Central mechanisms of itch: A systematic literature review and meta-analysis

Functional magnetic resonance imaging in prurigo nodularis: A call to study neural sensitization phenomena

Prurigo nodularis is a chronic pruritic, inflammatory skin condition characterized by nodular skin lesions in a typical distribution pattern caused by various dermatologic and/or nondermatologic conditions. In recent years, significant advances have been made in the understanding of the cutaneous pathophysiology of prurigo nodularis, resulting in novel treatment options such as interleukin-4, -13, -17, and -31 or Janus kinase inhibitors. Many aspects of the neurophysiology are largely unknown, including the processing in the central structural and functional network involved in prurigo nodularis. Functional neuroimaging allows noninvasive assessment of brain function and structure. Due to its high spatial resolution and temporal precision, functional magnetic resonance imaging has proven to be a suitable method for exploring neural mechanisms and assessing pharmacologic effects in dermatologic research. In this systematic review, the current knowledge of functional magnetic resonance imaging in the context of prurigo nodularis and its centrally active treatment options is summarized.

Higher-order sensorimotor circuit of the whole-brain functional network involved in pruritus regulation in atopic dermatitis

BACKGROUND

Little is known about the neural mechanisms underlying pruritus regulation in Atopic dermatitis (AD).

OBJECTIVE

To investigate the functional changes of the resting-state whole brain network of AD participants and the mechanisms by which they were involved in pruritus regulation.

METHOD

Based on the functional magnetic resonance imaging data from 19 AD participants and 37 healthy controls (HC), a graph-theoretical measure of degree centrality (DC) conjoined with a voxel-level seed-based functional connectivity (FC) method was used to identify abnormal higher-order nodes and the functionally relevant circuit in AD participants compared to healthy controls (HC).

RESULTS

Of 64 participants screened, 19 AD participants (12M/7F, median [IQR] age, 27 [14] years) and 36 HCs (13M/23F, median [IQR] age, 20 [1] years) were enrolled. DC values of the left superior frontal gyrus (LSFG) increased in AD participants and exhibited a negative correlation with the SCORAD score (r = -0.561, p = 0.012) compared with HC. In the FC analysis with LSFG as the seed, FC values of several sensory and motor regions increased in AD participants, highly overlapping with the anatomical distribution of the inferior fronto-occipital fascicle (IFOF). AD participants with severe pruritus exhibited lower levels of DC (T = -2.316, p = 0.033) and FC between the LSFG and left insula (T = -2.203, p = 0.042) than those with mild-to- moderate pruritus.

CONCLUSIONS AND RELEVANCE

LSFG was involved in pruritus regulation in AD by forming a high-order sensorimotor circuit through the IFOF, a white matter fascicle that proved to provide multimodal integration in motor control and sensory information processing. These results offer more mechanism-guided treatment targets for severe pruritus in AD.

Functional Brain Changes Following Burn Injury: A Narrative Review

BACKGROUND

Burn injuries cause significant motor and sensory dysfunctions that can negatively impact burn survivors' quality of life. The underlying mechanisms of these burn-induced dysfunctions have primarily been associated with damage to the peripheral neural architecture, however, evidence points to a systemic influence of burn injury. Central nervous system (CNS) reorganizations due to inflammation, afferent dysfunction, and pain could contribute to persistent motor and sensory dysfunction in burn survivors. Recent evidence shows that the capacity for neuroplasticity is associated with self-reported functional recovery in burn survivors.

OBJECTIVE

This review first outlines motor and sensory dysfunctions following burn injury and critically examines recent literature investigating the mechanisms mediating CNS reorganization following burn injury. The review then provides recommendations for future research and interventions targeting the CNS such as non-invasive brain stimulation to improve functional recovery.

CONCLUSIONS

Directing focus to the CNS following burn injury, alongside the development of non-invasive methods to induce functionally beneficial neuroplasticity in the CNS, could advance treatments and transform clinical practice to improve quality of life in burn survivors.

Basic mechanisms of itch

Pruritus (or itch) is an unpleasant sensation leading to a desire to scratch. In the epidermis, there are selective C or Aδ epidermal nerve endings that are pruriceptors. At their other ends, peripheral neurons form synapses with spinal neurons and interneurons. Many areas in the central nervous system are involved in itch processing. Although itch does not occur solely because of parasitic, allergic, or immunologic diseases, it is usually the consequence of neuroimmune interactions. Histamine is involved in a minority of itchy conditions, and many other mediators play a role: cytokines (eg, IL-4, IL-13, IL-31, IL-33, and thymic stromal lymphopoietin), neurotransmitters (eg, substance P, calcitonin gene-related peptide, vasoactive intestinal peptide, neuropeptide Y, NBNP, endothelin 1, and gastrin-releasing peptide), and neurotrophins (eg, nerve growth factor and brain-derived neurotrophic factor). Moreover, ion channels such as voltage-gated sodium channels, transient receptor potential vanilloid 1, transient receptor ankyrin, and transient receptor potential cation channel subfamily M (melastatin) member 8 play a crucial role. The main markers of nonhistaminergic pruriceptors are PAR-2 and MrgprX2. A notable phenomenon is the sensitization to pruritus, in which regardless of the initial cause of pruritus, there is an increased responsiveness of peripheral and central pruriceptive neurons to their normal or subthreshold afferent input in the context of chronic itch.

