Sensory endings in skin grafts and scars after extensive burns

The role of patient-reported outcomes in peripheral nerve surgery

Measuring the outcome of peripheral nerve surgery is challenging because of the spectrum of functional impairment is dependent on the level and severity of the lesion. There are no nerve-specific patient-reported outcome measures, and no universally accepted outcome measurement both in terms of the parameters to be assessed and the methods and timing of the assessment. Nevertheless, the use of patient-reported outcome measures is fundamental to better understand the needs and expectations of patients, to take advantage of all treatment opportunities to offer the best possible support to these patients. This paper outlines current concepts in the measurement of outcome in peripheral nerve surgery.

Dendritic atoh1a+ cells serve as transient intermediates during zebrafish Merkel cell development and regeneration

Sensory cells often adopt specific morphologies that aid in the detection of external stimuli. Merkel cells encode gentle touch stimuli in vertebrate skin and adopt a reproducible shape characterized by spiky, actin-rich microvilli that emanate from the cell surface. The mechanism by which Merkel cells acquire this stereotyped morphology from basal keratinocyte progenitors is unknown. Here, we establish that dendritic Merkel cells (dMCs) express atonal homolog 1a (atoh1a), extend dynamic filopodial processes, and arise in transient waves during zebrafish skin development and regeneration. We find that dMCs share molecular similarities with both basal keratinocytes and Merkel cells, yet display mesenchymal-like behaviors, including local cell motility and proliferation within the epidermis. Furthermore, dMCs can directly adopt the mature, microvilliated Merkel cell morphology through substantial remodeling of the actin cytoskeleton. Loss of Ectodysplasin A signaling alters the morphology of dMCs and Merkel cells within specific skin regions. Our results show that dMCs represent an intermediate state in the Merkel cell maturation program and identify Ectodysplasin A signaling as a key regulator of Merkel cell morphology.

Merkel Cells Are Multimodal Sensory Cells: A Review of Study Methods

Merkel cells (MCs) are rare multimodal epidermal sensory cells. Due to their interactions with slowly adapting type 1 (SA1) Aβ low-threshold mechanoreceptor (Aβ-LTMRs) afferents neurons to form Merkel complexes, they are considered to be part of the main tactile terminal organ involved in the light touch sensation. This function has been explored over time by ex vivo, in vivo, in vitro, and in silico approaches. Ex vivo studies have made it possible to characterize the topography, morphology, and cellular environment of these cells. The interactions of MCs with surrounding cells continue to be studied by ex vivo but also in vitro approaches. Indeed, in vitro models have improved the understanding of communication of MCs with other cells present in the skin at the cellular and molecular levels. As for in vivo methods, the sensory role of MC complexes can be demonstrated by observing physiological or pathological behavior after genetic modification in mouse models. In silico models are emerging and aim to elucidate the sensory coding mechanisms of these complexes. The different methods to study MC complexes presented in this review may allow the investigation of their involvement in other physiological and pathophysiological mechanisms, despite the difficulties in exploring these cells, in particular due to their rarity.

Programmed death ligand 1 signals in cancer cells

The paradigm of surface-expressed programmed death ligand 1 (PDL1) signalling to immune cell programmed death 1 (PD1) to inhibit antitumour immunity has helped to develop effective and revolutionary immunotherapies using antibodies blocking these cell-extrinsic interactions. The recent discovery of cancer cell-intrinsic PDL1 signals has broadened understanding of pathologic tumour PDL1 signal consequences that now includes control of tumour growth and survival pathways, stemness, immune effects, DNA damage responses and gene expression regulation. Many such effects are PD1-independent. These insights demonstrate that the prevailing cell-extrinsic PDL1 signalling paradigm is useful, but incomplete in important respects. This Perspective discusses historical and recent advances in understanding cancer cell-intrinsic PDL1 signals, mechanisms for signal controls and important immunopathologic consequences including resistance to cytotoxic agents, targeted small molecules and immunotherapies. Cancer cell-intrinsic PDL1 signals present novel drug discovery targets and also have potential as reliable treatment response biomarkers. Cancer cell-intrinsic PD1 signals and cell-intrinsic PDL1 signals in non-cancer cells are discussed briefly, as are PDL1 signals from soluble and vesicle-bound PDL1 and PDL1 isoforms. We conclude with suggestions for addressing the most pressing challenges and opportunities in this rapidly developing field.

