Clinical aspects and management of chronic itch

P2 purinergic signaling and pruritus

Pruritus is a common sensation that triggers scratching. Extracellular nucleotides and nucleosides, along with their receptors, primarily compose the purinergic signaling. The purinergic signaling mechanism in itch remains incompletely understood. Keratinocytes, fibroblasts, Langerhans cells, primary sensory nerve endings in the skin, and neurons and satellite glial cells in primary sensory ganglia (dorsal root ganglia and trigeminal ganglia) have been confirmed to express multiple subtypes of P2X and P2Y receptors. Purinergic signaling in the skin and primary sensory ganglia is involved in the pathological changes of skin pruritus, including atopic dermatitis, psoriasis, systemic sclerosis, diabetes complicated with pruritus, or other pruritus disorders. The interaction between P2 purinergic signaling and histamine receptors, transient receptor potential (TRP) channel receptors, and Mas-related G protein-coupled receptor member A3 (MrgprA3) receptors, which mediate itch signaling, is involved in the pathological process of skin pruritus. P2 purinergic receptor agonists can induce itching behaviors in animals. Targeted antagonism or inhibition of P2 purinergic receptors in the skin and primary sensory ganglia can alleviate pathological changes in skin pruritus. This review summarizes studies concluding that P2 receptors are involved in the pathogenesis of pruritus, with several showing potential as novel therapeutic options for alleviating pruritus.

Patient Needs and Treatment Goals in Chronic Atopic Pruritus: Does Eczema Make a Difference?

Chronic pruritus (≥ 6 weeks) is a frequent symptom in atopic diseases, with phenotypes ranging from non-lesional skin to inflammatory diseases like atopic dermatitis. Data on patients' needs and treatment goals depending on the skin phenotype and disease burden are limited. This study aimed to analyse the impact of distinct phenotypes of chronic atopic pruritus on disease burden and treatment goals. Another objective was to investigate whether the disease burden influences the treatment goals. Patient-reported outcomes of 1,086 adult patients (n = 529 with atopic dermatitis, n = 557 with chronic pruritus on non-lesional skin with atopic skin diathesis) were analysed age- and gender-matched (mean age 49.7 ± 19.0 years; n = 605 female [55.7%]), comparing pruritus intensity (Numeric Rating Scale), quality of life (Dermatology Life Quality Index, ItchyQol), anxiety and depression (Hospital Anxiety and Depression Scale), and patient needs (Patient Needs Questionnaire of the Patient Benefit Index-Pruritus). Although the disease burden was significantly higher in patients with atopic dermatitis (prolonged disease duration, increased quality of life impairment, higher pruritus intensity), the treat-ment goals of both phenotypes matched in 92.6%. The most important needs were to no longer experience itching, find a clear diagnosis and therapy, and have confidence in the therapy.

Beyond the classic players: Mas-related G protein-coupled receptor member X2 role in pruritus and skin diseases

Chronic spontaneous urticaria (CSU), atopic dermatitis (AD), psoriasis and rosacea are highly prevalent inflammatory skin conditions which impose a significant burden on patients' quality of life. Their pathophysiology is likely multifactorial, involving genetic, immune and environmental factors. Recent advancements in the field have demonstrated the key role of mast cells (MC) in the pathophysiology of these conditions. The Mas-related G protein-coupled receptor X2 (MRGPRX2) has emerged as a promising non-IgE-mediated MC activation receptor. MRGPRX2 is predominately expressed on MC and activated by endogenous and exogenous ligands, leading to MC degranulation and release of various pro-inflammatory mediators. Mounting evidence on the presence of endogenous MRGPRX2 agonists (substance P, cortistatin-14, LL37, PAMP-12 and VIP) and its high expression among patients with CSU, AD, rosacea, psoriasis and chronic pruritus emphasizes the pathogenic role of MRGPRX2 in these conditions. Despite the currently available treatments, there remains a pressing need for novel drug targets and treatment options for these chronic inflammatory skin conditions. Here, we reviewed the pathogenic role of MRGPRX2 and its potential as a novel therapeutic target and provided an update on future research directions.

