Disease of the lymphatics can be either developmental or acquired. Developmental disorders include heritable forms of lymphatic pathology, such as Milroy’s disease, as well as congenital vascular malformations, such as Klippel-Trenaunay syndrome (Table 40-1). Acquired forms of lymphatic disease may arise from damage to, or disruption of, the lymphatics by trauma, infection, neoplasms, or iatrogenic causes. These iatrogenic sources of lymphatic vascular insufficiency include trauma induced by surgery or radiotherapy. Heritable forms of lymphedema are uncommon, with the highest frequency associated with Klinefelter’s syndrome.¹ Globally, lymphatic filariasis contributes the greatest share of the lymphatic disease burden,² but in the developed world, where filariasis is distinctly uncommon, most lymphatic pathology arises as a direct consequence of the treatment of malignant melanoma and breast and pelvic malignancies. Lymphedema is a common complication of treatment and has increased as the number of cancer survivors has grown.³

Lymph is the product of interstitial fluid, originating from ultrafiltration of plasma at the capillary level. When interstitial fluid enters the lymphatic vascular compartment, it is designated as lymph; ultimately, lymph is returned to the blood circulation through the central lymphatics, including the thoracic duct, which empties into the left subclavian vein. Lymph is the body fluid compartment through which mass transport, intercellular signaling, and immune trafficking occur; in addition, it is the major route for lipids to be absorbed from the gut.⁴

The lymphatic vasculature represents the third component of the vascular system. These vessels are morphologically distinct from arteries and veins but are developmentally derived from a common progenitor of veins. Anatomic derangements of the lymphatic vasculature most commonly manifest clinically as lymphedema.

Lymphedema arises when the lymph transport from tissue is impaired. Lymphedema is characterized by tissue fluid accumulation that can arise from overproduction of lymph, reduced clearance of lymph, or varying degrees of both. Lymph overproduction may arise from enhanced capillary permeability induced by inflammatory states or burns; the increased venous pressure that results from thrombotic venous occlusion, venous insufficiency, or right-sided heart failure; or conditions that lead to diminished plasma oncotic pressure, such as the hypoproteinemia observed in nephrotic syndrome or severe malnutrition. Lymphedema impairs protein clearance from the tissues and, with chronicity, often leads to alterations in tissue architecture, manifest as cutaneous and subcutaneous fibrosis as well as lipodystrophy. The affected appendages or trunk becomes dysmorphic and may exhibit decreased range of motion, pain, or a combination thereof.

Treatment options for lymphedema exist but rarely, if ever, lead to complete resolution. Current treatment approaches focus on management of edema by means of manual decongestive therapy and external compression garments. Surgical options have had limited success in treatment of patients with lymphedema and have been most successful in those with discrete injuries to the lymphatics. Current research aims at elucidating the underlying pathophysiology of lymphedema as well as the mechanics of lymphatic development, with the hope that targets for molecular therapeutics will be identified.

The discovery of the lymphatics was first noted in the paper “De Lacteibus sive Lacteis Venis,” Quarto Vasorum Mesarai corum Genere novo invento, authored by Gasparo Aselli, a Milanese professor of anatomy and surgery, in 1627. Aselli was particularly noted for his vivisections. During a canine vivisection, he divided the mesenteric lymphatics, which produced chylous return. A subsequent vivisection did not produce the white, cream-like material that he had previously seen. He was able to deduce that production of chyle was dependent on the time of prior feedings. It was from this discovery that he proposed that the white network of cords originating from the intestines were not nerves. He was able to replicate this finding in other species. This work was furthered by Thomas Bartholin, a Danish anatomist; Jean Pecquet, a French scientist whose particular interest was the thoracic duct; and Olaus Rudbeck, a Swedish scientist and linguist.

The course of the lymphatics was further examined by Marie Philibert Constant Sappey, who co-authored the first French textbook combining anatomy with histology. He devised a technique to identify the lymphatics by injecting mercury directly into the vessels. Von Recklinghausen successfully stained the lymphatics with silver nitrate and was then able to differentiate blood vessel capillaries from lymphatic capillaries. However, it was not until the advent of electron microscopy and immunohistochemical staining that significant delineation of the structure of the lymphatic vessels could be accomplished.^5,6 Until recently, the lymphatics were not considered medically significant. It is through the more recent understanding of the role of lymphatics in the metastatic spread of solid tumors that interest has been stimulated in this heretofore unexplored realm of research and clinical care.