Neuropathic Itch

Neurologic insults as varied as inflammation, stroke, and fibromyalgia elicit neuropathic pain and itch. Noxious sensation results when aberrantly increased afferent signaling reaches percept-forming cortical neurons and can occur due to increased sensory signaling, decreased inhibitory signaling, or a combination of both processes. To treat these symptoms, detailed knowledge of sensory transmission, from innervated end organ to cortex, is required. Molecular, genetic, and behavioral dissection of itch in animals and patients has improved understanding of the receptors, cells, and circuits involved. In this review, we will discuss neuropathic itch with a focus on the itch-specific circuit.

Identification of Merkel cells associated with neurons in engineered skin substitutes after grafting to full thickness wounds

Engineered skin substitutes (ESS), prepared using primary human fibroblasts and keratinocytes with a biopolymer scaffold, were shown to provide stable closure of excised burns, but relatively little is known about innervation of ESS after grafting. This study investigated innervation of ESS and, specifically, whether Merkel cells are present in healed grafts. Merkel cells are specialized neuroendocrine cells required for fine touch sensation in skin. We discovered cells positive for keratin 20 (KRT20), a general marker for Merkel cells, in the basal epidermis of ESS after transplantation to mice, suggesting the presence of Merkel cells. Cells expressing KRT20 were not observed in ESS in vitro. However, widely separated KRT20-positive cells were observed in basal epidermis of ESS by 2 weeks after grafting. By 4 weeks, these cells increased in number and expressed keratins 18 and 19, additional Merkel cells markers. Putative Merkel cell numbers increased further between weeks 6 and 14; their densities varied widely and no specific pattern of organization was observed, similar to Merkel cell localization in human skin. KRT20-positive cells co-expressed epidermal markers E-cadherin and keratin 15, suggesting derivation from the epidermal lineage, and neuroendocrine markers synaptophysin and chromogranin A, consistent with their identification as Merkel cells. By 4 weeks after grafting, some Merkel cells in engineered skin were associated with immature afferents expressing neurofilament-medium. By 8 weeks, Merkel cells were complexed with more mature neurons expressing neurofilament-heavy. Positive staining for human leukocyte antigen demonstrated that the Merkel cells in ESS were derived from grafted human cells. The results identify, for the first time, Merkel cell-neurite complexes in engineered skin in vivo. This suggests that fine touch sensation may be restored in ESS after grafting, although this must be confirmed with future functional studies.

Interrater and intrarater reliability of the Semmes Weinstein aesthesiometer to assess touch pressure threshold in burn scars

Burn scars are frequently accompanied with sensory deficits often remaining present months or even years after injury. Clinimetric properties of assessment tools remain understudied within burn literature. Tactile sense of touch can be examined with the touch pressure threshold (TPT) method using the Semmes Weinstein monofilament test (SWMT). There is in recent research no consensus on the exact measurement procedure when using the SWMT. The aim of this paper was to determine the interrater and intrarater reliability of TPT within burn scars and healthy controls using the 'ascending descending' measurement procedure. We used the newly developed guidelines for reporting reliability and agreement studies (GRRAS) as a basis to report this reliability study. In total 36 individuals were tested; a healthy control group and a scar group. The interrater reliability was excellent in the scar group (ICC=0.908/SEM=0.21) and fair to good in the control group (ICC=0.731/SEM=0.12). In the scar group intrarater ICC value was excellent (ICC=0.822/SEM=0.33). Within the control group also an excellent intrarater reliability (ICC=0.807/SEM=0.27) was found. In conclusion this study shows that the SWMT with the 'ascending descending' measurement procedure is a feasible and reliable objective measure to evaluate TPT in (older) upper extremities burn scars as well as in healthy skin.

