Considerations for improving assay sensitivity in chronic pain clinical trials: IMMPACT recommendations.

A number of pharmacologic treatments examined in recent randomized clinical trials (RCTs) have failed to show statistically significant superiority to placebo in conditions in which their efficacy had previously been demonstrated. Assuming the validity of previous evidence of efficacy and the comparability of the patients and outcome measures in these studies, such results may be a consequence of limitations in the ability of these RCTs to demonstrate the benefits of efficacious analgesic treatments vs placebo ("assay sensitivity"). Efforts to improve the assay sensitivity of analgesic trials could reduce the rate of falsely negative trials of efficacious medications and improve the efficiency of analgesic drug development. Therefore, an Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials consensus meeting was convened in which the assay sensitivity of chronic pain trials was reviewed and discussed. On the basis of this meeting and subsequent discussions, the authors recommend consideration of a number of patient, study design, study site, and outcome measurement factors that have the potential to affect the assay sensitivity of RCTs of chronic pain treatments. Increased attention to and research on methodological aspects of clinical trials and their relationships with assay sensitivity have the potential to provide the foundation for an evidence-based approach to the design of analgesic clinical trials and expedite the identification of analgesic treatments with improved efficacy and safety.

NGX-4010, a high-concentration capsaicin patch, for the treatment of postherpetic neuralgia: a randomized, double-blind, controlled study with an open-label extension.

OBJECTIVES: To assess the efficacy, tolerability, and safety of NGX-4010, a high-concentration capsaicin dermal patch (capsaicin 640 microg/cm(2), 8%) in patients with postherpetic neuralgia (PHN). METHODS: Patients were randomized to receive NGX-4010 or control patch in a 4-week, double-blind study. This was followed by an open-label extension phase (up to 48 weeks total) where patients could receive up to three additional treatments no sooner than 12 weeks after initial treatment. The primary efficacy variable was mean change from baseline in mean morning and evening numerical pain rating scale (NPRS) scores. RESULTS: During days 8-28 after the double-blind treatment, NGX-4010 patients had a mean change in NPRS scores from baseline of -32.7% compared with -4.4% for control patients (P = 0.003). Mean NPRS scores decreased from baseline during week 1 in both treatment groups, remained relatively stable through week 12 in NXG-4010 patients, but returned to near baseline during weeks 2-4 in controls. Mean change in NPRS scores from baseline during weeks 2-12 was -33.8% for NGX-4010 and +4.9% for control recipients. A similar decrease in NPRS scores from baseline was maintained with subsequent NGX-4010 treatments, regardless of the number of treatments received. Transient increases in application site pain were adequately managed with analgesics. No increases in application site reactions or adverse events were observed with repeated treatments. No patients discontinued the study due to an adverse event. CONCLUSION: NGX-4010 is a promising topical treatment for PHN patients, which appears to be tolerable, generally safe, and effective.

Assessment of neuropathic pain in primary care.

Management of patients presenting with chronic pain is a common problem in primary care. Essentially, the classification of chronic pain falls into 3 broad categories: (1) pain owing to tissue disease or damage (nociceptive pain), (2) pain caused by somatosensory system disease or damage (neuropathic pain), and (3) pain without a known somatic background. Key challenges in developing a targeted holistic approach to treatment include appropriate diagnosis of the cause or causes of pain; identifying the type of pain and assessing the relative importance of its various components; and determining appropriate treatment. In clinical examination, sensory abnormalities are the crucial findings leading to a diagnosis of neuropathic pain, for which pharmacotherapy with antidepressants and anticonvulsants represents the cornerstone of medical treatment. Chronic neuropathic pain is underrecognized and undertreated, yet primary care physicians are uniquely placed on the frontlines of patient management, where they can play a pivotal role in treatment and prevention through diagnosis, therapy, follow-up, and referral. This review provides guidance in understanding and identifying the neuropathic contribution to pain presenting in primary care; assessing its severity through patient history, physical examination, and appropriate diagnostic tests; and establishing a rational treatment plan.

Quantitative sensory testing and mapping: a review of nonautomated quantitative methods for examination of the patient with neuropathic pain.

