Emerging Paradigms Integrating the Lymphatic and Integumentary Systems: Clinical Implications
This is an archived article reprinted with permission from Volume 9 Issue 2 of WCHM. Robyn Bjork and Heather Hettrick are leading experts in the field of wound care and lymphedema management.
Ernest Starling, in the late 1880s, introduced a model of capillary fluid exchange, based on hydrostatic and oncotic pressures (Starling, 1894). It was thought that increased hydrostatic pressure on the arterial side of the blood capillaries forced fluid into the interstitium, while lower hydrostatic pressure, coupled with higher oncotic pressure on the venous side of the blood capillaries, resulted in reabsorption of fluid. Until more recently, this was the prevailing understanding of fluid homeostasis, and formed the basis of wound, edema, and lymphedema education.
More recently, it has been established that the endothelial glycocalyx layer (EGL) controls the movement of proteins and fluid across the blood capillary wall. Despite prevailing principles regarding Starling’s Law, it is now understood that there is no reabsorption of fluid back into the venous side of blood capillaries. Rather, there is only diminishing net filtration across the capillary bed, and fluid and blood proteins are removed from tissues via reabsorption through lymphatic capillaries alone. Consequently, a new paradigm that all edemas are on a lymphedema continuum has emerged (Bjork, Hettrick, 2018).
Evidence suggests that areas of lymphatic failure produce regions of integumentary vulnerability subject to inflammation, infection and carcinogenesis, essentially, skin barrier failure (Carlson, 2014). This may be the basis behind most integumentary dysfunction and contribute to the development of various wound pathologies (complicated by underlying disease processes and comorbidities). These combined findings highlight the interconnectedness of the lymphatic and integumentary systems and the need for a more unified clinical approach for the management of patients with chronic wounds and lymphedema.
Structure & Function of the Endothelial Glycocalyx Layer
In 1940, Danielli first introduced the concept of a protein-based lining of all blood vessels that plays a vital role in fluid filtration. And, in 1966, Luft visualized this layer for the first time through electron microscopy. By 2007, the EGL gained recognition as controlling the movement of proteins and fluid across the blood capillary wall. Subsequently, Levick and Michel (2010) mathematically demonstrated that there is no net reabsorption of fluid back into the venous side of the blood capillaries, rather only diminishing net filtration into the interstitium. Then, in 2014, Mortimer and Rockson integrated this new understanding of “no net reabsorption” into their review “New developments in clinical aspects of lymphatic disease”, bringing it to the forefront of edema management.
A healthy EGL is approximately 0.5 um thick in the blood capillaries; it is progressively thicker in larger vessels, up to 4.5 um in carotid arteries (Reitsma et al., and Weinbaum, Tarbell, and Damiano, 2007). The EGL is made up of two continuous layers. The base is a slimy layer that coats the endothelial cells of the vessel wall. This slime or gel matrix is made up of chains of glycoproteins and proteoglycans that attach directly into the membranes of the endothelial cells, creating “backbones” (Reitsma et al., and Weinbaum, Tarbell, and Damiano, 2007). These backbones are linked together by a web of glycosaminoglycans that can absorb 10,000 times their weight in water (Biddle, 2013), thus creating a slimy gel layer. This sophisticated layer lines the endothelial cells of blood vessels and is integral in keeping fluid in or out, based on physiologic requirements. Within this base layer are wavy clefts, or channels, with tight junctions that control movement of fluid and protein through the EGL into the interstitium (Weinbaum, Tarbell, and Damiano, 2007). The EGL’s essential role is maintaining vascular homeostasis.
The second layer of the EGL is made up of soluble plasma components linked to each other in a direct way or via soluble proteoglycans and/or glycosaminoglycans (Reitsma et al., 2007). This layer is visualized as hair-like projections, extending into the lumen of the blood vessel, organized into a hexagonal matrix with roots that attach to the “backbone” proteins of the gel base layer (Weinbaum, Tarbell, and Damiano, 2007) (Figure 1). Since the “backbone” proteins are tethered into the endothelial cell membranes of the capillary wall, and crosslinked in the gel matrix, blood flow shear forces acting on the hairlike projections mechanically transmit this information into the endothelial cells themselves. The endothelial cells respond to the mechanical signals, such as producing and releasing nitric oxide, which dilates the vessel (Biddle, 2013).