Antipruritic Effects of Single Administration of Paroxetine on Acute and Chronic Itch

Itch is described as an unpleasant sensation, and chronic itch, such as that in atopic dermatitis (AD), often decreases a patient's QOL. There are few effective treatments for various chronic pruritic disorders that are not limited to inflammation. Selective serotonin reuptake inhibitors (SSRIs) are antidepressants used to treat some chronic pruritus disorders. However, there is little evidence from clinical and basic studies using animal models. In this study, we found that paroxetine suppressed acute and chronic itch in mouse models. Single administration of paroxetine (10 mg/kg) inhibited scratching behavior caused by histamine-dependent or histamine-independent itch. Moreover, paroxetine (10 mg/kg) inhibited spontaneous scratching behavior in AD model using NC/Nga mice without affecting locomotor function. These results suggest that paroxetine suppresses chronic itch caused by AD via histamine-dependent and -independent pathways. This study provides one of the few pieces of evidence that SSRIs suppress itch.

Consensus on the Diagnostic Algorithm for Chronic Nodular Prurigo

Chronic nodular prurigo (CNP) is a chronic dermatological disease characterized by the presence of chronic pruritus and pruritic nodular lesions. The aim of this study was to reach consensus among a group of experts based on a non-systematic literature review and an algorithm for the clinical diagnosis of CNP. The resulting algorithm is structured in 3 blocks: 1) early identification of the patient with a possible diagnosis of CNP; 2) diagnosis and assessment of CNP; and 3) categorization of CNP (identification of the underlying causes or associated comorbidities). We believe that this clinical algorithm can facilitate the correct diagnosis of patients with CNP. Additionally, it raises awareness on the need for a multidisciplinary approach and specific treatment of CNP, steps of paramount importance to make better therapeutic decisions.

[Translated article] Consensus on the Diagnostic Algorithm for Chronic Nodular Prurigo

Chronic nodular prurigo (CNP) is a chronic dermatological disease characterized by the presence of chronic pruritus and pruritic nodular lesions. The aim of this study was to reach consensus among a group of experts based on a non-systematic literature review and an algorithm for the clinical diagnosis of CNP. The resulting algorithm is structured in 3 blocks: 1) early identification of the patient with a possible diagnosis of CNP; 2) diagnosis and assessment of CNP; and 3) categorization of CNP (identification of the underlying causes or associated comorbidities). We believe that this clinical algorithm can facilitate the correct diagnosis of patients with CNP. Additionally, it raises awareness on the need for a multidisciplinary approach and specific treatment of CNP, steps of paramount importance to make better therapeutic decisions.

Spinal Nmur2-positive Neurons Play a Crucial Role in Mechanical Itch

The dorsal spinal cord is crucial for the transmission and modulation of multiple somatosensory modalities, such as itch, pain, and touch. Despite being essential for the well-being and survival of an individual, itch and pain, in their chronic forms, have increasingly been recognized as clinical problems. Although considerable progress has been made in our understanding of the neurochemical processing of nociceptive and chemical itch sensations, the neural substrate that is crucial for mechanical itch processing is still unclear. Here, using genetic and functional manipulation, we identified a population of spinal neurons expressing neuromedin U receptor 2 (Nmur2+) as critical elements for mechanical itch. We found that spinal Nmur2+ neurons are predominantly excitatory neurons, and are enriched in the superficial laminae of the dorsal horn. Pharmacogenetic activation of cervical spinal Nmur2+ neurons evoked scratching behavior. Conversely, the ablation of these neurons using a caspase-3-based method decreased von Frey filament-induced scratching behavior without affecting responses to other somatosensory modalities. Similarly, suppressing the excitability of cervical spinal Nmur2+ neurons via the overexpression of functional Kir2.1 potassium channels reduced scratching in response to innocuous mechanical stimuli, but not to pruritogen application. At the lumbar level, pharmacogenetic activation of these neurons evoked licking and lifting behaviors. However, ablating these neurons did not affect the behavior associated with acute pain. Thus, these results revealed the crucial role of spinal Nmur2+ neurons in mechanical itch. Our study provides important insights into the neural basis of mechanical itch, paving the way for developing novel therapies for chronic itch. PERSPECTIVE: Excitatory Nmur2+ neurons in the superficial dorsal spinal cord are essential for mechanical but not chemical itch information processing. These spinal Nmur2+ neurons represent a potential cellular target for future therapeutic interventions against chronic itch. Spinal and supraspinal Nmur2+ neurons may play different roles in pain signal processing.

[Recommended diagnostics for pruritus affecting primary non-lesional skin]

BACKGROUND

Chronic pruritus affecting primary non-lesional skin (CPNL) manifests as a common symptom across a spectrum of diseases spanning various medical specialties. Given the diverse etiological factors involved, diagnosing the underlying condition poses a significant challenge.

