Burn Progression in Human Skin—A Review of Current Knowledge and Opportunities for Future Research

Abstract

Treatment of a burn wound often depends on the vertical depth of injury, which is commonly determined by visual assessment. Burn progression is the concept that in the early postburn period, a partial-thickness burn may progress to a deeper burn requiring surgery. Therefore, the initial appearance of the wound may not be indicative of the eventual extent of injury. Several preclinical studies attribute burn wound progression to events such as vasoconstriction, inflammation, programmed cell death, free radical damage, and microvascular occlusion. Due to the concern for translatability of animal models for burn wounds, human studies are essential to understanding burn progression in patients. Unfortunately, only a few small human studies exploring mechanisms including apoptosis, ischemia, and infection exist. Inherent to determining burn progression is burn-depth determination and healing potential, an area of research that has many ongoing investigations without a clear standard method to replace visual evaluation. Treatments to prevent burn progression in humans, including the use of negative pressure wound therapy and the application of cooling dressings, have been studied with small sample sizes. Here, we aim to summarize the current data on human burn progression. In addition, we discuss novel methods that could be used in future research to define early burn wound progression. Future work in human tissue should focus on the assessment and timeline of progression, explore the reversibility and prevention of injury progression and use animal models in parallel as complementary tools for hypothesis-driven research based on findings in humans.

Graphical Abstract

Graphical Abstract.

INTRODUCTION

Burn injuries are classified by mechanism (ie, thermal, electrical, and chemical), percentage of total body surface area (TBSA) affected, and burn depth.¹ While TBSA percentage often correlates with the need for initial resuscitation, depth of burn injury determines the need for surgical intervention. Burn depth is classified as superficial, superficial partial thickness, deep partial thickness, and full thickness based on the extent of injury.² Superficial burns are confined to the epidermis and are painful but do not blister. Both superficial partial-thickness and deep partial-thickness burns extend into the dermis and have variable clinical appearances including hyperemic, blanchable, blistering, and pale, depending on factors such as the time elapsed since injury, body location, and mechanism. Full-thickness burns affect the entire dermis and may extend into underlying adipose or muscle layers, requiring surgical treatment to reduce healing time and avoid adverse functional and aesthetic outcomes.³ The primary technique used to determine burn depth is visual assessment, which relies heavily on surgeon expertise and is a subjective interpretation.⁴ This remains challenging early after injury as the initial appearance of the burn depth may not be indicative of the eventual depth of injury due to the progression of the injury over time (Figure 1).

Figure 1.

Visual Evolution of Burn Appearance in 4 Patients. All Patients Except A Proceeded to the Operating Room for Excision and Autografting. Patient A Underwent Hydrosurgical Debridement on Day 12 But Healed Without Autografting on Day 19. PBD = Postburn day.

Burn wound progression or “conversion” is the process, previously identified through multiple animal models, by which partial-thickness burns that could have healed without surgery progress to deeper partial-thickness or full-thickness burns that require surgery for optimal healing (Figure 2).^5–8 This concept relates to Jackson’s burn theory in which he described 3 concentric zones of burn injury: zone of coagulation, zone of stasis, and zone of hyperemia.⁹ The zone of coagulation is the area that sustained the most direct injury with irreversible tissue damage, whereas the zone of hyperemia is the surrounding area of increased perfusion that promotes tissue healing. The zone of stasis, or the “at-risk” zone, is the area of potentially reversible cellular damage and is the least understood.¹⁰ Most human burn conversion hypotheses, based on animal studies, stress the importance of the zone of stasis, proposing that this zone is most susceptible to local and systemic factors. Therefore, early intervention may allow this tissue to heal adequately, whereas insults to the area may result in the progression of irreversible tissue damage.

Figure 2.

