fluctuate and change over time, and be impacted by multiple and shifting factors. Studies of postoperative recovery patterns and transition from acute to chronic pain are needed for further investigation of preventive measures and treatments to modify unfavourable recovery paths.

METHODS

In this explorative study, 85 patients undergoing surgery due to either presumptive or confirmed lung cancer reported pain intensities bi-monthly for 12 months. Pain trajectories during recovery were investigated, using group-based trajectory modelling. Associations with possible risk factors for PPSP, including clinical variables and anxiety and depression score (HADS), were also explored.

RESULTS

A trajectory model containing three 12-month pain recovery groups was computed. One group without PPSP fully recovered (50%) within two to three months. Another group with mild-intensity PPSP followed a protracted recovery trajectory (37%), while incomplete recovery was observed in the last group (13%). Acute postoperative pain and younger age were associated with a less favourable recovery trajectory. More neuropathic pain symptoms were observed in patients with incomplete recovery.

CONCLUSIONS

Three clinically relevant recovery trajectories were identified, based on comprehensive pain tracking. Higher acute postoperative pain intensity was associated with an unfavourable pain recovery trajectory.

SIGNIFICANCE STATEMENT

Understanding the transition from acute to chronic postoperative pain and identifying preoperative risk factors is essential for the development of targeted treatments and the implementation of preventive measures. This study (1) identified distinct recovery trajectories based on frequent pain assessment follow-ups for 12 months after surgery and (2) evaluated risk factors for unfavourable postoperative pain recovery paths. Findings suggest that early higher postoperative pain intensity is associated with an unfavourable long-term recovery path.

Subgrouping of facilitatory or inhibitory conditioned pain modulation responses in patients with chronic knee pain. Explorative analysis from a multicentre trial

BACKGROUND

Facilitatory and inhibitory conditioned pain modulation (CPM) responses are observed in healthy volunteers and chronic pain patients, but the clinical implications for phenotyping are unknown. This study aimed to subgroup and compare chronic knee pain patients according to their CPM responses.

METHODS

This explorative, cross-sectional study included 127 patients with chronic knee pain (osteoarthritis or following total knee arthroplasty). Individual CPM responses were categorized as facilitatory (test stimuli pain intensity increased when conditioning stimuli were applied), as inhibitory (test stimuli pain intensity decreased) or as no change (defined as less than 5.3% change in pain intensity). Outcomes were clinical pain intensities, temporal summation, widespread pain, self-reported physical function, PainDETECT questionnaire and Pain Quality Assessment Scale. Data were analysed as comparisons between the inhibitory and the facilitatory groups and using multivariate linear regression models.

RESULTS

Fifty-four patients had facilitatory CPM responses, 49 had inhibitory CPM responses, and 24 showed no change in CPM response. A between-group difference was observed for self-reported physical function, with the facilitatory CPM group reporting better function (54.4 vs. 46.0, p = 0.028) and the facilitatory CPM group reported more deep pain sensations (3.2 vs. 2.0, p = 0.021). The remaining outcomes showed no between-group differences. Higher clinical pain intensity and facilitated temporal summation were associated in the facilitated CPM group but not in the inhibitory CPM group.

CONCLUSION

These explorative findings indicated that quantitative clinical and experimental differences exist between facilitatory or inhibitory CPM responses in a chronic knee pain patient population. Differences in patients' CPM responses should be further investigated to unravel possible clinical importance.

SIGNIFICANCE

Our findings confirm that conditioned pain modulation consist of inhibitory and facilitatory responders among a patient population with chronic knee pain. This explorative study indicates that patients with either facilitatory or inhibitory conditioned pain modulation could exhibit differences in pain outcomes. Subgrouping of chronic pain patients depending on individual conditioned pain modulation responses could be considered in phenotyping patients prior to inclusion in clinical trials or used for personalizing the management regime.

Quantitative sensory testing, psychological profiles and clinical pain in patients with psoriatic arthritis and hand osteoarthritis experiencing pain of at least moderate intensity

BACKGROUND

Chronic pain is the hallmark symptom of joint diseases. This study examined the differences in quantitative sensory testing between patients with psoriatic arthritis (PsA), hand osteoarthritis (hand-OA) and a pain-free control group and differences between patients with and without concomitant fibromyalgia (cFM).

METHODS

All patients and pain-free controls were assessed using pressure pain thresholds (PPT), temporal summation of pain (TSP), conditioned pain modulation (CPM) and clinical pain intensities. Psychological distress was assessed with the Hospital Anxiety and Depression Scale, Pain Catastrophizing Scale, and Pittsburgh Sleep Quality Index. Disability was assessed with the Health Assessment Questionnaire and pain quality with the painDETECT questionnaire. cFM was identified using the revised 2016 American College of Rheumatology diagnostic criteria.

RESULTS

Patients with hand-OA (n = 75) or PsA (n = 58) had statistically significant lower PPTs and CPM, greater TSP, and higher scores of psychological distress (p < 0.05) than controls (n = 20). Patients with cFM (58%) had higher scores of depression (p = 0.001), anxiety (p = 0.004), catastrophizing (p = 0.012), disability (p < 0.001), higher painDETECT score (p = 0.001), TSP (p = 0.027), and reduced sleep quality (p = 0.021) when compared to patients without cFM.

CONCLUSION

Patients with hand-OA and PsA exhibited signs of pain sensitization and a higher degree of psychological distress and disability than pain-free individuals. Patients with cFM had greater TSP, painDETECT score, disability, catastrophizing, and reduced sleep quality, than patients without, indicating greater degree of pain sensitization, psychological burden, and disability.

STATEMENT OF SIGNIFICANCE

This paper shows that a significant proportion of patients with hand osteoarthritis and psoriatic arthritis with moderate pain intensity have significantly increased signs of pain sensitization and markers of psychological distress. A large proportion of these patients fulfil the criteria for concomitant fibromyalgia and these patients show even greater propensity towards pain sensitization and psychological distress.

Dose-dependency of diclofenac's cardiovascular risks: a series of nationwide emulated trials

Background

The dose dependency of the adverse effects of diclofenac remains poorly understood.

Purpose

To examine the dose-related cardiovascular risks associated with diclofenac initiation

Methods

We used Danish nationwide health registries (1999–2018) to conduct a series of emulated trials (n=285). Eligible adults had no recent NSAID fillings, contraindications, or conditions with low adherence. Individuals eligible for inclusion were ≥18 years with (1) ≥90 days continuous prescription records prior to diclofenac initiation (baseline); (2) no NSAID prescriptions ≤90 days before enrollment, and (3) no exclusion criteria. Exclusion criteria reflected likelihood of low adherence to treatment (dementia, schizophrenia, or antipsychotic drug use) and labeled contraindications (ulcer disease/anti-ulcer drugs, gastrointestinal bleeding, inflammatory bowel disease, thrombocytopenia, or heart failure). Initiators of diclofenac were compared to healthcare-seeking non-initiators, but also head-to-head for initiators of high (≥75 mg pills as proxy for ≥150 mg/daily) vs. low dose (≤50 mg pills as proxy for &lt;150 mg/daily). Cox proportional-hazards regression was used to compute the intention-to-treat hazard ratio, as measure of the incidence rate ratio (IRR), of major adverse cardiac and cerebrovascular events (MACCE) within 30 days from initiation.

