Sensory functions in chronic neuralgia

The safety and feasibility of transcranial direct current stimulation combined with conservative treatment for patients with cervicogenic headaches: A double-blinded randomized control study protocol

Background

Cervicogenic headaches (CGH) are common following concussion and whiplash injuries and significantly reduce patient quality of life. Conservative therapies such as ET (ET) and physiotherapy combined with injection-based therapies are cornerstones of treatment for CGH but have shown limited efficacy. Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) has shown promise in treating other chronic pain conditions. The primary aim of this trial is to evaluate the feasibility and safety of tDCS when combined with ET for the treatment of CGH.

Methods

Adults (aged 18-65), blinded to treatment arm, will be randomized into one of two groups: active tDCS followed by ET or sham tDCS followed by ET. Transcranial direct current stimulation will be applied over M1 three times per week for 6-weeks and ET will be performed daily. The primary outcomes of this trial will be the feasibility and safety of the intervention. Feasibility will be defined as greater than 30 % recruitment, 70 % protocol adherence, and 80 % retention rate. Safety will be defined as no severe adverse events. Secondary exploratory outcomes will assess improvement in pain, strength, function, and quality of life.

Conclusions

This trial aims to demonstrate the safety and feasibility of tDCS in combination with ET for the treatment of CGH. Cervicogenic headaches can be difficult to treat contributing to significant impairments function and quality of life. Transcranial direct current stimulation is a potential novel treatment to improve health outcomes in these patients.

Registration

ClinicalTrials.gov-NCT05582616.

An interdisciplinary perspective on peripheral drivers of pain in rheumatoid arthritis

Pain is one of the most debilitating symptoms of rheumatoid arthritis (RA), and yet remains poorly understood, especially when pain occurs in the absence of synovitis. Without active inflammation, experts most often attribute joint pain to central nervous system dysfunction. However, advances in the past 5 years in both immunology and neuroscience research suggest that chronic pain in RA is also driven by a variety of abnormal interactions between peripheral neurons and mediators produced by resident cells in the local joint environment. In this Review, we discuss these novel insights from an interdisciplinary neuro-immune perspective. We outline a potential working model for the peripheral drivers of pain in RA, which includes autoantibodies, resident immune and mesenchymal cells and their interactions with different subtypes of peripheral sensory neurons. We also offer suggestions for how future collaborative research could be designed to accelerate analgesic drug development.

The distinctive role of menthol in pain and analgesia: Mechanisms, practices, and advances

Menthol is an important flavoring additive that triggers a cooling sensation. Under physiological condition, low to moderate concentrations of menthol activate transient receptor potential cation channel subfamily M member 8 (TRPM8) in the primary nociceptors, such as dorsal root ganglion (DRG) and trigeminal ganglion, generating a cooling sensation, whereas menthol at higher concentration could induce cold allodynia, and cold hyperalgesia mediated by TRPM8 sensitization. In addition, the paradoxical irritating properties of high concentrations of menthol is associated with its activation of transient receptor potential cation channel subfamily A member 1 (TRPA1). Under pathological situation, menthol activates TRPM8 to attenuate mechanical allodynia and thermal hyperalgesia following nerve injury or chemical stimuli. Recent reports have recapitulated the requirement of central group II/III metabotropic glutamate receptors (mGluR) with endogenous κ-opioid signaling pathways for menthol analgesia. Additionally, blockage of sodium channels and calcium influx is a determinant step after menthol exposure, suggesting the possibility of menthol for pain management. In this review, we will also discuss and summarize the advances in menthol-related drugs for pathological pain treatment in clinical trials, especially in neuropathic pain, musculoskeletal pain, cancer pain and postoperative pain, with the aim to find the promising therapeutic candidates for the resolution of pain to better manage patients with pain in clinics.

Spinal TRPA1 Contributes to the Mechanical Hypersensitivity Effect Induced by Netrin-1

