Diagnosis and Management of Neuropathic Itch

Unmet needs in treating itch: reaching beyond eczema

PURPOSE

Pruritus is an unpleasant sensation that creates the urge to scratch. In many chronic conditions, relentless pruritus and scratching perpetuates a vicious itch-scratch cycle. Uncontrolled itch can detrimentally affect quality of life and may lead to sleep disturbance, impaired concentration, financial burden, and psychological suffering. Recent strides have been made to develop guidelines and investigate new therapies to treat some of the most common severely pruritic conditions, however, a large group of diseases remains underrecognized and undertreated. The purpose of this article is to provide a comprehensive review of the challenges hindering the treatment of pruritus.

METHODS

An online search was performed using PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1994 to 2024. Included studies were summarized and assessed for quality and relevance in treating pruritus.

RESULTS

Several barriers to treating pruritus emerged, including variable presentation, objective measurement of itch, and identifying therapeutic targets. Itch associated with autoimmune conditions, connective tissue diseases, genodermatoses, cutaneous T-cell lymphoma, and pruritus of unknown origin were among the etiologies with the greatest unmet needs.

CONCLUSION

Treating pruritus poses many challenges and there are many itchy conditions that have no yet been addressed. There is an urgent need for large-scale controlled studies to investigate potential targets for these conditions and novel therapies.

Current and emerging drugs for the treatment of pruritus: an update of the literature

INTRODUCTION

Pruritus, particularly in its chronic form, often imposes significant suffering and reductions in patients' quality of life. The pathophysiology of itch is varied depending on disease context, creating opportunities for unique drug development and multimodal therapy.

AREAS COVERED

The purpose of this article is to provide an update of the literature regarding current and emerging therapeutics in itch. We review the multitudes of drug targets available and corresponding drugs that have shown efficacy in clinical trials, with a particular emphasis on phase 2 and 3 trials and beyond. Broadly, these targets include therapies directed against type 2 inflammation (i.e. Th2 cytokines, JAK/STAT, lipid mediators, T-cell mediators, and other enzymes and receptors) and neural receptors and targets (i.e. PARs, TRP channels, opioid receptors, MRGPRs, GABA receptors, and cannabinoid receptors).

EXPERT OPINION

Therapeutics for itch are emerging at a remarkable pace, and we are entering an era with more and more specialized therapies. Increasingly, these treatments are able to relieve itch beyond their effect on inflammation by directly targeting the neurosensory system.

Aetiology, pathogenesis and management of neuropathic itch: A narrative review with recent updates

Neuropathic itch is a relatively common yet under-reported cause of systemic pruritus. It is a debilitating condition often associated with pain, which impairs the patient's quality of life. Although much literature exists about renal and hepatic pruritus, there is a dearth of information and awareness about neuropathic itch. The pathogenesis of neuropathic itch is complex and can result from an insult at any point along the itch pathway, ranging from the peripheral receptors and nerves until the brain. There are several causes of neuropathic itch, many of which do not produce any skin lesions and are thus, often missed. A detailed history and clinical examination are necessary for the diagnosis, while laboratory and radiologic investigations may be needed in select cases. Several therapeutic strategies currently exist involving both non-pharmacological and pharmacological measures, the latter including topical, systemic, and invasive options. Further research is ongoing to clarify its pathogenesis and to design newer targeted therapies with minimal adverse effects. This narrative review highlights the current understanding of this condition, focusing on its causes, pathogenesis, diagnosis, and management, along with newer investigational drugs.

Understanding Neural Factors in Burn-related Pruritus and Neuropathic Pain

Post-burn pruritus and neuropathic pain significantly affect the quality of life of affected individuals in several domains including psychosocial well-being, sleep and general impairment in activities of daily living. Whilst neural mediators involved in itch in the non-burns setting have been well investigated, there remains a lacuna of literature examining the pathophysiological and histological changes unique to burn-related pruritus and neuropathic pain. The aim of our study was to conduct a scoping review into the neural factors that contribute to burn-related pruritus and neuropathic pain. A scoping review was conducted to provide an overview of the available evidence. The PubMed, EMBASE and Medline databases were searched for publications. Data regarding neural mediators implicated, population demographics, total body surface area (TBSA) affected and sex was extracted. In total, 11 studies were included in this review with a total of 881 patients. The most frequently investigated neurotransmitter was the Substance P (SP) neuropeptide which appeared in 36% of studies (n = 4), followed by calcitonin gene-related peptide (CGRP) in 27% of studies (n = 3). Post-burn pruritus and neuropathic pain are symptomatic experiences that are predicated upon a heterogeneous group of underlying mechanisms. What is clear from the literature, however, is that itch and pain may occur secondary to the influence of both neuropeptides, such as SP, and other neural mediators including Transient receptor protein channels. Of the articles included for review, they were characterized by small sample sizes and large differences in statistical methodology and reporting.

