Chronic itch and pain--similarities and differences

Irritable bowel syndrome in patients with chronic pruritus of undetermined origin

BACKGROUND

Chronic pruritus is an important distressing condition that is often refractory to treatment. Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that occurs without an organic cause.

OBJECTIVES

We aimed to investigate the relationship between chronic pruritus of undetermined origin (CPUO) and IBS.

METHODS

Eighty patients with chronic pruritus (CP) of undetermined origin and fifty healthy control subjects without pruritus were included into the study. All of the participants were examined by a gastroenterologist for concomitant IBS.

RESULTS

The frequency of IBS was found higher in patients with CP of undetermined origin (P: 0.02), but we did not observe any significant association between pruritus intensity and either presence of IBS (P: 0.08) or the subtypes of the syndrome (P: 0.40). Furthermore, patients with CP of undetermined origin between 40 and 60 years, female gender and longer duration of the disease were found to be significantly associated with the presence of IBS (P: 0.02, P: 0.01 and P < 0.001).

CONCLUSIONS

We found that the frequency of IBS was higher in patients with CP than in healthy controls. Our study is the first report about the relation between CP of undetermined origin and IBS. Further studies with larger numbers of the patients are needed to show association between IBS and CPUO using laboratory tests to define underlying diseases such as lactose intolerance, functional dyspepsia and emotional diseases.

Characteristics of chronic wounds that itch

OBJECTIVE

This study was designed to explore characteristics of chronic wounds that present with wound-related itch.

BACKGROUND

Although wound-related itch is recognized clinically, little is known about the phenomenon. Recent scientific advances have enabled the study of itch physiology, yet the clinical problem is not well described in the literature.

DESIGN

The study was observational, descriptive.

METHOD

Persons (N = 200) with wounds being followed up at a hospital-affiliated wound care center were interviewed and assessed. Instruments included a health history tool, Paul-Pieper Itching Questionnaire, Bates-Jensen Wound Assessment Tool, and 10-g monofilament for assessment of sensation in the area of the wound.

RESULTS

Participants were aged 21 to 98 years (mean, 66.82 [SD, 14.02] years); 56% of the participants were men, and 85% were white. One-fourth (56/200) of the participants reported wound-related itch. Wounds that itched were generally larger (t77.74 = -3.27; P = .002; d = 0.63; 95% confidence interval [CI], -1.01 to -0.25), had more tissue edema (t88.38 = -2.19; P = .031; 95% CI, -0.93 to -0.47), and demonstrated more granulation tissue in the wound base (t98.71 = 2.03; P = .045; 95% CI, 0.01-0.87), compared with wounds without itch. Greater itch was associated with wounds that had a moderate amount of exudate (P = .02) or necrotic tissue in the base.

CONCLUSIONS

Wound itch was present in more severe wounds as evidenced by larger size, more tissue edema, and necrotic tissue. Understanding wound itch could promote wound healing and improve quality of life for persons with chronic wounds.

Sex differences in itch perception and modulation by distraction--an FMRI pilot study in healthy volunteers

BACKGROUND

Even though itch is a common syndrome of many diseases there is only little knowledge about sex and gender differences in pruritus, especially in central itch perception and modulation. To our knowledge, this is the first fMRI study examining sex differences in perception and its modulation by distraction.

METHODS

Experimental itch was induced by application of histamine (0.1 mM) via microdialysis fibers twice at the left forearm and twice at the left lower leg in 33 healthy volunteers (17 females, 16 males). The brain activation patterns were assessed by fMRI during itch without and with distraction (Stroop task). Between the various conditions, subjects were asked to rate itch intensity, desire to scratch and pain intensity. In a second experiment in 10 of the 33 volunteers histamine was replaced by saline solution to serve as control for the 'Stroop' condition.

RESULTS

Women generally presented higher itch intensities compared to men during itch over the course of the experiment. A more specific analysis revealed higher itch intensities and desire to scratch in women during experimental induced itch that can be reduced by distraction at the lower legs when itch is followed by 'Stroop'. In contrast, men depicted significant reduction of 'itch' by 'Stroop' at the forearms. Women depicted higher brain activation of structures responsible for integration of sensory, affective information and motor integration/planning during 'itch' and 'Stroop' condition when compared to men. No sex differences were seen in the saline control condition.

