Mechanisms and treatment of opioid-induced pruritus: Peripheral and central pathways

Glutamatergic Projections from the Basolateral Amygdala to Medial Prefrontal Cortex Contribute to Acute Itch Sensation Processing

Itch refers to an aversive sensation that generates a desire to scratch. The basolateral amygdala (BLA) activity is crucial in driving motivation, sensation, and emotional responses. Excitatory projections from the BLA play a vital role in regulating neuronal activity throughout the brain, including the medial prefrontal cortex (mPFC). Nevertheless, whether the BLA neurons and BLA-mPFC circuit contribute to itch sensation remains elusive. In this study, fluoro-gold retrograde tracing, morphological staining, and neuronal manipulation approaches were employed to investigate the role of BLA-mPFC projections in itch processing. Results showed that glutamatergic neurons in the BLA were activated in response to histamine- and chloroquine-induced acute itch stimuli. Chemogenetic activation of these neurons significantly mitigated the scratching behavior, whereas their inhibition increased the number of scratching bouts. The percentages of fluoro-gold-labeled CaMKII+ neurons expressing FOS in the BLA, which project to the mPFC, were 40.10 ± 2.26% and 73.84 ± 6.48% in acute itch models induced by histamine and chloroquine, respectively. Optogenetic activation of the BLA-mPFC pathway reduced histamine- or chloroquine-induced scratching bouts, whereas its inhibition increased the scratching bouts. These results provide evidence that BLA-mPFC projections are implicated in the acute itch processing, expanding our understanding to the circuit mechanism underlying the modulation of itch.

Incidence and influential factors of postoperative pruritus in morphine-based intravenous patient-controlled analgesia

BACKGROUND

Pruritus is a distressing symptom of systemic opioid analgesia that responds poorly to conventional anti-pruritus treatments. This study aimed to determine the incidence and risk factors for postoperative pruritus using intravenous patient-controlled analgesia (IV-PCA).

METHODS

Opioid-naïve patients who underwent morphine-based IV-PCA for postoperative pain at a tertiary center between January 1, 2020 and June 30, 2023, were included retrospectively. The primary outcome was pruritus within 72 hours after surgery. Cumulative morphine consumption and pain numerical rating scores were measured to evaluate the potential impact of pruritus on postoperative pain control.

RESULTS

A total of 1696 patients were enrolled, of whom 119 (7.0%) developed pruritus during the study period. Five independent factors for pruritus were identified, including intraoperative uses of hydroxyethyl starch solutions (adjusted odds ratio [aOR]: 0.13, 95% CI, 0.04-0.43), lockout interval of IV-PCA (aOR: 0.50, 95% CI, 0.27-0.94, on base-2 logarithmic scale), droperidol addition to morphine solutions (aOR: 0.53, 95% CI, 0.35-0.81), cumulative morphine dose (aOR: 1.76, 95% CI, 1.47-2.12, on base-2 logarithmic scale), and postoperative uses of antihistamines (aOR: 2.90, 95% CI, 1.83-4.60) (c-statistic = 0.745). Patients with pruritus had higher postoperative morphine consumption (median: 67.5 mg, interquartile range: 38.3-94.0 vs 38.0 mg, 21.0-65.4, p < 0.001) but similar pain intensity compared to those without pruritus.

CONCLUSION

Increasing the lockout interval and the droperidol regimen may protect patients from morphine-induced pruritus after IV-PCA. Further studies are warranted to clarify the mechanisms underlying the anti-pruritus effects of hydroxyethyl starch.

Practical guide for the diagnosis and treatment of localized and generalized cutaneous pruritus (chronic itch with no underlying pruritic dermatosis)

Itch, also known as pruritus, is one of the most prevalent symptoms observed in dermatological practices. Itch frequently arises from primary pruritic dermatoses, although it may also manifest in the absence of a primary pruritic skin rash. The latter itchy condition is referred to as "cutaneous pruritus" in the Japanese guidelines published in 2020. Cutaneous pruritus can be classified into two categories based on its distribution: localized cutaneous pruritus and generalized cutaneous pruritus. Localized cutaneous pruritus is indicative of a neuropathic cause, whereas generalized cutaneous pruritus suggests underlying systemic disease(s), drug-induced itch, psychogenic itch (also known as functional itch disorder), or chronic pruritus of unknown origin (CPUO). Systemic diseases associated with cutaneous pruritus include disorders of iron metabolism, chronic kidney disease, chronic liver disease (especially cholestasis), endocrine/metabolic diseases, hematological disorders, and malignant solid tumors. CPUO is a term used to describe chronic itch that is often generalized and for which no underlying cause can be identified despite a comprehensive and careful diagnostic workup. A variety of treatment approaches are available for cutaneous pruritus, including device-based physical therapies (such as phototherapy) and medications that act on the itch-perception processing pathway from the skin, peripheral sensory nerves, the spinal cord, to the brain. This review presents an overview of the current knowledge regarding cutaneous pruritus, from its underlying pathophysiologic mechanisms to the diagnostic procedures and treatment approaches that are currently available.

