EXTRINSIC NON-IMMUNE

• Schistocytes


  Red blood cell fragments formed by mechanical shearing as cells rapidly pass through turbulent areas of small blood vessels
• Microangiopathic hemolytic anemia (MAHA)


  Group of potentially life-threatening disorders characterized by red blood cell fragmentation and thrombocytopenia
• Mechanism of red blood cell fragmentation in MAHA

  1. Mechanical shearing of RBC membranes as cells rapidly pass through turbulent areas of small blood vessels partially blocked by microthrombi or damaged endothelium

  2. Resulting fragments (schistocytes) become distorted and rigid

  3. Spleen clears the rigid RBC fragments from circulation through extravascular hemolysis
• Laboratory evidence of hemolytic anemia in MAHA

  • Decreased hemoglobin
  • Increased reticulocyte count
  • Increased serum indirect bilirubin
  • Increased serum lactate dehydrogenase activity
  • Decreased serum haptoglobin
  • Hemoglobinemia and hemoglobinuria (severe cases)
• Thrombocytopenia in MAHA


  Due to consumption of platelets in thrombi forming in microvasculature
• Major microangiopathic hemolytic anemias

  • Thrombotic thrombocytopenic purpura (TTP)
  • Hemolytic uremic syndrome (HUS)
  • HELLP syndrome
  • Disseminated intravascular coagulation (DIC)
• Thrombotic thrombocytopenic purpura (TTP)

  Rare, life-threatening disorder characterized by microangiopathic hemolytic anemia, severe thrombocytopenia, and markedly elevated serum lactate dehydrogenase activity
• Cause of TTP


  Deficiency of von Willebrand factor-cleaving protease ADAMTS13
• Role of ADAMTS13 in preventing thrombosis

  1. Cleaves ultra-large von Willebrand factor multimers into shorter segments with less hemostatic potential

  2. Prevents excessive binding and activation of platelets
• Pathogenesis of TTP


  • Deficiency of ADAMTS13 leads to accumulation of ultra-large von Willebrand factor multimers, platelet aggregation, and microthrombi formation in small blood vessels
  • Causes hemolytic anemia, thrombocytopenia, and organ ischemia
• Types of TTP


  • Idiopathic
  • Secondary
  • Inherited (Upshaw-Schulman syndrome)
• Typical laboratory findings in TTP

  • Hemoglobin 8-10 g/dL
  • Platelet count 10-30 x 10^9/L
  • Schistocytes on peripheral blood film
  • Polychromasia and nucleated RBCs (after bone marrow response)
  • Increased white blood cell count
  • Hemoglobinuria (with extensive intravascular hemolysis)
  • Proteinuria, hematuria, and casts (from renal damage)
• ADAMTS13 activity in TTP

  • Severely reduced (<5-10% of normal) in idiopathic and inherited TTP
  • Less severely reduced in secondary TTP
• Treatment of TTP


  • Plasma exchange therapy to remove autoantibodies and replace ADAMTS13 (for idiopathic TTP)
  • Corticosteroids to suppress autoimmune response
  • Plasma infusion (for inherited TTP)
• Hemolytic uremic syndrome (HUS)


  Characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure
• Types of HUS


  • Typical (Shiga toxin-associated) HUS
  • Atypical HUS
• Typical (Shiga toxin-associated) HUS


  Caused by bacteria producing Shiga toxin, often preceded by acute gastroenteritis with bloody diarrhea
• Atypical HUS


  Caused by unregulated activation of the alternative complement pathway
• Laboratory findings in HUS

  • Microangiopathic hemolytic anemia
  • Mild to moderate thrombocytopenia
  • Evidence of renal failure (elevated creatinine, proteinuria, hematuria, casts)
• Typical HUS comprises 90% of HUS cases
• Cause of typical HUS

  Infection with Shiga-like toxin-producing Escherichia coli (STEC), such as serotype O157:H7
• Pathogenesis of typical HUS

  1. Shiga toxins absorbed from intestines into plasma, have affinity for Gb3 glycolipid receptors on endothelial cells

  2. Toxins damage endothelial cells, leading to microthrombi formation and organ damage, especially kidneys
• Stx-HUS


  • Comprises 90% of cases of HUS
  • Most common cause is infection with Shiga-like toxin-1-producing Escherichia coli (STEC), such as serotype O157:H7
  • Can also be caused by toxin-producing Shigella
  • Occurs most often in young children but can be found in patients of all ages
  • Patients initially have acute gastroenteritis, often with bloody diarrhea, and after approximately 5 to 13 days develop oliguria and other symptoms of renal damage
  • About one fourth of patients also develop neurologic manifestations
• Stx-HUS pathogenesis


  1. E. coli and Shigella serotypes release Shiga toxins (Stx-1 and Stx-2, also called verotoxins) that are absorbed from the intestines into the plasma

  2. Toxins have an affinity for the Gb3 glycolipid receptors (CD77) on endothelial cells, particularly those in the glomerulus and brain

  3. Toxin is transported into the endothelial cells, where it inhibits protein synthesis and causes endothelial cell injury and eventual apoptosis

  4. Shiga toxin, together with many cytokines secreted as a result of the infection, also induces changes in endothelial cells that are prothrombotic, including expression of tissue factor, adhesion molecules, and secretion of increased amounts of ULVWF multimers