Jackson’s Zones of Injury and Burn Progression Schematic (Created in https://BioRender.com)

MECHANISM OF BURN PROGRESSION BASED ON PRECLINICAL STUDIES

There are several proposed mechanisms of burn progression, most of which have been studied exclusively in animal models. Ischemia, inflammation, reactive oxygen species, programmed cell death, and microvascular occlusion are believed to be contributors to burn wound progression (Figure 3). Furthermore, animal studies also suggest that progression can be prevented early after injury suggesting the zone of stasis can be protected. Comprehensive reviews on burn progression in animals have been published, therefore, we will summarize the current state (Table 1) and refer the reader to the reviews for more detail.^10,22,23

Figure 3.

Proposed Mechanisms of Burn Wound Progression Studied in Various Animal Models (Created in https://BioRender.com)

Table 1.

Understanding of Burn Progression in Animal Models

Mechanism	Model	Findings	Reference
Ischemia	Rabbit	Systemic administration of epinephrine to induce dermal vasoconstriction leads to progression of burn necrosis	Knabl et al.11
	Rodent (rat)	Immediate application of warm water to induce vasodilation delayed burn progression and reduced the surface area extension of tissue necrosis	Tobalem et al.12
Inflammation	Swine	Treatment with an autoantibody-inhibiting peptide decreased dermal injury and increased re-epithelialization compared to control	Sadeghipour et al.13
	Rodent (rat)	Inhibition of NLRP3 inflammasome activation with 3,4-methylenedioxy-β-nitrostyrene is associated with decreased burn depth, less severe collagen denaturation, increased residual hair follicles, and less inflammation at the site of injury	Xiao et al.14
	Rodent (rat)	Topical treatment with antitumor necrosis factor-alpha hyaluronic acid conjugates reduced burn progression by nearly 30%	Sun et al.15
Reactive oxygen species	Rodent (rat)	Oral ingestion of an herbal supplement, Savda Munziq, known for its antioxidative properties significantly decreased the number of apoptotic cells in the zone of stasis	Zhou et al.16
	Rodent (mice)	A bioactive hydrogel encapsulated with epigallocatechin gallate-copper capsules with ROS-scavenging properties accelerated burn wound healing by promoting epidermal and dermal regeneration	Li et al.17
	Swine	Application of metal chelation lotion to burn wounds reduced both horizontal and vertical progression likely via mechanism of attenuating oxidate stress	El Ayadi et al.18
Microvascular occlusion	Rodent (mice)	When treated systemically with an erythropoietin derivative helix beta surface peptide, the microvasculature within the burn wound remained patent and progression to a full-thickness burn was prevented	Bohr et al.19
	Swine	Dilation of peripheral microvasculature with a fibronectin-derived peptide reduced erythrocyte occlusion, increased re-epithelialization at 10 days, and decreased scar depth at 28 days	Asif et al.20
	Rodent (rat)	Subcutaneous injection of low molecular weight heparin increased fibroblast proliferation and angiogenesis of the zone of stasis compared to control	Uraloğlu et al.21

Abbreviations: NLRP3, nucleotide-binding domain, leucine-rich–containing family, pyrin domain-containing-3; ROS, reactive oxygen species.

Despite a large body of literature using animal models to investigate burn wound progression, their applicability to human burn progression is questionable given the inherent difference in skin anatomy and function (Figure 4). For example, small animal models such as mice or rats have significantly different histologic and physiologic characteristics of their skin compared to the epidermis, dermis, and dermal appendages of humans. While larger animal models, including pig, have skin architecture that more closely resembles human skin, the skin lacks widespread eccrine glands, the dermal and adipose tissue is much denser than humans, and the hair follicles penetrate the subcutaneous tissue. Furthermore, there are financial and space constraints that limit their widespread utilization.¹¹ For these reasons, confirmation of burn wound progression in humans is essential to facilitate the development of future therapeutic interventions that may prevent further injury.

Figure 4.