The lymphatics closely resemble the structure of blood vessels and have a common embryonic origin.⁷ The prototypical lymphatic capillary has a single-cell layer of endothelium in a blind-ended tube arrangement. In culture, lymphatic endothelial cells (LECs) form “cobblestone” monolayers, which then form tubules. These tubules express histologic markers identical to the blood vasculature, including F-actin, fibronectin, Weibel-Palade bodies, and von Willebrand factor. However, compared with blood capillaries, the basement membrane of lymphatic capillaries shows a higher degree of fenestration; indeed, the basement membrane may be entirely absent.

Lymphatic vessels do not show the level of organization typical of arterial or venous vessels. The larger, collecting lymphatic vessels have three distinct layers: intima, media, and adventitia. They possess a smooth musculature that confers intrinsic contractility. Analogously to veins, the lymphatics possess intraluminal valves that promote forward flow.

The lymphatics of the lower limbs are divided into a superficial anicial deep system. The superficial system, which originates within the skin, is composed of a medial and lateral channel. The medial channel begins at the dorsum of the foot, follows the course of the saphenous vein, and passes through the inguinal lymph nodes. The lateral channel begins at the lateral aspect of the foot, travels proximally, and then crosses to the medial side at the level of the mid-leg. From there, the lateral channel vessels travel with the medial channel lymphatics through the inguinal lymph nodes. The deep lymphatics chiefly communicate with the superficial system through the popliteal and inguinal lymph nodes. The deep lymphatics travel adjacent to the deep blood vessels before passing through the inguinal nodes.

The small and medium lymphatics empty into the main channels. These channels are often parallel to the spine and are seen near the inferior vena cava below the level of the diaphragm. In the mediastinum, the thoracic duct crosses to the left of the spine at approximately the fifth thoracic vertebrae, remains anterior to the spine, and ascends to the base of the neck. There it terminates in the left brachiocephalic or left subclavian vein. Other large main channels may join with the thoracic duct or empty directly into the great vessels.

The anatomy of the lymphatics can be understood from their common origin within the venous system. Budge and Huntington in chickens and cats, respectively, were able to localize the development of lymph sacs from vein buds.⁸ Frances Sabin posited that the origin of the lymphatics was within the cardinal vein.⁹ Zebrafish studies of the lymphatic development corroborate this hypothesis.¹⁰ A proposed mechanism for differentiation based on lineage-specific growth factors has been presented by Oliver.¹¹ It is presumed that a single type of endothelial progenitor cell (EPC) gives rise to all three classes of vessels. It is not known if there is a single type of EPC, but evidence points to a common origin because all endothelia cells (ECs) of the embryonic cardinal vein have the ability to respond to lymphatic growth factors.⁷ All three types of ECs have been induced from EPCs.¹²

Lymphatic Competence

Lymphatic competence precedes lymphatic commitment. The initiation of lymphatic development occurs after vasculogenesis. Expression of endothelial hyaluronan receptor-1 (LYVE-1) and vascular endothelial growth factor receptor-3 (VEGFR-3) define lymphatic competence.¹¹³ LYVE-1 has only recently undergone scrutiny as an important marker for tumor-induced lymphangiogenesis.¹⁴

Blood endothelial cells (BECs) express LYVE-1 in the lymphatic competence phase.¹¹⁵ LYVE-1 has been localized to tissues other than LECs, having been observed in macrophages, hepatic sinusoidal cells, and embryonic arterial endothelial cells in the yolk sac of chick embryos, an area devoid of lymphatic vessels.^16,17 Its expression is downregulated in hepatocellular carcinoma and cirrhosis. Antibodies to LYVE-1 have been successfully used in murine models and, in combination with antibodies to prospero-related homeobox (Prox-1), have immunohistochemically identified lymphatic channels in the skin.¹⁸ The ease of use of an anti–LYVE-1 antibody has allowed investigation into the hitherto poorly defined lymphatic microvasculature.^18,19 The physiological role of LYVE-1 is still not well understood.

