Beyond biopsychosocial: The keystone mechanism theory of pain.

Pain is a deeply personal experience, with interindividual differences in its chronification and treatment presenting a formidable healthcare challenge. The biopsychosocial model (BPSm) has been hugely influential within nascent attempts at precision pain medicine, steering the field away from a reductionist biomechanical viewpoint and emphasising complex interactions of biological, psychological, and social factors which shape the individuality of pain. However, despite offering a strong theoretical foundation and holistic perspective, we contend that the BPSm remains limited in its capacity to deliver truly mechanistically informed treatment of pain. We therefore propose the keystone model of pain which offers a pragmatic balance between the dimensionality expansive BPSm and overly reductive approaches, providing both theoretical and practical advantages for the transition from treating populations to individual people.

Capsaicin treatment in neuropathic pain: axon reflex vasodilatation after 4 weeks correlates with pain reduction.

ABSTRACT: Capsaicin, an agonist at the transient receptor potential vanilloid 1, is used for the topical treatment of peripheral neuropathic pain. Reversible receptor defunctionalization and degeneration and subsequent regeneration of cutaneous nociceptors are discussed as its mechanism of action. Here, we hypothesize an accelerated functional recovery of a subclass of nociceptive afferents, the peptidergic vasoactive nociceptors, as the potential cause of capsaicin analgesia. In this noninterventional exploratory trial, 23 patients with peripheral neuropathic pain were treated with one topical high-concentration capsaicin application. Baseline pain ratings, comorbidities, and quality of life were assessed. Functional laser speckle contrast analysis (heat-evoked neurogenic vasodilatation to assess functional properties of peptidergic nociceptors) and quantitative sensory testing were performed in the affected skin. Four weeks after treatment, functional laser speckle contrast analysis and questionnaires were repeated. Telephone interviews were conducted at weeks 2, 10, and 12. Topical capsaicin treatment induced a significant reduction in pain intensity with a maximum at 4 weeks. At the same time, heat-evoked neurogenic vasodilatation was on average similar to pretreatment values. Half of the patients not only showed a functional recovery but also an improvement in vasodilatation, indicating regeneration of nerve fibers. Patients with improved heat-evoked neurogenic vasodilatation at week 4 showed a greater pain reduction than those with deterioration. The degree of vasodilatation significantly correlated with pain reduction. These findings suggest that (1) regeneration of peptidergic nociceptors may be the mechanism behind capsaicin-induced analgesia and (2) that a disease-modifying effect of capsaicin on these fibers already occurs 4 weeks after application.

Chemotherapy-induced peripheral neuropathy (CIPN): current therapies and topical treatment option with high-concentration capsaicin.

Cancer diagnosis and treatment are drastic events for patients and their families. Besides psychological aspects of the disease, patients are often affected by severe side effects related to the cancer itself or as a result of therapeutic interventions. Particularly, chemotherapy-induced peripheral neuropathy (CIPN) is the most prevalent neurological complication of oral or intravenous chemotherapy. The disorder may require dose reduction of chemotherapy and is accompanied by multiple symptoms with long-term functional impairment affecting quality of life (QoL), e.g., sensory and functional deteriorations as well as severe pain. Although CIPN may reverse or improve after termination of the causative chemotherapy, approximately 30-40% of patients are faced with chronicity of the symptoms. Due to the advantages in cancer diagnosis and treatments, survival rates of cancer patients rise and CIPN may occur even more frequently in the future. In this review, we summarize current recommendations of leading national and international societies regarding prevention and treatment options in CIPN. A special focus will be placed on current evidence for topical treatment of CIPN with high-dose capsaicin. Finally, an algorithm for CIPN treatment in clinical practice is provided, including both pharmacologic and non-pharmacologic modalities based on the clinical presentation.

Up-date on Clinical Management of Postherpetic Neuralgia and Mechanism-Based Treatment: New Options in Therapy.

Patients with postherpetic neuralgia may experience various sensory signs and symptoms of pain. Despite this, the recommendations for medicinal treatment do not differ accordingly. In order to find the appropriate treatment options for postherpetic neuralgia, several attempts have been made in the past. The crucial obstacle to these attempts was insufficient or no subgrouping of patients according to their sensory phenotype, mostly resulting in an unsatisfactory treatment response. Recently, a new concept of retrospective stratification according to the patients' sensory phenotype has been made in a large cohort of pain patients. This new stratification tool allows a predictive validity for treatment response in subgroups of patients and might be of potential value in determining the optimal treatment in postherpetic neuralgia patients.