Results

Among 3,177,484 diclofenac initiatiors, 31% used high and 69% used low dose. Compared with non-initiators, diclofenac initiatiors had a 70% increased rate for MACCE (IRR 1.70, 95% CI: 1.55–1.86), reflecting for the individual MACCE components an increased IRRs of 1.66 (95% CI: 1.54–1.79) for myocardial infaction, 1.32 (1.20–1.45) for ischemic stroke, and 1.69 (1.54–1.86) for cardiac death. The effect for MACCE did not differ between initiators of high (IRR 1.73, 95% CI: 1.51–1.97) and low dose (IRR 1.68, 95% CI: 1.52–1.86) (Figure 1). When comparing high and low dose diclofenac head-to-head, we found no meaningful difference in the IRR for MACCE (1.03, 95% CI: 0.89–1.19), MI (0.99, 0.87–1.11), ischemic stroke (0.95, 0.81–1.11) or cardiac death (1.04, 0.90–1.21) (Figure 2).

Conclusion

Initiation of low- and high-dose diclofenac was associated with a consistent and comparable increase in cardiovascular risk.

Funding Acknowledgement

Type of funding sources: Foundation. Main funding source(s): Novo Nordisk Foundation

Functional and structural neuroplastic changes related to sensitization proxies in patients with Osteoarthritis: a systematic review

OBJECTIVE

Several reports in literature have identified sensitization as a possible basis for the enhanced pain reactions associated with Osteoarthritis (OA). The aim of this current systematic review is to summarize functional and structural brain changes associated with surrogate sensitization parameters assessed in patients with OA-related pain.

DESIGN

Systematic review.

SUBJECTS

Patients with OA related pain.

METHODS

A literature search was conducted systematically in MEDLINE, CINAHL, EMBASE databases for human studies up to December 2019. Articles were included if they assessed brain imaging and senzitisation parameters (quantitative sensory testing and questionnaires) in adults with OA related pain. Methodological quality was assessed using the Methodological Index for Non-Randomized Studies (MINORS) score.

RESULTS

Five studies reporting on 138 patients were included in this review. The MINORS scale yielded mean scores of 8.5/16 and 12.3/24, for the cohort and case-control studies respectively. Four low-quality studies suggest a greater pain matrix activation associated with clinical measures of sensitization in patients with OA, while another study underlined the presence of structural changes (reduced gray matter volume) in the cortical areas involved in the nociceptive processing possible also related to sensitization.

CONCLUSION

This review shows conflicting evidence for structural and functional neuroplastic brain changes related to sensitization proxies in patients with OA.

Sensory, Motor, and Psychosocial Characteristics of Individuals With Chronic Neck Pain: A Case-Control Study

OBJECTIVE

Given the complex and unclear etiology of neck pain, it is important to understand the differences in central sensitization as well as psychosocial factors in individuals with chronic neck pain and healthy controls. The purpose of this study was to benchmark differences in central sensitization, psychosocial factors, and range of motion between people with nonspecific chronic neck pain and healthy controls and to analyze the correlation between pain intensity, neck disability, and psychosocial factors in people with chronic neck pain.

METHODS

Thirty individuals with chronic neck pain and 30 healthy controls were included in this case-control study. Outcome measures were as follows: central sensitization (pressure pain threshold, temporal summation, and conditioned pain modulation), psychosocial factors (depressive symptoms, pain catastrophizing, and quality of life), and active cervical range of motion.

RESULTS

People with neck pain had lower local pressure pain threshold, a decrease in conditioned pain modulation, more depressive symptoms, greater pain catastrophizing, lower quality of life, and reduced range of motion for neck rotation when compared with healthy controls. In people with neck pain, moderate correlations were observed between pain intensity and quality of life (ρ = -0.479), disability and pain catastrophizing (ρ = 0.379), and disability and quality of life (ρ = -0.456).

CONCLUSIONS

People with neck pain have local hyperalgesia, impaired conditioning pain modulation, depressive symptoms, pain catastrophizing, low quality of life, and reduced active range of motion during neck rotation, which should be taken into account during assessment and treatment.

IMPACT

This study shows that important outcomes, such as central sensitization and psychosocial factors, should be considered during assessment and treatment of individuals with nonspecific chronic neck pain. In addition, pain intensity and neck disability are correlated with psychosocial factors.

Cost-effectiveness of 12 weeks of supervised treatment compared to written advice in patients with knee osteoarthritis: a secondary analysis of the 2-year outcome from a randomized trial

OBJECTIVE

To assess the 24-month cost-effectiveness of supervised treatment compared to written advice in knee osteoarthritis (OA).

DESIGN

100 adults with moderate-severe OA not eligible for total knee replacement (TKR) randomized to a 12-week individualized, supervised treatment (exercise, education, diet, insoles and pain medication) or written advice. Effectiveness was measured as change in quality-adjusted life years (QALYs) from baseline to 24 months, including data from baseline, 3, 6, 12 and 24 months, while healthcare costs and transfer payments were derived from national registries after final follow-up. Incremental costs, quality-adjusted life-years (QALYs), and incremental cost-effectiveness ratios (ICERs) were calculated. A sensitivity analysis resampling existing data was conducted and the probability of cost-effectiveness was estimated using a 22,665 Euros/QALY threshold. In a sensitivity analysis, cost-effectiveness was calculated for different costs of the supervised treatment (actual cost in study; cost in private practice; and in-between cost).

RESULTS

Average costs were similar between groups (6,758 Euros vs 6,880 Euros), while the supervised treatment were close to being more effective (incremental effect (95% CI) of 0.075 (-0.005 to 0.156). In the primary analysis excluding deaths, this led the supervised treatment to be cost-effective, compared to written advice. The sensitivity analysis demonstrated that the results were sensitive to changes in the cost of treatment, but in all scenarios the supervised treatment was cost-effective (ICERs of 6,229 to 20,688 Euros/QALY).

CONCLUSIONS

From a 24-month perspective, a 12-week individualized, supervised treatment program is cost-effective compared to written advice in patients with moderate-severe knee OA not eligible for TKR.

TRIAL REGISTRATION

ClinicalTrials.gov number: NCT01535001.

Quantitative sensory testing in feline osteoarthritic pain - a systematic review and meta-analysis

Quantitative sensory testing (QST) is a psychophysical test used to quantify somatosensory sensation under normal or pathological conditions including osteoarthritis (OA).

OBJECTIVE

This study aimed to conduct a systematic review and meta-analysis of studies using QST in healthy and osteoarthritic cats, registered at Systematic Review Research Facility (#26-06-2017).