VEGF is expressed in all lineages of the vasculature— arteries, veins, and lymphatics. Five isotypes have been identified. They include VEGF-A, VEGF-B, VEGF-C, VEGF-D, and VEGF-E as well as three known receptors, VEGFR-1 (also known as Fms-like tyrosine kinase receptor [Flt-1]), VEGFR-2 (Flk-1), and VEGFR-3 (Flt-4).²²⁰ In lymphatic endothelia, representative members of the VEGF family are the ligands VEGF-C and VEGF-D and their receptor VEGFR-3.^21,22,23,24

VEGFR-3 is broadly expressed during the development of the vasculature in both blood and lymphatic vessel lineages.²⁵ This receptor is necessary for the maturation of both lineages. Targeted inactivation of the gene encoding VEGFR-3 resulted in derangement of the large vessels. Vasculogenesis and angiogenesis occurs, but embryos develop pericardial effusions and resultant heart failure by embryonic day 9.5.²⁶ Assessment of the lymphatic vasculature is invariably inconclusive. VEGFR-3 is initially expressed in blood vascular endothelial cells (BECs) and subsequently downregulated in these cells; conversely, expression is maintained in lymphatic endothelial cell (LEC) progenitors and eventually becomes limited to the LECs. VEGFR-3 is primarily expressed in the lymphatic vasculature.^24,27 It is also expressed in wound granulation tissue and capillaries associated with tumor angiogenesis.^24,28 After differentiation, VEGF-C and VEGF-D continue to exert their influence and, in concert with angiopoietin-2 (Ang2) interactions with the Tie2 receptor, lead to formation of mature lymphatic vessels.^29,30,31

In addition, a mucin-type transmembrane glycoprotein, podoplanin/T1α, interacts with RhoA, a small GTPase. Selective blocking with small interfering RNA effectively silences podoplanin expression, preventing lymphatic endothelial capillary tube formation.³² Likewise, a cell-permeable inhibitor of Rho inhibits lymphatic endothelial capillary formation in the same manner. It can be concluded that early activation of RhoA depends on podoplanin expression. However, the nature of the interaction between podoplanin and VEGFR-3 has yet to be characterized.

Lymphatic Commitment

Nuclear transcription factor Prox-1 has been found to be expressed in lymph sacs originating from the cardinal vein.^15,26,27 Prox-1 has been found to be exclusive to committed lymphatic lineage cells. Prox-1 expression reciprocally suppresses blood vascular–specific genes in venous endothelial cells.³³ Moreover, Prox-1 must be constitutively expressed to maintain the lymphatic phenotype.^3,34 However, Prox-1 inactivation does not prevent lymphatic budding from the cardinal vein. As demonstrated in Prox 1 -/- knockout mice, LEC buds form but maintain blood vascular markers and fail to commit to the lymphatic cell markers. It is thought that Prox-1 induces lymphatic endothelial cell differentiation via integrin α9 and VEGFR-3.³⁵ Integrin α9 had previously been shown to function as a receptor for VEGF-C and VEGF-D.³⁶ Upregulation of integrin α9 and VEGFR-3 may lead to recruitment of ECs and subsequent maturation and proliferation of LECs. Prox-1 changes the fate of ECs to LECs from the default endpoint of blood vasculature.^19,27,34

Lymphatic Specification

Committed LECs continue to express Prox-1. They migrate peripherally from the cardinal vein and begin to express lymphatic-specific markers.^27,37 It has been shown that conditional deletion of Prox-1 leads to dedifferentiation of LECs into BECs, implying that continued Prox-1 expression is necessary to maintain the LEC phenotype.^3,11 BECs markers are suppressed with Prox-1 in a dose-dependent fashion, as seen in human LECs, in which knockdown of Prox-1 is accomplished after transfection with siRNA. The human LECs with reduced Prox-1 expression express little to no secondary lymphoid cytokine (SLC) or podoplanin. FOXC2 and β-tubulin, which are genes involved in lymphatic valve development, are also downregulated. BEC markers such as endoglin and CD34 are ectopically expressed. The implication is that Prox-1 activity is required to maintain lymphatic identity; thus, the EC phenotype is mutable and therefore not irreversible.