Chronic eyelid and ocular itch

Chronic eyelid and ocular itch affect many patients seeking dermatologic or ophthalmologic care and have a high burden on patient quality of life. Clinicians should consider the broad range of possible diagnoses when approaching the patient with itch of the eyes or eyelids lasting more than 6 weeks. Allergic conjunctivitis and allergic contact dermatitis are the most common causes of chronic itch of the eyes and eyelids, respectively. Other diagnoses to consider include atopic dermatitis, xerosis, neurogenic itch, dry eye syndrome, seborrheic dermatitis, blepharitis, rosacea, lichen simplex chronicus, and papulosquamous disorders. If no organic cause can be elucidated, diagnoses of psychogenic pruritus or chronic pruritus of unknown origin may be considered. Herein, we discuss the possible etiologies of chronic eyelid and ocular itch inclusive of clinical presentation, diagnostic considerations, and current therapies.

Clinical aspects and management of chronic itch

Chronic pruritus (CP) (ie, itch that persists for more than 6 weeks) poses significant challenges to patients' health and quality of life. It is a common reason for visits to dermatologists and general practitioners and can be caused by a range of conditions, including systemic diseases such as chronic kidney disease or liver diseases, malignancies, neuropathic conditions, and dermatoses such as atopic dermatitis. CP often does not develop in parallel with the course of the disease and can become an entity of its own, which must be treated with antipruritic drugs, even if the underlying cause is already under therapy. Depending on the etiology of CP, different pathways in the pathogenesis have been analyzed recently, following which new treatments have been developed and tested in randomized controlled trials. This article discusses the recent results of these studies and highlights how best to manage health care for patients with CP.

Mechanisms and treatments of neuropathic itch in a mouse model of lymphoma

Our understanding of neuropathic itch is limited due to a lack of relevant animal models. Patients with cutaneous T cell lymphoma (CTCL) experience severe itching. Here, we characterize a mouse model of chronic itch with remarkable lymphoma growth, immune cell accumulation, and persistent pruritus. Intradermal CTCL inoculation produced time-dependent changes in nerve innervations in lymphoma-bearing skin. In the early phase (20 days), CTCL caused hyperinnervations in the epidermis. However, chronic itch was associated with loss of epidermal nerve fibers in the late phases (40 and 60 days). CTCL was also characterized by marked nerve innervations in mouse lymphoma. Blockade of C-fibers reduced pruritus at early and late phases, whereas blockade of A-fibers only suppressed late-phase itch. Intrathecal (i.t.) gabapentin injection reduced late-phase, but not early-phase, pruritus. IL-31 was upregulated in mouse lymphoma, whereas its receptor Il31ra was persistently upregulated in Trpv1-expressing sensory neurons in mice with CTCL. Intratumoral anti-IL-31 treatment effectively suppressed CTCL-induced scratching and alloknesis (mechanical itch). Finally, i.t. administration of a TLR4 antagonist attenuated pruritus in early and late phases and in both sexes. Collectively, we have established a mouse model of neuropathic and cancer itch with relevance to human disease. Our findings also suggest distinct mechanisms underlying acute, chronic, and neuropathic itch.

Chronic pruritus in multiple sclerosis and clinical correlates

BACKGROUND

To date, little is known about the prevalence of itch in multiple sclerosis (MS) and its characteristics.

OBJECTIVES

In this cross-sectional study, we assessed the prevalence, intensity and characteristics of chronic pruritus in MS patients and its effect on quality-of-life and association with MS symptoms, clinical signs, comorbidities and MRI findings.

METHODS

MS patients presenting to an outpatient neurology clinic were asked about their current symptoms. Those who experienced chronic pruritus were administered the Standardized Itch Questionnaire and Itch Quality of Life forms. All patients' medical records were reviewed. Patients with any medical conditions associated with chronic itch were excluded.