DESIGN

Hierarchical models with random intercepts for each individual study extracted through the systematic review were fit to subject-level data; QST measures were contrasted between healthy and osteoarthritic cats. Four bibliographic databases were searched; quality and risk of bias assessment were performed using pre-established criteria.

RESULTS

Six articles were included; most were of high quality and low risk of bias. Punctate tactile threshold (n = 70) and mechanical temporal summation (n = 35) were eligible for analysis. Cats with OA have lower punctate tactile threshold [mean difference (95%HDI): -44 (-60; -26) grams] and facilitated temporal summation of pain [hazard ratio (95%HDI): 5.32 (2.19; 14) times] when compared with healthy cats. The effect of sex and body weight on sensory sensitivity remained inconclusive throughout all analyses. Due to the correlation between age and OA status, it remains difficult to assess the effect of OA on sensory sensitivity, independently of age.

CONCLUSIONS

Clear and transparent reporting using guidelines are warranted. Similar to people, centralized sensitization is a feature of OA in cats. Future studies should try to elucidate the age effect on feline OA. Research with natural OA in cats is promising with potential to benefit feline health and welfare, and improve translatability to clinical research.

Acquisition of a simple motor skill: task-dependent adaptation and long-term changes in the human soleus stretch reflex

Changing the H reflex through operant conditioning leads to CNS multisite plasticity and can affect previously learned skills. To further understand the mechanisms of this plasticity, we operantly conditioned the initial component (M1) of the soleus stretch reflex. Unlike the H reflex, the stretch reflex is affected by fusimotor control, comprises several bursts of activity resulting from temporally dispersed afferent inputs, and may activate spinal motoneurons via several different spinal and supraspinal pathways. Neurologically normal participants completed 6 baseline sessions and 24 operant conditioning sessions in which they were encouraged to increase (M1up) or decrease (M1down) M1 size. Five of eight M1up participants significantly increased M1; the final M1 size of those five participants was 143 ± 15% (mean ± SE) of the baseline value. All eight M1down participants significantly decreased M1; their final M1 size was 62 ± 6% of baseline. Similar to the previous H-reflex conditioning studies, conditioned reflex change consisted of within-session task-dependent adaptation and across-session long-term change. Task-dependent adaptation was evident in conditioning session 1 with M1up and by session 4 with M1down. Long-term change was evident by session 10 with M1up and by session 16 with M1down. Task-dependent adaptation was greater with M1up than with the previous H-reflex upconditioning. This may reflect adaptive changes in muscle spindle sensitivity, which affects the stretch reflex but not the H reflex. Because the stretch reflex is related to motor function more directly than the H reflex, M1 conditioning may provide a valuable tool for exploring the functional impact of reflex conditioning and its potential therapeutic applications. NEW & NOTEWORTHY Since the activity of stretch reflex pathways contributes to locomotion, changing it through training may improve locomotor rehabilitation in people with CNS disorders. Here we show for the first time that people can change the size of the soleus spinal stretch reflex through operant conditioning. Conditioned stretch reflex change is the sum of task-dependent adaptation and long-term change, consistent with H-reflex conditioning yet different from it in the composition and amount of the two components.

Potential diagnostic value of a type X collagen neo-epitope biomarker for knee osteoarthritis

OBJECTIVE

Phenotypic changes of chondrocytes toward hypertrophy might be fundamental in the pathogenesis of osteoarthritis (OA), of which type X collagen (Col10) is a well-known marker. The purpose was to develop a specific immunoassay for blood quantification of a newly identified neo-epitope of type Col10 to assess its diagnostic value for radiographic knee OA.

METHODS

A neo-epitope of Col10 was identified in urine samples from OA patients. A monoclonal antibody against the neo-epitope was produced in Balb/C mice. The enzyme responsible for the cleavage was identified. Immunohistochemical detection of this neo-epitope was performed on human OA cartilage. An immunoassay (Col10neo) was developed and quantified in two clinical studies: the C4Pain-003 and the NYU OA progression study. Receiver operating characteristic curve (ROC) curve analysis was carried out to evaluate the discriminative power of Col10neo between OA and rheumatoid arthritis (RA).

RESULTS

A neo-epitope specific mAb was produced. The Cathepsin K-generated neo-epitope was localized to the pericellular matrix of chondrocytes, while its presence was extended and more prominent in superficial fibrillation in the cartilage with advanced degradation. In the C4Pain study, a higher level of Col10neo was seen in subjects with greater KL grade. The group of the highest tertile of Col10neo included more subjects with KL3-4. In the NYU study, Col10neo was statistically higher in OA than control or RA. ROC curve analysis revealed area under the curve was 0.88 (95% CI 0.81-0.94).

CONCLUSION

Our findings indicate that Col10neo linked to hypertrophic chondrocytes could be used as a diagnostic biochemical marker for knee OA.

Antipruritic effects of transient heat stimulation on histaminergic and nonhistaminergic itch

BACKGROUND

Chronic itch is notoriously difficult to treat. Counterstimuli are able to inhibit itch, but this principle is difficult to apply in clinical practice, and the mechanisms behind counterstimulation-induced itch suppression in humans are unclear.

OBJECTIVES

Firstly, to analyse the stimulus-response effects of transient heat stimuli on histaminergic and nonhistaminergic itch, and secondly, to investigate whether the antipruritic effect depends on homotopic (peripheral mediation) or heterotopic (central mediation) counterstimulation relative to the itch provocation site.

METHODS

Eighteen healthy volunteers participated (eight female, mean age 25·7 ± 0·8 years). Itch was evoked on premarked areas of the volar forearms, by either histamine (1% solution) or cowhage (35-40 spicules). In addition to the itch provocations (experiment 1), 5-s homotopic heat stimuli at 32, 40, 45 or 50 °C were applied. In experiment 2, heat stimuli were applied either homotopically, intrasegmentally (next to the provocation site) or extrasegmentally (dorsal forearm). Itch intensity was evaluated throughout the procedures using a digital visual analogue scale.

RESULTS

Homotopic counterstimuli inhibited histaminergic itch by 41·3% at 45 °C (P < 0·01) and by 76·7% at 50 °C (P < 0·001). Cowhage-induced itch was less prone to counterstimulation and was significantly diminished only at 50 °C, by 43·6% (P = 0·009). Counterstimulations applied heterotopically were not able to inhibit itch significantly.

CONCLUSIONS

Itch pathway-specific effects of counterstimuli were observed between homo- and heterotopic stimulation. Histaminergic itch was robustly inhibited by short-term homotopic noxious heat stimuli for up to 10 min. Nonhistaminergic itch was only weakly inhibited. The inhibitory effects exerted by the short-term heat stimuli only occurred following homotopic counterstimulation.

Total knee replacement and non-surgical treatment of knee osteoarthritis: 2-year outcome from two parallel randomized controlled trials

OBJECTIVES

To compare 2-year outcomes of total knee replacement (TKR) followed by non-surgical treatment to that of non-surgical treatment alone and outcomes of the same non-surgical treatment to that of written advice.