OBJECTIVES

To provide a comprehensive overview of clinical, laboratory, and imaging diagnostics for CPNL.

MATERIALS AND METHODS

A thorough literature search on the diagnostics of chronic pruritus was conducted using PubMed with specific keywords "chronic pruritus AND non-lesional skin", "chronic itch AND non-lesional skin", "chronic pruritus AND diagnostics", "chronic itch AND diagnostics", "CKD-aP", "hepatic pruritus", "cholestatic pruritus", and "myeloproliferative neoplasms AND pruritus".

RESULTS

A systematic diagnostic approach is recommended for patients with CPNL, guided by the prevalence of pruritus-associated diseases. Initial basic diagnostics facilitate a cost-effective and focused evaluation during the initial medical assessment. Information pertaining to underlying diseases can be further refined through specialized diagnostic procedures.

CONCLUSIONS

CPNL often presents a diagnostic dilemma. Adopting a stepwise diagnostic strategy facilitates the identification of underlying etiologies, which is crucial for recognizing diseases and administering pruritus-specific pharmacotherapy.

Prurigo Nodularis: Pathogenesis and the Horizon of Potential Therapeutics

Chronic pruritus that lasts for over 6 weeks can present in various forms, like papules, nodules, and plaque types, with prurigo nodularis (PN) being the most prevalent. The pathogenesis of PN involves the dysregulation of immune cell-neural circuits and is associated with peripheral neuropathies, possibly due to chronic scratching. PN is a persistent and challenging condition, involving complex interactions among the skin, immune system, and nervous system. Lesional skin in PN exhibits the infiltration of diverse immune cells like T cells, eosinophils, macrophages, and mast cells, leading to the release of inflammatory cytokines and itch-inducing substances. Activated sensory nerve fibers aggravate pruritus by releasing neurotransmitters, perpetuating a vicious cycle of itching and scratching. Traditional treatments often fail, but recent advancements in understanding the inflammatory and itch transmission mechanisms of PN have paved the way for innovative therapeutic approaches, which are explored in this review.

Atopic dermatitis (eczema) guidelines: 2023 American Academy of Allergy, Asthma and Immunology/American College of Allergy, Asthma and Immunology Joint Task Force on Practice Parameters GRADE- and Institute of Medicine-based recommendations

BACKGROUND

Guidance addressing atopic dermatitis (AD) management, last issued in 2012 by the American Academy of Allergy, Asthma and Immunology/American College of Allergy, Asthma and Immunology Joint Task Force, requires updating as a result of new treatments and improved guideline and evidence synthesis methodology.

OBJECTIVE

To produce evidence-based guidelines that support patients, clinicians, and other decision-makers in the optimal treatment of AD.

METHODS

A multidisciplinary guideline panel consisting of patients and caregivers, AD experts (dermatology and allergy/immunology), primary care practitioners (family medicine, pediatrics, internal medicine), and allied health professionals (psychology, pharmacy, nursing) convened, prioritized equity, diversity, and inclusiveness, and implemented management strategies to minimize influence of conflicts of interest. The Evidence in Allergy Group supported guideline development by performing systematic evidence reviews, facilitating guideline processes, and holding focus groups with patient and family partners. The Grading of Recommendations Assessment, Development and Evaluation (GRADE) approach informed rating the certainty of evidence and strength of recommendations. Evidence-to-decision frameworks, subjected to public comment, translated evidence to recommendations using trustworthy guideline principles.

RESULTS

The panel agreed on 25 recommendations to gain and maintain control of AD for patients with mild, moderate, and severe AD. The eAppendix provides practical information and implementation considerations in 1-2 page patient-friendly handouts.

CONCLUSION

These evidence-based recommendations address optimal use of (1) topical treatments (barrier moisturization devices, corticosteroids, calcineurin inhibitors, PDE4 inhibitors [crisaborole], topical JAK inhibitors, occlusive [wet wrap] therapy, adjunctive antimicrobials, application frequency, maintenance therapy), (2) dilute bleach baths, (3) dietary avoidance/elimination, (4) allergen immunotherapy, and (5) systemic treatments (biologics/monoclonal antibodies, small molecule immunosuppressants [cyclosporine, methotrexate, azathioprine, mycophenolate, JAK inhibitors], and systemic corticosteroids) and UV phototherapy (light therapy).