RESULTS

Seventy-seven total MS patients were included, and 27 (35%) reported pruritus. The average itch NRS severity was 5.42 (range 0-10). The most affected body parts were the extremities, face or scalp, and trunk. Itch was characterized as acute (74%), paroxysmal (59%) and tingling (55%). Heat (52%) was the most common aggravating factor, while cold temperatures had no effect. Compared with MS patients without itch, itch patients reported more fatigue (77% vs 44%, p = 0.004), heat sensitivity (48% vs 20%, p = 0.0177), cognitive impairment (62% vs 26%, p = 0.0029) and depression or anxiety (48% vs 16%, p = 0.0063). Additionally, itch patients had more T2 hyperintensities in the posterior cervical cord and anterior pons/ventromedial medulla (74.1% vs 46.0%, p = 0.018 and 29.6% vs 8.0%, p = 0.020, respectively). Finally, T2 hyperintensities in the anterior pons/ventromedial medulla were strongly associated with itch localized to the face or scalp (OR 11.3, 95% CI 1.6-78.6, p = 0.025).

CONCLUSION

MS patients experience paroxysmal neuropathic pruritus that is most frequently localized to the extremities, face or scalp. Patients with itch were more likely to have MS-related comorbidities and demyelinating lesions in the spinal cord or brainstem.

Autoimmune Connective Tissue Diseases-Related Pruritus: Proper Diagnosis and Possible Mechanisms

Pruritus is a well-known bothersome symptom among skin disorders, especially inflammatory skin disorders. Lately, a high prevalence of pruritus in patients with autoimmune connective tissue diseases (ACTDs) has been revealed. Patients with ACTDs may suffer from varying degrees of pruritus, which affect their quality of life. However, it is rarely recognized both by patients and physicians. Meanwhile, pruritus is not only a symptom but is also related to the disease severity of some ACTDs. The pathophysiology of ACTD related pruritus is ambiguous. This review summarizes the features and possible mechanisms of ACTD-related pruritus, which might lead to proper diagnosis and treatment.

Scalp dysesthesia: a neuropathic phenomenon

Scalp dysesthesia is an abnormal sensation of the scalp in the absence of cutaneous disease. It is characterized by a burning and/or itching sensation and can be related to a variety of neurogenic or psychogenic causes. This condition is extremely bothersome and is also common- especially amongst the geriatric population, in women, in patients with diabetes mellitus and patients with psychiatric history. However, despite its prevalence in many populations, there is limited data about its causes and characteristics. Given its limited cutaneous manifestations it is also easily misdiagnosed and an underrecognized cause of scalp pruritus in the dermatological community. Therefore, education on scalp dysesthesia is paramount to helping physicians identify and provide appropriate treatment for these patients. This review focuses predominately on the neurogenic causes (with a brief review of psychogenic itch) of scalp dysesthesia and the therapeutics that have been found to be effective for this condition. Neurogenic causes of scalp dysesthesia occur with damage to the central or peripheral pathways of itch sensation, resulting in modification and heightened sensitivity of nerves that result in abnormal sensations in the absence of or out of proportion to external stimuli. A comprehensive review of etiologies is provided here, ranging from lesions to the central nervous system caused by cervical spine disease, trigeminal trophic syndrome, tumor, stroke, and multiple sclerosis, to small fiber neuropathies caused by diabetes, brow lifts, keloid and burn scarring. Recently, there have also been reports of scalp dysesthesias associated with post-infectious COVID-19. Treatment options tailored towards disease severity and different causes of disease will also be discussed. By elucidating the different mechanisms and therapeutic treatments of scalp dysesthesia, we hope to provide clinicians with the tools to identify and treat this condition as well as encourage further research into its etiologies and therapeutics.

Local Anesthetics Inhibit Transient Receptor Potential Vanilloid Subtype 3 Channel Function in Xenopus Oocytes

BACKGROUND

The transient receptor potential vanilloid subtype 3 (TRPV3) channel is activated by innocuous temperature and several chemical stimuli. It is proposed to be involved in pathological pain development and is therefore considered a potential target for treating pain. Local anesthetics have been used for patients with both acute and chronic pain. Although blockage of the voltage-gated sodium channel is the primary mechanism by which local anesthetics exert their effects, they cannot be explained by this mechanism alone, especially in pathologic states such as chronic pain. Indeed, the effects of local anesthetics on multiple targets involved in the pain pathway have been reported. It has also been suggested that modulating the function of transient receptor potential (TRP) channels (eg, TRPV1 and transient receptor potential ankyrin 1 [TRPA1]) is one of the mechanisms of action of local anesthetics. However, the effects of local anesthetics on TRPV3 have not been reported.

METHODS

We expressed TRPV3 in Xenopus oocytes and investigated the effects of local anesthetics on 2-aminoethoxydiphenyl borate (2APB)-induced currents using 2-electrode voltage-clamp techniques.