DESIGN

In two randomized trials, 200 (mean age 66) adults with moderate to severe knee osteoarthritis (OA), 100 eligible for TKR and 100 not eligible for TKR, were randomized to TKR followed by non-surgical treatment, non-surgical treatment alone, or written advice. Non-surgical treatment consisted of 12 weeks of supervised exercise, education, dietary advice, use of insoles, and pain medication. The primary outcome was the mean score of the Knee Injury and Osteoarthritis Outcome Score (KOOS) subscales, covering pain, symptoms, activities of daily living (ADL), and quality of life (QOL).

RESULTS

Patients randomized to TKR had greater improvements than patients randomized to non-surgical treatment alone (difference of 18.3 points (95% CI; 11.3 to 25.3)), who in turn improved more than patients randomized to written advice (difference of 7.0 points (95% CI; 0.4 to 13.5)). Among patients eligible for TKR, 16 (32%) from the non-surgical group underwent TKR during 2 years and among those initially ineligible, seven patients (14%) from the non-surgical group and ten (20%) from the written advice group underwent TKR.

CONCLUSIONS

TKR followed by non-surgical treatment is more effective on pain and function than non-surgical treatment alone, which in turn is more effective than written advice. Two out of three patients with moderate to severe knee OA eligible for TKR delayed surgery for at least 2 years following non-surgical treatment.

TRIAL REGISTRATION

ClinicalTrials.gov numbers NCT01410409 and NCT01535001.

Predicting drug efficacy in chronic low back pain by quantitative sensory tests

BACKGROUND

Drugs are prescribed for chronic low back pain without knowing in advance whether a patient will respond to them or not. Quantitative sensory tests (QST) can discriminate patients according to sensory phenotype, possibly reflecting underlying mechanisms of pain processing. QST may therefore be a screening tool to identify potential responders to a certain drug. The aim of this study was to investigate whether QST can predict analgesic effects of oxycodone, imipramine and clobazam in chronic low back pain.

METHODS

Oxycodone 15 mg (n = 50), imipramine 75 mg (n = 50) and clobazam 20 mg (n = 49) were compared to active placebo tolterodine 1 mg in a randomized, double-blinded, crossover fashion. Electrical, pressure and thermal QST were performed at baseline and after 1 and 2 h. Pain intensity was assessed on a 0-10 numeric rating scale every 30 min for up to 2 h. The ability of baseline QST to predict pain reduction after 2 h was analysed using linear mixed models. Genetic variants of drug-metabolizing enzymes and genes affecting pain sensitivity were examined as covariables.

RESULTS

No predictor of analgesic effect was found for oxycodone and clobazam. Thermal QST was associated with analgesic effect of imipramine: patients more sensitive to heat or cold were more likely to experience an effect of imipramine. Pharmacogenetic variants and pain-related candidate genes were not associated with drug efficacy.

CONCLUSIONS

Thermal QST have the potential to predict imipramine effect in chronic low back pain. Oxycodone and clobazam effects could not be predicted by any of the selected QST or genetic variants.

SIGNIFICANCE

Predicting drug efficacy in chronic low back pain remains difficult. There is some evidence that patients more sensitive to heat and cold pain respond better to imipramine.

Decomposition of Biomedical Signals in Spatial and Time-frequency Modes

Objectives: The purpose of this paper is to introduce a new method for spatial-time-frequency analysis of multichannel biomedical data. We exemplify the method for data recorded with a 31-channel Philips biomagnetometer.

Methods: The method creates approximations and decompositions of spatiotemporal signal distributions using elements (atoms) chosen from a very large and redundant set (dictionary). The method is based on the Matching Pursuit algorithm, but it uses atoms that are distributed both in time and space (instead of only time-distributed atoms in standard Matching Pursuit). The time-frequency distribution of signal components is described by Gabor atoms and their spatial distribution is modeled by spatial modes. The spatial modes are created with the help of Bessel functions. Two versions of the method, differing in the definition of spatial properties of the atoms, are presented.

Results: The technique was validated on simulated data and real magnetic field data. It was used for removal of powerline noise from multichannel magnetoencephalography data, extraction of high-frequency somatosensory evoked fields and for separation of partially overlapping T- and U-waves in magnetocardiography.

Conclusions: The method allows for parameterization of multichannel data in the time-frequency as well as in the spatial domains. It thus can be used for signal preserving filtering simultaneously in time, frequency, and space. Applications are e.g. the elimination of artifact components, extraction of components with biological meaning, and data exploration.

Evaluating the ability of non-rectangular electrical pulse forms to preferentially activate nociceptive fibers by comparing perception thresholds

Aims

Selective activation of nociceptive fibers is difficult using electrical stimulation as the activation threshold is higher than for non-nociceptive fibers. It remains unclear to what extent accommodation of non-nociceptive fibers during slowly rising electrical pulses can be utilized to reverse this activation order. The aim of this study was to evaluate the ability of different pulse forms to activate nociceptive fibers with minimal co-activation of non-nociceptive fibers by comparing subjective perception thresholds (PT).

Methods

Electrical pulses were applied on the volar forearm of 25 subjects with (1) small diameter pin electrodes providing high current density in the skin epidermis, where primarily nociceptive fibers terminate and (2) standard patch electrodes (2.63 cm2). PTs were obtained for exponential current increase, linear current increase, increasing form of exponential current decay (ED), and standard rectangular current pulses. All pulse forms were tested at two relatively long durations (5 and 50 ms). The PT ratio between patch- and pin electrode was calculated as an estimate of the ability of a pulse form to preferentially activate nociceptive fibers. The short form McGill pain questionnaire (SF-MPQ) was used to assess perceived quality of pain for all pulse forms.

Results

For the pin electrode, PT tended to decrease with increasing pulse area. Patch electrode PT tended to increase for increasing pulse area for non-rectangular 50 ms pulses, in contrast to 5 ms pulses, indicating accommodation of non-nociceptive fibers. Largest PT ratio was obtained for the 50 ms ED. SF-MPQ scores were higher for the pin- compared to the patch electrode. Pin electrode pain qualities were mainly described as stabbing and sharp. SF-MPQ scores did not differ between pulse forms.

Conclusions

Long duration ED pulses seem to activate nociceptive fibers better than regular, short duration pulses; most likely reflecting accommodation of non-nociceptive fibers.

Aquaporin 4 expression on trigeminal satellite glial cells under normal and inflammatory conditions

Aims

Limited information is currently available for the expression and role of Aquaporin 4 (AQP4) (AQ4) in the peripheral nervous system (PNS). It has been demonstrated that AQP4 is expressed in sensory ganglia. Immunohistochemistry has revealed that satellite glial cells (SGCs) surrounding the cell bodies of the primary afferent sensory neurons in these sensory ganglia exclusively express AQP4 at a considerably lower level than what is seen in astrocytes. The pathophysiological relevance of AQP4 in peripheral nociception; however, remains unclear. Hence, this study aimed at investigating AQP4 expression in trigeminal neurons and SGCs under normal and inflammatory conditions relevant to craniofacial pain conditions.

