Thrombotic microangiopathy

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Thrombotic microangiopathy
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Micrograph of thrombotic microangiopathy with the characteristic onion-skin layering seen in older lesions. PAS stain.
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Thrombotic microangiopathy (TMA) is a pathology that results in thrombosis in capillaries and arterioles, due to an endothelial injury. [1] It may be seen in association with thrombocytopenia, anemia, purpura and kidney failure.


The classic TMAs are hemolytic uremic syndrome and thrombotic thrombocytopenic purpura. Other conditions with TMA include atypical hemolytic uremic syndrome, disseminated intravascular coagulation, scleroderma renal crisis, malignant hypertension, antiphospholipid antibody syndrome, and drug toxicities, e.g. calcineurin inhibitor toxicity. [1]

Signs and symptoms

The clinical presentation of TMA, although dependent on the type, typically includes: fever, microangiopathic hemolytic anemia (see schistocytes in a blood smear), kidney failure, thrombocytopenia and neurological manifestations. Generally, renal complications are particularly predominant with Shiga-toxin-associated hemolytic uremic syndrome (STx-HUS) and atypical HUS, whereas neurologic complications are more likely with TTP. Individuals with milder forms of TTP may have recurrent symptomatic episodes, including seizures and vision loss. [2] With more threatening cases of TMA, and also as the condition progresses without treatment, multi-organ failure or injury is also possible, as the hyaline thrombi can spread to and affect the brain, kidneys, heart, liver, and other major organs. [3]


The specific cause is dependent of the type of TMA that is presented, but the two main pathways that lead to TMA are external triggers of vascular injury, such as viruses, bacterial Shiga toxins or endotoxins, antibodies, and drugs; and congenital predisposing conditions, including decreased levels of tissue factors necessary for the coagulation cascade. Either of these pathways will result in decreased endothelial thromboresistance, leukocyte adhesion to damaged endothelium, complement consumption, enhanced vascular shear stress, and abnormal vWF fragmentation. The central and primary event in this progression is injury to the endothelial cells, which reduces the production of prostaglandin and prostacyclin, ultimately resulting in the loss of physiological thromboresistance, or high thrombus formation rate in blood vessels. Leukocyte adhesion to the damaged endothelial wall and abnormal von Willebrand factor (or vWF) release can also contribute to the increase in thrombus formation. [4] More recently, researchers have attributed both TTP and HUS to targeted agents, such as targeted cancer therapies, immunotoxins, and anti-VEGF therapy. [2]

Bacterial toxins are the primary cause of one category of thrombotic microangiopathy known as HUS or hemolytic uremic syndrome. HUS can be divided into two main categories: Shiga-toxin-associated HUS (STx-HUS), which normally presents with diarrhea, and atypical HUS. The Shiga-toxin inhibits the binding of eEF-1-dependent binding of aminoacyl tRNA to the 60S subunit of the ribosome, thus inhibiting protein synthesis. The cytotoxicity from the lack of protein damages glomerular endothelial cells by creating voids in the endothelial wall and detaching the basement membrane of the endothelial layer, activating the coagulation cascade. Atypical HUS may be caused by an infection or diarrheal illness or it may be genetically transmitted. This category of TMA encompasses all forms that do not have obvious etiologies. Mutations in three of the proteins in the complement cascade have been identified in patients with atypical HUS. [3] Several chemotherapeutic drugs have also been shown to cause damage to the epithelial layer by reducing the ability for the cells to produce prostacyclin, ultimately resulting in chemotherapy-associated HUS, or C-HUS.[ citation needed ]

The second category of TMAs is TTP thrombotic thrombocytopenic purpura, which can be divided into 3 categories: congenital, idiopathic, and non-idiopathic. [5] Congenital and idiopathic TTP are generally associated with deficiencies in ADAMTS13, a zinc metalloprotease responsible for cleaving Very Large vWF Multimers in order to prevent inappropriate platelet aggregation and thrombosis in the microvasculature. Natural genetic mutations resulting in the deficiency of ADAMTS13 have been found in homozygous and heterozygous pedigrees in Europe. [3] Researchers have identified common pathways and links between TTP and HUS, [2] [6] while other sources express skepticism about their common pathophysiology. [7]

The repression of the vascular endothelial growth factor (VEGF) can also cause glomerular TMA (damage to the glomerular microvasculature). It is likely that the absence of VEGF results in the collapse of fenestrations in the glomerular endothelium, thus causing microvascular injury and blockages associated with TMA. [8]