Committed LECs begin to bud from the cardinal vein on murine E10.5. Budding and migration lead to development of lymphatic sacs in the periphery that will eventually become the nascent lymphatic structures throughout the embryo. These structures are first known as primary lymphatic sacs. During this stage, LECs express neuropilin 2 (Nrp2), a component of the VEGFR-VEGF pathway,^38,39 as well as podoplanin/T1α.^40,41 From the lymphatic sacs, secondary budding and migration occur, leading to capillary formation in a centrifugal fashion. These newly formed vessels eventually constitute the lymphatic vasculature throughout the tissues.¹³

Neuropilin 2 is a receptor for class III semaphorins and is also one of the possible mediators for the signaling pathways VEGFR-2 and VEGFR-3.^7,42,43 Inactivation of Nrp2 leads to reduced LEC proliferation and decreased number of lymphatic capillaries.⁴³ Nrp1 is limited to arteries⁴³ and may have the ability to compensate for the loss of Nrp2. Nrp2-/- pups survive until maturity, but Nrp1-/- pups survive until E13.5. Nrp1/Nrp2 double-null pups survive until around E8.5 and have severe vascular defects.⁷

Podoplanin/T1α expression occurs some time after Prox-1 expression. Podoplanin/T1α pups die after birth secondary to respiratory failure⁴⁰ and develop severe lymphedema from defective lymphatic function and patterning. Podoplanin/T1α was not found to affect lymphatic development in the intestines. Deficiency of podoplanin/T1α had no effect on LYVE-1 expression. The changes in lymphatic formation appear to be related to podoplanin/T1α’s role in lumen formation and migration of LECs.⁴⁰

The process of lymphovenous separation may be directed by three additional signaling molecules: Slp-76, Syk, and PLCg2.^11,44 Slp-76 is not expressed on ECs but rather on T cells, macrophages, mast cells, natural killer cells, neutrophils, immature B cells, and platelets.⁴⁴ Syk is expressed by ECs and both hematopoietic lineages, myeloid and leukemoid.⁴⁵ PLCg2 is essential to B-cell maturation and several Fc receptors.⁴⁶ Inactivation of any of the three signaling molecules leads to phenotype characterized by blood-filled lymphatics. Slp-76-/- and Syk-/- embryonic stem cells develop blood cell lymphatics when localized to lymphatics in chimeras.⁴⁷ The implication is that loss of endothelial cell expression of either signaling molecule or expression on hematopoietic cells is responsible for the abnormal phenotype.

By E14.5, LECs have matured into differentiated lymphatic vessels and will continue to organize into the network that will lead to the mature lymphatic vasculature.³⁷ Desmoplakin and β-chemokine D6 receptor are expressed in the late stage of terminal differentiation. The mature lymphatic network expresses the complete adult profile of lymphatic markers soon after birth.⁷

Secondary lymphangiogenesis denotes generation of lymphatic tissue in any context not directly associated with embryonic lymphatic development. Secondary lymphangiogenesis originates de novo from lymphatic capillaries in proximity to, but still independent from, blood vessels.^28,48 Lymphangiogenesis can be seen in inflammatory states, such as psoriasis, and in persistent airway infection with Mycoplasma pulmonis in a rodent model.⁴⁹ The latter may represent a mechanism for bronchial lymphedema. The exact contribution of the newly formed lymphatics to the persistence or resolution of chronic inflammation is undefined.¹³

Metastatic disease through the lymphatic system was historically thought to be a passive process. Lymphatic metastasis is common to many tumor types, and its clinical identification has become an integral part of cancer staging. Recent research has focused on the implications of lymphangiogenesis, its role in the biology of metastasis, and the implications of anti-lymphangiogenic therapeutic strategies. Systematic investigation of tumors has shown increased expression of VEGFR-3 and VEGF-C in many tumor types, suggesting lymphangiogenic induction and metastatic transformation.^50,51,52,53 Blocking of VEGFR-3 signaling suppresses cancer spread by preventing lymphangiogenesis and lymph node metastasis in a mouse model of lung cancer.⁵⁴ Breast, prostate, cervical, pancreatic, and other cancers have demonstrated VEGFR-3 agonist expression or utilization of the VEGF-VEGFR-3 signaling pathway.^{50,51,55,56,57,58} Evidence suggests that tumors expressing VEGF-C or VEGF-D, or lymphatic tissue with upregulated VEGFR-3, predict metastasis to regional lymph nodes, tumor progression, increased mortality, and decreased patient survival.^59,60,61,62 VEGFR-3 blockade has not been shown to affect preexisting lymphatic vessel morphology or function,^54,63,64 but clinical application of anti–VEGFR-3 therapeutics is not yet available.