Methods

Rat trigeminal ganglia (TG) were isolated from adult male Sprague-Dawley rats subjected to a model of trigeminal inflammation evoked by unilateral complete Freund’s adjuvant (CFA) injection in temporomandibular joint. Immunohistochemistry was performed on TG sections of CFA-treated animals. NeuN and GS markers were used for identification of neurons and SGCs, respectively. AQP4 expression was investigated in both ipsilateral and contralateral TG sections. The study protocol was approved by the local ethics committee.

Results

Co-localization of NeuN-AQP4 and GS-AQP4 were identified in both ipsi and contralateral trigeminal ganglia of the CFA-treated rats. However, we did not detect any difference between the ipsi- and contralateral side in terms of alteration in AQP4 receptor expression.

Conclusions

AQP4 was expressed both on trigeminal neurons and SGCs and CFA did not cause a remarkable change in AQP4 expression, when ipsilateral and contralateral TG of the test animals was compared. Previously, it has been shown that in a neuropathic pain model no difference is detectable between wild type and AQP4-deficient mice, for mechanical and thermal perception; however, in formalin pain model AQP4-deficient mice have higher thermal pain thresholds. Further investigation is required to clarify role of AQP4 in pain.

Combined electric and pressure cuff pain stimuli for assessing conditioning pain modulation (CPM)

Aims

Traditionally, conditioning pain modulation (CPM) can be assessed by applying a test stimulus (TS) before and after application of a conditioning stimulus (CS), which is normally applied extra-segmental. Currently, no studies have attempted to apply the TS and CS to the same site using different stimuli modalities. The aim of this study was to evaluate electrical TS and cuff pressure CS applied to the same experimental site for studying CPM.

Methods

20 male volunteers participated in this study, which consisted of stimulations applied by a cuff-algometer (NociTech and Aalborg University, Denmark) and current stimulator (Digitimer DS5, UK), through two Ag/AgCl electrodes (Ambu® Neuroline 700, Denmark). The cuff was wrapped around the lower leg and stimulation electrodes were placed under the cuff and to the same location on the contralateral leg. Electrical TS were applied to the non-dominant leg with or without cuff pressure CS on the dominant (CS1) or the same (non-dominant) leg (CS2, electrode under cuff). The subjects were instructed to rate the electrical evoked pain intensity on a 10-cm continuous visual analog scale (VAS, “0” represented “no pain”, and “10” represented “maximal pain”). The pain detection threshold (PDT) was defined as “1” on the VAS scale.

Results

There was no significant deference in PDT for neither CS1 nor CS2. A median split subanalysis on CPM-responders versus CPM-nonresponders to the TS + CS1 combination. Using this grouping, there was significant increase in PDT when comparing TS to TS + CS1 or TS + CS2 (4.0 mA vs 5.6 mA; P < 0.05, 4.0 mA vs 5.1 mA; P < 0.05).

Conclusions

The study indicates that CPM can be evoked in a subgroup of subjects by applying the electrical test stimulus and cuff pressure conditioning stimuli to the same experimental site.

Assessing Offset Analgesia through electrical stimulations in healthy volunteers

Background and aims

Offset Analgesia (OA) is a disproportionally large decrease of pain perception evoked by a slight decrease of a painful cutaneous heat stimulus, resulting in a lower pain perception compared to a simple constant stimulus at the same temperature. This study aimed to investigate the possibility of evoking the same disproportional analgesic effect by applying electrical stimuli.

Methods

24 subjects underwent two control-trials of 30 s constant intensity by applying either heat stimulation at 48 °C or an electrical stimulation at 150% of the individual electrical Pain Detection (ePD) on the volar forearm. OA-trials consisted of a 30 s stimulation, divided into three periods: T1 (5 s), T2 (5 s), and T3 (20 s), with stimuli intensities of 48 °C, 49°C and 48°C or 150% of ePD, 180% of ePD, and 150% of ePD.

Subjects were asked to rate the pain intensity on an electronic Visual Analog Scale (VAS; 0: no pain; 10: worst imaginable pain), and were categorized as responders if they showed more than 30% lower VAS at heat OA-trials compared to heat control-trial. Repeated measures Analysis of Variance was applied to investigate the difference in pain intensity to the electrical OA-trials, compared with the electrical control-trials.

Results

Responders to the heat OA-trial also responded to the electrical OA-trial compared to the electrical control-trial, with an analgesic effect of 3.3±0.5 VAS points (P < 0.001). However, when analyzing all subjects, no difference was found comparing the electrical OA-trial (VAS 3.8±0.5) to the electrical control-trial (VAS 6.2±0.4; P > 0.5).

Conclusions

This study suggests that responders to the heat OA-paradigm also respond to the electrical OA-paradigm.

The effect of UVB-induced skin inflammation on histaminergic and non-histaminergic evoked itch and pain

Aims

Itch often occurs in cutaneous conditions characterized by some degree of inflammation, e.g. atopic dermatitis, psoriasis or urticaria. It is unclear to which extent cutaneous inflammation causes sensitization of pruriceptive primary afferent C-fibers. The aim of this study was to explore if inflammation induced by UVB (B-ultraviolet rays) modify neurogenic inflammation and itch associated with experimental itch provocations.

Methods

Twenty healthy volunteers (10F/10M, 26.2 ± 1.6 years) were included. Eight circles (diameter=2cm), four on each volar forearm were studied. Two spots were irradiated with 0.5 × Minimal Erythema Dose (MED), two with 1× MED and two with 2× MED, and two acted as controls. Itch provocations were conducted using histamine (1%) percutaneously introduced and 35–45 cowhage spicules (non-histaminergic itch). The duration and intensity of itch and pain, sensitivity to touch-evoked itch (STI), mechanical pain thresholds (MPT) and sensitivity (MPS), and superficial blood perfusion where measured in UVB-irradiated- and control areas 24-h after UVB-irradiation and following each itch provocation.

Results

UVB induced dose-dependent hyperalgesia validated by decreased MPT, increased MPS and neurogenic inflammation (all P < 0.01). UVB-induced inflammation did not increase the magnitude of itch reported following any itch provocation. However, cowhage was associated with more pain in UVB-irradiated areas and the proportional ratings of the mixed itch and pain sensation was shifted towards increased pain dominance (P < 0.01). Itch provocations did not increase the mechanical hyperalgesia induced by UVB, whereas it provoked an increase in superficial blood perfusion compared to UVB alone (P < 0.05).

Conclusions

(1) The UVB model induces sensitization to pain but not itch stimuli and independently increases the nociceptive sensations associated with non-histaminergic itch provocation. (2) The inflammatory UVB-perturbation does not mimic the sensitization associated with inflammatory dermatoses where lesioned skin is more receptive to pruritogens, suggesting that more specific or prolonged inflammatory processes are involved in clinical itch conditions.

