Plasma leakage and intrinsic coagulopathy are the pathological hall marks in dengue haemorrhagic fever (DHF). Viral virulence, infection enhancing antibodies, cytokines and chemical mediators in the setting of intense immune activation are the key players implicated in the pathogenesis of DHF; the exact nature of which is yet to be fully understood. The pathophysiological changes the attended clinical features of plasma leakage necessitate recognition of changing physiological parameters for the early recognition of plasma leakage and appropriate fluid therapy. On the other hand, the changes in the haematological indices resulting from coagulopathy can tempt the clinician to initiate other modalities of therapy. A clearer understanding of the pathogenesis of DHF and the appreciation that both of these fundamental pathological changes share common pathogenic mechanisms would facilitate the appropriateness of management decisions and the early recognition of severe disease. Thus, thrombocytopaenia, reduced fibrinogen, and prolonged partial thromboplastin time early in the disease course connoted severe disease and attended plasma leakage rather than clinical bleeding. The detection of plasma cytokine profile by a multiple bead immunoassay could also complement clinical parameters in predicting severe disease early in the disease course. Thus, MIP-
Infection by any one of the four serotypes of dengue virus (DENV) remains asymptomatic in the vast majority. Clinical spectrum among symptomatic infection ranges from undifferentiated fever (viral syndrome), dengue fever (DF), and dengue haemorrhagic fever (DHF) to the expanded dengue syndrome with isolated organopathy (unusual manifestations). DF can be without haemorrhage or have unusual haemorrhage, while DHF can be without shock or with shock, that is, dengue shock syndrome [
The WHO criteria for the clinical diagnosis of DHF requires the presence of acute and continuous fever of 2 to 7 days, haemorrhagic manifestations associated with thrombocytopenia (100,000 cells/c.mm or less) and haemoconcentration (haematocrit >20% from baseline of patient or population of same age). Haemorrhagic manifestations could be mucosal and or skin or even a positive tourniquet test which is the commonest. Hepatomegaly occurs at some stage of DHF and often precedes plasma leakage and hence a valuable early predictor of plasma leakage [
DHF is most commonly seen in children with secondary dengue infection but has been documented in primary infection with DENV-1 and DENV-3, as well as in infants. These infants had acquired maternal dengue antibody and subsequently experienced a dengue infection [
Abnormal haemostasis and plasma leakage are the main pathophysiological hall marks in DHF. Even though more than half a century has elapsed since plasma leakage was first identified its precise mechanism remains elusive. The main factor implicated in the development of DHF rather than the relatively innocuous DF in dengue infection is secondary dengue infection but other factors like viral virulence and host characteristics are also important. Severe disease is the result of a complex interaction between the virus and the immune response evoked by the host with secondary infection [
Plasma leakage is specific to the pleural and peritoneal surfaces. In DHF there is no vasculitis and hence no injury to the vessel walls, and plasma leakage results from cytokine mediated increase in vascular permeability. The ensuing movement of albumin and the resultant reduction of intravascular oncotic pressure facilitate further loss of fluid from the intravascular compartment. The basic Starling principle still holds true in explaining microvascular ultrafiltration based on the balance of the oncotic and hydrostatic pressures. However the glycocalyx, which is a gelatinous layer lining the vascular endothelium is also implicated in controlling fluid movement by the adherence of albumin molecules in to its matrix, damage of which, leads to loss of albumin into the extravascular compartment [
The immune system is implicated in the pathogenesis of DHF owing to the increased propensity to develop DHF with secondary dengue infection. The innate immune mechanisms comprising the complement pathway and NK cells as well as humoral and cell-mediated immune mechanisms launched in response to antigenic stimulation are involved in the clinical manifestations. Complement activation as well as vascular permeability may be influenced by viral products like NS1. Different immune mechanisms in the form of antibody enhanced viral replication leading to an exaggerated cytokine response impacts vascular permeability [
Infection with one dengue serotype elicits immunity to that serotype but does not provide long-term cross-protective immunity to the remaining serotypes. Subsequent infection with a different serotype results in the binding of the new virus to cross reactive nonneutralising antibody from the previous infection facilitating the uptake by mononuclear phagocytes enabling amplified viral replication
Neutralising antibodies are key factors in the aetiopathogenesis of the disease. However, the cellular immune response is also important. It has been demonstrated that memory dengue T lymphocyte response after a primary infection includes both serotype-specific and serotype-cross-reactive T lymphocytes [
Cytokines that may induce plasma leakage such as interferon g, interleukin (IL) 2, and tumour necrosis factor (TNF)
A recent study has demonstrated the plasma cytokine profile in dengue fever from a Brazilian population which was detected by a multiplex bead immunoassay. MIP-
Complement activation as a result of immune complexes (virus-antibody) or immune activation and cytokine production could also be involved in the mechanism of plasma leakage. Certain complement fragments such as C3a and C5a are known to enhance permeability. NS1 antigen in dengue virus has been shown to regulate complement activation and hence could play a role in the pathogenesis of DHF [
In summary monocytes, macrophages, and dendritic cells are the major targets for DENV.