Central medial thalamic nucleus dynamically participates in acute itch sensation and chronic itch-induced anxiety-like behavior in male mice

Itch is an annoying sensation consisting of both sensory and emotional components. It is known to involve the parabrachial nucleus (PBN), but the following transmission nodes remain elusive. The present study identified that the PBN-central medial thalamic nucleus (CM)-medial prefrontal cortex (mPFC) pathway is essential for itch signal transmission at the supraspinal level in male mice. Chemogenetic inhibition of the CM-mPFC pathway attenuates scratching behavior or chronic itch-related affective responses. CM input to mPFC pyramidal neurons is enhanced in acute and chronic itch models. Specifically chronic itch stimuli also alter mPFC interneuron involvement, resulting in enhanced feedforward inhibition and a distorted excitatory/inhibitory balance in mPFC pyramidal neurons. The present work underscores CM as a transmit node of the itch signal in the thalamus, which is dynamically engaged in both the sensory and affective dimensions of itch with different stimulus salience.

Efficiency of cutaneous heat diffusion after local hyperthermia for the treatment of itch

BACKGROUND

Today, itching is understood as an independent sensory perception, which is based on a complex etiology of a disturbed neuronal activity and leads to clinical symptoms. The primary afferents (pruriceptors) have functional overlaps with afferents of thermoregulation (thermoceptors). Thus, an antipruritic effect can be caused by antagonizing heat-sensitive receptors of the skin. The ion channel TRP-subfamily V member 1 (TRPV1) is of particular importance in this context. Repeated heat application can induce irreversible inactivation by unfolding of the protein, causing a persistent functional deficit and thus clinically and therapeutically reducing itch sensation.

MATERIAL AND METHODS

To demonstrate relevant heat diffusion after local application of heat (45°C to 52°C for 3 and 5 seconds) by a technical medical device, the temperature profile for the relevant skin layer was recorded synchronously on ex vivo human skin using an infrared microscope.

RESULTS

The results showed that the necessary activation temperature for TRPV1 of (≥43°C) in the upper relevant skin layers was safely reached after 3 and 5 seconds of application time. There were no indications of undesirable thermal effects.

CONCLUSION

The test results show that the objectified performance of the investigated medical device can be expected to provide the necessary temperature input for the activation of heat-sensitive receptors in the skin. Clinical studies are necessary to prove therapeutic efficacy in the indication pruritus.

Functional connectivity impairment of thalamus-cerebellum-scratching neural circuits in pruritus of chronic spontaneous urticaria

Pruritus of chronic spontaneous urticaria (CSU) is one of the most common and irritating sensations that severely affects the quality of life. However, the changes in the functional connectivity (FC) between thalamic subregions and other brain regions have not been fully elucidated. This study aimed to explore the potential changes in brain neural circuits by focusing on various subregions of the thalamus in patients with CSU pruritus to contribute to the understanding of chronic pruritus from the perspective of central mechanisms. A total of 56 patients with CSU and 30 healthy controls (HCs) completed the data analysis. Urticaria Activity Score 7 (UAS7), pruritus visual analog score (VAS-P), Dermatological Life Quality Index (DLQI), and immunoglobulin E (IgE) values were collected to assess clinical symptoms. Seed-based resting-state functional connectivity (rs-FC) analysis was used to assess relevant changes in the neural circuits of the brain. Compared to HCs, seeds within the caudal temporal thalamus (cTtha) on the right side of patients with CSU showed increased rs-FC with the cerebellum anterior lobe (CAL). Seeds within the lateral prefrontal thalamus (lPFtha) on the right side showed increased rs-FC with both CAL and pons, while those within the medial prefrontal thalamus (mPFtha) on the right side showed increased rs-FC with both CAL and the dorsal lateral prefrontal cortex (dlPFC) on the right side. Seeds within the posterior parietal thalamus (PPtha) on the right side showed increased rs-FC with the cerebellum posterior lobe (CPL) on the left side. The UAS7 values and IgE levels were positively correlated with the rs-FC of the right dlPFC. Our results suggest that patients with CSU may exhibit stronger rs-FC alterations between certain thalamic subregions and other brain regions. These changes affect areas of the brain involved in sensorimotor and scratching.