Netrin-1, a chemoattractant expressed by floor plate cells, and one of its receptors (deleted in colorectal cancer) has been associated with pronociceptive actions in a number of pain conditions. Here, we addressed the question of whether spinal TRPC4/C5 or TRPA1 are among the downstream receptors contributing to pronociceptive actions induced by netrin-1. The experiments were performed on rats using a chronic intrathecal catheter for administration of netrin-1 and antagonists of TRPC4/C5 or TRPA1. Pain sensitivity was assessed behaviorally by using mechanical and heat stimuli. Effect on the discharge rate of rostral ventromedial medullary (RVM) pain control neurons was studied in lightly anesthetized animals. Netrin-1, in a dose-related fashion, induced mechanical hypersensitivity that lasted up to three weeks. Netrin-1 had no effect on heat nociception. Mechanical hypersensitivity induced by netrin-1 was attenuated by TRPA1 antagonist Chembridge-5861528 and by the control analgesic compound pregabalin both during the early (first two days) and late (third week) phase of hypersensitivity. TRPC4/C5 antagonist ML-204 had a weak antihypersensitivity effect that was only in the early phase, whereas TRPC4/C5 antagonist HC-070 had no effect on hypersensitivity induced by netrin-1. The discharge rate in pronociceptive ON-like RVM neurons was increased by netrin-1 during the late but not acute phase, whereas netrin-1 had no effect on the discharge rate of antinociceptive RVM OFF-like neurons. The results suggest that spinal TRPA1 receptors and pronociceptive RVM ON-like neurons are involved in the maintenance of submodality-selective pronociceptive actions induced by netrin-1 in the spinal cord.

Bedside testing for precision pain medicine

PURPOSE OF REVIEW

In recent years, the identification of therapy responders has become an increasing focus of pain research. On the basis of laboratory quantitative sensory testing, subgroups of patients were identified, which have been shown to predict treatment response. However, the high cost and time expenditure limits the use of these lab-QST protocols in clinical practice and large clinical trials.

RECENT FINDINGS

Recently, different bedside testing protocols were developed as easy-to-use alternative for lab-QST. In addition, patients can be subgrouped based on their symptoms by use of patient-reported outcome measures. First results suggest that these approaches can be used to stratify patients into pathophysiological-plausible subgroups predictive for treatment response.

SUMMARY

This review presents recently developed bedside approaches that can be implemented as stratification tools in future clinical trials to realize individualized pain medicine. Being complementary rather than replaceable, future studies should combine questionnaires and sensory testing and apply them prospectively in large clinical trials.

Pinprick Evoked Potentials-Reliable Acquisition in Healthy Human Volunteers

OBJECTIVE

Pinprick evoked potentials (PEPs) represent a novel tool to assess the functional integrity of mechano-nociceptive pathways with a potential toward objectifying sensory deficits and gain seen in neurological disorders. The aim of the present study was to evaluate the feasibility and reliability of PEPs with respect to age, stimulation site, and skin type.

METHODS

Electroencephalographic responses evoked by two pinprick stimulation intensities (128 mN and 256 mN) applied at three sites (hand dorsum, palmar digit II, and foot dorsum) were recorded in 30 healthy individuals. Test-retest reliability was performed for the vertex negative-positive complex amplitudes, N-latencies, and pain ratings evoked by the 256mN stimulation intensity.

RESULTS

Feasibility of PEP acquisition was demonstrated across age groups, with higher proportions of evoked potentials (>85%) for the 256mN stimulation intensity. Reliability analyses, that is, Bland-Altman and intraclass correlation coefficients, revealed poor to excellent reliability upon retest depending on the stimulation sites.

CONCLUSIONS

This study highlights the reliability of PEP acquisition from cervical and lumbar segments across clinically representative age groups. Future methodological improvements might further strengthen PEP reliability in order to complement clinical neurophysiology of sensory nerve fibers by a more specific assessment of mechano-nociceptive pathways. Beyond looking at sensory deficits, PEPs may also become applicable to revealing signs of central sensitization, complementing the clinical assessment of mechanical hyperalgesia.

Identification of spinal circuits involved in touch-evoked dynamic mechanical pain

Mechanical hypersensitivity is a debilitating symptom associated with millions of chronic pain patients. It exists in distinct forms, including brush-evoked dynamic and filament-evoked punctate. Here we report that dynamic mechanical hypersensitivity induced by nerve injury or inflammation was compromised in mice with ablation of spinal VT3^Lbx1 neurons defined by coexpression of VGLUT3^Cre and Lbx1^Flpo, as indicated by the loss of brush-evoked nocifensive responses and conditional place aversion. Electrophysiological recordings show that VT3^Lbx1 neurons form morphine-resistant polysynaptic pathways relaying inputs from low-threshold Aβ mechanoreceptors to lamina I output neurons. Meanwhile, the subset of somatostatin (SOM) lineage neurons preserved in VT3^Lbx1 neuron-ablated mice is largely sufficient to mediate von Frey filament-evoked punctate mechanical hypersensitivity, including both morphine-sensitive and morphine-resistant forms. Furthermore, acute silencing of VT3^Lbx1 neurons attenuated pre-established dynamic mechanical hypersensitivity induced by nerve injury, suggesting these neurons as a potential cellular target for treating this form of neuropathic pain.