CONCLUSION

Women and men exhibited localisation dependent differences in their itch perception with women presenting higher itch intensities and desire to scratch. Our findings parallel clinical observations of women reporting higher itch intensities depending on itch localisation and suffering more from itch as compared to men.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Neuropathic mechanisms in the pathophysiology of burns pruritus: redefining directions for therapy and research

Pruritus in burn wounds is a common symptom affecting patient rehabilitation. Over the last decades, there has been a resurgence of interest into more effective strategies to combat this distressing problem; nevertheless, no reports exist in the literature to propose pathophysiological mechanisms responsible for the generation and persistence of pruritic symptoms in the late phases of burns rehabilitation. Neuronal pathways mediating pruritic and painful stimuli share striking similarities, which allows the comparative exploration of the less extensively studied pruritic mechanisms using pain models. Furthermore, emerging anatomical, neurophysiological, and pharmacological evidence supports the involvement of neuropathic mechanisms in chronic burns pruritus. This work updates the conceptual framework for the pathophysiology of burns itch by embracing the contribution of the central nervous system in the maintenance of symptoms into a chronic state. The proposed pathophysiological model paves new avenues in burns pruritus research and is likely to have implications in the quest for more effective therapeutic regimens in clinical practice.

Use of pregabalin in the management of chronic uremic pruritus

CONTEXT

Uremic pruritus (UP) affects many patients suffering from chronic kidney disease (CKD) and has a negative impact on quality of life and survival. It has become increasingly evident that central transmission and sensitization processes similar to those observed in chronic pain are important mechanisms of pruritus.

OBJECTIVES

To test the potential role of pregabalin in reducing the intensity of UP in CKD patients.

METHODS

We prospectively collected data on CKD patients who suffered from severe intractable pruritus. Patients were asked to record the intensity of pruritus on a visual analogue scale.

RESULTS

Twelve patients were studied. The average pretreatment pruritus score was 9.7 ± 0.9 and decreased to 3.7 ± 2.35, 3.2 ± 1.75, and 3 ± 1.5 after one, four, and 24 weeks of treatment, respectively (P < 0.05). The positive effect of pregabalin was demonstrated during the first week of therapy in six patients. Most patients required 25mg a day. Pregabalin was well tolerated, with somnolence and dizziness developing in two patients.

CONCLUSION

We demonstrated dramatic improvement of long-standing UP after the initiation of pregabalin. We suggest that pregabalin can be used safely in CKD but careful titration of the dose is required to obtain an optimal response and minimize the possible adverse effects.

Cutting-edge issues in primary biliary cirrhosis

Several crucial issues remain open in our understanding of primary biliary cirrhosis (PBC), an autoimmune liver disease targeting the small- and medium-sized intrahepatic bile ducts. These issues include the high tissue specificity of the autoimmune injury despite the nontraditional autoantigens found in all mitochondria recognized by PBC-associated autoantibodies, the causes of the commonly observed pruritus, and the disease etiology per se. In all these fields, there has been recent interest secondary to the use of large-scale efforts (such as genome-wide association studies) that were previously considered poorly feasible in a rare disease such as PBC as well as other intuitions. Accordingly, there are now fascinating theories to explain the onset and severity of pruritus due to elevated autotaxin levels, the peculiar apoptotic features of bile duct cells to explain the tissue specificity, and genomic and epigenetic associations contributing to disease susceptibility. We have arbitrarily chosen these four aspects as the most promising in the PBC recent literature and will provide herein a discussion of the recent data and their potential implications.

[Pain and pruritus : differences and similarities as revealed by the PainDetect questionnaire parameters]

Chronic pruritus and chronic pain are frequent symptoms of a variety of underlying diseases. Painful sensations usually suppress acute itching. In chronic states, both may be present in parallel and be a part of one event. Patients with chronic pruritus should be asked for the presence of pain, which can be identified and characterized using specific and validated questionnaires. The early detection of (neuropathic) pain in patients with chronic pruritus can be done using the PainDetect questionnaire. In general, patients suffering from both itch and pain have a highly impaired quality of life, high degree of objective health burden, need a more intensive health care and a complex analgetic and antipruritic therapy.

Role of induced negative and positive emotions in sensitivity to itch and pain in women

BACKGROUND

Itch and pain are common symptoms in skin disease. It has been suggested that negative emotions may play a role in itch and pain. To date, however, the role of emotions has only been studied for pain in experimental studies, not yet for itch.

OBJECTIVES

To investigate the effects of negative and positive emotions on the sensitivity to itch and pain.

METHODS

Film fragments were used to induce a negative or positive emotional state in healthy women. Itch and pain were induced using the following somatosensory stimuli: electrical stimulation, histamine iontophoresis and the cold pressor test.

RESULTS

Results showed that the scores for itch and pain evoked by histamine and the cold pressor test, respectively, were significantly higher in the negative than in the positive emotion condition, whereas tolerance thresholds to electrical stimulation and the cold pressor test, and stimulus unpleasantness scores did not differ between the two conditions.

CONCLUSIONS

These findings for the first time indicate in an experimental design that emotions play a role in sensitivity to somatosensory sensations of both itch and pain.