Neuropathic mechanisms in the pathophysiology of burns pruritus: redefining directions for therapy and research

Pruritus in burn wounds is a common symptom affecting patient rehabilitation. Over the last decades, there has been a resurgence of interest into more effective strategies to combat this distressing problem; nevertheless, no reports exist in the literature to propose pathophysiological mechanisms responsible for the generation and persistence of pruritic symptoms in the late phases of burns rehabilitation. Neuronal pathways mediating pruritic and painful stimuli share striking similarities, which allows the comparative exploration of the less extensively studied pruritic mechanisms using pain models. Furthermore, emerging anatomical, neurophysiological, and pharmacological evidence supports the involvement of neuropathic mechanisms in chronic burns pruritus. This work updates the conceptual framework for the pathophysiology of burns itch by embracing the contribution of the central nervous system in the maintenance of symptoms into a chronic state. The proposed pathophysiological model paves new avenues in burns pruritus research and is likely to have implications in the quest for more effective therapeutic regimens in clinical practice.

Concise review: tissue-engineered skin and nerve regeneration in burn treatment

Burns not only destroy the barrier function of the skin but also alter the perceptions of pain, temperature, and touch. Different strategies have been developed over the years to cover deep and extensive burns with the ultimate goal of regenerating the barrier function of the epidermis while recovering an acceptable aesthetic aspect. However, patients often complain about a loss of skin sensation and even cutaneous chronic pain. Cutaneous nerve regeneration can occur from the nerve endings of the wound bed, but it is often compromised by scar formation or anarchic wound healing. Restoration of pain, temperature, and touch perceptions should now be a major challenge to solve in order to improve patients' quality of life. In addition, the cutaneous nerve network has been recently highlighted to play an important role in epidermal homeostasis and may be essential at least in the early phase of wound healing through the induction of neurogenic inflammation. Although the nerve regeneration process was studied largely in the context of nerve transections, very few studies have been aimed at developing strategies to improve it in the context of cutaneous wound healing. In this concise review, we provide a description of the characteristics of and current treatments for extensive burns, including tissue-engineered skin approaches to improve cutaneous nerve regeneration, and describe prospective uses for autologous skin-derived adult stem cells to enhance recovery of the skin's sense of touch.

Postburn Scars: An Update

Burn wounds give rise to the largest scars we can find in human pathology, influencing patients' quality of life. Despite the improved knowledge on pathophysiology, efficacy of the various treatments remains unsatisfactory. In this short review recent literature is examined with a focus on recent data on postburn pathological scars epidemiology and risk factors, which underline the high prevalence and the long evolution, pointing to identify this illness as a systemic inflammatory one, more frequent in women and in those of younger age, regulated by local factors relevant in wound healing.

The forearm flap: assessment of functional and aesthetic outcomes and quality of life

PURPOSE

Microvascular surgery frequently represents the only possibility to reach satisfactory cosmetic and functional outcomes after tumor resection in the head and neck. We present the functional and aesthetic outcomes and quality of life (QOL) of our patients who underwent a forearm flap transfer.

PATIENTS AND METHODS

Thirty-one patients who were operated on in the departments of oral and maxillofacial surgery and otorhinolaryngology of the University of Essen participated in the study. Grip power, pinch power, and range of motion at the wrist as well as that in the metacarpophalangeal joints were measured. A sensory assessment including 2-point discrimination, temperature discrimination, and documentation of numbness, paresthesia, and scar pain was performed. The changes in hemodynamics were evaluated with digital optical blood pressure measurement and digital electronic pulse oscillography. Aesthetic satisfaction was investigated with an ordinal scale. Forearm circumference and hair growth on the flap were documented. Quality of life was evaluated with the EORTC QLQ-C30 (European Organization for Research and Treatment of Cancer Quality of Life Questionnaire version 3) and H&N35 (Head and Neck Module) after medical treatment and wound healing.

RESULTS

Strength, range of motion, and forearm circumference measurements showed no impairment in comparison with the opposite forearm. Subjective sensory disturbances were found in 31% of the patients but were all regarded as nondisturbing. On the split-thickness graft at the forearm, diminished sensory function could be proven and missing temperature discrimination was revealed in 93%. Evaluation of the hemodynamics showed clinical unremarkable vascularization changes, although slight differences were detectable in the collective. Female (64%) and male (77%) patients rated the aesthetic outcome in their forearm in most cases as good. Hair growth on the transplant could be observed, but it decreased with time and disturbed only a few patients in the oral cavity (14%). In comparison with normative data from a general German population, the QOL assessment showed that patients with tumors stated a similar QOL with regained emotional, cognitive, social, and role functioning. There were significant differences in the symptom scales and single items of the EORTC QLQ-C30 and H&N35. In comparison with a collective of patients with tumors of the oral cavity that were treated with various therapies, no difference was detectable.