RESULTS

Clinically used local anesthetics inhibited the 2APB-activated currents from the TRPV3 channel in a concentration-dependent manner at pharmacologically relevant concentrations with half maximal inhibitory concentration (IC50) values of 2.5 (lidocaine), 1.4 (mepivacaine), 0.28 (ropivacaine), and 0.17 (bupivacaine) mmol/L, respectively. Conversely, these local anesthetics also directly induced currents at higher concentrations, although these currents were quite small compared to the 2APB-induced currents. We found that the inhibition of TRPV3 by lidocaine is noncompetitive and independent of intracellular signaling cascades. 2APB-induced TRPV3 currents were reduced by extracellular N-(2,6-dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX-314) but not by intracellular QX-314 nor benzocaine. Moreover, lidocaine showed a use-dependent block in TRPV3 inhibition. Finally, QX-314 appeared to slightly permeate the activated TRPV3 channel pore based on examination of oocytes coexpressing TRPV3 and a sodium channel. These results suggest that local anesthetics could inhibit TRPV3 channel function by extracellular interactions of their charged forms with the channel pore.

CONCLUSIONS

Local anesthetics inhibited TRPV3 2APB-induced currents at pharmacologically relevant concentrations when TRPV3 was expressed in Xenopus oocytes. These effects seem to occur via an extracellular interaction between the charged form of the anesthetic with the TRPV3 channel pore. These results help to elucidate the mechanisms of action of local anesthetics.

Approach to the Patient with Chronic Pruritus

Chronic pruritus (itch lasting ≥6 weeks) is a bothersome chief complaint that may present in a broad variety of diseases. Most itch-causing diagnoses fit into 1 of 5 categories (inflammatory, secondary to systemic disease, neuropathic, chronic pruritus of undetermined origin, and psychogenic itch) and this broad differential can be narrowed using key findings in the history and physical. In this article, we discuss which key findings are most pertinent for narrowing this differential and guiding further workup and treatment, as well as how to treat many itchy conditions.

The Role of Gut Microbiota in Chronic Itch-Evoked Novel Object Recognition-Related Cognitive Dysfunction in Mice

The high incidence of patients with chronic itch highlights the importance of fundamental research. Recent advances in the interface of gut microbiota have shed new light into exploring this phenomenon. However, it is unknown whether gut microbiota plays a role in chronic itch in rodents with or without cognitive dysfunction. In this study, the role of gut microbiota in diphenylcyclopropenone (DCP)-evoked chronic itch was investigated in mice and hierarchical cluster analysis of novel object recognition test (ORT) results were used to classify DCP-evoked itch model in mice with or without cognitive dysfunction (CD)-like phenotype and 16S ribosomal RNA (rRNA) gene sequencing was used to compare gut bacterial composition between CD (Susceptible) and Non-CD phenotypes (Unsusceptible) in chronic itch mice. Results showed that the microbiota composition was significantly altered by DCP-evoked chronic itch and chronic itch induced novel object recognition-related CD. However, abnormal gut microbiota composition induced by chronic itch may not be correlated with novel object recognition-related CD.

Genodermatoses with itch as a prominent feature

A number of inherited conditions cause chronic itch as a part of the recognized phenotype. Advances in the understanding of the genetic factors that cause these diseases elucidate the molecular underpinning of itch as a symptom. Our knowledge of the causes of chronic itch has also advanced, providing an opportunity to integrate the genetic pathophysiology with the molecular landscape of chronic itch mediators. This article reviews select genodermatoses that have itch as a predominant feature with a focus on the pathophysiology of the disease, how it may lead to itch and potential therapeutic targets.

Current and emerging systemic treatments targeting the neural system for chronic pruritus

INTRODUCTION

Pruritus is a debilitating symptom that significantly affects the quality of life of patients who suffer from it. Many current and emerging systemic treatments targeting the neural system have been successful in treating itch of various underlying etiologies.

AREAS COVERED

A complete search of the PUBMED and Google Scholar databases was completed and literature pertinent to current and emerging systemic anti-pruritic drugs which target the neural system was compiled. The purpose of this review is to give the reader with an overview of the current and emerging systemic therapeutic options which target the neural system for chronic pruritus. The authors then provide the reader with their expert perspectives on the future of these therapies.

EXPERT OPINION

Exciting new anti-pruritic therapies targeting the neural system which show promise include NK-1 inhibitors, opioid receptor modulators, and drugs targeting specific itch receptors such as Mrgpr, Na_v1.7, and PAR2, as well as selective GABA modulators. Future studies should be conducted in order to fully understand these exciting therapeutic options.