The itchy scalp--scratching for an explanation

Scalp pruritus is a common complaint that is considered a diagnostically and therapeutically challenging situation. Scalp skin has a unique neural structure that contains densely innervated hair follicles and dermal vasculature. In spite of the recent advances in our understanding of itch pathophysiology, scalp itching has not been studied as yet. In this review, we summarize the current knowledge on the neurobiology of scalp and hair follicles as well as itch mediators and provide a putative mechanism for scalp itch with special emphasis on neuroanatomy and pathophysiology.

Management of itch in atopic dermatitis

Atopic dermatitis is a common, pruritic, inflammatory skin disorder. Chronic, localized, or even generalized pruritus is the diagnostic hallmark of atopic dermatitis, and its management remains a challenge for physicians. The threshold for itch and alloknesis is markedly reduced in these patients, and infections can promote exacerbation and thereby increase the itch. Modern management consists of anti-inflammatory, occasionally antiseptic, as well as antipruritic therapies to address the epidermal barrier as well as immunomodulation or infection. Mild forms of atopic dermatitis may be controlled with topical therapies, but moderate-to-severe forms often require a combination of systemic treatments consisting of antipruritic and immunosuppressive drugs, phototherapy, and topical compounds. In addition, patient education and a therapeutic regimen to help the patient cope with the itch and eczema are important adjuvant strategies for optimized long-term management. This review highlights various topical, systemic, and complementary and alternative therapies, as well as provide a therapeutic ladder for optimized long-term control of itch in atopic dermatitis.

Basic mechanisms of itch

Chronic itch represents a burdensome clinical problem that can originate from a variety of aetiologies. Pruriceptive itch originates following the activation of peripheral sensory nerve endings following damage or exposure to inflammatory mediators and ascends to the brain through the spinal thalamic tract. Much insight has been gained into the understanding of the mechanisms underlying pruriceptive itch through studies using humans and experimental animals. More than one sensory nerve subtype is thought to subserve pruriceptive itch which includes both unmyelinated C-fibres and thinly myelinated Aδ nerve fibres. There are a myriad of mediators capable of stimulating these afferent nerves leading to itch, including biogenic amines, proteases, cytokines, and peptides. Some of these mediators can also evoke sensations of pain and the sensory processing underlying both sensations overlaps in complex ways. Studies have demonstrated that both peripheral and central sensitization to pruritogenic stimuli occur during chronic itch.

[Comparative psychological aspects of itching and pain]

Itching is a major symptom of chronic skin diseases such as atopic dermatitis and leads to considerable psychological strain. Chronic itching lowers patient's quality of life similar to chronic pain and influences the medical treatment. The frequently resulting scratching behavior (short-term avoidance of itch) leads to continuation and exacerbation of the disease, just as with specific pain behavior. For the development of itching and pain psychosocial factors have been identified in addition to somatic ones. However, recent data suggest that there is a complex interaction between pain and itching and comparable mechanisms of neuronal sensitization. In contrast to traditional biomedical one-dimensional models which focus mainly on physical and not psychological factors of a disease, recent data support a biopsychosocial model of development and maintenance for itching and pain. Biopsychosocial understanding of a disease should consequently be taken as the basis for treatment and the importance of interdisciplinary treatment is emphasized. This article will focus on chronic itching and pain with particular consideration of psychological factors.

Induction of nocebo and placebo effects on itch and pain by verbal suggestions

Physical complaints, such as pain, can be effectively reduced by placebo effects through induction of positive expectations, or increased by nocebo effects through induction of negative expectations. In the present study, verbally induced nocebo and placebo effects on itch were experimentally investigated for the first time. In part 1, the role of verbal suggestions in inducing nocebo effects on itch and pain was investigated. All subjects received the same somatosensory quantitative sensory testing stimuli, that is, mechanical and electrical stimuli and application of histamine, and verbal suggestions to manipulate expectations regarding the stimuli. The suggestions were designed to produce either high expectations for itch (itch nocebo) or pain (pain nocebo) or low expectations for itch (itch nocebo control) or pain (pain nocebo control). Results showed that high itch and pain expectations resulted in higher levels of itch and pain, respectively. When comparing nocebo effects, induced by verbal suggestions, results were more pronounced for itch than for pain. In part 2, verbal suggestions designed to produce a placebo effect on itch (itch placebo) or pain (pain placebo), or neutral suggestions (itch placebo control and pain placebo control) were given regarding a second application of histamine and compared with the first application applied in part 1. Results of placebo effects only showed a significantly larger decrease in itch in the itch placebo condition than in the pain placebo condition. In conclusion, we showed for the first time that nocebo and possibly placebo responses can be induced on itch by verbal suggestions.

