Itch and pain

Breaking the Itch-Scratch Cycle: Topical Options for the Management of Chronic Cutaneous Itch in Atopic Dermatitis

Chronic itch is an unpleasant sensation that triggers a desire to scratch that lasts for six weeks or more. It is a major diagnostic symptom of myriad diseases, including atopic dermatitis for which it is the most prominent feature. Chronic itch can be hugely debilitating for the sufferer, damaging in terms of both the monetary cost of treatment and its socioeconomic effects, and few treatment options exist that can adequately control it. Corticosteroids remain the first line treatment strategy for atopic dermatitis, but due to the risks associated with long-term use of corticosteroids, and the drawbacks of other topical options such as topical calcineurin inhibitors and capsaicin, topical options for itch management that are efficacious and can be used indefinitely are needed. In this review, we detail the pathophysiology of chronic pruritus, its key features, and the disease most commonly associated with it. We also assess the role of the skin and its components in maintaining a healthy barrier function, thus reducing dryness and the itch sensation. Lastly, we briefly detail examples of topical options for the management of chronic pruritus that can be used indefinitely, overcoming the risk associated with long-term use of corticosteroids.

The Impact of Pain and Itch on Functioning and Health-Related Quality of Life in Systemic Sclerosis: An Exploratory Study

CONTEXT

Pain and itch are common symptoms reported by patients with systemic sclerosis (SSc; scleroderma), which can markedly diminish function and health-related quality of life (HRQL).

OBJECTIVES

The aim of this exploratory study was to examine the impact that pain, itch, and the interaction of both have on function (depressive symptoms, overall disability, fatigue, sleep disturbance) and HRQL in patients with SSc.

METHODS

A total of 964 patients from the Canadian Scleroderma Reserch Group Registry completed questionnaires measuring itch and pain severity, function, and HRQL. Multiple regression analyses were performed to examine the impact that pain, itch, and pain × itch interaction have on each outcome variable while controlling for demographic measures. A P-value of ≤0.01 was required for a difference to be deemed statistically significant.

RESULTS

Our results revealed that patients with SSc who reported higher pain and itch severity were also more likely to have greater depressive symptoms, overall disability, sleep and fatigue problems, even when demographic measures were controlled for (P-values ≤0.001). Similar results were obtained for HRQL, regardless of the domains (P-values ≤0.001). A significant association between pain × itch interaction and sleep (P = 0.002), physical functioning (P = 0.003), and general health (P ≤ 0.001) variables also was found. Further investigation of the nature of the pain × itch interaction showed that the effect of pain severity on outcome variables diminishes as itch severity increases.

CONCLUSION

Both pain and itch appear to have a detrimental impact on functioning and HRQL in patients with SSc, suggesting that more targeted approaches to symptom management are warranted.

Voltage-gated potassium channels involved in regulation of physiological function in MrgprA3-specific itch neurons

Itch is described as an unpleasant or irritating skin sensation that elicits the desire or reflex to scratch. MrgprA3, one of members of the Mrgprs family, is specifically expressed in a subpopulation of dorsal root ganglion (DRG) in the peripheral nervous system (PNS). These MrgprA3-expressing DRG neurons have been identified as itch-specific neurons. They can be activated by the compound, chloroquine, which is used as a drug to treat malaria. In the present study, we labeled these itch-specific neurons using the method of molecular genetic markers, and then studied their electrophysiological properties. We also recorded the cutaneous MrgprA3(-) neurons retrogradely labeled by Dil dye (MrgprA3(-)-Dil). We first found that MrgprA3(+) neurons have a lower excitability than MrgprA3(-) neurons (MrgprA3(-)-non-Dil and MrgprA3(-)-Dil). The number of action potential (AP) was reduced more obviously in MrgprA3(+) neurons than that of in MrgprA3(-) neurons. In most cases, MrgprA3(+) neurons only generated single AP; however, in MrgprA3(-) neurons, the same stimulation could induce multiple AP firing due to the greater voltage-gated potassium (Kv) current existence in MrgprA3(+) than in MrgprA3(-) neurons. Thus, Kv current plays an important role in the regulation of excitability in itch-specific neurons.