Topical allyl-isothiocyanate (mustard oil) as a TRPA1-dependent human surrogate model of pain, hyperalgesia, and neurogenic inflammation – A dose response study

Aims

Transient receptor potential ion channel A1 (TRPA1) has been shown to play a pathoetiological role in several painful and inflammatory conditions and therefore several new drug candidates, targeting TRPA1 are currently being developed [1]. While the natural TRPA1-agonist allyl-isothiocyanate (AITC, known as “mustard oil”) is an exceedingly common animal pain model, it has only been sparsely investigated as a potential human surrogate model of pain and neurogenic inflammation. This study aimed to evaluate dose-response features of AITC as a sensitizing, algogenic irritant in human skin.

Methods

Three concentrations of AITC (10%, 50%, 90%) and vehicle (100% paraffin) were applied for 5 min to 3 cm × 3 cm areas on the volar forearms in 14 healthy volunteers, and evoked pain (visual analog scale 0–100 mm) and pain quality were assessed. Following the application, a battery of quantitative sensory tests was conducted including assessment of mechanical and thermal sensitivity. Neurogenic inflammation was evaluated using Laser Perfusion Imaging (FLPI). Erythema and pigmentation were assessed before, immediately after and≈64 h after AITC exposure.

Results

Topical application of AITC induced significant dose-dependent, moderate-to-severe spontaneous pain mostly described as burning as well as mechanical and heat hyperalgesia (p < 0.05). The model also produced robust dynamic mechanical allodynia (p < 0.05). Only modest increases in pain hypersensitivity were observed between the 50% and 90% concentrations. Neurogenic inflammation was evoked by all concentrations and assessments by FLPI demonstrated a significant dose dependent increase from 10% to 50% ATIC, with a ceiling effect from 50% to 90%.

Conclusions

Topical AITC-application evokes dose-dependent rapid pain and somatosensory sensitization with optimal concentrations recommended to be above 10% and equal to or below 50%. The model is translatable and could be useful in pharmacological proof-of-concept studies of TRPA1-antagonists, analgesics and anti-inflammatory compounds or for exploratory clinical purposes (e.g. loss- or gain-of-function in peripheral neuropathies).

The effect of facilitated temporal summation of pain, widespread pressure hyperalgesia and pain intensity in patients with knee osteoarthritis on the responds to Non-Steroidal Anti-Inflammatory Drugs – A preliminary analysis

Background

Non-Steroidal Anti-Inflammatory Drug (NSAID) treatment is recommended as the first step in the treatment of knee osteoarthritis (KOA). Due to the risk of side-effects of NSIADs and low responds rate, methods for selection of NSAID responders are highly warranted. Recent studies suggest that pain sensitization in KOA might be predictive for the effect of surgical treatment, why the purpose of the present study was to evaluated whether quantitative sensory testing (QST) could be predictive for the effect of NSAID treatment in KOA.

Material and methods

100 patients were enrolled and assessed using temporal summation of pain (TSP), pressure pain thresholds (PPT) and pain intensity (visual analog scale, VAS; 0: no pain and 10: worst imaginable pain) before and 8 weeks after daily treatment of Ibuprofen 400 mg × 3, Paracetamol 1 g × 3 and Pantoprazole 20 mg × 1. TSP was assessed by the difference in pain scores to one PinPrick stimulus followed by 10 PinPrick stimuli. PPT was assessed at the extensor carpi radialis longus muscle. Responders were categorized as decrease in pain intensity of at least three VAS-points.

Results

80 patients had complete data at follow-up. 28 patients were categorized as responders and 52 patients as non-responders. Pre-treatment pain intensity (responders: 7.5 (SD: 1.6) vs. nonresponders: 6.5 (SD: 2.3), P < 0.03) and TSP (responders: 1.9 (SD: 2.5) vs. non-responders: 1.0 (SD: 1.5), P < 0.05) were significantly higher in responders compared with non-responders. Pre-treatment PPT at the arm trended towards significantly lower in responders (293 kPa (SD: 85)) compared with no responders (339 kPa (SD: 124), P < 0.06).

Conclusions

This preliminary analysis indicate that KOA patients with high pain intensities, facilitated TSP and widespread pressure hyperalgesia respond to NSAIDs.

The number of active trigger points is associated with sensory and emotional aspects of health-related quality of life in tension type headache

Aims

Some evidence supports that referred pain elicited by active trigger points (TrPs) reproduces some features of tension type headache (TTH). Our aim was to investigate the association between the number of active TrPs and health-related quality of life TTH.

Methods

Patients with TTH diagnosed by experienced neurologists according to the last International Headache Classification (ICHD-III) were included. Exclusion criteria included other primary headaches, medication overuse headache, whiplash injury or fibromyalgia. TrPs were bilaterally explored within the masseter, temporalis, trapezius, sternocleidomastoid, splenius capitis, and suboccipital. Health-related quality of life was assessed with the SF-36 questionnaire including 8 domains: physical functioning, physical role, bodily pain, general health, vitality, social functioning, role-emotional, and mental health. Higher scores represent better quality of life. Spearman correlation coefficients were used to determine correlations between the active TrPs and SF-36.

Results

Two hundred and two patients (mean age: 45±12 years) with a headache frequency of 17±7 days/month participated. Each patient with TTH exhibited 4.7±2.9 active TrPs. The number of active TrPs showed moderate weak negative associations with bodily pain (r s: −0.216; P =0.002), emotional role (r s: -0.185; P = 0.008) and vitality (r s: –0.161; P = 0.02), but not with the remaining domains: the higher the number of active TrPs, the worse the emotional role and vitality and the higher the pain interference with daily life. These results were similar in both frequent episodic and chronic TTH.

Conclusions

The number of active TrPs was associated with sensory and emotional aspects of quality of life in a cohort of subjects with TTH.

Assessment of deep dynamic mechanical sensitivity in individuals with tension-type headache: The dynamic pressure algometry

BACKGROUND

To explore the validity of dynamic pressure algometry for evaluating deep dynamic mechanical sensitivity by assessing its association with headache features and widespread pressure sensitivity in tension-type headache (TTH).

METHODS

One hundred and eighty-eight subjects with TTH (70% women) participated. Deep dynamic sensitivity was assessed with a dynamic pressure algometry set (Aalborg University, Denmark^© ) consisting of 11 different rollers including fixed levels from 500 g to 5300 g. Each roller was moved at a speed of 0.5 cm/s over a 60-mm horizontal line covering the temporalis muscle. Dynamic pain threshold (DPT-level of the first painful roller) was determined and pain intensity during DPT was rated on a numerical pain rate scale (NPRS, 0-10). Headache clinical features were collected on a headache diary. As gold standard, static pressure pain thresholds (PPT) were assessed over temporalis, C5/C6 joint, second metacarpal, and tibialis anterior muscle.