Manifestations resembling thrombotic microangiopathy have been reported in clinical trials evaluating high doses of valacyclovir (8000 mg/day) administered for prolonged periods (months to years) for prophylaxis of cytomegalovirus (CMV) infection and disease, particularly in persons with HIV infection. A number of factors may have contributed to the incidence of thrombotic microangiopathy in those trials including profound immunosuppression, underlying diseases (advanced HIV disease, graft-versus-host disease), and other classes of drug, particularly antifungal agents. There were no reports of thrombotic microangiopathy among the 3050 subjects in the four trials evaluating Valacyclovir for suppression of recurrent genital herpes. Although one of the trials was in HIV-infected subjects, the patients did not have advanced HIV disease. The implication is that the occurrence of thrombotic microangiopathy is restricted to severely immunosuppressed persons receiving higher Valacyclovir dosages than are required to control HSV infection. [9]


CBC and blood film: decreased platelets and schistocytes PT, aPTT, fibrinogen: normal markers of hemolysis: increased unconjugated bilirubin, increased LDH, decreased haptoglobin negative Coombs test. Creatinine, urea, to follow renal function ADAMSTS-13 gene, activity or inhibitor testing (TTP).[ citation needed ]


The course of treatment and the success rate is dependent on the type of TMA. Some patients with atypical HUS and TTP have responded to plasma infusions or exchanges, a procedure which replaces proteins necessary for the complement cascade that the patient does not have; however, this is not a permanent solution or treatment, especially for patients with congenital predispositions. [3] Monoclonal antibodies like eculizumab and caplacizumab can assist with atypical hemolytic uremic syndrome and acquired thrombotic thrombocytopenic purpura respectively whilst dexamethasone can help with immune thrombotic thrombocytopenic purpura and low molecular weight heparin can help with disseminated intravascular coagulation.[ citation needed ]

See also

Related Research Articles

Hemolysis Rupturing of red blood cells and release of their contents

Hemolysis or haemolysis, also known by several other names, is the rupturing (lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid. Hemolysis may occur in vivo or in vitro.

Thrombotic thrombocytopenic purpura Medical condition

Thrombotic thrombocytopenic purpura (TTP) is a blood disorder that results in blood clots forming in small blood vessels throughout the body. This results in a low platelet count, low red blood cells due to their breakdown, and often kidney, heart, and brain dysfunction. Symptoms may include large bruises, fever, weakness, shortness of breath, confusion, and headache. Repeated episodes may occur.

Microangiopathic hemolytic anemia (MAHA) is a microangiopathic subgroup of hemolytic anemia caused by factors in the small blood vessels. It is identified by the finding of anemia and schistocytes on microscopy of the blood film.

Thrombocytopenia Medical condition

Thrombocytopenia is a condition characterized by abnormally low levels of platelets, also known as thrombocytes, in the blood. It is the most common coagulation disorder among intensive care patients and is seen in 20% of medical patients and a third of surgical patients.

Hemolytic–uremic syndrome Group of blood disorders related to bacterial infection

Hemolytic–uremic syndrome (HUS) is a group of blood disorders characterized by low red blood cells, acute kidney failure, and low platelets. Initial symptoms typically include bloody diarrhea, fever, vomiting, and weakness. Kidney problems and low platelets then occur as the diarrhea progresses. Children are more commonly affected, but most children recover without permanent damage to their health, although some children may have serious and sometimes life-threatening complications. Adults, specially the elderly, may present a more complicated presentation. Complications may include neurological problems and heart failure.

Von Willebrand factor

von Willebrand factor (VWF) is a blood glycoprotein involved in hemostasis, specifically, platelet adhesion. It is deficient and/or defective in von Willebrand disease and is involved in many other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic–uremic syndrome. Increased plasma levels in many cardiovascular, neoplastic, metabolic, and connective tissue diseases are presumed to arise from adverse changes to the endothelium, and may predict an increased risk of thrombosis.

Hemoglobinuria Abnormally increased hemoglobin in urine

Hemoglobinuria is a condition in which the oxygen transport protein hemoglobin is found in abnormally high concentrations in the urine. The condition is caused by excessive intravascular hemolysis, in which large numbers of red blood cells (RBCs) are destroyed, thereby releasing free hemoglobin into the plasma. Excess hemoglobin is filtered by the kidneys, which excrete it into the urine, giving urine a purple color. Hemoglobinuria can lead to acute tubular necrosis which is an uncommon cause of a death of uni-traumatic patients recovering in the ICU.