Pharmacokinetics of nalbuphine hydrochloride extended release tablets in hemodialysis patients with exploratory effect on pruritus

BACKGROUND

Uremic pruritus is a common and deleterious condition among hemodialysis (HD) patients. Central gating of μ/κ opiate circuitry plays an important role in mediating and countering pruritogenic sensation. The objective of this study was to assess the safety and pharmacokinetics (PK) of the mixed μ-antagonist/κ-agonist nalbuphine, administered orally as nalbuphine HCl extended release (ER) tablets in HD patients, and explore its effect on pruritus.

METHODS

In this open-label multiple escalating dose study, 15 HD patients with pruritus and 9 matched healthy subjects were enrolled. Nalbuphine HCl ER dose was escalated from 30 mg QD to 240 mg BID over 15 days. A full PK profile was obtained under dialysis and non-dialysis conditions as a function of dose. Clearance during dialysis was determined by sampling dialysate and arterial/venous blood during dialysis. Pruritus severity was assessed twice daily using a Visual Analog Scale (VAS). Safety monitoring included extensive monitoring of EKG, blood pressure, and pulse oximetry.

RESULTS

In HD patients, nalbuphine concentration peaked within 4-9 hours and attained steady state within 2-3 days, with no significant accumulation. Mean half-life was 14.2 hours, mean Cmax and AUCtau ranged between 13 and 83 ng/mL and 118 and 761 ng∙h/mL, respectively, with exposure increasing in a nearly dose-proportional fashion. Exposure in HD patients was about 2-fold higher than in healthy subjects. There was no meaningful difference between exposure on dialysis and non-dialysis days with 1% or less of the dose removed by dialysis. Nalbuphine suppressed itch in a dose-dependent manner, reducing mean VAS score from 4.0 to 1.2 at 180 mg and 0.4 at 240 mg.

CONCLUSIONS

Nalbuphine HCl ER tablets can be safely administered to HD patients without dose adjustment up to 240 mg BID and may hold promise in treating uremic pruritus.

How to Psychologically Minimize Scratching Impulses

Research in recent years, especially in the analgesic field, has intensively studied the placebo effect and its mechanisms. It has been shown that physical complaints can be efficiently reduced via learning and cognitive processes (conditioning and expectancies). However, despite evidence demonstrating a large variety of physiological similarities between pain and itch, the possible transfer of the analgesic placebo model to itch has not yet been widely discussed in research. This review therefore aims at highlighting potential transfers of placebo mechanisms to itch processes by demonstrating the therapeutic issues in pharmacological treatments for pruritus on a physiological basis and by discussing the impact of psychological mechanisms and psychological factors influencing itch sensations.

"It stings a bit but it cleans well": venoms of Hymenoptera and their antimicrobial potential

Venoms from Hymenoptera display a wide range of functions and biological roles. These notably include manipulation of the host, capture of prey and defense against competitors and predators thanks to endocrine and immune systems disruptors, neurotoxic, cytolytic and pain-inducing venom components. Recent works indicate that many hymenopteran species, whatever their life style, have also evolved a venom with properties which enable it to regulate microbial infections, both in stinging and stung animals. In contrast to biting insects and their salivary glands, stinging Hymenoptera seem to constitute an under-exploited ecological niche for agents of vector-borne disease. Few parasitic or mutualistic microorganisms have been reported to be hosted by venom-producing organs or to be transmitted to stung animals. This may result from the presence of potent antimicrobial molecules in venoms, histological features of venom apparatuses and selective effects of venoms on immune defenses of targeted organisms. The present paper reviews for the first time the venom antimicrobial potential of solitary and social Hymenoptera in molecular, ecological, and evolutionary perspectives.

Itch pathophysiology may differ among ethnic groups

The prevalence of itch is higher in individuals with darker skin types. In this paper, we review the systems involved in the physiology of itch and how they may differ across the races. Current data point out that the differences may be explained by barrier function, mast cell physiology, and itch receptor polymorphisms.