RESULTS

Side-to-side consistency between DPT (r = 0.843, p < 0.001) and pain evoked (r = 0.712; p < 0.001) by dynamic algometer was observed. DPT was moderately associated with widespread PPTs (0.526 > r > 0.656, all p < 0.001). Furthermore, pain during DPT was negatively associated with widespread PPTs (-0.370 < r < -0.162, all p < 0.05).

DISCUSSION

Dynamic pressure algometry was a valid tool for assessing deep dynamic mechanical sensitivity in TTH. DPT was associated with widespread pressure sensitivity independently of the frequency of headaches supporting that deep dynamic pressure sensitivity within the trigeminal area is consistent with widespread pressure sensitivity. Assessing deep static and dynamic somatic tissue pain sensitivity may provide new opportunities for differentiated diagnostics and possibly a new tool for assessing treatment effects.

SIGNIFICANCE

The current study found that dynamic pressure algometry in the temporalis muscle was associated with widespread pressure pain sensitivity in individuals with tension-type headache. The association was independent of the frequency of headaches. Assessing deep static and dynamic somatic tissue pain sensitivity may provide new opportunities for differentiated diagnostics and possibly a tool for assessing treatment effects.

Tissue mechanics during temporal summation of sequentially cuff pressure-induced pain in healthy volunteers and patients with painful osteoarthritis

BACKGROUND

The phenomenon of temporal summation to repetitive pressure pain stimuli is an important central neural mechanism for pain intensity encoding. This study evaluated the time-dependent behaviour of mechanical characteristics of soft tissue during repeated cuff stimulation used for eliciting temporal summation of cuff pressure-evoked pain. Such information of tissue mechanics is important for the interpretation of the pain response evoked during sequential stimulations.

METHODS

Temporal summation was assessed in 16 subjects separated into two groups (healthy controls and severe knee osteoarthritis patients) using a visual analogue scale during 10 repetitive painful cuff stimuli (1-s duration, 1-s break) of the lower leg. The geometry of the lower leg was constructed based on magnetic resonance image (MRI) data. The loading boundary condition of the finite element model was defined according to the parabolic pattern of the interface pressure around the limb and the time-dependent profile of the cuff pressure during repetitive stimuli.

RESULTS

The pain intensity significantly increased with an increasing number of stimuli (p < 0.001), and facilitated temporal summation of pain was observed in patients compared with healthy controls (p < 0.001). The maximal deep tissue stress and strain during stimuli 1-4 varied 43% and 9%, respectively. No variation was observed for stimuli 5-10.

CONCLUSIONS

The study concludes that the temporal summation of pain response during sequential cuff pressure is not explicable by a specific time-dependent behaviour of stress and strain in the activated deep tissue and hence not due to changes in tissue biomechanics.

SIGNIFICANCE

The temporal summation of pain during sequential cuff stimulation is inexplicable by the time-dependent response of mechanical stress and strain in soft tissue.

The association between histological, macroscopic and magnetic resonance imaging assessed synovitis in end-stage knee osteoarthritis: a cross-sectional study

OBJECTIVES

To investigate the association between magnetic resonance imaging (MRI), macroscopic and histological assessments of synovitis in end-stage knee osteoarthritis (KOA).

METHODS

Synovitis of end-stage osteoarthritic knees was assessed using non-contrast-enhanced (CE), contrast-enhanced magnetic resonance imaging (CE-MRI) and dynamic contrast-enhanced (DCE)-MRI prior to (TKR) and correlated with microscopic and macroscopic assessments of synovitis obtained intraoperatively. Multiple bivariate correlations were used with a pre-specified threshold of 0.70 for significance. Also, multiple regression analyses with different subsets of MRI-variables as explanatory variables and the histology score as outcome variable were performed with the intention to find MRI-variables that best explain the variance in histological synovitis (i.e., highest R²). A stepped approach was taken starting with basic characteristics and non-CE MRI-variables (model 1), after which CE-MRI-variables were added (model 2) with the final model also including DCE-MRI-variables (model 3).

RESULTS

39 patients (56.4% women, mean age 68 years, Kellgren-Lawrence (KL) grade 4) had complete MRI and histological data. Only the DCE-MRI variable MExNvoxel (surrogate of the volume and degree of synovitis) and the macroscopic score showed correlations above the pre-specified threshold for acceptance with histological inflammation. The maximum R²-value obtained in Model 1 was R² = 0.39. In Model 2, where the CE-MRI-variables were added, the highest R² = 0.52. In Model 3, a four-variable model consisting of the gender, one CE-MRI and two DCE-MRI-variables yielded a R² = 0.71.

CONCLUSION

DCE-MRI is correlated with histological synovitis in end-stage KOA and the combination of CE and DCE-MRI may be a useful, non-invasive tool in characterising synovitis in KOA.

User-independent assessment of conditioning pain modulation by cuff pressure algometry

BACKGROUND

The use of conditioning pain modulation (CPM) is hampered by poor reproducibility and lack of user-independent paradigms. This study refined the CPM paradigm by applying user-independent cuff algometry.

METHODS

In 20 subjects, the CPM effect of conditioning with cuff stimulation on the arm was investigated by pain test stimuli on the contralateral leg before and in parallel with different cuff conditionings (10, 30, 60 kPa/60 s; 30, 60 kPa/10 s). As test stimulus, another cuff was inflated (1 kPa/s) until the subjects detected the pain tolerance threshold (PTT) during which the pain detection threshold (PDT) and the pressure at a pain intensity of 6 cm on a 10-cm visual analogue scale (PVAS6) were extracted. For comparison, pressure pain thresholds (PPTs) as test stimuli were recorded by the user-dependent handheld pressure algometry. Combinations of cuff locations for conditioning (pain intensity standardized) and contralateral test stimuli were additionally evaluated (leg-arm, leg-leg, arm-thigh). The test-retest reliability in two sessions 1 month apart was assessed in five CPM protocols.

RESULTS

In all protocols, the PDT, PVAS6 and PTT increased during conditioning compared with baseline (p < 0.05). The CPM effect (i.e. conditioning minus baseline) for PVAS6, PTT and PPT increased for increasing conditioning intensities (p < 0.05). The CPM effects were not significantly different for changes in conditioning durations or conditioning/test stimulus locations. In two sessions, the CPM effects for PVAS6 and PTT assessed after 60 s of conditioning on the leg/thigh showed the highest intra-class correlations (0.47-0.73), where they were 0.04-0.6 for PPTs.

CONCLUSIONS

The user-independent cuff algometry is reliable for CPM assessment and for supra-pain threshold test stimuli better than the user-dependent technology.

SIGNIFICANCE

A user-independent CPM technique where the conditioning is controlled by one cuff stimulation, and the test-stimulus is provided by another cuff stimulation. This study shows that cuff algometry is reliable for CPM assessment.

Intra-articular onabotulinumtoxinA in osteoarthritis knee pain: effect on human mechanistic pain biomarkers and clinical pain

OBJECTIVES

OnabotulinumtoxinA (onabotA) attenuates peripheral nociceptive transduction and consequently neuronal firing. The aim of this mechanistic study was to evaluate the effect of intra-articular (IA) onabotA in patients with painful knee osteoarthritis (OA).