Human, Pig, and Mouse Skin in Cross-Section Stained for Hematoxylin and Eosin Demonstrating the Differences in Thickness and Dermal Appendages Between Species. Scale Bars = 200 microns

PATHOGENESIS OF BURN CONVERSION IN HUMANS

While there have been several studies examining the pathophysiology underlying burn wound progression in animal models as noted above, equivalent studies in humans are scarce. This is due to the limitations of studying human subjects, including restrictions on the timing of obtaining blood and wound samples, lack of baseline values, and interindividual heterogeneity due to factors such as age, comorbidities, and initial care provided, as well as intraindividual heterogeneity of the wounds. Additional challenges in studying burn progression in humans include the variability in clinical practice across burn centers, the need for pragmatic multicenter trials to enroll the large number of subjects needed to detect significance given the vast variability in real-world practice, as well as the lack of funding to support these types of studies. Furthermore, a recent scoping review assessed obstacles in conducting human studies early after traumatic injury, including the logistical challenges in obtaining consent in this patient population.¹² Proposed mechanisms of progression that have been studied in humans include apoptosis, ischemia, and infection (Table 2). Cutaneous biopsies collected within the first week of hospitalization for burn injury were evaluated for dermal apoptotic rates and a significantly higher rate of apoptosis was found in deep partial-thickness burns compared to superficial partial-thickness burns, full-thickness burns, and unburned skin. The authors postulate that dermal cell apoptosis in deep partial-thickness burns could explain the progression of these wounds into full-thickness burns.^13,14 In one study, human tissue samples collected from burned regions that varied according to time from injury, mechanism, and body location were analyzed for connexin 43, a gap junction protein that allows for the spread of pro-apoptotic signals between cells. They found that in early and intermediate burned samples, dermal fibroblasts in the zone of stasis have increased expression of connexin 43 as well as cleaved-caspase 3, indicating high levels of apoptosis.¹⁵ This study supports prior preclinical studies showing that apoptotic signaling in the zone of stasis may contribute to burn progression.

Table 2.

Human Studies Evaluating Burn Progression

Objective	Model	Proposed mechanism	Study results	Reference
Mechanism	Human burn excisional samples	Apoptosis	Upregulation of Connexin 43 in the zone of stasis may promote apoptosis-induced burn progression	Feng et al.24
	Human burn tissue samples	Apoptosis	Deep partial-thickness burns have a higher rate of apoptosis compared to superficial partial-thickness and full-thickness burns	Gravante et al.13
	Human burn patients	Ischemia-reperfusion	Initial standard resuscitation in burn patients is associated with ischemia-reperfusion injury and progression of burn wounds as evaluated by LDI	Jaskille et al.25
Mechanism/Treatment	Human burn patients	Infection	Clinically observed reduction in burn conversion from deep dermal to full thickness with the introduction of topical silver sulphadiazine into practice	Sawhney et al.26
Treatment	Human burn patients	N/A	Use of negative pressure therapy improved perfusion, decreased edema formation, and was associated with a lower rate of required skin grafting	Kamolz et al.27
	Human in vivo burn samples	N/A	Treatment with ex-vivo cooling device prevented burn progression at 3 hours following injury	Wright et al.28
	Human ex-vivo burn samples	N/A	Application of a non-pre-cooled bacterial nanocellulose dressing decreases wound temperature and burn necrosis via evaporative cooling	Holzer et al.29

Abbreviation: LDI, laser Doppler imaging.