Post-Burn Pruritus

Post-burn pruritus is the pruritus that occurs after burn during the rehabilitation and healing process of burn wounds. The post-burn pruritus is a common and serious complication of burn injury, which severely lowers the quality of life of the patient. Many potential treatments are available for pruritus but there is no consensus of the best single treatment yet. The precise mechanism of post-burn pruritus has not been elucidated, but it appears to have pruritogenic and neuropathic aspects. Clinically, post-burn pruritus tends to be intractable to conventional treatment but rather responds to neuroleptic agents, such as gabapentin and pregabalin. During wound healing, various neuropeptides secreted from the nerves of the skin control epidermal and vascular proliferation and connective tissue cells. When keratinocytes are activated by an itch-inducing substance, they secrete a variety of inflammatory substances that increase the susceptibility of the itch receptor. There are two mechanisms underlying post-burn neuropathic pruritus. The first one is peripheral sensitization. The second one is the intact nociceptor hypothesis. An effective treatment for post-burn pruritus will also be effective in other neuropathic and intractable itching. In this review, we summarized the interaction and mechanism of keratinocytes, immune cells, and nerve fibers related to post-burn pruritus.

Safety considerations when using drugs to treat pruritus

Introduction: Treatment for chronic pruritus ranges from use of topical formulations to newer biologic agents. Targeting treatment to the underlying etiology is key in reducing the burden of disease while avoiding systemic or adverse effects.Areas covered: This review details the effective medical treatments used in various etiologies of chronic itch with a focus on the potential adverse effects and safety data available for each.Expert opinion: New drug developments in the areas of neural signaling and immune targeting show great promise for the future of chronic itch treatment. These new therapies broaden the available treatment options but also pose new considerations for safety and adverse effects.

Management of Itch in the Elderly: A Review

Chronic itch is common in the elderly patient and may be caused by a variety of known dermatologic and non-dermatologic conditions and can have a significant effect on quality of life. Age-related changes in barrier function, immunosenescence, and neuronal changes and neuropathies are common predisposing factors to chronic itch in this age group. Certain primary dermatologic conditions are more common in the elderly and can cause chronic itch. Also, co-morbid diseases particularly of the renal, hepatobiliary, or hematologic systems, psychologic conditions, or medications may contribute to chronic itch in this population. Thus, medical workup for an elderly patient with chronic itch requires special attention to the patient's medical history, current health status, and medications. Topical treatments and emollients may be recommended for elderly patients, with consideration of specific adverse effects and alternatives. Systemic medications pose a higher risk of adverse effects and many are contraindicated in the elderly for this reason. In addition, management in the elderly may be complicated by differential pharmacokinetics of medications, the presence of co-morbid health conditions, cognitive disorders, physical limitations, and polypharmacy. New and emerging treatment modalities hold promise for use in the elderly due to these special considerations.

Itch: From mechanism to (novel) therapeutic approaches

Itch is a common sensory experience that is prevalent in patients with inflammatory skin diseases, as well as in those with systemic and neuropathic conditions. In patients with these conditions, itch is often severe and significantly affects quality of life. Itch is encoded by 2 major neuronal pathways: histaminergic (in acute itch) and nonhistaminergic (in chronic itch). In the majority of cases, crosstalk existing between keratinocytes, the immune system, and nonhistaminergic sensory nerves is responsible for the pathophysiology of chronic itch. This review provides an overview of the current understanding of the molecular, neural, and immune mechanisms of itch: beginning in the skin, proceeding to the spinal cord, and eventually ascending to the brain, where itch is processed. A growing understanding of the mechanisms of chronic itch is expanding, as is our pipeline of more targeted topical and systemic therapies. Our therapeutic armamentarium for treating chronic itch has expanded in the last 5 years, with developments of topical and systemic treatments targeting the neural and immune systems.

Chronic itch management: therapies beyond those targeting the immune system

OBJECTIVE

Chronic itch can be a debilitating, unrelenting symptom. Over the years, we have advanced our knowledge about immune-mediated itch (eg, atopic dermatitis) and have developed several treatments targeting these immune pathways. Chronic itch that is noninflammatory in nature is more elusive. However, we have gained some understanding of the neural component mediating itch and have made progress in treating this challenging symptom.

DATA SOURCES

A comprehensive literature search was conducted, and data and literature were reviewed on the topics of chronic itch, its pathophysiology, and nonimmunological treatments for chronic itch that work on the nerves.

STUDY SELECTIONS

Basic science papers, review articles, case reports, and clinical trial data were considered.

RESULTS

A variety of topical and systemic therapies targeting the nerves have shown effectiveness in treating patients with chronic itch of different types.

CONCLUSION

Treating chronic itch may be challenging, but an arsenal of treatments are available and many are on the horizon as we expand our knowledge of mechanisms of itch and the neural receptors involved.