Multimodal distribution of human cold pain thresholds

Background

It is assumed that different pain phenotypes are based on varying molecular pathomechanisms. Distinct ion channels seem to be associated with the perception of cold pain, in particular TRPM8 and TRPA1 have been highlighted previously. The present study analyzed the distribution of cold pain thresholds with focus at describing the multimodality based on the hypothesis that it reflects a contribution of distinct ion channels.

Methods

Cold pain thresholds (CPT) were available from 329 healthy volunteers (aged 18 – 37 years; 159 men) enrolled in previous studies. The distribution of the pooled and log-transformed threshold data was described using a kernel density estimation (Pareto Density Estimation (PDE)) and subsequently, the log data was modeled as a mixture of Gaussian distributions using the expectation maximization (EM) algorithm to optimize the fit.

Results

CPTs were clearly multi-modally distributed. Fitting a Gaussian Mixture Model (GMM) to the log-transformed threshold data revealed that the best fit is obtained when applying a three-model distribution pattern. The modes of the identified three Gaussian distributions, retransformed from the log domain to the mean stimulation temperatures at which the subjects had indicated pain thresholds, were obtained at 23.7 °C, 13.2 °C and 1.5 °C for Gaussian #1, #2 and #3, respectively.

Conclusions

The localization of the first and second Gaussians was interpreted as reflecting the contribution of two different cold sensors. From the calculated localization of the modes of the first two Gaussians, the hypothesis of an involvement of TRPM8, sensing temperatures from 25 – 24 °C, and TRPA1, sensing cold from 17 °C can be derived. In that case, subjects belonging to either Gaussian would possess a dominance of the one or the other receptor at the skin area where the cold stimuli had been applied. The findings therefore support a suitability of complex analytical approaches to detect mechanistically determined patterns from pain phenotype data.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Reduced epidermal thickness, nerve degeneration and increased pain-related behavior in rats with diabetes type 1 and 2

To examine the mechanisms contributing to pain genesis in diabetic neuropathy, we investigated epidermal thickness and number of intraepidermal nerve fibers in rat foot pad of the animal model of diabetes type 1 and type 2 in relation to pain-related behavior. Male Sprague-Dawley rats were used. Diabetes type 1 was induced with intraperitoneal injection of streptozotocin (STZ) and diabetes type 2 was induced with a combination of STZ and high-fat diet. Control group for diabetes type 1 was fed with regular laboratory chow, while control group for diabetes type 2 received high-fat diet. Body weights and blood glucose levels were monitored to confirm induction of diabetes. Pain-related behavior was analyzed using thermal (hot, cold) and mechanical stimuli (von Frey fibers, number of hyperalgesic responses). Two months after induction of diabetes, glabrous skin samples from plantar surface of the both hind paws were collected. Epidermal thickness was evaluated with hematoxylin and eosin staining. Intraepidermal nerve fibers quantification was performed after staining skin with polyclonal antiserum against protein gene product 9.5. We found that induction of diabetes type 1 and type 2 causes significant epidermal thinning and loss of intraepidermal nerve fibers in a rat model, and both changes were more pronounced in diabetes type 1 model. Significant increase of pain-related behavior two months after induction of diabetes was observed only in a model of diabetes type 1. In conclusion, animal models of diabetes type 1 and diabetes type 2 could be used in pharmacological studies, where cutaneous changes could be used as outcome measures for predegenerative markers of neuropathies.

Reappraising neuropathic pain in humans--how symptoms help disclose mechanisms

Neuropathic pain--that is, pain arising directly from a lesion or disease that affects the somatosensory system--is a common clinical problem, and typically causes patients intense distress. Patients with neuropathic pain have sensory abnormalities on clinical examination and experience pain of diverse types, some spontaneous and others provoked. Spontaneous pain typically manifests as ongoing burning pain or paroxysmal electric shock-like sensations. Provoked pain includes pain induced by various stimuli or even gentle brushing (dynamic mechanical allodynia). Recent clinical and neurophysiological studies suggest that the various pain types arise through distinct pathophysiological mechanisms. Ongoing burning pain primarily reflects spontaneous hyperactivity in nociceptive-fibre pathways, originating from 'irritable' nociceptors, regenerating nerve sprouts or denervated central neurons. Paroxysmal sensations can be caused by several mechanisms; for example, electric shock-like sensations probably arise from high-frequency bursts generated in demyelinated non-nociceptive Aβ fibres. Most human and animal findings suggest that brush-evoked allodynia originates from Aβ fibres projecting onto previously sensitized nociceptive neurons in the dorsal horn, with additional contributions from plastic changes in the brainstem and thalamus. Here, we propose that the emerging mechanism-based approach to the study of neuropathic pain might aid the tailoring of therapy to the individual patient, and could be useful for drug development.