METHOD

We conducted a double-blind, randomized, placebo-controlled, 12-week trial using a single ultrasound-guided IA injection of onabotA (200 U). Patients (N = 121) were randomized to receive onabotA (n = 61) or placebo (n = 60). Mechanistic pain biomarkers and clinical outcomes were used for profiling the effect. The biomarkers were pressure pain thresholds (PPTs) from the knee joint (localized sensitization) and extra-articular sites (widespread sensitization), and wind-up pain (central sensitization). Clinical assessments included the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), average daily pain (ADP), patient global impression of change (GIC), and rescue medication. The painDETECT questionnaire (PD-Q) was used for subgrouping patients (nociceptive, neuropathic, and mixed/uncertain).

RESULTS

The nociceptive and non-nociceptive groups were identical with respect to all baseline data. No significant differences in clinical efficacy parameters were found between onabotA and placebo in the entire population. The nociceptive group showed significant improvement after IA onabotA at week 8 for all WOMAC outcomes, ADP at weeks 9 and 10, and patient GIC at week 12, and significant reduction in rescue medication counts within each 14-day period at weeks 9 and 10. After 4, 8, and 12 weeks, significant correlations were obtained in the onabotA group between ADP (both the entire group and the nociceptive group) and various sensitization parameters. The nociceptive group showed pronounced effects on widespread sensitization.

CONCLUSIONS

Intra-articular onabotA given to patients with nociceptive knee OA reduced pain sensitization together with improvement in pain and function.

The interaction between NGF-induced hyperalgesia and acid-provoked pain in the infrapatellar fat pad and tibialis anterior muscle of healthy volunteers

BACKGROUND

Tissue pH is lowered in inflamed tissues, and the increased proton concentration activates acid-sensing ion channels (ASICs), contributing to pain and hyperalgesia. ASICs can be upregulated by nerve growth factor (NGF). The aim of this study was to investigate two new human experimental pain models combining NGF- and acid-induced pain in a randomized, controlled, double-blind study.

METHODS

In experiment 1, volunteers (N = 16) received an injection of either NGF or isotonic saline in each infrapatellar fat pad (IFP). One day after 5 mL of phosphate-buffered acidic saline was infused into each IFP at a rate of 20 mL/h. In experiment 2, the tibialis anterior (TA) muscle of additional volunteers (N = 16) was examined, following the same procedure except that the volume and infusion rate of acid were different (10 mL, 30 mL/h). Continuous pain ratings were recorded during and after acid infusions. In addition, soreness scores on a Likert scale and pressure pain thresholds (PPTs) were assessed.

RESULTS

The PPT of the IFP was significantly decreased at the NGF injection site on day 1, but acid-provoked pain ratings and the change in PPT from pre- to postinfusion between the knees were similar. In the muscle pain model, local mechanical hyperalgesia developed 3 h after the NGF injection and a significant additional decrease in PPT was found after acid infusion compared to preinfusion.

CONCLUSIONS

NGF sensitization in the IFP was not facilitated by acid, whereas an acid-provoked enhancement of muscle hyperalgesia was found. NGF sensitization of adipose tissue responds differently to acid provocation compared to muscle tissue.

SIGNIFICANCE

Quantification of two novel pain models combining NGF and acid. Hyperalgesia developed after NGF injection in the infrapatellar fat pad, but it was not facilitated by acid provocation. Contrary, NGF-induced hyperalgesia in muscle tissue was enhanced by acid.

Interaction between ultraviolet B-induced cutaneous hyperalgesia and nerve growth factor-induced muscle hyperalgesia

BACKGROUNDS AND OBJECTIVES

Clinical observations indicate that cutaneous hyperalgesia may arise from pain located in deep structures. The objective of this study was to investigate whether combined sensitization of deep and superficial somatic tissues facilitates skin hyperalgesia.

METHODS

The interaction between muscle and cutaneous hyperalgesia was investigated in 16 healthy volunteers. Skin sensitization was induced unilaterally on the same randomly selected part of the body by ultraviolet B (UVB) irradiation above the upper trapezius and low back muscles. The next day, muscle hyperalgesia was induced bilaterally in low back muscles by injections of nerve growth factor (NGF). Thus, 1 day after irradiation there was skin sensitization, whereas after 2 days both skin and muscle sensitizations were present. Cutaneous blood flow, pin-prick thresholds, pressure pain thresholds (PPTs), temporal summation to repetitive painful pressure stimulation, and stimulus-response functions of graded pressure stimulations and pain intensity were assessed within the irradiated skin area and in the surrounding area before and 1, 2 and 3 days after irradiation.

RESULTS

Comparing baseline with 1 day after irradiation, UVB and UVB+NGF locations demonstrated: (1) Increased superficial blood flow inside the irradiated area (p < 0.01); (2) Reduced pin-prick (p < 0.01) and PPTs (p < 0.05) within the irradiated area and in the surrounding area; (3) Left-shifted pressure stimulus-response function within the irradiated area (p < 0.01); (4) Facilitated temporal summation inside the irradiated area (p < 0.01).

CONCLUSIONS

Using skin and deep tissue pain sensitization models simultaneously, no significant synergistic effects were found within the 3-day investigation suggesting little integration between the two phenomena in this period.

The efficacy of 12 weeks non-surgical treatment for patients not eligible for total knee replacement: a randomized controlled trial with 1-year follow-up

OBJECTIVE

To compare the efficacy of a 12-week non-surgical treatment program with usual care in patients with knee osteoarthritis (OA) not eligible for total knee replacement (TKR).

METHOD

This two-arm parallel group assessor-blinded randomized controlled trial (RCT) included 100 adults from secondary care with knee OA, confirmed by radiography (Kellgren-Lawrence grade ≥1), but not eligible for a TKR. The 12-week non-surgical treatment program consisted of individualized progressed neuromuscular exercise, patient education, insoles, dietary advice and prescription of pain medication if indicated, while usual care comprised two leaflets with information and advice on knee OA and recommended treatments. The primary outcome was the change from baseline to 12 months in the Knee injury and Osteoarthritis Outcome Score (KOOS)4 defined as the average score for the KOOS subscales of pain, symptoms, activities of daily living (ADL), and quality of life (QOL).

RESULTS

91% of the patients completed the 12 months follow-up on the primary outcome. Compared with usual care, patients undergoing the treatment program improved more in KOOS4 (adjusted mean difference (95% CI) of 9.6 (4.4-14.8)) with no serious treatment-related adverse events (AE). The number needed to treat (NNT), defined as the number of patients needed to treat for one person to improve 15% was 7.2. Secondary outcomes supported the primary findings.

CONCLUSION

In patients with mostly moderate to severe knee OA not eligible for TKR, a 12-week individualized, non-surgical treatment program is more efficacious at 12 months compared with usual care and has few treatment-related AE.

TRIAL REGISTRATION

ClinicalTrials.gov (NCT01535001).