Gabapentin and pregabalin inhibit the itch-associated response induced by the repeated application of oxazolone in mice

We investigated the effects of gabapentin and pregabalin on the itch-associated response in a mouse model of chronic dermatitis induced by the repeated application of 4-ethoxymethylene-2-phenyl-2-oxazolin-5-one (oxazolone). Challenging the mice with oxazolone-induced chronic dermatitis with the oxazolone evoked severe and transient scratching behavior until 1 h after the application of oxazolone. Thereafter, a more mild and continuous scratching behavior was also observed for at least 8 h. Both severe and continuous scratching behaviors were suppressed by systemic injection of gabapentin and pregabalin. This effect of these compounds was correlated with its affinity for the α₂δ subunit of voltage-gated Ca²(+) channels. Intrathecal injection, but not peripheral treatment, with gabapentin inhibited the scratching behavior in this model. Gabapentin failed to suppress the scratching behavior induced by the intradermal injection of compound 48/80 in normal mice. The expression of the α₂δ-1 subunit in dorsal root ganglion (DRG) from mice following repeated application of oxazolone was significantly higher than that from normal mice. These results suggest that gabapentin and pregabalin show an anti-pruritic activity through α₂δ-subunit binding, and the up-regulation of the α₂δ-1 subunit in DRG may therefore play an important role in its anti-pruritic activity.

Chronic pruritus in the absence of specific skin disease: an update on pathophysiology, diagnosis, and therapy

Chronic pruritus is a major and distressing symptom of many cutaneous and systemic diseases and can significantly impair the patient's quality of life. Pruritus perception is the final result of a complex network involving dedicated nerve pathways and brain areas, and an increasing number of peripheral and central mediators are thought to be involved. Itch is associated with most cutaneous disorders and, in these circumstances, its management overlaps with that of the skin disease. Itch can also occur without associated skin diseases or primary skin lesions, but only with nonspecific lesions secondary to rubbing or scratching. Chronic itch with no or minimal skin changes can be secondary to important diseases, such as neurologic disorders, chronic renal failure, cholestasis, systemic infections, malignancies, and endocrine disorders, and may also result from exposure to some drugs. The search for the cause of pruritus usually requires a meticulous step-by-step assessment involving careful history taking as well as clinical examination and laboratory investigations. Few evidence-based treatments for pruritus are available. Topical therapy, oral histamine H(1) receptor antagonists, and phototherapy with UV radiation can target pruritus elicitation in the skin, whereas antiepileptic drugs, opioid receptor antagonists, and antidepressants can block signal processing in the CNS.

Lysophosphatidic acid is a potential mediator of cholestatic pruritus

BACKGROUND & AIMS

Pruritus is a common and disabling symptom in cholestatic disorders. However, its causes remain unknown. We hypothesized that potential pruritogens accumulate in the circulation of cholestatic patients and activate sensory neurons.

METHODS

Cytosolic free calcium ([Ca(2+)](i)) was measured in neuronal cell lines by ratiometric fluorometry upon exposure to serum samples from pruritic patients with intrahepatic cholestasis of pregnancy (ICP), primary biliary cirrhosis (PBC), other cholestatic disorders, and pregnant, healthy, and nonpruritic disease controls. Putative [Ca(2+)](i)-inducing factors in pruritic serum were explored by analytical techniques, including quantification by high-performance liquid chromatography/mass spectroscopy. In mice, scratch activity after intradermal pruritogen injection was quantified using a magnetic device.

RESULTS

Transient increases in neuronal [Ca(2+)](i) induced by pruritic PBC and ICP sera were higher than corresponding controls. Lysophosphatidic acid (LPA) could be identified as a major [Ca(2+)](i) agonist in pruritic sera, and LPA concentrations were increased in cholestatic patients with pruritus. LPA injected intradermally into mice induced scratch responses. Autotaxin, the serum enzyme converting lysophosphatidylcholine into LPA, was markedly increased in patients with ICP versus pregnant controls (P < .0001) and cholestatic patients with versus without pruritus (P < .0001). Autotaxin activity correlated with intensity of pruritus (P < .0001), which was not the case for serum bile salts, histamine, tryptase, substance P, or mu-opioids. In patients with PBC who underwent temporary nasobiliary drainage, both itch intensity and autotaxin activity markedly decreased during drainage and returned to preexistent levels after drain removal.

CONCLUSIONS

We suggest that LPA and autotaxin play a critical role in cholestatic pruritus and may serve as potential targets for future therapeutic interventions.