CONCLUSIONS

Correct planning and elevation presupposed, there will be no clinically relevant finding in strength, motion, and hemodynamics in the forearm and hand after free forearm flap transfer. Sensory findings are present after the operation but decrease with wound healing or are nondisturbing. In this study, the patients with tumors judged the aesthetic result in their forearm in most cases as good. They stated a high QOL with regained emotional, cognitive, social, and role functioning, although they perceived symptoms caused by the tumor and therapy.

Enhanced inflammatory hyperalgesia after recovery from burn injury

Severe burn induces severe pain. While chronic as well as acute pain syndromes are reported, the peripheral mechanisms of burn-induced chronic pain syndromes have not been studied. We tested the hypothesis that burn induces plastic changes in primary afferent nociceptors that predispose to chronic pain states. Mechanical nociceptive thresholds were measured using the Randall-Selitto paw-withdrawal test in male Sprague-Dawley rats, before and following a small (<1% total body surface area) partial-thickness thermal injury to the dorsal surface of one hind paw. This burn induced mechanical hyperalgesia, which lasted over 2 weeks. After recovery, local injection of prostaglandin E2 (PGE2), to mimic re-injury, induced an enhanced and markedly prolonged mechanical hyperalgesia compared to the hyperalgesic effect of PGE2 in the control contralateral paw. This prolonged PGE2-induced hyperalgesia was reversed by a selective inhibitor of protein kinase C-epsilon (PKCepsilon). Our findings suggest PKCepsilon as a peripheral mechanism for burn-induced chronic pain syndromes.

Burn injury pain: the continuing challenge

The development of more effective methods of relieving pain associated with burn injury is a major unmet medical need. Not only is acute burn injury pain a source of immense suffering, but it has been linked to debilitating chronic pain and stress-related disorders. Although pain management guidelines and protocols have been developed and implemented, unrelieved moderate-to-severe pain continues to be reported after burn injury. One reason for this is that the intensity of pain associated with wound care and rehabilitation therapy, the major source of severe pain in this patient population, varies widely over the 3 phases of burn recovery, making it difficult to estimate analgesic requirements. The effects of opioids, the most commonly administered analgesics for burn injury procedural pain, are difficult to gauge over the course of burn recovery because the need for an opioid may change rapidly, resulting in the overmedication or undermedication of burn-injured patients. Understanding the mechanisms that contribute to the intensity and variability of burn injury pain over time is crucial to its proper management. We provide an overview of the types of pain associated with a burn injury, describe how these different types of pain interfere with the phases of burn recovery, and summarize pharmacologic pain management strategies across the continuum of burn care. We conclude with a discussion and suggestions for improvement. Rational management, based on the underlying mechanisms that contribute to the intensity and variability of burn injury pain, is in its infancy. The paucity of information highlights the need for research that explores and advances the identification of mechanisms of acute and chronic burn injury pain.

PERSPECTIVE

Researchers continue to report that burn pain is undertreated. This review examines burn injury pain management across the phases of burn recovery, emphasizing 3 types of pain that require separate assessment and management. It provides insights and suggestions for future research directions to address this significant clinical problem.

The reinnervation pattern of wounds and scars may explain their sensory symptoms

Anaesthesia, pruritus and pain are common in cutaneous scars. The reinnervation pattern of healing wounds and scars might help to explain these symptoms, as sensory neurotransmitters are known to be mediators of inflammation and healing. We quantified the regeneration patterns of blood vessels and nerves in excisional skin wounds as they matured into scars. Mice underwent 1cm(2) full thickness skin excisions. Wounds were harvested between five and 84 days. Sections underwent immunohistochemical staining for protein gene product 9.5 (PGP9.5) a pan-neuronal marker, and the sensory neuropeptides calcitonin gene related peptide (CGRP) and substance P (SP). The endothelial marker von Willebrand factor (VWF) was used to allow co-localisation and quantification of blood vessels. Nerve fibre density was quantified at multiple sites within wounds. There was no difference in the reinnervation/revascularisation pattern between peripheral and central sites. The density of PGP9.5, CGRP, SP and VWF peaked between 14 and 42 days, and levels of PGP9.5, CGRP and VWF all decreased to approximately those found in unwounded skin by 84 days (mature scar). SP levels, however, remained elevated at approximately twice the density found in unwounded skin. Increased densities of SP and CGRP in healing wounds could explain the unpleasant sensory symptoms of healing wounds.