The role of the lymphatics was hypothesized by William Hunter, William Cruikshank, and William Hewson. They proposed that the role of the lymphatics was to remove fluid from the limbs and that they take part in the body’s response to infection.⁶⁵ The experiments of Karl Ludwig and Ernest Starling later lent credence to these theories. Ludwig cannulated the lymphatic vessels and was able to drain lymph directly from them. He analyzed the lymph and thought it to be a filtrate of plasma. Starling thought that the forces of hydrostatic and oncotic pressure bore the same relationship in the lymphatics as they did in the blood vessels.

Starling was one of the first to propose that interstitial fluid is an ultrafiltrate of blood. He postulated that lymph is produced as a function of the opposing forces of capillary hydrostatic pressure, tissue oncotic pressure, interstitial hydrostatic pressure, and plasma oncotic pressure. The first two forces drive filtration, and the latter two support absorption. Filtration typically exceeds reabsorption by 2 to 4 L/d. Net filtration of protein from the bloodstream, approximately 100 g/d, may occur, with albumin representing the primary protein component. The interstitial fluid also receives waste products, including protein and ammonia, as well as foreign matter, including bacteria and viruses. Lymph nodes are strategically placed within the lymphatic vasculature, with the ability to sample the lymphatic fluid and thereby promote immune surveillance.

The functions of the lymphatic system can be conceptualized as providing a parallel system of mass transport separate from the bloodstream, thereby supporting homeostasis of the interstitial fluid and providing a system of immune trafficking. As a mass transport system, its three functions are (1) reabsorption of excess interstitial fluid, protein, and waste products; (2) filtration and removal of foreign material from the interstitial fluid; and (3) absorption of lipids from the intestine. As an immune trafficking system, it provides surveillance of the interstitial space and acts to distribute immune cells and substances from lymphoid tissue to the circulation.

The actual transport of fluid is driven by a combination of autocontractility of the vasculature and extrinsic compression in addition to negative thoracic pressure. These forces are exploited by an intraluminal valve system in the larger vessels as well as a probable two-valve, unidirectional system in the capillaries.^66,67 Flow within the lymphatic system may be modified by circulating hormones and prostanoids.^{68,69,70,71,72,73}

Lymphedema arises when the net flow of fluid into the tissues is positive. The implication is that the influx of fluid is greater than the ability of the lymphatics to remove the fluid. This may occur when the production of lymph increases, when transport by the lymphatics is reduced, or both. Increases in lymph production may arise when Starling forces shift net pressure to favor flow of fluid into the interstitium. Increases in venous pressure result in increased venular and capillary hydrostatic pressure and an increase the driving force for ultrafiltration; diminution in oncotic pressure, such as that seen hypoproteinemic states such as malnutrition, have a similar effect. Elevated venous pressure occurs in individuals with disease states such as right heart failure, deep vein thrombosis, and venous insufficiency. Local inflammation increases capillary permeability, thereby accelerating loss of fluid and plasma proteins to the interstitium. Lymph production may increase 10- to 20-fold in inflammatory states.⁷⁴ Hypoproteinemia, seen in states such as cirrhosis and nephrotic syndrome, leads to development of lower extremity edema through the same net effect of fluid flow into the interstitium.

Primary Lymphedema

Primary lymphedema is the term applied to an inborn defect in the lymphatic anatomy or function. This may or may not present clinically early in life. Its prevalence is estimated at 1.15 in 100,000 individuals.⁷⁵ The prototype of primary lymphedema is Milroy’s disease, a condition that has more recently been associated with mutations with inactivating mutations of the tyrosine kinase domain of VEGFR-3.^76,77 Given the numerous causes of lymphedema, both known and unknown, there is no unified classification scheme. Two useful classification systems can be considered, based upon anatomic patterns vasculature or age of onset.

