Phantom limb syndrome: from pathogenesis to treatment. A narrative review

Phantom Limb Syndrome (PLS) can be defined as the disabling or painful sensation of the presence of a body part that is no longer present after its amputation. Anatomical changes involved in Phantom Limb Syndrome, occurring at peripheral, spinal and brain levels and include the formation of neuromas and scars, dorsal horn sensitization and plasticity, short-term and long-term modifications at molecular and topographical levels. The molecular reorganization processes of Phantom Limb Syndrome include NMDA receptors hyperactivation in the dorsal horn of the spinal column leading to inflammatory mechanisms both at a peripheral and central level. At the brain level, a central role has been recognized for sodium channels, BDNF and adenosine triphosphate receptors. In the paper we discuss current available pharmacological options with a final overview on non-pharmacological options in the pipeline.

A systematic review of the prevalence of postamputation and chronic neuropathic pain associated with combat injury in military personnel

ABSTRACT

Combat trauma can lead to widespread tissue damage and limb loss. This may result in chronic neuropathic and post amputation pain, including phantom limb pain (PLP) and residual limb pain (RLP). The military population is distinct with respect to demographic, injury, and social characteristics compared with other amputation and trauma cohorts. We undertook a systematic review of studies of military personnel, with a history of combat injury, that reported a prevalence of any type of postamputation pain or chronic neuropathic pain, identified from Embase and MEDLINE databases.Using the inverse variance method with a random-effects model, we undertook a meta-analysis to determine an overall prevalence and performed exploratory analyses to identify the effect of the type of pain, conflict, and time since injury on prevalence. Pain definitions and types of pain measurement tools used in studies were recorded. Thirty-one studies (14,738 participants) were included. The pooled prevalence of PLP, RLP, and chronic neuropathic pain were 57% (95% CI: 46-68), 61% (95% CI: 50-71), and 26% (95% CI: 10-54), respectively. Between-study heterogeneity was high (I 2 : 94%-98%). Characterisation of duration, frequency, and impact of pain was limited. Factors reported by included studies as being associated with PLP included the presence of RLP and psychological comorbidity. The prevalence of postamputation pain and chronic neuropathic pain after combat trauma is high. We highlight inconsistency of case definitions and terminology for pain and the need for consensus in future research of traumatic injury.

Nerve-myeloid cell interactions in persistent human pain: a reappraisal using updated cell subset classifications

ABSTRACT

The past 20 years have seen a dramatic shift in our understanding of the role of the immune system in initiating and maintaining pain. Myeloid cells, including macrophages, dendritic cells, Langerhans cells, and mast cells, are increasingly implicated in bidirectional interactions with nerve fibres in rodent pain models. However, our understanding of the human setting is still poor. High-dimensional functional analyses have substantially changed myeloid cell classifications, with recently described subsets such as epidermal dendritic cells and DC3s unveiling new insight into how myeloid cells interact with nerve fibres. However, it is unclear whether this new understanding has informed the study of human chronic pain. In this article, we perform a scoping review investigating neuroimmune interactions between myeloid cells and peripheral nerve fibres in human chronic pain conditions. We found 37 papers from multiple pain states addressing this aim in skin, cornea, peripheral nerve, endometrium, and tumour, with macrophages, Langerhans cells, and mast cells the most investigated. The directionality of results between studies was inconsistent, although the clearest pattern was an increase in macrophage frequency across conditions, phases, and tissues. Myeloid cell definitions were often outdated and lacked correspondence with the stated cell types of interest; overreliance on morphology and traditional structural markers gave limited insight into the functional characteristics of investigated cells. We therefore critically reappraise the existing literature considering contemporary myeloid cell biology and advocate for the application of established and emerging high-dimensional proteomic and transcriptomic single-cell technologies to clarify the role of specific neuroimmune interactions in chronic pain.

Analysis of Macrophages and Peptidergic Fibers in the Skin of Patients With Painful Diabetic Polyneuropathy

BACKGROUND AND OBJECTIVES

The mechanisms of pain in patients with diabetic polyneuropathy are unknown. Studies have suggested a role of inflammation and increased neuropeptides peripherally in pain generation. This study examined the possible skin markers of painful diabetic polyneuropathy (P-DPN): macrophages, substance P (SP), and calcitonin gene-related peptide (CGRP).

METHODS

The participants were included from a large Danish cross-sectional clinical study of type 2 diabetes. We diagnosed definite diabetic polyneuropathy using the Toronto criteria and used the Neuropathic Pain Special Interest Group classification for defining P-DPN. We included 60 skin biopsies from patients with diabetic polyneuropathy-30 with P-DPN and 30 with nonpainful diabetic polyneuropathy (NP-DPN)-and 30 biopsies from healthy controls of similar age and sex. The biopsies were stained using PGP 9.5, IbA1, and SP and CGRP primary markers.

RESULTS

There was increased macrophage density in patients with P-DPN (8.0%) compared with that in patients with NP-DPN (5.1%, p < 0.001), and there was increased macrophage density in patients with NP-DPN (5.1%) compared with that in healthy controls (3.1%, p < 0.001). When controlling for neuropathy severity, body mass index, age, and sex, there was still a difference in macrophage density between patients with P-DPN and patients with NP-DPN. Patients with P-DPN had higher median nerve fiber length density (274.5 and 155 mm^-2 for SP and CGRP, respectively) compared with patients with NP-DPN (176 and 121 mm^-2 for SP and CGRP, respectively, p = 0.009 and 0.04) and healthy controls (185.5 and 121.5 mm^-2 for SP and CGRP, respectively), whereas there was no difference between patients with NP-DPN and controls without diabetes (p = 0.64 and 0.49, respectively). The difference between P-DPN and NP-DPN for SP and CGRP was significant only in female patients, although a trend was seen in male patients.

DISCUSSION

The findings point to a possible involvement of the innate immune system in the pathogenesis of neuropathic pain in patients with DPN, although markers of activated macrophages were not measured in this study.

Increased peptidergic fibers as a potential cutaneous marker of pain in diabetic small fiber neuropathy

ABSTRACT

Diabetic polyneuropathy (DPN) is a common complication of diabetes and is often associated with neuropathic pain. The mechanisms underlying development and maintenance of painful DPN are largely unknown, and quantification of intraepidermal nerve fiber density from skin biopsy, one of the neuropathological gold standard when diagnosing DPN, does not differentiate between patients with and without pain. Identification of possible pain pathophysiological biomarkers in patients with painful DPN may increase our knowledge of mechanisms behind neuropathic pain. Animal models of painful DPN have been shown to have an increased density of peptidergic nerve fibers (substance P and calcitonin gene-related peptide). In this study, we performed a detailed skin biopsy analysis in a well-characterized group of DPN patients with primarily small fiber involvement, with and without pain, and in healthy controls and test for correlation between skin biopsy findings and pain intensity and quantitative sensory testing. We found that although there was no difference in intraepidermal nerve fiber density using protein gene product 9.5 between patients with and without pain, patients with pain had increased density of dermal peptidergic fibers containing substance P and calcitonin gene-related peptide compared with patients with painless DPN and healthy controls. Peptidergic nerve fiber density correlated with pain ratings in patients with pain (R = 0.33; P = 0.019), but not with quantitative sensory testing results. In this article, we show, for the first time in humans, an increased density of dermal peptidergic fibers in painful DPN. These findings provide new insight in the pathophysiological mechanisms of pain in diabetes and open the research towards new therapeutic targets.