OBJECTIVES: Despite a growing interest in neuropathic pain, neurologists and pain specialists do not have a standard, validated, office examination for the evaluation of neuropathic pain signs to complement the neurologic, musculoskeletal, and general physical examinations. An office neuropathic pain examination focused on quantifying sensory features of neuropathic pain, ranging from deficits to allodynia and hyperalgesia, and evoked by a physiologically representative array of stimuli, will be an essential tool to monitor treatment effectiveness and for clinical investigation into the mechanisms and management of neuropathic pain. Such an examination should include mapping of areas of stimulus-evoked neuropathic pain and standardized, reproducible quantitative sensory testing (QST) of tactile, punctuate, pressure, and thermal modalities. METHODS: We review quantitative sensory testing methodology in general and specific tests for the evaluation of neuropathic pain phenomena. RESULTS: Numerous quantitative sensory testing techniques for dynamic mechanical, pressure, vibration, and thermal sensory testing and mapping have been described. We propose a comprehensive neuropathic pain evaluation protocol that is based upon these available techniques. CONCLUSIONS: A comprehensive neuropathic pain evaluation protocol is essential for further advancement of clinical research in neuropathic pain. A protocol that uses tools readily available in clinical practice, when established and validated, can be used widely and thus accelerate data collection for clinical research and increase clinical awareness of the features of neuropathic pain.

A randomized, double-blind, placebo-controlled trial evaluating the efficacy and safety of ABT-594 in patients with diabetic peripheral neuropathic pain.

ABT-594 is a neuronal nicotinic acetylcholine receptor (NNR) agonist that exhibits potent analgesic activity in preclinical models of acute, chronic, and neuropathic pain. The purpose of this phase 2, randomized, multicenter, double-blind, placebo-controlled study was to evaluate the safety and analgesic efficacy of ABT-594 in patients with diabetic peripheral neuropathic pain (DPNP). A total of 266 DPNP patients were randomized 1:1:1:1 to receive placebo, ABT-594 150 microg BID, ABT-594 225 microg BID, or ABT-594 300 microg BID. Patients were titrated to a fixed-dose of ABT-594 over 7 days and remained at this dose for another 6 weeks. Compared to placebo, all three ABT-594 treatment groups showed significantly greater decreases on the average diary-based 0-10 Pain Rating Scale (PRS) score from baseline to final evaluation, the primary efficacy measure (placebo, -1.1; 150 microg BID, -1.9; 225 microg BID, -1.9; 300 microg BID, -2.0). The proportion of patients achieving at least a 50% improvement in the average diary-based PRS was greater in all three ABT-594 treatment groups. However, adverse event (AE) dropout rates were significantly higher in all three ABT-594 treatment groups (28% for 150 microg BID, 46% for 225 microg BID, and 66% for 300 microg BID) than for the placebo group (9%). Consistent with the expected side-effect profile of NNR agonists, the most frequently reported AEs were nausea, dizziness, vomiting, abnormal dreams, and asthenia. This study establishes proof of concept for NNR agonists as a new class of compounds for treating neuropathic pain.

Voluntary facial displays of pain increase suffering in response to nociceptive stimulation.

UNLABELLED: Facial expressions of pain are an important part of the pain response, signaling distress to others and eliciting social support. To evaluate how voluntary modulation of this response contributes to the pain experience, 29 subjects were exposed to thermal stimulation while making standardized pain, control, or relaxed faces. Dependent measures were self-reported negative effect (valence and arousal) as well as the intensity of nociceptive stimulation required to reach a given subjective level of pain. No direct social feedback was given by the experimenter. Although the amount of nociceptive stimulation did not differ across face conditions, subjects reported more negative effects in response to painful stimulation while holding the pain face. Subsequent analyses suggested the effects were not due to preexisting differences in the difficulty or unpleasantness of making the pain face. These results suggest that voluntary pain expressions have no positively reinforcing (pain attenuating) qualities, at least in the absence of external contingencies such as social reinforcement, and that such expressions may indeed be associated with higher levels of negative affect in response to similar nociceptive input. PERSPECTIVE: This study demonstrates that making a standardized pain face increases negative affect in response to nociceptive stimulation, even in the absence of social feedback. This suggests that exaggerated facial displays of pain, although often socially reinforced, may also have unintended aversive consequences.