Sex differences in pain-related behavior and expression of calcium/calmodulin-dependent protein kinase II in dorsal root ganglia of rats with diabetes type 1 and type 2

Sex differences in pain-related behavior and expression of calcium/calmodulin dependent protein kinase II (CaMKII) in dorsal root ganglia were studied in rat models of Diabetes mellitus type 1 (DM1) and type 2 (DM2). DM1 was induced with 55mg/kg streptozotocin, and DM2 with a combination of high-fat diet and 35mg/kg of streptozotocin. Pain-related behavior was analyzed using thermal and mechanical stimuli. The expression of CaMKII was analyzed with immunofluorescence. Sexual dimorphism in glycemia, and expression of CaMKII was observed in the rat model of DM1, but not in DM2 animals. Increased expression of total CaMKII (tCaMKII) in small-diameter dorsal root ganglia neurons, which are associated with nociception, was found only in male DM1 rats. None of the animals showed increased expression of the phosphorylated alpha CaMKII isoform in small-diameter neurons. The expression of gamma and delta isoforms of CaMKII remained unchanged in all analyzed animal groups. Different patterns of glycemia and tCaMKII expression in male and female model of DM1 were not associated with sexual dimorphism in pain-related behavior. The present findings do not suggest sex-related differences in diabetic painful peripheral neuropathy in male and female diabetic rats.

Neuropathic pain: is quantitative sensory testing helpful?

Neuropathic pain arises as a consequence of a lesion or disease affecting the somatosensory system and is characterised by a combination of positive and negative sensory symptoms. Quantitative sensory testing (QST) examines the sensory perception after application of different mechanical and thermal stimuli of controlled intensity and the function of both large (A-beta) and small (A-delta and C) nerve fibres, including the corresponding central pathways. QST can be used to determine detection, pain thresholds and stimulus-response curves and can thus detect both negative and positive sensory signs, the second ones not being assessed by other methods. Similarly to all other psychophysical tests QST requires standardised examination, instructions and data evaluation to receive valid and reliable results. Since normative data are available, QST can contribute also to the individual diagnosis of neuropathy, especially in the case of isolated small-fibre neuropathy, in contrast to the conventional electrophysiology which assesses only large myelinated fibres. For example, detection of early stages of subclinical neuropathy in symptomatic or asymptomatic patients with diabetes mellitus can be helpful to optimise treatment and identify diabetic foot at risk of ulceration. QST assessed the individual's sensory profile and thus can be valuable to evaluate the underlying pain mechanisms which occur in different frequencies even in the same neuropathic pain syndromes. Furthermore, assessing the exact sensory phenotype by QST might be useful in the future to identify responders to certain treatments in accordance to the underlying pain mechanisms.

Heterotopic noxious conditioning stimulation (HNCS) reduced the intensity of spontaneous pain, but not of allodynia in painful peripheral neuropathy

In 15 patients with painful peripheral neuropathy and dynamic mechanical allodynia, the influence of spontaneous ongoing neuropathic pain on pain sensitivity in a remote pain-free area was examined, as was the influence of ischemia-induced heterotopic noxious conditioning stimulation (HNCS) on the intensity of ongoing pain and brush-evoked allodynia. In addition, the modulating effect of HNCS on pain sensitivity in a pain-free area was investigated. Pain thresholds to pressure and heat as well as the sensitivity to suprathreshold pressure- and heat pain were assessed in the pain-free area. Dynamic mechanical allodynia was induced by a recently developed semi-quantitative brushing technique and the patients continuously rated the intensity of the allodynia using a computerized visual analogue scale (VAS). The total brush-evoked pain intensity was calculated as the area under the VAS curve. At baseline, no significant difference in pain sensitivity was found between patients and their healthy controls in the pain-free area, indicating a lack of activation of pain modulatory systems from the spontaneous pain. Compared to baseline, the patients rated the ongoing neuropathic pain intensity significantly lower during the HNCS-procedure (p<0.05). In contrast, there was no influence from HNCS on the total brush-evoked pain intensity. In the pain-free area higher pressure pain thresholds were demonstrated during conditioning stimulation in patients and controls alike (p<0.01). In controls only, a significantly higher heat pain threshold was found during the HNCS-procedure (p<0.01). The main finding of the present study was that HNCS altered differentially spontaneous and brush-provoked pain in patients with painful peripheral neuropathy.