During secondary infection with a different DENV serotype cross-reactive nonneutralising antibodies bind to DENV and facilitate uptake via Fc receptors resulting in enhanced viral replication. The resultant higher viral antigen load leads to an exaggerated activation of cross-reactive dengue specific T cells. Biological mediators released by the activated T cells as well as virus infected cells along with complement activation by viral proteins, and immune complexes are implicated in increasing vascular permeability and coagulopathy.
These biological mediators influence clinical outcomes to a variable extent. Thus IL-1
The pathogenesis of bleeding in DHF is unclear even though well-recognised coagulation disturbances do exist. The clinical haemorrhagic manifestations range from a mere positive tourniquet test, skin petechiae and ecchymoses to epistaxis, and gum bleeding to severe gastrointestinal haemorrhages. Thrombocytopaenia is a consistent finding, while prolonged partial thromboplastin time and reduced fibrinogen concentration are the other abnormal haemostatic indices evident from early in the disease course. These haematological abnormalities seem to correlate better with the timing and severity of plasma leakage rather than the clinical haemorrhagic manifestations [
These recent findings raise the possibility for common pathogenic mechanisms responsible for both plasma leakage and abnormalities in the haemostatic indices. The true nature of the intrinsic coagulopathy evident early in the disease course and in mild forms of dengue can be confounded by the advent of hypovolemic shock and hypoxia in DHF with severe plasma leakage with less than optimal correction.
Thrombocytopaenia is initially due to bone marrow suppression during the febrile viraemic phase of the illness. Progressive thrombocytopaenia with defervescence result from immune mediated platelet destruction. Virus-antibody complexes have been detected on the platelet surface of DHF patients suggesting a role for immune-mediated destruction of platelets [
The role of the glycocalyx rather than the endothelial cells per se in controlling ultrafiltration in the microvasculature is increasingly recognised and
The low plasma fibrinogen detected in DHF could thus be a reflection of loss into the interstitial spaces in the setting of increased vascular permeability. Heparan sulphate forms an integral part of the glycocalyx which when damaged by the initial cytokine response in DHF gets liberated to the circulation and acts like an anticoagulant which could explain the prolonged APTT [
Development of antibodies potentially cross-reactive to plasminogen could have a role in causing haemorrhage in DHF [
Precise knowledge on the extent to which DENV infects endothelial cells is lacking as few studies have addressed the issue in the viraemic phase of the illness. Even though DENV has infected endothelial cells
Expression of ICAM-1 (intercellular adhesion molecule-1) and beta-integrin on micro vascular endothelium by DENV has been reported. DENV also affects the expression of cytokine receptors. These may contribute to the mechanisms involved in plasma leakage in DHF.
The role of DENV infected endothelial cells in the pathogenesis of coagulopathy in DHF is equally intriguing. There is upregulation of tissue plasminogen, thrombomodulin, protease activated receptor-1, and tissue factor receptor, while there is downregulation of tissue factor inhibitor and activated protein C.