Individual differences in the effects of perceived controllability on pain perception: critical role of the prefrontal cortex.

The degree to which perceived controllability alters the way a stressor is experienced varies greatly among individuals. We used functional magnetic resonance imaging to examine the neural activation associated with individual differences in the impact of perceived controllability on self-reported pain perception. Subjects with greater activation in response to uncontrollable (UC) rather than controllable (C) pain in the pregenual anterior cingulate cortex (pACC), periaqueductal gray (PAG), and posterior insula/SII reported higher levels of pain during the UC versus C conditions. Conversely, subjects with greater activation in the ventral lateral prefrontal cortex (VLPFC) in anticipation of pain in the UC versus C conditions reported less pain in response to UC versus C pain. Activation in the VLPFC was significantly correlated with the acceptance and denial subscales of the COPE inventory [Carver, C. S., Scheier, M. F., & Weintraub, J. K. Assessing coping strategies: A theoretically based approach. Journal of Personality and Social Psychology, 56, 267-283, 1989], supporting the interpretation that this anticipatory activation was associated with an attempt to cope with the emotional impact of uncontrollable pain. A regression model containing the two prefrontal clusters (VLPFC and pACC) predicted 64% of the variance in pain rating difference, with activation in the two additional regions (PAG and insula/SII) predicting almost no additional variance. In addition to supporting the conclusion that the impact of perceived controllability on pain perception varies highly between individuals, these findings suggest that these effects are primarily top-down, driven by processes in regions of the prefrontal cortex previously associated with cognitive modulation of pain and emotion regulation.

Long-term use of controlled-release oxycodone for noncancer pain: results of a 3-year registry study.

OBJECTIVE: To evaluate the outcomes associated with the use of controlled-release (CR) oxycodone for up to 3 years in the treatment of noncancer pain. METHODS: Adult patients who previously participated in controlled trials of CR oxycodone for osteoarthritis pain, diabetic neuropathy pain, or low back pain, and who continued to require opioid analgesia for moderate or severe pain, were enrolled in an open-label, uncontrolled, registry study. Data collected over time included dose, pain severity on a numeric scale, treatment acceptability, adverse events, and descriptions of problematic drug-related behavior. RESULTS: Two hundred thirty-three patients were enrolled. When the study closed, 141, 86, and 39 patients had taken CR oxycodone for at least 1, 2, and 3 years, respectively; mean duration of treatment was 541.5 days. Among the 219 intent-to-treat patients (received at least 1 dose and provided at least 1 postdose study observation), the mean (SD, range) daily dose was 52.5 (+/-38.5, 10.0 to 293.5) mg. Before the end of month 3, 44% required an increase in total daily dose; this dropped to 23% during months 4 to 6, to 17% during months 10 to 12, and remained at approximately 10% for each time interval thereafter (range 8% to 13%). Among the large majority of patients with stable or lower dose requirements after the initial 3 months of treatment, the average pain intensity ratings were unchanged or improved for approximately 70% to 80% of patients at all subsequent time points through month 33, and for 54% (7/13 patients) at month 36. A decrease in pain was initially seen by the end of month 3, and for the majority of patients, the Average Pain Intensity score remained the same, better, or minimally worse (<3 points) for the remainder of the 3-year study period. The most common adverse events were constipation and nausea, and the incidence of these events declined over time on treatment. Investigators reported 6 cases (2.6%) of possible drug misuse but no evidence of de novo addiction was observed. DISCUSSION: These registry data demonstrate that a subgroup of patients with noncancer pain experienced prolonged relief with tolerable side effects and modest need for dose escalation during long-term therapy with CR oxycodone.

Assessment: use of epidural steroid injections to treat radicular lumbosacral pain: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology.

Based on the available evidence, the Therapeutics and Technology Assessment subcommittee concluded that 1) epidural steroid injections may result in some improvement in radicular lumbosacral pain when assessed between 2 and 6 weeks following the injection, compared to control treatments (Level C, Class I-III evidence). The average magnitude of effect is small and generalizability of the observation is limited by the small number of studies, highly selected patient populations, few techniques and doses, and variable comparison treatments; 2) in general, epidural steroid injection for radicular lumbosacral pain does not impact average impairment of function, need for surgery, or provide long-term pain relief beyond 3 months. Their routine use for these indications is not recommended (Level B, Class I-III evidence); 3) there is insufficient evidence to make any recommendation for the use of epidural steroid injections to treat radicular cervical pain (Level U).