Differences in sensory processing between chronic cervical zygapophysial joint pain patients with and without cervicogenic headache

Background: It is not known why some patients with underlying chronic nociceptive sources in the neck develop cervicogenic headache (CEH) and why others do not. This quantitative sensory testing (QST) study systematically explores the differences in sensory pain processing in 17 CEH patients with underlying chronic cervical zygapophysial joint pain compared to 10 patients with chronic cervical zygapophysial joint pain but without CEH.

Methods: The QST protocol comprises pressure pain threshold testing, thermal detection threshold testing, electrical pain threshold testing and measurement of descending inhibitory modulation using the conditioned pain modulation (CPM) paradigm.

Results: The main difference between patients with or without CEH was the lateralization of pressure hyperalgesia to the painful side of the head of CEH patients, accompanied by cold as well as warm relative hyperesthesia on the painful side of the head and neck.

Discussion: From this hypothesis-generating study, our results suggest that rostral neuraxial spread of central sensitization, probably to the trigeminal spinal nucleus, plays a major role in the development of CEH.

Guidelines and recommendations for assessment of somatosensory function in oro-facial pain conditions--a taskforce report

The goals of an international taskforce on somatosensory testing established by the Special Interest Group of Oro-facial Pain (SIG-OFP) under the International Association for the Study of Pain (IASP) were to (i) review the literature concerning assessment of somatosensory function in the oro-facial region in terms of techniques and test performance, (ii) provide guidelines for comprehensive and screening examination procedures, and (iii) give recommendations for future development of somatosensory testing specifically in the oro-facial region. Numerous qualitative and quantitative psychophysical techniques have been proposed and used in the description of oro-facial somatosensory function. The selection of technique includes time considerations because the most reliable and accurate methods require multiple repetitions of stimuli. Multiple-stimulus modalities (mechanical, thermal, electrical, chemical) have been applied to study oro-facial somatosensory function. A battery of different test stimuli is needed to obtain comprehensive information about the functional integrity of the various types of afferent nerve fibres. Based on the available literature, the German Neuropathic Pain Network test battery appears suitable for the study of somatosensory function within the oro-facial area as it is based on a wide variety of both qualitative and quantitative assessments of all cutaneous somatosensory modalities. Furthermore, these protocols have been thoroughly described and tested on multiple sites including the facial skin and intra-oral mucosa. Standardisation of both comprehensive and screening examination techniques is likely to improve the diagnostic accuracy and facilitate the understanding of neural mechanisms and somatosensory changes in different oro-facial pain conditions and may help to guide management.

TRPM8 in health and disease: cold sensing and beyond

This review focuses on TRPM8, one of the approximately 30 members of the diverse family of transient receptor potential (TRP) ion channels. Initially identified from the prostate, TRPM8 has been studied more extensively in the sensory system and is best established as a major transducer of environmental cold temperatures. An increasing body of evidence suggests that it may also be an important player in various chronic conditions, such as inflammatory/neuropathic pain and prostate cancer. Small molecule compounds that selectively modulate TRPM8 are beginning to emerge and will be critically valuable for better understanding the role of this channel in both physiological and pathological states, on which the prospects of TRPM8 as a viable therapeutic target rest.

Quantitative sensory testing: a diagnostic tool for painful neuropathy

Quantitative sensory testing (QST) analyzes sensory perceptions of external stimuli of controlled intensity. Both large and small fiber function can be evaluated by applying thermal and mechanical stimuli, thus closing the diagnostic gap for the conventional electrophysiology in the examination of thin and unmyelinated nerve fibers. Detection, pain thresholds and stimulus–response curves can be determined; therefore, QST is a valuable diagnostic tool for identifying both sensory loss (i.e., hypoesthesia and hypoalgesia) and gain (i.e., hyperalgesia and allodynia) in patients with painful or painless neuropathy. Every psychophysical approach QST requires standardized stimuli administration, instructions and data evaluation to achieve valid results. QST can be used to evaluate distinct somatosensory profiles and thus give hints to the underlying mechanisms that occur with different frequencies in different pain syndromes. This might be helpful for the future establishment of mechanism-based pharmacotherapy. Since normative data are available, QST also contributes to the individual diagnosis of neuropathy. The present article gives an outline of QST application in diagnosis and its limitations for the evaluation of neuropathic pain.

The contribution of TRPM8 and TRPA1 channels to cold allodynia and neuropathic pain

Cold allodynia is a common feature of neuropathic pain however the underlying mechanisms of this enhanced sensitivity to cold are not known. Recently the transient receptor potential (TRP) channels TRPM8 and TRPA1 have been identified and proposed to be molecular sensors for cold. Here we have investigated the expression of TRPM8 and TRPA1 mRNA in the dorsal root ganglia (DRG) and examined the cold sensitivity of peripheral sensory neurons in the chronic construction injury (CCI) model of neuropathic pain in mice.