A C-fiber-mediated neuropathic brachioradial pruritus

A 56-year-old man presented to the outpatient clinic with a 3-year history of itch within the innervation territory of C-6 of the left arm. Sudden neck movements induced intermittent paresthesias in the same dermatome. No dermatological diseases, allergies, or trauma to the affected extremity or the spine or a history of familial pruritus were reported. Neurological physical examination and electromyography revealed normal findings. Quantitative sensory testing demonstrated selective C-fiber dysfunction at C6-8 on the left, and cervical MR imaging revealed multilevel degenerative cervical spine pathology with neuroforaminal stenoses. Brachioradial neuropathic pruritus caused by cervical neuroforaminal stenosis was the final diagnosis. Treatment consisted of 2 cervical epidural steroid applications that resulted in clinical disappearance of the itch and improvement in C-fiber function on quantitative sensory testing.

Antipruritic treatment with systemic μ-opioid receptor antagonists: a review

During the past two decades, systemic μ-opioid receptor antagonists (MORA) have been used in the treatment of various forms of chronic pruritus. In a number of case reports, case series, and controlled trials, treatment with MORA has demonstrated considerable antipruritic effects. In double-blind controlled studies, significant antipruritic relief has been achieved by MORA in cholestatic pruritus, chronic urticaria, and atopic dermatitis. In case reports and case series, antipruritic efficacy of MORA has been reported in prurigo nodularis, mycosis fungoides, postburn pruritus, aquagenic pruritus, hydroxyethyl starch-induced pruritus, and pruritus of unknown origin. However, most of the evidence remains anecdotal, the design of these trials varies, and comparison of results is difficult. In this review we aim to present an overview of these reports and to assess the evidence for the antipruritic action of the drugs naloxone, nalmefene, and naltrexone, which are currently in use for the treatment of chronic pruritus of different origins. We will also evaluate recommendations for the use of MORA in daily medical practice.

Effects of virtual reality immersion and audiovisual distraction techniques for patients with pruritus

BACKGROUND

Virtual reality immersion (VRI), an advanced computer-generated technique, decreased subjective reports of pain in experimental and procedural medical therapies. Furthermore, VRI significantly reduced pain-related brain activity as measured by functional magnetic resonance imaging. Resemblance between anatomical and neuroendocrine pathways of pain and pruritus may prove VRI to be a suitable adjunct for basic and clinical studies of the complex aspects of pruritus.

OBJECTIVES

To compare effects of VRI with audiovisual distraction (AVD) techniques for attenuation of pruritus in patients with atopic dermatitis and psoriasis vulgaris.

METHODS

Twenty-four patients suffering from chronic pruritus - 16 due to atopic dermatitis and eight due to psoriasis vulgaris - were randomly assigned to play an interactive computer game using a special visor or a computer screen. Pruritus intensity was self-rated before, during and 10 min after exposure using a visual analogue scale ranging from 0 to 10. The interviewer rated observed scratching on a three-point scale during each distraction program.

RESULTS

Student's t tests were significant for reduction of pruritus intensity before and during VRI and AVD (P=0.0002 and P=0.01, respectively) and were significant only between ratings before and after VRI (P=0.017). Scratching was mostly absent or mild during both programs.

CONCLUSIONS

VRI and AVD techniques demonstrated the ability to diminish itching sensations temporarily. Further studies on the immediate and late effects of interactive computer distraction techniques to interrupt itching episodes will open potential paths for future pruritus research.

Adjuvant topical therapy with a cannabinoid receptor agonist in facial postherpetic neuralgia

BACKGROUND

Postherpetic neuralgia is a frequent adverse event in herpes zoster patients and difficult to treat. Conventional analgetic therapy often fails to reduce the burning pain transmitted by unmyelinated nerve fibers. These nerves express cannabinoid receptors which exert a role in modulation of nociceptive symptoms. Therefore, topical therapy with cannabinoid receptor agonist seems likely to suppress local burning pain.

PATIENTS AND METHODS

In an open-labeled trial, 8 patients with facial postherpetic neuralgia received a cream containing the cannabinoid receptor agonist N-palmitoylethanolamine. The course of symptoms was scored with the visual analog scale.

RESULTS

5 of 8 patients (62.5 %) experienced a mean pain reduction of 87.8 %. Therapy was tolerated by all patients. No unpleasant sensations or adverse events occurred.

CONCLUSIONS

Topical cannabinoid receptor agonists are an effective and well-tolerated adjuvant therapy option in postherpetic neuralgia.