[Burned palm reconstruction. Current concepts regarding grafting techniques, sensibility and hand function]

BACKGROUND

The purpose of this study was to evaluate the results of conservative and operative treatment for burn injuries in the palmar region of the hand. METHODS AND CLINICAL MATERIAL: One hundred ten patients from the severe burn center in Ludwigshafen, Germany were evaluated a mean of 28 months postoperatively. Sixty-one had been treated with split-thickness skin grafts (43 sheet and 18 mesh grafts). In addition to subjective evaluation (including pain assessment and aesthetic outcome rating by visual analog scale), objective outcome analysis included clinical tests, measurement of active range of motion and grip strength, and sensibility testing with the two-point discrimination and Semmes-Weinstein monofilament tests.

RESULTS

Of the patients, 90% were satisfied with the results, showing excellent pain relief with an average score under 13 on the visual analog scale. Aesthetic outcome was rated good with sheet grafts; in appearance, mesh grafts tended to be rated average or insufficient. Functional outcome tests demonstrated a significant correlation between depth of injury and range of motion. Grip strength analysis revealed superior results with sheet grafts. Sensibility in the injured areas was lower than on the contralateral hands. The Semmes-Weinstein test average was 3.4 degrees at the burned area vs 3.0 degrees in healthy hands, underscoring lower sensibility after burns.

CONCLUSION

Surgical treatment of burned palms leads to good subjective and objective results, if specialized burn units are involved. Overall sheet transplantation seems to be the better choice for surgical reconstruction of the palmar burned hand.

Sensory perception and neuroanatomical structures in normal and grafted skin of burn survivors

BACKGROUND

This study compared the neural structures found in grafted skin of burn survivors with neural structures found in site-matched normal skin and correlated these structures with psychophysical measures of sensation.

METHODS

Fifteen skin-grafted male burn survivors (47.7+/-10.4 years old) with deep partial- or full-thickness thermal burn injuries covering an average of 11+/-5.6% of their total body surface and with normal skin at a matching, unburned, contralateral site were recruited into this study. Threshold determinations and magnitude estimations for touch, cold, warmth and heat-pain were performed at sites with grafted and normal skin, using Semmes-Weinstein monofilaments and the Medoc TSA 2001 thermal stimulator. Skin biopsies from both the grafted and normal sites were stained with antibodies for protein gene product 9.5 (PGP) and neurofilament 200 kDa. Nerve fibers in the epidermis and nerve fibers or bundles of nerve fibers in the superficial and deep dermis as well as innervated blood vessels, hair follicles and sweat glands were counted.

RESULTS

On average, the data were collected 43.1+/-10.4 months after grafting. When thresholds on grafted skin were compared to thresholds on normal skin, they showed elevated sensory thresholds [touch (p<0.003), cold (p<0.031), warmth (p<0.009)]. Magnitude estimates of touch, cold and warmth differed on the two sides with sensations elicited from grafts being smaller than those from normal skin. Heat-pain thresholds and heat-pain magnitude estimations were not statistically different on the two sites. By comparison to the normal side, and consistent with the attenuated sensory functions of the grafts, counts of neural structures showed a reduction in innervation density; PGP-immunoreactive nerve fibers/bundles were reduced in grafted epidermis (p<0.026) and superficial dermis (p<0.001). The numbers of sweat glands (p<0.006) and hair follicles (p<0.001) were also reduced. The number of innervated blood vessels did not differ significantly on the two sides. There were significant correlations between sensory thresholds and the neuroanatomical variables: thresholds of cold and touch were correlated with the number of sweat glands in both grafted and normal skin (r2=0.56 and 0.50, respectively; p<0.001), while warmth thresholds were significantly correlated with the number of innervated blood vessels in grafted skin (r2=0.62, p<0.001). Encapsulated mechanoreceptors were not encountered in this study of hairy skin.

CONCLUSIONS

Touch, cold and warmth thresholds and magnitude estimations do not return to normal levels after skin grafting in burn survivors. The elevation of thresholds and reduction of sensory intensity is accompanied by a general decrease in the density of nerve terminals. The lack, or numerical reduction, of sweat glands and innervated blood vessels was also indicative of diminished sensation on grafted skin.