  5. Endothelial cell damage can cause stenosis (narrowing) of small blood vessels, which can be exacerbated by the activation of platelets and formation of platelet-fibrin thrombi

  6. Resultant blockages in the microvasculature of the glomeruli result in acute renal failure

  7. Endothelial damage and microthrombi can also occur in the microvasculature of the brain and other organs
• Atypical HUS (aHUS)


  • Comprises about 10% of cases of HUS
  • Characterized by uncontrolled activation of the alternative complement system, which causes endothelial cell injury, activation of platelets and coagulation factors, and formation of platelet-fibrin thrombi that obstruct the microvasculature in the glomerulus and other organs
• Atypical HUS (aHUS)


  • Approximately 50% to 70% of patients have inherited mutations in genes that code for components of the alternative complement pathway or its regulatory proteins
  • Inactivating mutations have been identified in genes for complement regulatory proteins, including complement factor H, complement factor I, membrane cofactor protein, and thrombomodulin
  • Activating mutations have been identified in the genes for complement factors B and C3
  • An acquired form of aHUS is associated with autoantibodies to complement factor H and accounts for approximately 5% to 10% of cases
  • In the remaining cases no mutation or autoantibodies have been identified
  • May be triggered by hematopoietic stem cell therapy, pregnancy, infection, inflammation, surgery, or trauma
• HELLP syndrome


  A serious complication in pregnancy, named for its characteristic presentation of hemolysis, elevated liver enzymes, and low platelet count
• HELLP syndrome


  • Occurs in approximately 0.5% of all pregnancies but develops in approximately 4% to 12% of pregnancies with preeclampsia and 30% to 50% of pregnancies with eclampsia, most often in the third trimester
  • In preeclampsia, abnormalities in the development of placental vasculature result in poor perfusion and hypoxia, leading to release of anti-angiogenic proteins from the placenta which bind to and inactivate placental and endothelial growth factors
  • Continued vascular insufficiency of the placenta results in maternal endothelial cell dysfunction, which leads to platelet activation and fibrin deposition in the microvasculature, particularly in the liver
• Disseminated Intravascular Coagulation (DIC)


  Characterized by the widespread activation of the hemostatic system, resulting in fibrin thrombi formation throughout the microvasculature
• Disseminated Intravascular Coagulation (DIC)

  • Major clinical manifestations are organ damage due to obstruction of the microvasculature and bleeding due to the consumption of platelets and coagulation factors and secondary activation of fibrinolysis
  • DIC is a complication of many disorders, such as metastatic cancers, acute leukemias, infections, obstetric complications, crush or brain injuries, acute hemolytic transfusion reactions, extensive burns, snake or spider envenomation, and chronic inflammation
• Traumatic Cardiac Hemolytic Anemia

  Mechanical hemolysis can occur in patients with prosthetic cardiac valves due to the turbulent blood flow through and around the implanted devices
• Traumatic Cardiac Hemolytic Anemia

  • Hemolysis is usually mild, and anemia does not generally develop due to compensation by the bone marrow
  • Severe hemolysis is rare and is usually due to paravalvular leaks in prosthetic cardiac valves
  • Hemolysis can also occur in patients with cardiac valve disease prior to corrective surgery
• Exercise-Induced Hemoglobinuria


  RBC lysis, with an increase in free plasma hemoglobin and a decrease in the serum haptoglobin level, has been demonstrated in some individuals after long-distance running and, to a lesser extent, after intensive cycling and swimming
• Exercise-Induced Hemoglobinuria


  • Various causes have been proposed, including mechanical trauma from the forceful, repeated impact of the feet or hands on hard surfaces, increased RBC susceptibility to oxidative stress, and exercise-induced alterations in membrane cytoskeletal proteins
  • Exercise-induced hemoglobinuria does not usually cause anemia unless the hemoglobinuria is particularly severe and recurrent
  • Exercise-induced hemoglobinuria is a diagnosis of exclusion, and other possible causes of hemolysis and hemoglobinuria should be investigated and ruled out
• Malaria


  A potentially fatal condition caused by infection of RBCs with protozoan parasites of the genus Plasmodium
• Malaria


  • Most human infections are caused by P. falciparum and P. vivax, but P. ovale, P. malariae, and a fifth species, P. knowlesi, also infect humans
  • P. knowlesi, a natural parasite of macaque monkeys, is easily misdiagnosed as P. malariae by microscopy
  • Approximately 3.4 billion people live in areas in which malaria is endemic and are at risk for the disease
  • Worldwide in 2016, there were an estimated 216 million cases of malaria, with 445,000 deaths, mostly in children younger than 5 years of age
• Malaysia were found actually to be caused by P. knowlesi when polymerase chain reaction (PCR) assays were used, including tests on archival blood films from 1996.
• With the use of molecular techniques, P. knowlesi malaria is now known to be widespread in Malaysia, and cases have been reported in other areas of Southeast Asia, although the US Centers for Disease Control and Prevention considers this to be a zoonotic malaria.
• Approximately 3.4 billion people live in areas in which malaria is endemic and are at risk for the disease.
• Worldwide in 2016, there were an estimated 216 million cases of malaria, with 445,000 deaths, mostly in children younger than 5 years of age.