DHF cases have increased in the recent past and will continue to increase in numbers in time to come as DHF is commoner in secondary dengue infection. The probability of secondary dengue infection in a given population is expected to increase owing to the presumed high prevalence of previous exposure to clinical or asymptomatic dengue infection based on epidemiological data particularly in dengue endemic regions in the world. Despite the complexity of the immunopathogenic mechanism involved in severe disease, what is inexorable is that all patients with DHF have plasma leakage, the magnitude and progression of which will impact outcome. Dengue infection must be diagnosed early and in all such patients clinicians need to be alert and vigilant to identify DHF patients early at the inception of plasma leakage before shock sets in. Appropriate interventions with judicious fluid therapy at this stage could offset adverse outcomes and ensure a favourable outcome. Immunopathogenic mechanisms implicated in DHF could serve to meet the challenges of identifying in the febrile phase patients who could behave as DHF during the disease course. In this context assay of specific biomarkers identified in dengue could be useful. Thus while MIP-1
Similarly thrombocytopaenia is best used as a marker of severe disease particularly when it is <100,000 cells/c.mm or when there is a rapid drop. Its usefulness is as an indicator of prognosis during the disease course rather than a parameter for therapeutic interventions. Recognising the poor correlation of thrombocytopaenia with bleeding should caution the clinician against the futility albeit danger of prophylactic platelet transfusions.
Clinicians should also bear in mind that cytokines play different roles in the pathogenesis of DHF. Some been stimulatory while others tend to downregulate the immunological network.
Critical alterations in the cytokine balance with attended adverse, rather than beneficial outcomes could be expected if corticosteroids are used for immunosuppression when such management decisions are based on the superficial consideration of immunological mechanisms as the underlying basis of DHF pathogenesis. Even though cytokines are implicated in the pathogenesis of increased vascular permeability absence of inflammation and the transient nature of altered permeability with a tendency for spontaneous cessation of plasma leakage also raises the irrationality of using steroids and other anti-inflammatory agents.
Plasma leakage and coagulopathy are the fundamental pathological changes responsible for clinical manifestations, morbidity, and mortality in DHF. A complex interplay between immunological mechanisms with viral and host factors are implicated in the pathogenesis. Both humoral and cell-mediated immune mechanisms eventually result in the release of cytokines responsible for changes in the selective microvascular permeability and the resultant plasma leakage. Plasma leakage progresses either rapidly or slowly to cease completely and predictably after 24 to 48 hours of onset, raising the possibility of existence of underlying functional change rather than structural damage and inflammation in the vasculature. The influence of DENV on endothelial cells may be direct or indirect via release of mediators from infected or activated immune cells. Changes in the expression of adhesion molecules, enzymes, and cytokine receptors on endothelial cells are implicated in increasing the vascular permeability as well as activation of the coagulating system.
The two fundamental pathological attributes in DHF are plasma leakage and intrinsic coagulopathy.
The balance of hydrostatic and oncotic pressures is important in plasma leakage. However the glycocalyx also plays a crucial role in fluid fluxes. The permeation of fibrinogen apart from albumin into its matrix, as well as the release of heparan sulphate from its brush surface impacts both plasma leakage and intrinsic coagulopathy. The recognition of the role of biological markers on these pathogenic mechanisms can have far reaching diagnostic and therapeutic implications.
Clinicians should strive to predict severe disease before the advent of shock. Clinical predictors such as tender hepatomegaly and tachycardia after defervescence are exceedingly useful to suspect incipient plasma leakage. Technological advances and the availability of multiplex cytokine profile would facilitate these efforts. It could also open up new vistas in developing interventions targeting specific cytokines to reduce plasma leakage. However the importance of diligent and accurate monitoring of heart rate, pulse pressure, urine output, and haematocrit for the early detection of plasma leakage and adjustments to fluid therapy should not be overlooked and constitute an essential and integral part of case management. Our understanding of the pathogenesis of DHF and the availability of biological markers could serve to complement the clinician’s efforts. Prevention of immune enhanced viral replication is another area to focus specific therapeutic interventions. Use of fresh frozen plasma for this purpose is an exciting area of research [