Additional studies have investigated the role of hypoperfusion and tissue hypoxia on the progression of burn wounds. A single study of patients with large burns (TBSA ranged from 40% to 82%) found that during initial resuscitation as guided by the Parkland formula, there were episodes of repetitive ischemia and reperfusion with associated changes in the base deficit. Though not explicitly evaluated, the authors suggest that this ischemia-reperfusion injury and production of oxygen-free radicals could be responsible for the deepening of burn wounds seen in the first several days after injury.¹⁶ Infection of burn wounds is not only a significant cause of burn sepsis but is also thought to play a role in burn progression. Although not directly concluded to be due to a reduction in infection rates, early studies of topical silver sulphadiazine showed that there was a correlation between daily use of the cream and a reduction in the conversion rate of deep dermal to full-thickness burns at a single center.¹⁷ Studies have also shown that the pH of the burn wound surface is associated with burn healing potential and progression. Burns with higher pH were more likely to require excision and grafting, and while the exact mechanism is unknown, a higher pH has been associated with increased bacterial colonization while a lower pH can be a marker of restoration of the stratum corneum in more superficial burns.¹⁸

TREATMENTS TO PREVENT BURN CONVERSION IN HUMANS

Fluid resuscitation and local wound care, with special attention to preventing desiccation and infection, is crucial in the early postburn period to reduce the degree of progression. Early excision of necrotic tissue, typically defined as 24-72 hours following injury, has been the primary treatment to prevent burn wound progression; benefits include decreased risk of wound infection and mortality in large burns.¹⁹ However, since this intervention risks overexcision of potentially regenerative tissue and progression can continue despite the removal of necrotic tissue, alternative approaches to prevent burn progression have been explored. The use of subatmospheric pressure therapy in the form of a negative pressure wound therapy (NPWT) device was studied on patients with bilateral partial-thickness hand burns. Negative pressure wound therapy was applied to the burned hand which clinically appeared to be a more extensive injury. The hand treated with an NPWT was found to have hyperperfusion as detected by Indocyanine Green (ICG) microangiography as well as a reduction in edema formation. Ultimately, only 2 burned regions in the NPWT-treated group required grafting whereas 4 regions in the control group required grafting, suggesting that NPWT has the potential to prevent burn progression.²⁰ However, with small numbers and without precise objective measurements of burn depth, it is challenging to know if comparisons to assess progression started with an equal baseline.

Additional studies have been conducted to examine the effect of cooling applications on burn wound progression. One study utilized tissue that would be discarded in women who were anesthetized for breast reconstruction surgery and created burns on the tissue that were subsequently cooled for 20 minutes. Uncooled burns demonstrated progression over a 3-hour period following injury, whereas burns treated with the cooling apparatus were found to have a larger area of viable dermis after 3 hours.²¹ Similarly, in a human ex-vivo skin model, burns were created followed by the application of a bacterial nanocellulose dressing to cool the burn via evaporation. This wound dressing resulted in less necrosis and less dermal-epidermal separation, consistent with the prior studies that suggest immediate cooling of the wound prevents further burn progression.³⁰ A prospective cohort study of patients with less than 10% TBSA burns found that the application of cool, running water for 20 minutes within 3 hours of injury was associated with a reduction in burn wound depth but not the need for grafting.³¹ The authors suggest that initial treatment of burns with cool water can prevent the progression of superficial and superficial partial-thickness burns, but is unable to prevent the progression of deep partial-thickness injury. Potential mechanisms for these findings include decreased cell death as hypothermia has been shown to decrease cell energy requirements, and attenuation of the inflammatory response through inhibition of histamine and bradykinin-induced vasodilation.

GAPS IN KNOWLEDGE

There are current limitations in the evaluation of burn-depth progression in humans starting with challenges in determining the depth of injury itself. The primary technique used to determine burn depth is visual assessment, relying heavily on surgeon expertise and subjective interpretation.⁴ This assessment is accurate in less than 70% of cases, necessitating more sophisticated techniques for evaluating burn depth.² Histology from a wound biopsy has long been regarded as the “gold standard.” However, even this method has challenges in that it is invasive and only samples a small area at a single time point. Furthermore, dermatopathologists who are experts in evaluating skin diseases, lack the training in burn injury to reliably interpret what the morphology means for regenerative capacity. One group created a burn biopsy algorithm using over 800 patient samples at the time of excision and grafting to determine if a simplified method could detect whether a burn would heal by 21 days. The study confirmed past literature on the misclassification rates of burn depth. Unfortunately, the ability to heal wounds within 21 days was unknown as many patients underwent surgical excision of their burns well before this time.²⁴