Correlating an 11-point verbal numeric rating scale to a 4-point verbal rating scale in the measurement of pruritus

Pruritus, the subjective sensation of itch, can be a difficult symptom to assess and treat. Evaluating levels of patient discomfort across populations has proven to be challenging because of a lack of a standardized measurement tool to quantify levels of itch. The most widely used tool is the 4-point verbal rating scale (VRS-4), but others use an 11-point verbal numeric rating scale (VNRS-11). Therefore, the purpose of our study was to correlate a VNRS-11 to a VRS-4 for the assessment of pruritus. Fifty parturients were enrolled in this correlational study. Each subject's level of pruritus was measured at five specific intervals after administration of intrathecal opioids. The scales were administered one minute apart in random order. Data analysis demonstrated a strong correlation between these two scales, (r=.910 to .853) indicating that each verbal descriptor on the VRS-4 could readily be substituted with a quantifiable range on the VNRS-11.

Botulinum toxin type A reduces histamine-induced itch and vasomotor responses in human skin

BACKGROUND

Clinical evidence has revealed the antipruritic effect of botulinum toxin type A (BoNT/A). BoNT/A is believed to be effective against itch as it inhibits the release of acetylcholine as well as some other substances that may be involved in itch.

OBJECTIVES

To investigate the effect of subcutaneous administration of BoNT/A on experimentally histamine-induced itch in human skin.

METHODS

In this double-blind, placebo-controlled study, 14 healthy men (mean +/- SD age 26.3 +/- 2.6 years) received BoNT/A (Botox; Allergan, Irvine, CA, U.S.A.; 5 U) and isotonic saline on the volar surface of either forearm. Histamine prick tests were performed four times at the treatment sites (before treatment, and days 1, 3 and 7 after treatment). The itch intensity (as rated on a 0-10 visual analogue scale), itch area, neurogenic inflammation (visible flare area), blood flow (laser Doppler) and cutaneous temperature (thermographic images) were measured over the course of the trials.

RESULTS

BoNT/A reduced the histamine-induced itch intensity (F(1,39) = 30.2, P < 0.001) and itch area (F(1,39) = 8.8, P = 0.011) compared with saline at all time points after treatment. The duration of itch was also shorter for BoNT/A-treated areas (F(1,39) = 19.4, P < 0.001), with a peak effect at day 7. The flare area was smaller in the BoNT/A-treated arm compared with the saline-treated arm at all time points after treatment (F(1,39) = 15.4, P = 0.002). Findings from blood flow (F(1,26) = 177.3, P < 0.001) and temperature measurements (F(1,26) = 27.6, P < 0.001) clearly showed the suppressive effect of BoNT/A on vasomotor reactions, with the maximal effect on days 3 and 7.

CONCLUSIONS

BoNT/A reduced the itch intensity, blood flow and neurogenic inflammation in response to the histamine prick test in human skin. The findings could be applicable in the treatment of some pruritic conditions that can be difficult to treat with conventional treatments.

[Opioid-induced pruritus. Mechanisms and treatment regimens]

Substantial progress has been achieved in recent years in research on the interaction between pain and pruritus. Over and above the known inhibition of pruritus by painful stimuli (e.g. scratching), a foundation for the explanation of opioid-induced pruritus was laid through the discovery of pruritus-specific neuronal processing channels. Although traditionally the degranulating effect of opioids on mast cells was assumed to be the essential mechanism, it is now clear that opioids can also induce itching at the spinal level. Neurons of the dorsal horn of the pain system inhibit spinal itch neurons. If this inhibition is weakened by opioids, the disinhibited itch neurons become active and mediate itching, without stimulation of the primary afferent peripheral nerves. Spinal triggering of itching is observed in particular by activation of mu-opioid receptors (mu-OR), while kappa-OR surprisingly suppress itch. The therapeutic implications of this interaction will be described.

Are pruritus and scratching the cough of the skin?

Pruritus is not the equivalent of the cough of the skin, but itch and scratch can certainly be defined as such. In physiological conditions, they share the same function: to exclude a foreign body. Itch/scratching and cough could be selective responses for the same diseases, mainly atopic diseases, and their pathophysiology is similar (role of C fibers and mast cells; role of histamine, substance P and tachykinins). This is an intriguing analogy rather than a pathophysiological identity. It may be inappropriate for many disease settings. Itch and cough can be triggered or enhanced by stress. This similarity is very interesting because it could give rise to many new research ideas.

Generalized and symptom-specific sensitization of chronic itch and pain

BACKGROUND

Physicians are frequently confronted with patients reporting severe itch and pain. Particularly in patients suffering from persistent itch and pain, central and peripheral sensitization processes are assumed to be involved in the long-term maintenance and aggravation of the symptoms. The present study explores generalized and symptom-specific sensitization processes in patients suffering from persistent itch and pain. Specifically, it examines whether patients with chronic itch and pain are more sensitive to somatosensory stimuli (generalized sensitization) and simultaneously perceive somatosensory stimuli as a symptom of their main physical complaint, e.g. pain in chronic pain patients (symptom-specific sensitization).