Anatomic Patterns. This scheme relies on an anatomic description of the lymphatic vasculature:^78,79,80

1. 
   

   Aplasia: No collecting vessels seen

   
   
2. 
   

   Hypoplasia: Decreased number of vessels

   
   
3. 
   

   Numeric hyperplasia: Increase in the number of vessels

   
   
4. 
   

   Hyperplasia (megalymphatics): Increase in the number of vessels and vessels with demonstrated tortuosity and dilatation and have valvular incompetence

Age of Onset. This scheme divides primary lymphedema into three categories:²

1. 
   

   Congenital lymphedema: At birth or near birth

   
   
2. 
   

   Lymphedema praecox: Onset after birth up to age 35 years; typically presents during the peripubertal years

   
   
3. 
   

   Lymphedema tarda: Onset after age 35 years

Involvement may be limited to the distal or proximal lymphatic vessels and nodes but may involve both. More than half of all cases of primary lymphedema involve the proximal lymphatics or proximal nodes. Intranodal fibrosis has been demonstrated in histopathology.⁶⁷ These cases tend to be predominantly female.^79,80 Proximal cases tend to be unilateral in presentation and greater in severity. Also, the extent and severity of abnormality are more likely to progress. Chronic proximal disease may eventually affect the uninvolved distal lymphatics.

About one-third of all primary lymphedema cases involve the distal lymphatic vessels with grossly normal proximal vessels.^80,81 Unlike proximal disease, distal disease tends to bilateral and rarely progressive to the same limb or uninvolved limbs after 1 year since initial presentation. The girth of the limb may continue to increase in 40% of patients. The overall prognosis is thought to be good.

Bilateral hyperplasia or tortuous dilatation of the lymphatic channels is seen in a minority of patients. There is a slight male predominance. The prognosis is worse, and involvement is greater in patients with mega-lymphatics.

Parsing the Phenotypic Classification. Primary lymphedema disorders are gradually being characterized with the help of molecular diagnostics. Studies have begun to elucidate the derangements found in abnormal development that give rise to diseases such as Milroy’s disease and lymphedema–distichiasis. Meige’s disease, also known as lymphedema praecox, is relatively more common than the autosomal dominant Milroy’s disease.⁸² Sporadic cases of primary lymphedema are more frequent than hereditary lymphedema.⁸⁰ Familial cases of lymphedema are not trivial and represent opportunities to understand the genetic mechanism implicit in lymphedema.

The discovery of the genetic mechanisms that predispose to primary lymphedema has accelerated. The most studied signaling pathway, involved in Milroy’s disease, or hereditary lymphedema type I, is the VEGF–VEGFR-3 pathway. At least 21 mutations have been described in VEGFR-3 (or the flt4 gene), and all of them have been found to involve a missense mutation involving the tyrosine–kinase domain.^{76,83,84,85,86,87,88} A murine model of Milroy’s disease (the Chy mouse) is characterized by poorly functioning lymphatics and may be a candidate for gene therapy.⁸⁹ Chy mice develop hypoplastic lymphatic vasculature and pathognomonic chylous ascites. Overexpression of ligands to VEGFR-3 induces the development of functional lymphatic vessels.⁸⁹ This implies that VEGF-C and VEGF-D signaling may promote lymphangiogenesis. An IgG-based neutralizing antibody composed of an Fc domain and the fusion of the ligand portion of VEGFR-3 competitively inhibits VEGF-C and VEGF-D in a dose-dependent manner.⁹⁰ A blocking antibody consisting of the constitutive keratin-14 promoter and VEGFR-3-Ig was able to prevent development of lymphatic vasculature and causes edema in a similar manner to primary lymphedema.