In behavioral experiments, chronic constriction injury (CCI) of the sciatic nerve induced a hypersensitivity to both cold and the TRPM8 agonist menthol that developed 2 days post injury and remained stable for at least 2 weeks. Using quantitative RT-PCR and in situ hybridization we examined the expression of TRPM8 and TRPA1 in DRG. Both channels displayed significantly reduced expression levels after injury with no change in their distribution pattern in identified neuronal subpopulations. Furthermore, in calcium imaging experiments, we detected no alterations in the number of cold or menthol responsive neurons in the DRG, or in the functional properties of cold transduction following injury. Intriguingly however, responses to the TRPA1 agonist mustard oil were strongly reduced.

Our results indicate that injured sensory neurons do not develop abnormal cold sensitivity after chronic constriction injury and that alterations in the expression of TRPM8 and TRPA1 are unlikely to contribute directly to the pathogenesis of cold allodynia in this neuropathic pain model.

Ectopic discharge in Abeta afferents as a source of neuropathic pain

Ectopic discharge in axotomized dorsal root ganglion neurons is a key driver of neuropathic pain. However, the bulk of this activity is generated and carried centrally in large diameter myelinated Abeta afferents, a cell type that normally signals touch and vibration sense. Evidence is considered suggesting that following axotomy, Abeta afferents undergo a change in their electrical characteristics and also in the neurotransmitter complement that they express. This dual phenotypic switching renders them capable of (1) directly driving postsynaptic pain signaling pathways in the spinal cord, and (2) triggering and maintaining central sensitization.

Neuropathic pain: a clinical perspective

Neuropathic pain syndromes, i.e., pain after a lesion or disease of the peripheral or central nervous system, are clinically characterized by spontaneous pain (ongoing, paroxysms) and evoked types of pain (hyperalgesia, allodynia). A variety of distinct pathophysiological mechanisms in the peripheral and central nervous system operate in concert: In some patients the nerve lesion triggers molecular changes in nociceptive neurons that become abnormally sensitive and develop pathological spontaneous activity (upregulation of sodium channels and receptors, e.g., vanilloid TRPV1 receptors, menthol-sensitive TRPM8 receptors, or alpha-receptors). These phenomena may lead to spontaneous pain, shooting pain sensations, as well as heat hyperalgesia, cold hyperalgesia, and sympathetically maintained pain. Spontaneous activity in damaged large nonnociceptive A-fibers may lead to paresthesias. All these changes may also occur in uninjured neurons driven by substances released by adjacent dying cells and should receive more attention in the future. The hyperactivity in nociceptors in turn induces secondary changes (hyperexcitability) in processing neurons in the spinal cord and brain. This central sensitization causes input from mechanoreceptive A-fibers to be perceived as pain (mechanical allodynia). Neuroplastic changes in the central descending pain modulatory systems (inhibitory or facilitatory) may lead to further hyperexcitability. Neuropathic pain represents a major neurological problem and treatment of patients with such pain has been largely neglected by neurologists in the past. The medical management of neuropathic pain consists of five main classes of oral medication (antidepressants with reuptake blocking effect, anticonvulsants with sodium-blocking action, anticonvulsants with calcium-modulating actions, tramadol, and opioids) and several categories of topical medications for patients with cutaneous allodynia and hyperalgesia (capsaicin and local anesthetics). In many cases an early combination of compounds effecting different mechanisms is useful. At present existing trials only provide general pain relief values for specific causes, which in part may explain the failure to obtain complete pain relief in neuropathic pain conditions. In general, the treatment of neuropathic pain is still unsatisfactorily. Therefore, a new hypothetical concept was proposed in which pain is analyzed on the basis of underlying mechanisms. The increased knowledge of pain-generating mechanisms and their translation into symptoms and signs may in the future allow a dissection of the mechanisms that operate in each patient. If a systematic clinical examination of the neuropathic pain patient and a precise phenotypic characterization is combined with a selection of drugs acting against those particular mechanisms, it should ultimately be possible to design optimal treatments for the individual patient.

Neuropathic pain: translational research and impact for patient care

Neuropathic pain syndromes (ie, pain after a lesion or disease of the peripheral or central nervous system) are clinically characterized by spontaneous pain (ongoing, paroxysms) and evoked types of pain (hyperalgesia, allodynia). Different pathophysiologic mechanisms occur solitarily or combined at peripheral nociceptors, spinal cord, or in the brain, which cause a broad variety of signs and symptoms. Currently, the medical management is still arduous. Therefore, a new concept was proposed in which pain is analyzed on the basis of underlying mechanisms. The increased knowledge of pain-generating mechanisms and their translation into signs and symptoms may allow for a dissection of the individual mechanisms, and it ultimately should be possible to design optimal treatments for each patient.