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

Efficacy and Safety of Pregabalin and Gabapentin for Pruritus: A Systematic Review and Meta-Analysis

CONTEXT

Limited data existed on the efficacy and safety of novel antiepileptic drugs (pregabalin and gabapentin) in treating pruritus.

OBJECTIVES

To assess their role in managing either acute or chronic pruritus.

METHODS

A systematic search was conducted in PubMed, EMBASE, the Cochrane Library, and Web of Science databases for relevant randomized controlled trials. Pooled odd ratio (OR) with 95% CI were performed using RevMan5.4 and R4.3.1.

RESULTS

Analysis of 27 articles involving 2,016 patients showed significant reduction in pruritus incidence (OR, 0.30 [CI, 0.22-0.4]; I2=1%) and improvements in VAS (MD, 2.76 [CI, 0.95-4.57]; I2=98%) and 5-D scores (MD, 3.42 [CI, 2.10-4.75]; I2=92%) with pregabalin/gabapentin compared to controls. Adverse effects mainly included dizziness, somnolence, nausea and vomiting, dry mouth, constipation, and anxiety, with no significant difference between the groups (OR, 1.08 [CI, 0.32-3.59]; I2=76%).

CONCLUSION

The novel antiepileptic drugs pregabalin and gabapentin demonstrated significant therapeutic value in the treatment of pruritus, with a favorable safety profile. Compared to commonly used pruritus treatments such as antihistamines and antidepressants, these medications offered a promising alternative.

Current and emerging opioids for the treatment of chronic cough: a mini review

INTRODUCTION

Chronic cough has increasingly been recognized as a distinct clinical entity that affects a significant portion of the global population. Despite advancements in understanding its pathophysiology, treatment options remain limited. Opioid analgesics have long been used for cough, and some have proven clear antitussive potential. However, these have yet to be approved by regulatory authorities for the treatment of chronic cough. Several novel synthetic opioid modulators that demonstrated antitussive effects in early-stage studies also failed to translate into clinical practice.

AREAS COVERED

This mini review aims to summarize the implications of opioid receptors in the development of cough medicines and highlight recent advances in opioid analgesics in cough trials. PUB MED/CINAHL/Web of Science/Scopus was searched (September 2024).

EXPERT OPINION

Our understanding of the precise sites of action and the involvement of peripheral opioid receptors in cough remains limited. Despite these gaps in knowledge, opioids remain a viable option for some patients until more novel effective treatments are available. Due to the frequent opioid side effects, new opioid derivatives with improved properties are needed. The development of tailored or biased delta-opioid receptor ligands and mixed agonists of opioid receptor-like 1/mu receptors may offer hope for new opioid-based drug discovery for chronic cough.

Epidural injection of hydromorphone for postoperative pain after episiotomy: a randomized controlled trial

Perineal incision resulting in post-partum pain and impact on puerperal life was the focus of our study. We recruited postpartum mothers who had undergone vaginal delivery to receive labor analgesia and episiotomy with an epidural injection of 0.5 mg hydromorphone to assess the therapeutic effects of postpartum analgesia. The participants were randomly allocated into two groups: the control group (Group NS) received an epidural injection of an equal amount of saline, while the study group (Group HY) received an epidural injection of 0.5 mg hydromorphone. We collected relevant data from electronic medical records to compare the differences between the two groups. The intervention group demonstrated lower pain scores at 4, 8, 12, 16, and 24 h compared to the control group (p < 0.001). Additionally, a higher number of patients in the control group required pain medication (7 (15.9%) compared to 2 (4.7%)). The time to first analgesia request in group NS was earlier than that in group HY (8.94 ± 1.27 h compared to 16.96 ± 3.38 h). The study group experienced higher rates of vomiting (P = 0.002) and itching (P < 0.001). However, there were no differences between the two groups in terms of urinary retention, dyskinesia, respiratory depression, dizziness, or neonatal feeding. The epidural injection of 0.5 mg hydromorphone proved to be effective in alleviating pain caused by maternal episiotomy and did not negatively affect neonatal feeding.Clinical trial registration: http://www.chictr.org.cn/usercenter.aspx identifier: ChiCTR2200064687.