The composite EGL, including soluble proteins and other components that bind to it, has a negative charge that repels red blood cells (RBCs) and platelets so they do not touch the vessel wall. This space between the RBCs and the EGL is called the “exclusion zone” (Reitsma et al., 2007). The EGL is dynamic and can “shed” in response to stimuli, such as during inflammation or disease states. During inflammation, this shedding of the hair-like projections allows more fluid to escape through the EGL. Shedding also exposes adhesion molecules (Reitsma et al., 2007) to which platelets or white blood cells (WBCs) attach. WBCs are squeezed tightest in the blood capillary where they enter the venule and are known to crush the EGL temporarily by 20% (Weinbaum, Tarbell, and Damiano, 2007). It is here that WBCs tether to exposed adhesion molecules and then remain tethered as they roll across the venule wall to exit into the tissues, known as diapedesis.
The EGL is particularly sensitive to ischemia, which can result in rapid shedding as a protective mechanism. A high fat, high cholesterol diet, oxidative low-density lipoproteins, and hyperglycemia also cause shedding of the EGL, and the EGL has been found to be thinner in areas prone to atherosclerosis (Reitsma et al., 2007). The EGL plays an important role in diabetes mellitus, peripheral arterial disease, reperfusion injury, intravenous fluid mismanagement, renal disease, and dialysis (Biddle, 2013). It also has an antithrombotic effect due to “enzyme docking” and plays an important role in reducing oxidative stress (Biddle, 2013). With this, it is important to appreciate the role and implications the EGL has with respect to integumentary dysfunction—inclusive of lymphatic and cutaneous disease.
To summarize, the functional importance of the vascular endothelial glycocalyx layer cannot be overemphasized. (See Figure 1.) Biddle, 2013, eloquently details the EGL’s functions as: regulation of vascular permeability, mechanotransducer regulating vascular tone, moderator for leukocyte and platelet adhesion, provides antithrombotic effect in vasculature, repulses red blood cells from the vascular endothelium, and reduces oxidative stress.
FIGURE 1. Vascular endothelial glycocalyx (Biddle, 2013). This figure is used with permission from the author.
No Net Reabsorption Exception
Once the interstitial fluid enters the lymphatic capillary, the lymph is funneled through pre-collectors and into collectors that propel the lymph toward lymph nodes via sequenced contraction of lymphangions coupled with one-way valves. Under normal conditions, ~4L of lymph re-enters the venous system at the venous angles in the neck. However, a sum of ~8L/day of fluid moves out of the blood capillaries and into the tissues (Levick, 2010; Renkin, 1986). The structure and function of the lymph nodes is essential to reconciling this apparent discrepancy.
In 1983, Knox et al. found that ~50% of the fluid portion of lymph is reabsorbed into the venous circulation via the blood capillaries within canine lymph nodes. The same year, Adair and Guyton demonstrated that increasing the venous pressure in canine lymph nodes resulted in movement of fluid back into the node, thereby reducing the concentration of proteins in the efferent lymph vessels. This highlights the role of the lymph nodes and lymphatics in fluid homeostasis, as well as the impact of chronic venous hypertension. Elevated venous pressure not only results in ultrafiltration from the blood capillaries but also slows reabsorption of fluid from the lymph nodes back into the venous circulation. The dense, capsular design of the lymph nodes, their placement in joint areas that are mechanically compressed by movement, and the presumed absence of EGL, all work synergistically to facilitate fluid reabsorption back into the venous system. Conversely, immobility and decreased joint movement through the full range of motion, lymph node removal, or venous hypertension can have a significant impact on fluid retention in the dermis and subcutaneous tissues. Ultimately, this stagnant fluid may lead to fibrosclerosis and deleterious alterations within these tissues.
New Paradigm: All Edemas Are on a Lymphedema Continuum
All edemas are on a lymphedema continuum. Since we now know that all swelling is managed by reabsorption by the lymphatic capillaries alone, the patency of dermal lymphatics and the efficiency of lymphatic drainage are paramount to edema management and wound healing. Interventions to help prevent damage to lymphatic capillaries, and techniques to facilitate lymphatic drainage and lymphangiogenesis need to be considered as part of wound management. As early as 1994 (Scelsi, et al.), damage to dermal lymphatics was observed in skin biopsies from patients affected by chronic venous insufficiency (CVI). In more recent studies using near-infrared fluorescence lymphatic imaging (NIRFLI) technology, baseline imaging showed impaired lymphatic function and bilateral dermal backflow in all subjects with chronic venous insufficiency, even those without ulcer formation (Rasmussen et al., 2016). As edema progresses to chronic edema, pathophysiological changes occur as a result of localized lymphatic insufficiency or failure. For example, swelling post-orthopedic surgery or traumatic injury or chronic edema surrounding a venous leg ulcer can lead to localized protein accumulation and degradation, resulting in localized inflammation and connective tissue proliferation. According to Foldi, “. . . stagnating high protein edema develops a pathohistological state of chronic inflammation, with infiltration of the tissue by mononuclear cells, angiogenesis, proliferation of connective tissue, fibrosis and fibrosclerosis . . .” He further goes on to describe how oxidation and degradation of interstitial proteins attracts monocytes (macrophages) that, in turn, ingest proteins and activate fibroblasts and adipocytes. This activation results in connective tissue and adipose proliferation. As such, wounds and impaired cutaneous function are highly associated with inflammation and fibrosis associated with lymphatic dysfunction.