Classification of patients with pain based on neuropathic pain symptoms: comparison of an artificial neural network against an established scoring system.

Wider use of pain assessment tools that are specifically designed for certain types of pain--such as neuropathic pain--contribute an increasing amount of information which in turn offers the opportunity to employ advanced methods of data analysis. In this manuscript, we present the results of a study where we employed artificial neural networks (ANNs) in an analysis of pain descriptors with the goal of determining how an approach that uses a specific symptoms-based tool would perform with data from the real world of clinical practice. We also used traditional statistics approaches in the form of established scoring systems as well as logistic regression analysis for the purpose of comparison. Our results confirm the clinical experience that groups of pain descriptors rather than single items differentiate between patients with neuropathic and non-neuropathic pain. The accuracy obtained by ANN analysis was only slightly higher than that of the traditional approaches, indicating the absence of nonlinear relationships in this dataset. Data analysis with ANNs provides a framework that extends what current approaches offer, especially for dynamic data, such as the rating of pain descriptors over time.

Duloxetine for the management of diabetic peripheral neuropathic pain: evidence-based findings from post hoc analysis of three multicenter, randomized, double-blind, placebo-controlled, parallel-group studies.

OBJECTIVE: This post hoc analysis was aimed to summarize the efficacy and tolerability of duloxetine as represented by number needed to treat (NNT) and number needed to harm (NNH) to provide a clinically useful assessment of the position of duloxetine among current agents used to treat diabetic peripheral neuropathic pain (DPNP). METHODS: Data were pooled from three 12-week, multicenter, randomized, double-blind, placebo-controlled, parallel-group studies in which patients received 60 mg duloxetine either QD or BID or placebo. NNT was calculated based on rates of response (defined as >or=30% and >or=50% reductions from baseline in the weekly mean of the 24-hour average pain severity scores); NNH was calculated based on rates of discontinuation due to adverse events (AEs). RESULTS: Patients receiving duloxetine 60 mg QD and 60 mg BID had NNTs (95% CI) of 5.2 (3.8-8.3) and 4.9 (3.6-7.6), respectively, based on last observation carried forward; NNTs of 5.3 (3.8-8.3) for 60 mg QD and 5.7 (4.1-9.7) for 60 mg BID were obtained based on baseline observations carried forward. The NNHs (95% CI) based on discontinuation due to AEs were 17.5 (10.2-58.8) in the duloxetine 60-mg QD group and 8.8 (6.3-14.7) in the 60-mg BID group. CONCLUSION: These post hoc results suggest that duloxetine was effective and well tolerated for the management of DPNP and further support the importance of duloxetine as a treatment option for clinicians and patients to assist with the management of DPNP.

Quantitative sensory testing for spinal cord stimulation in patients with chronic neuropathic pain.

OBJECTIVE: A prospective pilot study was conducted, attempting to identify objective tests that would help clinicians to assess the efficacy of spinal cord stimulation (SCS) trial preceding permanent device implantation. SETTING: Four university hospitals in the United States and Israel. PARTICIPANTS: Thirteen patients with radicular leg pain due to failed back surgery syndrome (FBSS) or leg pain due to complex regional pain syndrome (CRPS) who were candidates for SCS. METHODS: PARTICIPANTS underwent a series of quantitative sensory tests prior to, and seven days after the initiation of SCS trial. These tests included: vibration threshold (conducted using the VSA 3000; Medoc Inc., Ramat Ishay, Israel), cold threshold, warm threshold, heat pain threshold, phasic heat pain threshold, tonic heat pain threshold (conducted using the TSA 2001; Medoc Inc.), and electrical pain tolerance at 5, 250 and 2000 Hz (administered using the NerveScan 2000; Neurotron, Inc., Baltimore, MD, USA). RESULTS: Useful data were obtained from 12 patients. The results of the vibration threshold and the tolerance to electrical stimulation at 5 and 250 Hz changed with an SCS trial. These results also correlated with the decision regarding the permanent implantation, which was made independently of them. In contrast, the results of thermal thresholds and tolerance to electrical stimulation at 2000 Hz tests did not change with the SCS trial. CONCLUSIONS: Our findings, which agree with those of a few other studies, suggest that the vibration threshold and the tolerance to electrical stimulation at 5 and 250 Hz tests can assist the clinician to select the right patients for permanent stimulation.