Complement induction in spinal cord microglia results in anaphylatoxin C5a-mediated pain hypersensitivity

Microarray expression profiles reveal substantial changes in gene expression in the ipsilateral dorsal horn of the spinal cord in response to three peripheral nerve injury models of neuropathic pain. However, only 54 of the 612 regulated genes are commonly expressed across all the neuropathic pain models. Many of the commonly regulated transcripts are immune related and include the complement components C1q, C3, and C4, which we find are expressed only by microglia. C1q and C4 are, moreover, the most strongly regulated of all 612 regulated genes. In addition, we find that the terminal complement component C5 and the C5a receptor (C5aR) are upregulated in spinal microglia after peripheral nerve injury. Mice null for C5 had reduced neuropathic pain sensitivity, excluding C3a as a pain effector. C6-deficient rats, which cannot form the membrane attack complex, have a normal neuropathic pain phenotype. However, C5a applied intrathecally produces a dose-dependent, slow-onset cold pain in naive animals. Furthermore, a C5aR peptide antagonist reduces cold allodynia in neuropathic pain models. We conclude that induction of the complement cascade in spinal cord microglia after peripheral nerve injury contributes to neuropathic pain through the release and action of the C5a anaphylatoxin peptide.

Facial altered sensation and sensory impairment after orthognathic surgery

The aim of this study was to determine whether impairment of sensory functions after trigeminal nerve injury differs in severity among patients who report qualitatively different altered sensations. Data were obtained from 184 patients. Before and at 1, 3 and 6 months after orthognathic surgery, patients were grouped as having no altered sensation, negative sensations only (hypoaesthetic), mixed sensations (negative+active), or active sensations only (paraesthetic or dysaesthetic). Bias-free estimates of contact detection and two-point discrimination were obtained to assess, via ANOVA, whether patients in the four groups exhibited different levels of sensory impairment. Impairment in contact detection and two-point discrimination was found to differ significantly among the groups at 6 months but not at 1 month. At 6 months, patients who reported negative sensations only exhibited the greatest impairment, on average, in contact detection; in contrast, patients who reported mixed sensations exhibited the greatest impairment in two-point discrimination. The least residual impairment at 6 months was observed in patients who reported no altered sensation. It is recommended that clinical judgments regarding nerve injury-associated sensory dysfunction should not be based on threshold testing results without consideration of patients' subjective reports of altered sensation.

Mapping the spinal and supraspinal pathways of dynamic mechanical allodynia in the human trigeminal system using cardiac-gated fMRI

Following injury and inflammation, pain to light stroking (dynamic mechanical allodynia) might develop at the damaged site (primary area) or in adjacent normal tissue (secondary area). Using fMRI we mapped changes in the spinal trigeminal nucleus (spV), and supraspinal brainstem nuclei following heat/capsaicin-induced primary and secondary dynamic mechanical allodynia in the human trigeminal system. The role of these structures in dynamic mechanical allodynia has not been clarified yet in humans. During the control session we applied the same mechanical stimuli to the same untreated trigeminal area. Primary and secondary mechanical allodynia showed equal levels of perceived pain intensity, and compared to control mechanical stimulation exhibited similar responses in the ipsilateral spV and contralateral ventrolateral periaqueductal gray (vlPAG). Activity in the spV was significantly higher during both conditions versus the control mechanical stimulation, indicating that central sensitization of second-order neurons is similar for primary and secondary mechanical allodynia. The vlPAG showed decreased activity that inversely correlated with pain ratings during primary allodynia, i.e. the more deactivated the vlPAG the higher the pain intensity (p<0.05, Pearson's correlation). Primary and secondary dynamic mechanical allodynia were also characterized by significant differences involving distinct supraspinal structures mainly involved in pain modulation and including the rostroventromedial medulla, pons reticular formation, dorsolateral PAG, all more active during primary versus secondary allodynia, and the medial reticular formation of the caudal medulla that was more active during secondary versus primary allodynia. These results indicate that the pain modulatory system is involved to a different extent during primary versus secondary mechanical allodynia.