Using VEGF-C as a therapeutic target via gene or growth factor modalities may prove to be fruitful as a method of promoting lymphangiogenesis.^89,91,92 An adenoviral vector of recombinant human VEGF-C was used to transfect a mouse. This transgenic mouse was then crossed with a Chy mouse. Overexpression of this specific ligand, VEGF-C156S, was able to rescue lymphatic function in offspring.⁸⁹

Forkhead-related transcription factor FOXC2+/- knock-out mice develop lymphedema–distichiasis syndrome.⁹³ This heritable lymphedema leads to bilateral lower limb and distichiasis, an abnormal growth of eyelashes from meibomian glands on the tarsal plate of the eyelids. The latter component can lead to corneal scarring. Inheritance is autosomal dominant and is related to truncating mutations in FOXC2.⁹⁴ Other lymphedema disorders are linked to FOXC2 mutations and do not necessarily share the features of distichiasis.⁹⁵ A number of metabolic syndromes, such as diabetes mellitus, dyslipidemia, and hypertriglyceridemia, have been linked to FOXC2,^96,97,98 as have venous valve failure and cardiac outflow tract abnormalities.^99,100

Microsatellite analysis has been used to confirm transcription factor gene SOX18 as the mutation present in a family with hypotrichosis–lymphedema–telangiectasia.⁷⁷ SOX18 has been found to be expressed in atherosclerotic lesions and pancreatic tumors.^77,101,102 SOX18’s role in lymphatic development is poorly understood, but it may be fundamental to early, common growth mechanisms as it is expressed in granulation tissue during angiogenesis.¹⁰³

Other rare types of primary lymphedema include Aagenaes’ syndrome, or lymphedema–cholestasis. This defect has been mapped to chromosome 15q.¹⁰⁴ Aneuploidy, as seen in Turner’s syndrome and Klinefelter’s syndrome, shows association with lymphedema, as do neurofibromatosis and Noonan’s syndrome. Lymphedema is also associated with a number of vascular malformation disorders, including yellow nail syndrome, lymphangiomyomatosis, arteriovenous malformations, and intestinal lymphangiectasia.

Secondary Lymphedema

Secondary lymphedema may be acquired by damage to the lymphatics through surgical procedures, irradiation, trauma, infection, and direct malignant invasion.¹⁰⁵ Secondary, or acquired, lymphedema in the developed world is most commonly iatrogenic and directly related to therapies that cause some insult to the lymphatic system.¹⁰⁶ The burden of lymphatic disease is best underscored by the 250,000 of the estimated 2 million breast cancer survivors who develop lymphatic dysfunction.²

The pathophysiology stems from the accumulation of protein-enriched fluid in the interstitial spaces. Impaired lymphatic function invokes a cascade of pathologic processes, ultimately leading to a state of fibrosis, regional immunosuppression, and inflammation. Architectural remodeling in the affected tissues leads to hypercellularity of adipocytes, fibroblasts, and keratinocytes followed by the accumulation of inflammatory cells, consisting chiefly of polymorphonuclear cells. Macrophages and monocytes migrate into the tissue, accelerating the tissue expansion, fibrotic changes, lipid deposition, and matrix degeneration. Chronic lymphedema evolves from the early soft swelling of the tissues to the dense, fibrotic scarring of late disease.¹⁰⁵ Structural derangements in dermal and subcutaneous tissues are marked.^{105,107,108,109} Inflammation may represent a future target for therapeutics.

Infection. Lymphatic filariasis, also known as elephantiasis, is a nematode infection endemic to tropical and subtropical regions in Asia, Africa, North America, and South America. It remains the most common cause of secondary lymphedema in the world. Wucheria bancrofti as well as Brugia malayi and Brugia timori use mosquitoes as a vector while in the microfilarial form.^110,111 Subsequent infection induces lymphangitis and resultant fibrosis of the lymph nodes. The World Health Organization estimates that more than 120 million people worldwide are infected. More than 40 million have severe disfiguration and are severely incapacitated. One-third of the infected are located in India, and another third are in Africa. The remainders are located in other parts of South Asia, the Pacific, and the Americas. Diagnosis is made clinically but may be confirmed by Giemsa-stained, thick blood film made from a blood sample taken at night because filaremia typically occurs during the nocturnal phase. Polymerase chain reaction and antigenic assays have also been used for confirmation. There is an associated eosinophilia that can be seen in tissue biopsy or on blood smear.