In 1976, Robert Stemmer defined a test used for differential diagnosis of lymphedema, which later was corroborated sonographically, macroscopically and microscopically by Brenner, Putz, and Moriggl in 2007. In its original description by Stemmer, “a thickened longitudinal skinfold when pinching the toes is a clinical sign for the early diagnosis of a lymphoedema, and delimits it from a pure venous oedema” (Stemmer, 1976). In the Best Practice for the Management of Lymphoedema (Framework, 2006), the Stemmer test is described as being performed on the second toe or middle finger, attempting to pinch and lift the skin. The test is considered positive for lymphedema when a skin fold cannot be raised, but a negative sign does not exclude lymphedema. In 2007, Brenner, Putz, and Moriggl showed that in individuals with lymphedema and a positive Stemmer sign, both the dermis and subcutaneous tissue of the second toe were thickened and the structure of the dermal layers destroyed, as well as an accumulation of edematous fluid in free spaces within the subcutaneous tissue.
In light of the new paradigm that all edemas are on a lymphedema continuum, co-author Robyn Bjork proposes an expanded version of the Stemmer test. This new test, named the “Bjork Bow Tie Test,” can be performed anywhere on the body to assess for inflammation and thickening of the integument that can occur with lymphatic dysfunction, such as around chronic wounds. To perform the test, in one maneuver gently pinch, lift and rotate the skin between the thumb and pointer finger, noting quality of tissue texture and thickness. Healthy skin can be lifted and pinched, should feel slippery between the layers, and produce a “bow tie” of wrinkles when rotated. (See Figures 2 and 3.) Skin that is positive for lymphedema will be thickened, less pliable, less able to be pinched and lifted, more difficult to rotate, and produce limited or no “bow tie” of wrinkles.
FIGURES 2 and 3. Pictured are Bjork Bow Tie Tests performed on different areas of skin with varying thicknesses. Both tests are negative for lymphedema, exemplified by the distinguishing “bow tie” of wrinkles.
For skin that may be distended from edema and cannot be lifted because of it, still observe for the “bow tie” of wrinkles when performing the technique. Areas of skin should be demarcated on a body map to indicate where the test is positive (+) or negative (-). Even a slightly positive test should be marked as positive (+). The subcutaneous tissue should also be assessed in a similar fashion. A negative test does not exclude lymphedema but means that the dermis and subcutaneous tissues have not yet developed the pathohistological changes described previously.
Relationship Between Lymphatic and Integumentary Systems
It is well established that there is a paucity and lack of standardization with respect to wound and lymphedema education in traditional medical and health professions education. If such education is provided, it is likely segmented and taught in separation rather than in parallel or unison. In 2008, a study by Patel et al. compared wound education in medical school curricula between the United States, Germany and the United Kingdom. The results of this retrospective indicated that the “total hours of required wound education received in the United States was 9.2 hours in the 4 years of medical school. In the United Kingdom, the total time devoted to wound-related issues equaled 4.9 hours over 5 years. In Germany, a total of 9 hours of wound education was provided over 6 years.” This study concluded that there is a deficiency with respect to wound education in preparing future physicians to manage wounds.
With respect to lymphedema, the education is even more sparse. A survey study published in 2011 by Vuong, Nguyen and Piller regarding the level of lymphatic education provided in medical schools around the United States, indicated that most programs devoted 30 minutes or less to teaching lymphatic function in the first two years of medical school. Further, nearly 40% of respondents indicated that 1-3 hours of time was devoted to the lymphatic system, while 25% indicated that 15 minutes or less was spent on the topic. The apparent lack of dedicated time in traditional medical education is further compounded by the fact that these two systems are highly interdependent, meaning impairment in one system directly impacts the other.