Neuropathic pain.

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aß, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Phenotyping neuropathic pain patients: implications for individual therapy and clinical trials.

PURPOSE OF REVIEW: The sensory phenotype can be used as a surrogate marker of underlying mechanisms of pain generation and is assessed by tools like the Quantitative Sensory Testing, Patient Reported Outcomes or the Capsaicin Response Test. In order to establish an individualized, mechanism-based treatment of pain, it has to be demonstrated that subgroups of patients with a distinct sensory phenotype respond differently to a certain treatment. RECENT FINDINGS: Retrospective analyses of several clinical trials revealed that the presence of certain somatosensory abnormalities in the painful area was associated with a better treatment outcome. Examples will be discussed in this article, showing that somatosensory phenotyping of patients with neuropathic pain is a promising method to subgroup patients in order to predict their response to treatment. SUMMARY: The discussed trials show the importance of the development of an individualized pain therapy. Up to now, no clinical trial has prospectively used the sensory phenotype as an inclusion or stratification criterion. Academic researchers and pharmaceutical industry should be encouraged to implement this approach in future trial designs.

High concentration capsaicin for treatment of peripheral neuropathic pain: effect on somatosensory symptoms and identification of treatment responders.

BACKGROUND: Pain is usually assessed by spontaneous pain ratings. Time-dependent (brief attacks) or evoked (allodynia) phenomena, common in neuropathic pain, are not captured. To evaluate the overall effectiveness of a treatment, improvement of all sensory symptoms should be measured. Since the pattern of sensory abnormalities might hint at the underlying mechanisms of pain, this baseline information may aid in predicting the treatment effect. Data on sensory neuropathic abnormalities (painDETECT questionnaire) were analyzed aiming to (1) evaluate the frequency of neuropathic symptoms in different peripheral neuropathic pain syndromes, (2) assess the effect of capsaicin 8% patch on neuropathic symptoms and (3) identify treatment responders based on baseline values. METHODS: Data analysis of a prospective 12 week non-interventional trial in peripheral neuropathic pain treated with capsaicin 8% cutaneous patch. Average pain intensity during the past 24 hours, pain descriptors and qualities of neuropathic pain were assessed to characterize the patients' sensory symptoms at baseline and to document changes. RESULTS: (1) Characteristic symptoms of neuropathic pain were present in all peripheral neuropathic pain syndromes, but frequencies varied in the individual syndromes. (2) Topical capsaicin 8% treatment significantly reduced the overall pain intensity and resulted in a reduction of sensory abnormalities. (3) Short disease duration predicted a better treatment effect. High painDETECT scores, the presence of burning and pressure-evoked pain were weakly associated with treatment response. CONCLUSIONS: Topical capsaicin 8% treatment effectively reduced sensory abnormalities in peripheral neuropathic pain. The association of sensory symptoms and treatment response aids in understanding the mechanism of action of high concentration capsaicin. It is, however, not possible to use sensory symptom patterns to predict treatment response to capsaicin on an individual level. LIMITATIONS: Completion of painDETECT was optional and therefore data was not available for all patients. Further studies for confirmation of these results are needed.

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.

A role for peripheral afferents in the pathophysiology and treatment of at-level neuropathic pain in spinal cord injury? A case report.

At-level neuropathic pain is a frequent symptom following spinal cord injury, but the underlying pathophysiology is not completely understood. We report a patient suffering from treatment-resistant at-level pain characterized by ongoing pain and mechanical allodynia for three years after an incomplete spinal lesion. Quantitative sensory testing revealed severe thermosensory deficits in the neuropathic pain area. However, topical application of capsaicin in the neuropathic pain area induced a burning pain sensation, a marked decrease in heat pain threshold and an increase in mechanical allodynia. Treatment with topical lidocaine patches (5%) led to considerable pain relief. These results indicate a functional connection between peripheral, spinal and supraspinal nociceptive pathways and that peripheral afferents may contribute to at-level neuropathic pain after spinal cord injury in this patient. Lesioned peripheral afferents in combination with central neuronal hyperexcitability are discussed as a likely underlying pain mechanism.