Less invasive techniques that would allow serial assessment of burns have been studied in animals and humans.^25,26 Despite numerous studies of advanced imaging techniques such as laser Doppler imaging, laser speckle contrast imaging, and ICG microangiography (reviewed extensively elsewhere^27–29,32), to our knowledge none of these techniques have been adopted as early diagnostic tools to assess burn progression in humans. There are several other technologies that have been proposed to evaluate burn wounds including optical camera imaging, photoacoustic imaging, and ultrasonic techniques that have also been reviewed previously.^27,33 Ultimately, the lack of a true “gold standard” method to determine burn depth and regenerative capacity limits the ability to optimize these technologies in humans as there is no ground truth for the images.

While some studies have evaluated burn wound fluid or serum to predict wound healing potential, to date there is no clinically acceptable biomarker.^34,35 Further investigations into the burn wound microenvironment at the transcriptional and translational level are being studied and could provide the detail necessary for further refinement of future advanced imaging techniques that, in combination, may allow prognostication of healing.³⁶ For example, with the increasing sophistication and availability of spatial transcriptomics, understanding the burn wound microenvironment with the spatial context within the tissue would allow direct identification (ground truth) of what is happening within the different depths within injury.³⁷ Importantly, additional studies are needed to understand how these techniques can be applied to characterize burn depth and predict burn progression.

CONSIDERATION OF CLINICAL CORRELATES TO BURN PROGRESSION

There are other clinical pathologies that may provide insight into our understanding of necrosis and progression. For example, the neurologic disorder, transient ischemic attack (TIA), was classically defined as a focal neurologic deficit that persists for less than 24 hours.³⁸ That definition has since been challenged with a shift to a tissue-based definition. Transient ischemic attack is now understood to be a transient episode of neurologic dysfunction caused by focal ischemia.³⁹ The model of ischemia in the brain is similar to that of a burn wound: the ischemic core is the region of irreversibly damaged tissue in the center, the penumbra refers to the surrounding tissue that is at risk for further injury but is still salvageable, and benign oligemia refers to the outermost zone with a reduction in blood flow (note: this is in contrast to the zone of hyperemia, in which there is increased perfusion).⁴⁰ Diffusion-weighted imaging will show restricted diffusion in the infarcted core, whereas a perfusion abnormality represents the penumbra. With this definition, TIA will show evidence of the penumbra on imaging, and not infarction.⁴¹ The ischemic penumbra, or “at-risk” tissue, is the target of early stroke reperfusion therapy and has been the focus of many studies since its conception over 40 years ago. Current work is underway to identify advanced imaging techniques, such as diffusion kurtosis imaging, that can identify the boundaries of the penumbra.⁴² Additional studies are underway to explore circular RNA derived from oxoglutarate dehydrogenase as a potential biomarker for the penumbra in patients with an acute ischemic stroke.⁴³ The emphasis on the understanding of the “at-risk” tissue in ischemic strokes suggests the focus of current burn injury research could be shifted as well. Perhaps understanding and identifying the potentially salvageable tissue in the zone of stasis could better inform clinical practice than determining total burn depth at the time of injury.