METHODS

Thresholds for different mechanical and electrical sensory stimuli of Quantitative Sensory Testing were determined in 15 female patients suffering from chronic itch associated with atopic dermatitis, 15 female chronic pain patients diagnosed with fibromyalgia, and 19 female healthy controls. Intensities of itch and pain sensations were rated on a visual analogue scale.

RESULTS

As expected, the patient groups had significantly lower tolerance thresholds for the somatosensory stimuli applied than the healthy controls, supporting generalized sensitization. Moreover, patients with chronic itch consistently reported more itch, while patients with chronic pain partly reported more pain in response to analogous somatosensory stimuli than the healthy controls and the other patient group, indicating symptom-specific sensitization.

CONCLUSION

The present study provides preliminary support that both generalized and symptom-specific sensitization processes play a role in the regulation and processing of somatosensory stimulation of patients with chronic itch and pain.

Significant differences in central imaging of histamine-induced itch between atopic dermatitis and healthy subjects

BACKGROUND/AIM

This is the first investigation of the central processing of itch in the brain in 8 subjects with atopic dermatitis (AD) in comparison to 6 healthy controls (HC), comparing histamine-induced itch related activations in the frontal, prefrontal, parietal, cingulate cortex, thalamus, basal ganglia and cerebellum.

METHODS

We employed 1% histamine-dihydrochlorid-iontophoresis of the left hand, recorded H2(15)O-PET-scans and perception of itch intensity on a numeric rating scale.

RESULTS

There was no significant difference in perceived itch intensity between AD and HC. Significant increase in rCBF was found in HC in the contralateral somatosensory and motor cortex, midcingulate gyrus, and ipsilateral prefrontal cortex; in AD: in the contralateral thalamus, somatosensory, motor and prefrontal cortex and cerebellum, in the ipsilateral precentral, prefrontal, orbitofrontal cortex, insula, pallidum and cerebellum. More brain sites were activated in AD than in HC. Activation in AD was significantly higher in the contralateral thalamus, ipsilateral caudate and pallidum.

CONCLUSIONS

We interpret our findings as possible central correlates of changes in the motor system in subjects with chronic itch, with activation of the basal ganglia possibly correlating to the vicious itch-scratch-circle in subjects with chronic itching skin diseases. However, further neuroimaging studies in healthy subjects and also in different skin diseases are needed to understand the complex mechanisms of the processing of itch.

Neurophysiological and neurochemical basis of modern pruritus treatment

Chronic pruritus of any origin is a frequent discomfort in daily medical practice, and its therapy is challenging. Frequently, the underlying origin may not be identified and symptomatic therapy is necessary. Conventional treatment modalities such as antihistamines often lack efficacy, and hence new therapeutic strategies are necessary. The neuronal mechanisms underlying chronic pruritus have been partly identified during the past years and offer new therapeutic strategies. For example, mast cell degranulation, activation of neuroreceptors on sensory nerve fibres and neurogenic inflammation have been identified to be involved in induction and chronification of the symptom. Accordingly, controlling neuroreceptors such as cannabinoid receptors by agonists or antagonists showed high antipruritic efficacy. Pruritus is transmitted to the central nervous system by specialized nerve fibres and sensory receptors. It has been demonstrated that pruritus and pain have their own neuronal pathways with broad interactions. Accordingly, classical analgesics for neuropathic pain (gabapentin, antidepressants) also exhibit antipruritic efficacy upon clinical use. In summary, these recent developments show that highlighting the basis of pruritus offers modern neurophysiological and neurochemical therapeutic models and the possibility to treat patients with refractory itching of different origin.

Itch induced by a novel method leads to limbic deactivations a functional MRI study

Functional brain imaging studies on itch usually use histamine as a stimulus and, in consequence, have to cope with the highly variable time course of this particular itch sensation. In this study, we describe a novel method of histamine application. To provoke itch, a mixture of histamine and codeine was applied through intradermally positioned microdialysis fiber. The itch was terminated by lidocaine application through the same fiber. During one fMRI session, this procedure was repeated four times in four different microdialysis fibers, including one placebo control. Itch ratings of the subjects were correlated with blood-oxygen-level-dependent (BOLD) effects. In a subsequent experiment performed in the same fMRI session, heat pain was provoked in the right forearm with a Peltier thermode. During both experiments, activation clusters were found in brain areas that have been described previously to be frequently activated in response to painful stimuli. This includes prefrontal areas, supplementary motor areas (SMA), premotor cortex, anterior insula, anterior midcingulate cortex, S1, S2, thalamus, basal ganglia, and cerebellum. In general, itch stimulation entailed more activation clusters, in particular on the contralateral brain side. Only on itch, but not on heat pain, negative BOLD signals were found in the subgenual anterior cingulate cortex and the amygdala. The latter results may be associated with the itch induced urge to scratch. Amygdala deactivation may be related to the preparation of scratching by aiming to dissolve the otherwise aversive effects of the noxious scratch stimuli. These negative BOLD effects may also be attributed to the stressful character of itch stimulation.