Carlson describes in his 2014 review article how lymphatic failure produces a cutaneous region susceptible to infection, inflammation and carcinogenesis, which he describes as a locus minoris resistentiae or path of least resistance. He explains in his article how lymphatic failure causes a disruption of adaptive immunity by “decreasing or obstructing immune trafficking by antigen, lymphocytes, macrophages and dendritic/antigen presenting cells (Langerhans cells) to the lymph node crating a cutaneous region of immunosuppression. All these abnormalities create a region of immunosuppression . . . or a condition called lymphatic dermopathy, which is failure of the skin as an immune organ.” In essence, lymphatic impairment can lead to integumentary dysfunction and integumentary dysfunction can exacerbate lymphatic dysfunction. For clinicians, it is important to recognize the interdependence these systems have on one another so proper diagnosis and interventions can be delivered.
To summarize, the EGL is the gatekeeper for blood capillary fluid exchange. There is only diminishing net fluid filtration, and no reabsorption across the blood capillaries of the dermis and subcutaneous tissues. All fluid and blood proteins moving into the interstitium must be removed via reabsorption through the lymphatic capillaries alone. Thus, all edemas are on a lymphedema continuum and are at risk of developing chronic inflammation, dermal thickening and connective tissue proliferation. The new, “Bjork Bow Tie Test” can be used to test for these integumentary changes anywhere on the body, including around wounds. Skin that is positive for lymphedema will be thickened, less pliable and produce limited or no “bow tie” of wrinkles.
Assessment of the lymphatics is important in chronic wound management, as impairment in one system indicates impairment in the other with varying levels of complexity and clinical presentation. Improved collaboration is needed between physicians, vascular/vein specialists, wound specialists, lymphedema therapists and other health care professionals, to establish cohesiveness of paradigms and common language. By working more closely together, progress toward effective, multi-disciplinary care, particularly for individuals with lower extremity lymphedemas and chronic wounds, is achievable.
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About the Authors
ROBYN BJORK, PT, CLWT, CWS, CLT-LANA is the founder and CEO of the International Lymphedema & Wound Training Institute (ILWTI). She is a physical therapist who is an expert in the field of wound care and lymphedema management and has over 20 years experience, spanning outpatient, inpatient, transitional, long-term, home-care and private practice settings.
Bjork is a featured speaker at national and international conferences, such as the Symposium on Advanced Wound Care, European Wound Management Association, Canadian Association of Wound Care, Wild on Wounds, National Lymphedema Network International Conferences, the Lymphatic Education & Research Network’s Symposium Series, and numerous regional lymphedema and wound conferences. Bjork has written, produced, and implemented over 155 hours of advanced, accredited curriculum for certifying wound and lymphedema specialists in the United States and abroad (www.ilwti.com). She is co-author on the chapter on lymphedema in Textbook of Chronic Wound Care: An Evidence-Based Approach to Diagnosis and Treatment and has authored articles in Wounds International, Pathways and Wound Care Advisor.
Bjork is a Certified Lymphedema and Wound Therapist through the International Lymphedema & Wound Training Institute, a Certified Wound Specialist through the American Board of Wound Management, and a Certified Lymphedema Therapist through the Lymphology Association of North America. She is a proud member of the American Physical Therapy Association, the Lymphology Association of North America, the Association for the Advancement of Wound Care, the World Alliance of Wound and Lymphedema Care, and the American College of Phlebology.
Bjork is a board member and past secretariat of the World Alliance of Wound and Lymphedema Care. She is an international expert in the morbidity management of podoconiosis and lymphatic filariasis and a chapter author on podoconiosis for a soon-to-be-published global wound and lymphedema management handbook for WAWLC and the World Health Organization. In recognition of her international work, in 2011 Bjork received the Central Florida Humanitarian Award and was featured in the commemorative edition of Space Coast Medicine magazine. She also received a Certificate of Special Congressional Recognition in appreciation of excellence in medicine.
HEATHER HETTRICK PT, PHD, CWS, CLT-LANA, CLWT is an associate professor in the Physical Therapy Program at Nova Southeastern University in Ft. Lauderdale, Florida. As a physical therapist, her expertise resides in integumentary dysfunction where she holds three board certifications.
Heather has diverse work experience in academia and the private sector. She is a key opinion leader and is actively involved in numerous professional organizations, conducts research, and publishes, presents and teaches nationally and internationally on integumentary related issues.
Dr. Hettrick is a past president of the American Board of Wound Management and is an active board member of the Association for the Advancement of Wound Care and World Alliance of Wound and Lymphedema Care.