Gabapentin for Pruritus in Palliative care

Itch/pruritus can be very distressing in palliative care population and often is difficult to treat. Conventional antihistamines lack efficacy. Cutaneous and central pathogenesis of itch is extremely complex and unclear, making its treatment challenging. Neuronal mechanisms have been identified in the pathophysiology of itch hence providing a myriad of therapeutic options. It has been established that pruritus and pain neuronal pathway interact with each other, hence neuropathic analgesics like gabapentin has shown to be efficacious antipruritic therapeutic option. Gabapentin impedes transmitting nociceptive sensations to brain, thus also suppressing pruritus. Gabapentin is safe and found to be effective in uremic pruritus, cancer/hematologic causes, opiod-induced itch, brachioradial pruritis, burns pruritus, and pruritus of unknown origin. Further research is required in this area to establish whether gabapentin is consistently effective.

Uraemic pruritus: clinical characteristics, pathophysiology and treatment

Pruritus is a common complication of end-stage renal disease (ESRD), affecting about one-third of dialysis patients. It is a chronic, unpleasant symptom with a strong negative impact on patients' quality of life, often inducing sleeplessness and mood disorders. Recent data show that it is also associated with increased mortality. The pathogenesis of uraemic pruritus (UP) is multifactorial. Triggering factors may include uraemia-related abnormalities (particularly involving calcium, phosphorus and parathyroid hormone metabolism), accumulation of uraemic toxins, systemic inflammation, cutaneous xerosis, and common co-morbidities such as diabetes mellitus and viral hepatitis. Recent findings suggest that the neurophysiology of itch is similar to that of pain; this has led to the hypothesis that the two phenomena also closely interact in ESRD patients, who often also experience uraemic neuropathy. The management of UP needs to address several different issues, such as optimization of dialysis efficacy and skin hydration, and correction of calcium-phosphorus metabolism abnormalities. A wide range of antipruritic drugs have been suggested for the treatment of UP, although most of them have only been tested in small, uncontrolled trials, which have yielded conflicting results. Antihistamines are now known to have little or no efficacy, although they are still often prescribed. Novel neurotropic drugs such as gabapentin, along with opioid receptor modulators such as nalfurafine, appear to be effective and well tolerated, but their efficacy has not yet been directly compared. Finally, physical therapies, including UV radiation, may also have a role in patients with refractory symptoms.

Pathogenesis and treatment of pruritus in cholestasis

Pruritus is an enigmatic, seriously disabling symptom accompanying cholestatic liver diseases and a broad range of other disorders. Most recently, novel itch-specific neuronal pathways, itch mediators and their relevant receptors have been identified. In addition, new antipruritic therapeutic strategies have been developed and/or are under evaluation. This review highlights recent experimental and clinical findings focusing on the pathogenesis and actual treatment of pruritus in cholestatic liver disease. Evidence-based therapeutic recommendations, including the use of anion exchange resins cholestyramine, colestipol and colesevelam, the microsomal enzyme inducer rifampicin, the opioid receptor antagonists naltrexone and naloxone, and the serotonin reuptake inhibitor sertraline, are provided.

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.

Repetitive scratching and noxious heat do not inhibit histamine-induced itch in atopic dermatitis

BACKGROUND

Repetitive scratching is the most common behavioural response to itch in atopic dermatitis (AD). Patients with chronic itch often report that very hot showers inhibit itch. We recently reported that scratching and noxious heat stimuli inhibit histamine-induced itch in healthy subjects. However, no psychophysical studies have been performed in AD to assess the effects of repetitive heat pain stimuli and scratching on histamine-induced itch.

OBJECTIVES

To examine the effects of repetitive noxious heat and scratching on itch intensity in patients with AD using quantitative sensory testing devices.

METHODS

Itch was induced with histamine iontophoresis in 16 patients with AD in both lesional and nonlesional skin as well as in 10 healthy subjects. Repetitive noxious heat and scratching were applied 3 cm distal to the area of histamine iontophoresis. Subjects rated their perceived intensity of histamine-induced itch with a computerized visual analogue scale.

RESULTS

Our results demonstrate that repetitive noxious heat and scratching do not inhibit itch intensity in lesional and nonlesional AD skin but do so in healthy skin. Of note, both these stimuli increase itch intensity in lesional AD skin.

CONCLUSIONS

Our results strongly suggest that scratching and noxious thermal stimuli have a different effect upon histamine-induced itch perception in patients with AD when compared with healthy controls. This difference may be associated with both peripheral and central sensitization of nerve fibres in AD.