Trial registration number

[http://www.chictr.org.cn], identifier [ChiCTR1900022994].

Altered resting-state functional connectivity of default mode network in brachioradial pruritus

BACKGROUND

Patients with chronic pruritus (CP) have a low quality of life, thus it is important to gain a better understanding of the underlying processes. Previous functional magnetic resonance imaging studies at rest (rs-fMRI) have shown that mainly areas associated with the default mode network (DMN), sensorimotor (SMN), frontoparietal (FPN) and salience networks (SN) are involved in the processing of itch in patients with chronic pruritus (CP), as well as the cortico-striatal circuit, which is involved in the motoric preparation of scratching. rs-fMRI studies on functional connectivity (FC) patterns of resting-state networks (RSNs) in patients with inflammatory atopic dermatitis (AD) or with neuropathic brachioradial pruritus (BRP) compared to healthy controls (HC) are lacking.

OBJECTIVES

The main goals of this study were to investigate whether functional connectivity within networks and areas associated with itch detection and processing are altered in patients with AD and BRP compared to matched healthy controls by rsfMRI, respectively.

METHODS

Patients with AD (n = 28) and with BRP (n = 28) were compared to corresponding matched healthy controls by rsfMRI. Group specific RSNs were identified by independent component analysis (ICA) and between-group differences in the RSNs were analyzed by dual regression technique. Seed-based functional connectivity was analyzed in several itch-related brain regions belonging to the DMN, SN and FPN, respectively.

RESULTS

ICA and seed-based analyses revealed decreased functional connectivity in BRP compared to HC specially within the DMN including the precuneus and cingulate cortex. For AD patients in comparison to HC, as well as when BRP and AD patients when compared directly, no significant FC differences at rest were seen.

CONCLUSIONS

Our findings point towards decreased FC particularly in the DMN at rest in patients with BRP. These results seem to indicate that central connectivity patterns at rest differentially encode itch in BRP and AD.

Functional Magnetic Resonance Imaging and Applications in Dermatology

As a noninvasive imaging modality able to show the dynamic changes in neurologic activity, functional magnetic resonance imaging has revolutionized the ability to both map and further understand the functional regions of the brain. Current applications range from neurosurgical planning to an enormous variety of investigational applications across many diverse specialties. The main purpose of this article is to provide a foundational understanding of how functional magnetic resonance imaging is being used in research by outlining the underlying basic science, specific methods, and direct investigational and clinical applications. In addition, the use of functional magnetic resonance imaging in current dermatological research, especially in relation to studies concerning the skin‒brain axis, is explicitly addressed. This article also touches on the advantages and limitations concerning functional magnetic resonance imaging in comparison with other similar techniques.

Chronic itch: emerging treatments following new research concepts

Until recently, itch pathophysiology was poorly understood and treatments were poorly effective in relieving itch. Current progress in our knowledge of the itch processing, the numerous mediators and receptors involved has led to a large variety of possible therapeutic pathways. Currently, inhibitors of IL-31, IL-4/13, NK₁ receptors, opioids and cannabinoids, JAK, PDE4 or TRP are the main compounds involved in clinical trials. However, many new targets, such as Mas-related GPCRs and unexpected new pathways need to be also explored.

Differential roles of prelimbic and anterior cingulate cortical region in the modulation of histaminergic and non-histaminergic itch

Itch is an unpleasant sensation that evokes a desire to scratch. Itch processing in the peripheral and spinal cord has been studied extensively, but the mechanism of itch in the central nervous system is still unclear. Anterior cingulate cortex (ACC) and prelimbic cortex (Prl), two subregions of the prefrontal cortex closely related to emotion and motivation, have been reported to be activated during itching in a series of functional imaging studies. However, the exact role of Prl and the differences between ACC and Prl in itch modulation remains unknown. To directly test the differential roles of ACC and Prl in itch processing, we chemogeneticlly inhibited the caudal ACC and Prl, respectively. We found that inhibition of caudal ACC reduced histaminergic but not non-histaminergic itch-induced scratching behaviors. In contrast, inhibition of Prl reduced both histaminergic and non-histaminergic itch-induced scratching behaviors. Our study provided direct evidence of Prl involvement in itch modulation and revealed the differential roles of caudal ACC and Prl in regulating histaminergic and non-histaminergic itch.