TRP channels as novel players in the pathogenesis and therapy of itch

Itch (pruritus) is a sensory phenomenon characterized by a (usually) negative affective component and the initiation of a special behavioral act, i.e. scratching. Older studies predominantly have interpreted itch as a type of pain. Recent neurophysiological findings, however, have provided compelling evidence that itch (although it indeed has intimate connections to pain) rather needs to be understood as a separate sensory modality. Therefore, a novel pruriceptive system has been proposed, within which itch-inducing peripheral mediators (pruritogens), itch-selective receptors (pruriceptors), sensory afferents and spinal cord neurons, and defined, itch-processing central nervous system regions display complex, layered responses to itch. In this review, we begin with a current overview on the neurophysiology of pruritus, and distinguish it from that of pain. We then focus on the functional characteristics of the large family of transient receptor potential (TRP) channels in skin-coupled sensory mechanisms, including itch and pain. In particular, we argue that - due to their expression patterns, activation mechanisms, regulatory roles, and pharmacological sensitivities - certain thermosensitive TRP channels are key players in pruritus pathogenesis. We close by proposing a novel, TRP-centered concept of pruritus pathogenesis and sketch important future experimental directions towards the therapeutic targeting of TRP channels in the clinical management of itch.

A gastrin-releasing peptide receptor mediates the itch sensation in the spinal cord

Itching, or pruritus, is defined as an unpleasant cutaneous sensation that serves as a physiological self-protective mechanism to prevent the body from being hurt by harmful external agents. Chronic itch represents a significant clinical problem resulting from renal diseases and liver diseases, as well as several serious skin diseases such as atopic dermatitis. The identity of the itch-specific mediator in the central nervous system, however, remains elusive. Here we describe that the gastrin-releasing peptide receptor (GRPR) plays an important part in mediating itch sensation in the dorsal spinal cord. We found that gastrin-releasing peptide is specifically expressed in a small subset of peptidergic dorsal root ganglion neurons, whereas expression of its receptor GRPR is restricted to lamina I of the dorsal spinal cord. GRPR mutant mice showed comparable thermal, mechanical, inflammatory and neuropathic pain responses relative to wild-type mice. In contrast, induction of scratching behaviour was significantly reduced in GRPR mutant mice in response to pruritogenic stimuli, whereas normal responses were evoked by painful stimuli. Moreover, direct spinal cerebrospinal fluid injection of a GRPR antagonist significantly inhibited scratching behaviour in three independent itch models. These data demonstrate that GRPR is required for mediating the itch sensation rather than pain, at the spinal level. Our results thus indicate that GRPR may represent the first molecule that is dedicated to mediating the itch sensation in the dorsal horn of the spinal cord, and thus may provide a central therapeutic target for antipruritic drug development.

Chronischer Pruritus

Chronic pruritus (starting from 6 weeks duration) is symptom of dermatological, internal, neurological or psychiatric disease. Identification and treatment of the underlying diseases is of great importance especially in the initial phase of chronic pruritus in order to prevent peripheral and central sensitization processes and thus chronification. Application of the redefined clinical classification, newly defined clinical algorithms and inquiry of clinical characteristics of pruritus is helpful in finding the underlying disease. In chronic pruritus existing for several years, clarifying the underlying origin is difficult and therapies are often ineffective. Next to conventional therapies such as antihistamines and corticosteroids, central effective substances can be applied preventing pruritus sensation on spinal or cerebral level.

Functional somatic syndrome: how it could be relevant to rheumatologists

Functional somatic syndrome (FSS) is defined as a group of related syndromes characterized more by symptoms, suffering, and disability than by structural or functional abnormality. The diagnostic criteria and/or symptoms of FSS often overlap, and co-morbidity is commonly found among the diseases of FSS. For example, patients with irritable bowel syndrome often suffer from chronic pain, and a high percentage of co-morbidity can be found with fibromyalgia. Accumulating evidence indicates the presence of visceral and somatic hyperalgesia in FSS as a common feature, and the central sensitization mechanism has been suggested to play an important role in the pathophysiology of FSS. In the present article, the authors introduce the concept of FSS focusing on its possible relevance to rheumatology in terms of pain perception. A possible implication of mast cells and proteinase-activated receptor-2 (PAR-2) in FSS is also reviewed.