Patterns of cortical reorganization parallel impaired tactile discrimination and pain intensity in complex regional pain syndrome

In the complex regional pain syndrome (CRPS), several theories proposed the existence of pathophysiological mechanisms of central origin. Recent studies highlighted a smaller representation of the CRPS-affected hand on the primary somatosensory cortex (SI) during non-painful stimulation of the affected side. We addressed the question whether reorganizational changes can also be found in the secondary somatosensory cortex (SII). Moreover, we investigated whether cortical changes might be accompanied by perceptual changes within associated skin territories. Seventeen patients with CRPS of one upper limb without the presence of peripheral nerve injuries (type I) were subjected to functional magnetic resonance imaging (fMRI) during electrical stimulation of both index fingers (IFs) in order to assess hemodynamic signals of the IF representation in SI and SII. As a marker of tactile perception, we tested 2-point discrimination thresholds on the tip of both IFs. Cortical signals within SI and SII were significantly reduced contralateral to the CRPS-affected IF as compared to the ipsilateral side and to the representation of age- and sex-matched healthy controls. In parallel, discrimination thresholds of the CRPS-affected IF were significantly higher, giving rise to an impairment of tactile perception within the corresponding skin territory. Mean sustained, but not current pain levels were correlated with the amount of sensory impairment and the reduction in signal strength. We conclude that patterns of cortical reorganization in SI and SII seem to parallel impaired tactile discrimination. Furthermore, the amount of reorganization and tactile impairment appeared to be linked to characteristics of CRPS pain.

Quantitative sensory testing: a comprehensive protocol for clinical trials

We have compiled a comprehensive QST protocol as part of the German Research Network on Neuropathic Pain (DFNS) using well established tests for nearly all aspects of somatosensation. This protocol encompasses thermal as well as mechanical testing procedures. Our rationale was to test for patterns of sensory loss (small and large nerve fiber functions) or gain (hyperalgesia, allodynia, hyperpathia), and to assess both cutaneous and deep pain sensitivity. The practicality of the QST protocol was tested in 18 healthy subjects, 21-58 years, half of them female. All subjects were tested bilaterally over face, hand and foot. We determined thermal detection and pain thresholds including a test for the presence of paradoxical heat sensations, mechanical detection thresholds to von Frey filaments and a 64-Hz tuning fork, mechanical pain thresholds to pinprick stimuli and blunt pressure, stimulus-response-functions for pinprick and dynamic mechanical allodynia (pain to light touch), and pain summation (wind-up ratio) using repetitive pinprick stimulation. The full protocol took 27+/-2.3 min per test area. The majority of QST parameters were normally distributed only after logarithmic transformation (secondary normalization) except for the frequency of paradoxical heat sensations, cold and heat pain thresholds, and for vibration detection thresholds. Thresholds were usually lowest over face, followed by hand, and then foot. Only thermal pain thresholds, wind-up ratio and vibration detection thresholds were not significantly dependent on the body region. There was no significant right-to-left difference for any of the QST parameters; left-to-right correlation coefficients ranged between 0.78 and 0.97, thus explaining between 61% and 94% of the variance. This study has shown that a complete somatosensory profile of one affected area and one unaffected control area, which will be necessary to characterize patients with a variety of diseases, can be obtained within 1 h. Case examples of selected patients illustrate the value of z-transformed QST data for an easy survey of individual symptom profiles.

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Dynamic mechanical allodynia: On the relationship between temporo-spatial stimulus parameters and evoked pain in patients with peripheral neuropathy

The relationship between temporo-spatial stimulus parameters and evoked pain intensity as well as duration was examined in patients with peripheral neuropathy and brush-evoked allodynia, i.e. dynamic mechanical allodynia. Brush-evoked allodynia was induced in the innervation territory of the lesioned nervous structure in 18 patients by lightly stroking different distances of the skin (20, 40, 60 mm) two or four times with brushes of different widths (4, 8, 16 mm). Pain intensity and duration of brush-evoked allodynia was recorded using a computerized visual analogue scale. The total brush-evoked pain intensity, including painful aftersensation was calculated as the area under the curve. Following each stimulus, the patients selected pain descriptors from a validated instrument. Significantly increased total brush-evoked pain intensity was demonstrated with increased brushing length and number of strokes (P<0.001), but not while altering brush width. Lack of influence of brush width was further underlined by the finding that activation of equivalent areas (e.g. 160 mm2) resulted in higher total brush-evoked pain intensity if brushing the skin with a thin brush (4 mm) over a longer distance (40 mm) than a thick brush (8 mm) over a shorter distance (20 mm). Significantly increased duration of aftersensation was demonstrated only following increased brushing length (P<0.008). The most commonly used sensory-discriminative pain descriptors were pricking, burning and sore and for the affective descriptors, annoying and troublesome. This is the first study demonstrating a relationship between evoked pain and some temporo-spatial stimulus parameters during brush-evoked allodynia.