New perspectives on the management of diabetic peripheral neuropathic pain.

Peripheral neuropathy affects about 30% of people with diabetes mellitus. Between 16% and 26% of diabetes patients experience chronic pain. This may be referred to as diabetic neuropathic pain (DNP) or diabetic peripheral neuropathic pain (DPNP). Minimum requirements for diagnosis of DPNP should include assessment of pain and symptoms and neurological examination, with the accent on sensory examination. Given that depression and other co-morbidities are commonly associated with this condition, a broad approach to management is essential. Lifestyle intervention and optimisation of glycaemic control are recommended as initial steps in management. An evidence-based treatment algorithm for DPNP has been proposed, recommending initial use of either a tricyclic antidepressant, selective serotonin noradrenaline re-uptake inhibitor or alpha-2-delta agonist, depending on patient co-morbidities and contra-indications. Addition of an opioid agonist may be required in the event of inadequate pain control. Irrespective of which treatment is offered, only about one third of patients are likely to achieve more than 50% pain relief. Further research to improve the diagnosis and management of DPNP is needed.

Consensus guidelines: treatment planning and options. Diabetic peripheral neuropathic pain.

Despite the number of patients affected by diabetic peripheral neuropathic pain (DPNP), little consensus exists about the pathophysiology, best diagnostic tools, and primary treatment choices. Theories about the causes of DPNP are inextricably linked with the causes of diabetic neuropathles, yet most patients with such neuropathies do not experience pain. The factors that differentiate patients with pain from those without remain unknown and are the subject of much research. When choosing treatment for patients with DPNP, physicians are confronted with a myriad of choices, none of which has been shown to be effective for all patients. This article reviews the evidence for these treatments and attempts to guide physicians in choosing those treatments based on evidence from well-designed clinical trials to support their use. Two agents, duloxetine and pregabalin, are formally approved by the Food and Drug Administration for the treatment of DPNP. In addition, several other agents, including the tricyclic class of antidepressants, have been effective in clinical trials. Ultimately, treatment choice must also Include consideration of adverse effects, individual patient factors such as comorbidities, and often cost.

A randomized, double-masked, placebo-controlled pilot trial of extended IV lidocaine infusion for relief of ongoing neuropathic pain.

OBJECTIVES: To determine the dose-response effect and safety of IV lidocaine at different dose infusion rates on spontaneous ongoing neuropathic pain. METHODS: In this double-masked, placebo-controlled, parallel study conducted in an outpatient clinical research center, patients with peripheral neuropathic pain received a 6-hour infusion of three doses (1, 3, and 5 mg/kg) of lidocaine or placebo. The main outcome measure was relief of pain intensity (percentage pain intensity difference [PID %]). Other measures were responder rate, adverse events, and correlation between lidocaine levels and PID %. RESULTS: There was a significant difference in the median PID % between the group treated with lidocaine 5 mg/kg/h (-34.60) and the placebo group (-11.96, P=0.012). Such effect began 4 hours after the onset of treatment and lasted until the end of the study. Lidocaine at lower infusion rates was no better than placebo in relieving pain. A modest but significant correlation was found between methylethylglycinexylidide (MEGX) levels and pain relief (R=0.60). There were no serious adverse events, but in two patients lidocaine was stopped prematurely. CONCLUSIONS: Lidocaine at 5 mg/kg/h was more effective than placebo at relieving neuropathic pain. The effect started 4 hours after the onset of treatment and continued for at least 4 hours after the end of the infusion. Additional research is needed using higher infusion rates with larger sample sizes to confirm these results and to explore the role of MEGX in the relief of neuropathic pain.

Rational multidrug therapy in the treatment of neuropathic pain.