HELLP syndrome is a complication of pregnancy; the acronym stands for Hemolysis, Elevated Liver enzymes, and Low Platelet count. It usually begins during the last three months of pregnancy or shortly after childbirth. Symptoms may include feeling tired, retaining fluid, headache, nausea, upper right abdominal pain, blurry vision, nosebleeds, and seizures. Complications may include disseminated intravascular coagulation, placental abruption, and kidney failure.

Schistocyte Fragmented portion of a red blood cell

A schistocyte or schizocyte is a fragmented part of a red blood cell. Schistocytes are typically irregularly shaped, jagged, and have two pointed ends.

Nephritic syndrome Medical condition

Nephritic syndrome is a syndrome comprising signs of nephritis, which is kidney disease involving inflammation. It often occurs in the glomerulus, where it is called glomerulonephritis. Glomerulonephritis is characterized by inflammation and thinning of the glomerular basement membrane and the occurrence of small pores in the podocytes of the glomerulus. These pores become large enough to permit both proteins and red blood cells to pass into the urine. By contrast, nephrotic syndrome is characterized by proteinuria and a constellation of other symptoms that specifically do not include hematuria. Nephritic syndrome, like nephrotic syndrome, may involve low level of albumin in the blood due to the protein albumin moving from the blood to the urine.

ADAMTS13 Metalloprotease enzyme

ADAMTS13 —also known as von Willebrand factor-cleaving protease (VWFCP)—is a zinc-containing metalloprotease enzyme that cleaves von Willebrand factor (vWf), a large protein involved in blood clotting. It is secreted into the blood and degrades large vWf multimers, decreasing their activity.

Hematologic diseases are disorders which primarily affect the blood & blood-forming organs. Hematologic diseases include rare genetic disorders, anemia, HIV, sickle cell disease & complications from chemotherapy or transfusions.

The term cryosupernatant refers to plasma from which the cryoprecipitate has been removed. It is used to treat thrombocytopenic purpura.

Shigatoxigenic Escherichia coli (STEC) and verotoxigenic E. coli (VTEC) are strains of the bacterium Escherichia coli that produce either Shiga toxin or Shiga-like toxin (verotoxin). Only a minority of the strains cause illness in humans. The ones that do are collectively known as enterohemorrhagic E. coli (EHEC) and are major causes of foodborne illness. When infecting humans, they often cause gastroenteritis, enterocolitis, and bloody diarrhea and sometimes cause a severe complication called hemolytic-uremic syndrome (HUS). The group and its subgroups are known by various names. They are distinguished from other strains of intestinal pathogenic E. coli including enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), and diffusely adherent E. coli (DAEC).

Atypical hemolytic uremic syndrome (aHUS) is an extremely rare, life-threatening, progressive disease that frequently has a genetic component. In most cases it can be effectively controlled by interruption of the complement cascade. Particular monoclonal antibodies, discussed later in the article, have proven efficacy in many cases.

Onconephrology is a specialty in nephrology that deals with the study of kidney diseases in cancer patients. A nephrologist who takes care of patients with cancer and kidney disease is called an onconephrologist. This branch of nephrology encompasses nephrotoxicity associated with existing and novel chemotherapeutics, kidney disease as it pertains to stem cell transplant, paraneoplastic kidney disorders, paraproteinemias, electrolyte disorders associated with cancer, and more as discussed below.

Upshaw–Schulman syndrome Medical condition

Upshaw–Schulman syndrome (USS) is the recessively inherited form of thrombotic thrombocytopenic purpura (TTP), a rare and complex blood coagulation disease. USS is caused by the absence of the ADAMTS13 protease resulting in the persistence of ultra large von Willebrand factor multimers (ULVWF), causing episodes of acute thrombotic microangiopathy with disseminated multiple small vessel obstructions. These obstructions deprive downstream tissues from blood and oxygen, which can result in tissue damage and death. The presentation of an acute USS episode is variable but usually associated with thrombocytopenia, microangiopathic hemolytic anemia (MAHA) with schistocytes on the peripheral blood smear, fever and signs of ischemic organ damage in the brain, kidney and heart.

Ravulizumab, sold under the brand name Ultomiris, is a humanized monoclonal antibody complement inhibitor medication designed for the treatment of paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome. It is designed to bind to and prevent the activation of Complement component 5 (C5).

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder characterized by thrombocytopenia and microangiopathic hemolytic anemia accompanied by variable neurological dysfunction, kidney failure, and fever. It is caused by severely reduced activity of the von Willebrand factor-cleaving protease ADAMTS13. Hereditary TTP, caused by ADAMTS13 gene mutations, is much less common.


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