Diethylcarbamazine is the preferred agent for treatment. It requires a 2-week dosing regimen.^111,112 Ivermectin has become an alternative regimen because it has a shortened treatment time (single dose) and a better side effect profile.¹¹ More recently, Wolbachia bacteria have been implicated as integral to inducing the host immune response necessary for the pathogenicity of filarial.¹¹⁴ Elimination of Wolbachia spp. from nematodes with doxycycline leads to abnormal larval development and markedly decreased adult worm fertility and viability. Novel strategies for elimination of filariasis have included an 8-week treatment course with doxycycline.

Lymphedema may also occur from bacterial infections that originate from the skin flora. Recurrent episodes of lymphangitis result in thrombosis and fibrosis of the lymphatic channels. Cellulitis, one of the most common causes of lymphedema, is typically caused by streptococcus. Bacteria gain access to the body via fissures induced by trichophytosis or defects in the skin.¹¹⁵ Lymphedema itself leads to recurrent infections because there is decreased immunosurveillance and impaired host response within the affected limb. Tuberculosis may also present as lymphedema. Mycobacteria may spread in the lymphatics. The initial presentation may be as a scrofula that progresses to lymphedema, often affecting the face.^116,117

Cancer-related Lymphedema. Lymphedema typically arises after surgeries involving lymph node excision or irradiation.¹¹⁸ In patients who have undergone cervical, axillary, or inguinal lymphadenectomy, there is at least a 14.9% incidence of late lymphedema. Subgroup analysis shows that up to 40% patients undergoing groin dissections developed lymphedema. Malignancies that typically involve structures adjacent to the lymphatics or were particularly prone to lymphatic spread and thusly tumor nodal staging and lymph node dissection are strongly associated with lymphedema. These include breast cancer, gynecologic cancers, urologic cancers, soft tissue sarcomas, and malignant melanoma.

Breast Cancer. The most morbid and perhaps most dreaded effect of breast cancer treatment, beyond recurrence, is lymphedema. It is also the most common form of lymphedema encountered in developed countries. The risk of lymphedema increases with the extent of surgical dissection and if the lymph bed is included in the radiation treatment volume. Observational studies report a 20% incidence of lymphedema after axillary lymph node dissection.^119,120 A recent self-reported prospective population study of 433 breast cancer patients reported a 40% cumulative incidence at 5 years.¹²¹ Breakdown of the study group by type of surgery was not included.

Risk factors for lymphedema include radiation therapy to the lymph nodes^{120,122,123,124,125,126} and the extent of axillary lymph node dissection.^{120,125,127,128,129,130,131} The greatest risk in seen in patients who receive both treatment modalities.^{128,132,133,134,135,136,137} In patients with node-negative disease, 5% developed lymphedema after sentinel node biopsy. Patients who received standard axillary dissection had a 13% incidence of lymphedema. In patients who received both surgery and radiotherapy, the incidence of lymphedema was 28.3%. Recent surgical techniques have focused on preservation of the lymphatics¹³⁸ and appropriately reducing the extent of axillary lymph node dissection.¹³⁹ Sentinel lymph node biopsy still carries a significant risk of lymphedema.^140,141 Other risk factors include obesity.

Gynecologic Malignancies. Cervical, uterine, endometrial, and vulvar malignancies are all associated with lymphedema. Patients with cervical cancer treated with radical hysterectomy alone have a lymphedema incidence that ranges from 5% to 25%.^142,143 Uterine and endometrial cancers carry a somewhat lower risk. The reported incidence of lymphedema in endometrial cancer after total abdominal hysterectomy and bilateral salpingo- oophorectomy (TAH/BSO) was 11% when there was concomitant lymphadenectomy.¹⁴⁴ No cases of lymphedema were reported in patients who underwent TAH/BSO alone. Of those who underwent lymphadenectomy, 13 had adjuvant radiotherapy. Patients in this study group who underwent lymphadenectomy and developed lymphedema had significantly more pelvic lymph nodes removed (P = .016).¹⁴⁴ Patients with uterine malignancies who underwent therapy have reported frequencies as high as 17.7%.^{144,145,146,147,148} Vulvar malignancies have the highest incidence of lymphedema of all the gynecologic cancers. Radical vulvectomy is associated with the highest incidence lymphedema at 48%.¹⁴⁹