Multidrug therapy (MDT) has been widely accepted and used as a standard of practice in most areas of medical practice, including neuropathic pain. Because neuropathic pain is a new field of medical science and practice, standards for its treatment including MDT are still evolving. In this article, we present rationale and principals for the MDT of neuropathic pain based on our best understandings of the underlying mechanisms of the disease processes and the actions of drugs, the goal being to maximize benefits and minimize adverse effects. MDT for neuropathic pain is based on a comprehensive clinical neuropathic pain assessment and ongoing monitoring of the drug therapy's efficacy and adverse effects, administering one drug at the time.

Pain ratings at the thresholds are necessary for interpretation of quantitative sensory testing.

Published databases of quantitative sensory testing (QST) for sensory thresholds provide a means for detecting deficits of the thermonociceptive sensory nervous system. These databases, however, do not assist in the assessment of neuropathic pain, which is characterized by pain or hyperalgesia, or both. We utilized the method of levels for innocuous thermal stimuli, warm and cool, and the method of limits for noxious thermal stimuli, hot pain and cold pain, to determine QST thresholds. Stimuli were applied to distal and proximal sites in the upper and lower limbs of 50 healthy volunteers, ranging in age from 19 to 59 years. Thresholds for innocuous and noxious stimuli in this study were similar to previously published results. The mean pain rating across all sites at thresholds for noxious heat and cold stimuli was 4.10, as rated on a 0-10 numeric scale. Suggestions are provided for combining threshold information for innocuous and noxious stimuli and related pain ratings for the evaluation of sensory nervous system function and, specifically, neuropathic pain.

Neuropathic pain symptoms relative to overall pain rating.

UNLABELLED: The influence of sensory symptoms on overall simple pain ratings in neuropathic pain is not well understood. The goal of this study was to determine this relationship by using the Neuropathic Pain Questionnaire (NPQ) and Neuropathic Pain Scale (NPS) in patients who had neuropathic pain. Overall pain intensity ratings were assessed by means of Average and Worst Pain ratings from the Brief Pain Inventory. Ongoing average pain was rated as 5.7 and worst pain as 7.7 on 0 to 10 scale, and it was present in 96% of patients, whereas symptoms that are commonly studied in the laboratory, such as increased pain due to touch and due to heat, were much less frequent (64% and 38%, respectively). Worst pain was most highly correlated with shooting, stabbing, and distress symptom complex, which was in contrast to Average pain, which was most highly correlated with symptom complex consisting of a large number of items. Analysis of conceptually related thermal heat sensation types of pain, spontaneous "burning" pain and the evoked pain "increased due to heat," showed that spontaneous burning pain was reported at much higher frequency and intensity than the evoked increased pain due to heat. There was no statistically significant difference between descriptors from NPS and NPQ among subgroups of neuropathic pain, such as polyneuropathy, radiculopathy, or posttraumatic neuralgia, but this could be in part due to relatively small number of patients in these subgroups of neuropathic pain. Quantitative analysis, as performed in this study, is one of the steps in developing an approach for elucidating the relationship between neuropathic pain symptoms and underlying mechanisms. PERSPECTIVE: Assessment of neuropathic pain symptoms by means of specifically designed questionnaires provides significant insight into patients' pain experience, including pain overall, which is under many influences. Further research of this type can contribute to advances in mechanism-based diagnosis and treatment.

Perceived controllability modulates the neural response to pain.

The response to painful stimulation depends not only on peripheral nociceptive input but also on the cognitive and affective context in which pain occurs. One contextual variable that affects the neural and behavioral response to nociceptive stimulation is the degree to which pain is perceived to be controllable. Previous studies indicate that perceived controllability affects pain tolerance, learning and motivation, and the ability to cope with intractable pain, suggesting that it has profound effects on neural pain processing. To date, however, no neuroimaging studies have assessed these effects. We manipulated the subjects' belief that they had control over a nociceptive stimulus, while the stimulus itself was held constant. Using functional magnetic resonance imaging, we found that pain that was perceived to be controllable resulted in attenuated activation in the three neural areas most consistently linked with pain processing: the anterior cingulate, insular, and secondary somatosensory cortices. This suggests that activation at these sites is modulated by cognitive variables, such as perceived controllability, and that pain imaging studies may therefore overestimate the degree to which these responses are stimulus driven and generalizable across cognitive contexts.