Another analogous disease process lies in the cardiac system, where myocardial infarction is characterized by an area of ischemic myocardial tissue surrounded by adjacent, viable tissue known as the “border zone.”⁴⁴ Typically due to atherosclerotic plaque formation causing vessel occlusion, the affected tissue becomes hypoxic resulting in cardiomyocyte cell death. The border zone contains normoxic tissue, and the oxygen gradient between these 2 areas is thought to increase the susceptibility of the border zone to pathophysiologic remodeling.⁴⁵ This abnormal remodeling leads to hypocontractility and arrhythmias of the border zone cardiac tissue.⁴⁶ Tissue heterogeneity of the border zone is often evaluated by cardiac magnetic resonance imaging, and prior studies have shown that quantification of the infarct core and border zones is associated with a greater incidence of future cardiovascular events.⁴⁷ Furthermore, heterogeneity of the infarcted and surrounding peri-infarct tissue is an independent predictor of postmyocardial infarction mortality.⁴⁸ Importantly, the postinfarction border zone has been found to extend into the surrounding myocardium, resulting in the progressive loss of contractility during cardiac remodeling.⁴⁹ In contrast to burn-depth progression that likely occurs over the first hours to days following injury, myocardial border zone progression occurs over several weeks as the heart remodels.⁵⁰ It is possible that early progression in burn wounds and the inflammatory signaling that occurs in the microenvironment may affect the ultimate “remodeling” of the remaining viable cells over the next several weeks to years. Future studies are needed to explore the relationship between the early extent of progression and burn healing potential. Through examination of analogous processes in the neurologic and cardiac symptoms, one may gain insights into the understanding of the “at-risk” region in burn injury (Figure 5). Specifically, it may be prudent to focus on imaging detection and identification of the zone of stasis in humans to target further interventions.

Figure 5.

Neurologic, Cardiac, and Dermatologic Conditions With Analogous “At-Risk” Tissue Zones (Created in https://BioRender.com)

CONCLUSIONS AND FUTURE DIRECTIONS

Evaluation of burn depth is a major focus of research in the burn field and includes methods such as visual inspection, wound fluid analysis, wound pH monitoring, advanced imaging, and artificial intelligence as described above. The goal of these diagnostic technologies should be to determine burn depth, prognosticate healing potential, and objectively guide treatment decisions. Ideally, improving the precision of burn-depth determination and healing potential would allow treatments to be tailored to utilize the least complex or invasive technology most efficiently and cost-effectively. For example, mid to deep dermal-depth burns potentially have many treatment options such as synthetic dressings, acellular and cellular skin substitutes that depend upon the amount of viable dermis remaining after surgical excision. Previous dogma pushed early excision and grafting in deep dermal and full-thickness burns to reduce inflammation and scarring. However, a new paradigm is possible where wounds containing enough viable dermis after early precise excision or enzymatic debridement may be able to heal with minimal scarring using a “skin substitute” with or without widely meshed autograft underlay in a donor site-sparing approach. The challenge in adopting this approach is the gap in knowledge of the microenvironmental changes that occur in human burns in the first hours to days after the injury, including progression to deeper injury and how this impacts the effectiveness of each treatment. To understand the impact of progression, there are several important areas of study, such as the timeline of progression, the types of burns that are prone to progress, and the reversibility or prevention of injury progression. Ultimately, characterization of burn progression at a microenvironmental level in humans is needed to truly understand the implications of a specific burn depth (Figure 6).

Figure 6.

Areas for Future Research and Development (Created in https://BioRender.com)

There has been substantial progress in preclinical studies to understand the underlying mechanisms and possible treatments to prevent burn wound progression. Unfortunately, the inherent physiologic and anatomic differences of animal models limit the translation of these findings in humans. Therefore, there is a critical need to further explore these proposed mechanisms in human patients. To address these concerns, a method of determining burn depth and healing potential in a reliable, reproducible, and ideally noninvasive fashion must be identified. Furthermore, a deeper understanding of the effects of both systemic and local inflammation, burn-associated microvasculature, and cell death signaling pathways is necessary to identify intervenable targets to prevent burn progression.

AUTHOR CONTRIBUTIONS

Angela Gibson (Conceptualization [lead], Investigation, Methodology, Resources [supporting], Supervision, Writing—review & editing [lead]) Mary Junak (Investigation, Methodology, Resources, Writing—original draft [lead], Writing—review & editing [supporting])

CONFLICT OF INTEREST STATEMENT

All authors declared that there are no conflicts of interest.