Substance P axons and sensory threshold increase in burn-graft human skin

BACKGROUND

Our knowledge of afferent nerve fiber reinnervation of grafted skin following third-degree burn is limited by a lack of quantitative histological and psychophysical assessment from the same cutaneous area. The current study compares fiber profile and functional recovery measurements in injured and control skin from the same subject.

MATERIALS AND METHODS

Nerve regeneration and modality-specific sensory thresholds were compared using immunocytochemical labeling with protein gene product 9.5 antibody to stain all axons and anti-substance P to label substance P axons (which are predominantly unmyelinated), as well as computerized instrumentation to obtain psychophysical estimates.

RESULTS

Compared to control skin, threshold measures of pinprick (P < 0.001), warming (P < 0.001), touch (P < 0.001), and vibration (P < 0.01) were significantly elevated in burn-graft skin and correlated with histological analysis of skin biopsies obtained from the same site. Immunohistochemical staining of all axons innervating the dermis and epidermis revealed a significant reduction in burn-graft relative to control skin (54% decrease, P < 0.0001). In contrast, the incidence of substance P nerve fibers was significantly elevated in burn-graft (177% increase, P < 0.05) and appeared to correlate with patient reports of pruritus and pain.

CONCLUSIONS

Observations support the hypothesis that sensory regeneration is fiber-size-dependent in burn-graft skin. The findings that substance P fiber growth increased while total fiber count decreased and that thermal threshold showed the greatest degree of functional recovery suggest that unmyelinated neurons have the greater ability to transverse scar tissue and reinnervate grafted skin following third-degree burn injury.

Nerve regeneration in a collagen-chitosan tissue-engineered skin transplanted on nude mice

A reconstructed skin made of a collagen-chitosan sponge seeded with human fibroblasts and keratinocytes and grown in vitro for 31 days was developed for the treatment of deep and extensive burns. The aim of this study was to assess whether this tissue-engineered skin could promote nerve regeneration in vivo, since recovery of sensation is a major concern for burnt patients. The human reconstructed skin was transplanted on the back of nude mice and the growth of nerve fibres within it was assessed 40, 60, 90 and 120 days after graft. Nerve growth was monitored by confocal microscopy using immunohistochemical staining of PGP 9.5 and 150 kD neurofilament, while Schwann cell migration was observed using protein S100 expression and laminin deposition. Nerve growth was first detected 60 days after transplantation and was more abundant 90 and 120 days after graft. Linear arrangements of Schwann cells were observed in the graft as early as 40 days after graft. Nerve growth was observed along these Schwann cell extensions 60 days after transplantation. We conclude that the three-dimensional architecture of the collagen-chitosan tissue-engineered skin sponge encourages nerve growth. This result provides new perspectives to increase nerve regeneration within the tissue-engineered skin by linkage of neurotrophic factors in the sponge before transplantation.

Changes in the density of protein gene product 9.5-immunoreactive nerve fibres in human oral mucosa under implant-retained overdentures

Reduced oral sensitivity and impaired masticatory cycles have been demonstrated in edentulous humans wearing removable dentures, and there is some evidence that these patients have a decreased innervation of the oral mucosa. Clinical and electrophysiological evidence shows that sensory performance improves after oral rehabilitation with implant-retained overdentures. The aim of this study was to compare the density of mucosal innervation in edentulous patients with that in dentate controls and to evaluate changes in the number or type of sensory receptors following placement of endosseous implants in these edentulous individuals. The mucosal innervation was evaluated by immunohistochemical assays for the neurospecific marker protein gene product 9.5, and the innervation pattern was compared with that of dentate controls. Morphometric analysis of the immunohistochemical material demonstrated a decrease in numbers of sensory receptors in the mucosa of edentulous patients and a significant increase in the number of nerve fibres in the mucosa covering the distal edentulous mandibular ridges supporting the prostheses after implant-retained rehabilitation. In contrast, there were only minor increases in the number of nerves in the peri-implant mucosa. These changes in innervation appear to be related either to the new biomechanical situation created by implant support, which favours more physiological tissue conditions, or to an adaptive mechanism in the peripheral processing of sensory stimuli. These changes may explain, at least partially, the clinically observed differences in sensory skills before and after implant placement.