Neural Oscillation Associated with Contagious Itch in Patients with Atopic Dermatitis

Objective: Itch is an unpleasant sensation associated with an urge to scratch and is a major health care issue associated with atopic dermatitis (AD). Contagious itch, i.e., subjective feelings of itchiness induced by watching others’ scratching behavior, is common in patients with AD. Using electroencephalography, we examined alpha (8–13 Hz) oscillations in sensorimotor areas associated with the desire to scratch in patients with AD. Methods: Thirty-six patients with AD and 34 healthy controls (HCs) participated in this study. They evaluated their itch levels after watching short videos of a model scratching or tapping parts of his body. Neural oscillations were recorded from nine electrodes, including those placed over sensorimotor areas. Time–frequency analysis was used to compare mu rhythm suppression over the sensorimotor areas in response to these videos between patients with AD and HCs. Results: The behavioral test showed that the visual stimuli induced increased feelings of itchiness in patients with AD relative to HCs under the tapping and scratching conditions. The time–frequency analysis revealed that mu rhythm suppression in response to scratching images was significantly prominent in patients with AD, but not in HCs. Conclusion: Patients with AD exhibited increased susceptibility to contagious itch. This phenomenon might be related to enhanced mu rhythm suppression in sensorimotor areas of the brain in these patients. Our findings provide new insight into the neurophysiological basis of itch sensations in patients with AD.

Increased grey matter volume and associated resting-state functional connectivity in chronic spontaneous urticaria: A structural and functional MRI study

BACKGROUND AND PURPOSE

Chronic itch is one of the most common irritating sensations, yet its mechanisms have not been fully elucidated. Although some studies have revealed relationships between itching and brain function, the structural changes in the brain induced by chronic itching, such as those accompanying chronic spontaneous urticaria (CSU), remain unclear. In this study, we aimed to explore the potential changes in brain structure and the associated functional circuitry in CSU patients to generate insights to aid chronic itch management.

METHODS

Forty CSU patients and forty healthy controls (HCs) were recruited. Seven-day urticaria activity score (UAS7) values were collected to evaluate clinical symptoms. Voxel-based morphometry (VBM) and seed-based resting-state functional connectivity (rs-FC) analysis were used to assess structural changes in the brain and associated changes in functional circuitry.

RESULTS

Compared with HCs, CSU patients had significantly increased grey matter (GM) volume in the right premotor cortex, left fusiform cortex, and cerebellum. UAS7 values were positively associated with GM volume in the left fusiform cortex. In CSU patients relative to HCs, the left fusiform cortex as extracted by VBM analysis demonstrated decreased functional connectivity with the right orbitofrontal cortex, medial prefrontal cortex (mPFC), premotor cortex, primary motor cortex (MI), and cerebellum and increased functional connectivity with the right posterior insular cortex, primary somatosensory cortex (SI), and secondary somatosensory cortex (SII). The left cerebellum as extracted from VBM analysis demonstrated decreased functional connectivity with the right supplementary motor area (SMA) and MI in CSU patients relative to HCs.

CONCLUSIONS

Our findings indicate that patients suffering from chronic itching conditions, such as CSU, are likely to demonstrate altered GM volume in some brain regions. These changes may affect not only the sensorimotor area but also brain regions associated with cognitive function.

Pruriplastic Itch-A Novel Pathogenic Concept in Chronic Pruritus

The International Association for the Study of Pain (IASP) defined three descriptors for pain: nociceptive pain is “pain that arises from actual or threatened damage to non neural tissue and is due to the activation of nociceptors”; neuropathic pain is “pain caused by a lesion or disease of the somatosensory nervous system”; and nociplastic pain is “pain that arises from altered nociception despite no clear evidence of actual or threatened tissue damage causing the activation of peripheral nociceptors or evidence for disease or lesion of the somatosensory system causing the pain.” Based on clinical and pathophysiological arguments, a similar definition of “pruriplastic pruritus” should be made. Pruriplastic pruritus would include psychogenic pruritus, as well as some cases of pruritus ani, vulvar pruritus, sensitive skin or other poorly understood cases of pruritus. This new descriptor of itch could serve as systematic screening for altered pruriceptive function in patients who suffer from chronic itch and it may also help in defining better tailored treatment by identifying patients who are likely to respond better to centrally rather than to peripherally targeted therapies.

Itch processing in the brain

Itch is a sensation defined as the urge to scratch. The central mechanisms of itch are being increasingly studied. These studies are usually based on experimental itch induction methods, which can be classified into the following categories: histamine-induced, induction by other non-histamine chemicals (e.g. cowhage), physically induced (e.g. electrical) and mentally induced (e.g. audio-visual). Because pain has been more extensively studied, some extrapolations to itch can be proposed and verified by experiments. Recent studies suggest that the itch-processing network in the brain could be disrupted in certain diseases. This disruption could be related to the implication of new regions or the exclusion of already engaged brain regions from itch-processing network in the brain.