A transient unresponsive state of self-scratching behaviour is induced in mice by skin-scratching stimulation

When mice were scratched with brushes on their dorsal skins, they began to scratch themselves with their hind paws. Thus, self-scratching behaviour was induced in mice in response to skin-scratching stimulation. If the second skin-scratching stimulation was given within a few days, the induction of the second self-scratching behaviour was significantly suppressed compared with the first one. Thereafter, mice gradually recovered from this unresponsive state within a week. Thus, a transient unresponsive state of self-scratching behaviour is induced by skin-scratching stimulation. Pretreatment with a tachykinin receptor NK-1R antagonist L-703606 or capsaicin significantly suppressed self-scratching behaviour, while pretreatment with a neutral endopeptidase inhibitor phosphoramidon significantly enhanced it. Pretreatment with a calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37) did not affect the following self-scratching behaviour. From these results, it is suggested that substance P (SP) signalling through its receptor NK-1R at least in part mediates the induction of self-scratching behaviour. After skin-scratching stimulation, immunoreactivity of SP both in the peripheral nerve fibres and in the dorsal root ganglion (DRG) neurons was significantly decreased and was well-correlated with suppression of self-scratching behaviour. From these findings, it is suggested that the induction of unresponsive states of self-scratching behaviour may be at least in part caused by the depleted states of SP in peripheral nerve fibres and/or in DRG neurons. The induction of a transient unresponsive state after skin-scratching may possibly happen also in patients with pruritus. Thus, further studies to elucidate the precise mechanisms are required.

Deletion of μ- and κ-Opioid Receptors in Mice Changes Epidermal Hypertrophy, Density of Peripheral Nerve Endings, and Itch Behavior

The mu- (MOR) and kappa- (KOR) opioid receptors have been implicated in the regulation of homeostasis of non-neuronal cells, such as keratinocytes, and sensations like pain and chronic pruritus. Therefore, we have studied the phenotype of skin after deletion of MOR and KOR. In addition, we applied a dry skin model in these knockout mice and compared the different mice before and after induction of the dermatitis in terms of epidermal thickness, epidermal peripheral nerve ending distribution, dermal inflammatory infiltrate (mast cells, CD4 positive lymphocytes), and scratching behavior. MOR knockout mice reveal as phenotype a significantly thinner epidermis and a higher density of epidermal fiber staining by protein gene product 9.5 than the wild-type counterparts. Epidermal hypertrophy, induced by the dry skin dermatitis, was significantly less developed in MOR knockout than in wild-type mice. Neither mast cells nor CD4 T(h)-lymphocytes are involved in the changes of epidermal nerve endings and epidermal homeostasis. Finally, behavior experiments revealed that MOR and KOR knockout mice scratch less after induction of dry skin dermatitis than wild-type mice. These results indicate that MOR and KOR are important in skin homeostasis, epidermal nerve fiber regulation, and pathophysiology of itching.

Are Itch and Chronic Pain Associated in Adults? Results of a Large Population Survey in Norway

BACKGROUND

Few large-scale epidemiological studies have been performed on the prevalence of itch. Itch and pain are common and complex symptoms which contribute to the burden of disease. Although there is antagonistic interaction between itch and pain, there are also many similarities in their pathophysiology.

OBJECTIVE

To investigate possible associations between chronic itch and chronic pain in a large population.

METHODS

The design was cross-sectional. 18,770 adults completed a self-administered questionnaire addressing sociodemographic factors, psychosocial factors and self-reported health including chronic itch and chronic pain.

RESULTS

Individuals reporting pain and itch were more likely to be women (80 and 60%, respectively, compared to 55% in the total sample), had a lower income (49 and 37% compared to 32%), were more likely to be depressed (36 and 20% compared to 11%) and reported poorer well-being (74 and 34% compared to 25%). In an adjusted logistic regression, chronic pain was strongly associated with chronic itch (OR = 1.79, 95% CI = 1.43-2.24).

CONCLUSION

This study demonstrates the association of chronic itch and chronic pain and points out the need of further studies that focus on both symptoms in dermatological diseases.