Thrombotic Thrombocytopenic Purpura
This devastating platelet disorder described by Moschowitz in 1925 is acute and nonpredictable. More prevalent in women than in men, thrombotic thrombo-cytopenic purpura (TTP) can occur in women postpar-tum or near delivery8 in those who have suffered from other immune disorders like SLE (systemic lupus ery-thematosus), and in those with previous viral infections or gastric carcinomas. Platelet counts are less than 20 X 109/L but other coagulation testing such as PT and PTT are within reference range. Platelet thrombi are dispersed throughout the arterioles and capillaries subsequent to the accumulation of large von Willebrand multimers made by the endothelial cells and platelets. The etiology for this pathological accumulation of multimers and subsequent thrombocytopenia is thought to be related to a deficiency of ADAMTS-13, a large metal-loprotease.9,10 This protein is responsible for cleaving large von Willebrand factor (vWF) multimers into smaller proteins. Large vWF multimers have increased binding sites for platelets as compared to smaller vWF portions. If large vWF are not cleaved and allowed to circulate, then excessive platelet clots may be formed. Schistocytes are seen in the peripheral smear and are directly related to shear stress as fragments of red cells are removed once the cells try to sweep past the thrombi. Patients experience a severe anemia with a high level of hemolysis, with increased lactate dehydrogenase (LDH). Some of the hemolysis may be intravascular with hemoglobinuria. Decreased haptoglobin may be seen. Microangiopathic hemolytic anemia (MHA) is the term used to describe this process of severe anemia combined with schistocytes (Fig. 16.1). Oftentimes patients will present with neurological complications. These complications may include mild presentations of visual impairment and intense headache ranging to more severe presentations such as coma and paresthesias. Renal dysfunction may occur, and patients with renal impairment experience an increased protein and possibly some blood in the urine. Treatment for TTP patients presents a dilemma for most physicians, as they watch their patients spiral rapidly downhill. Diagnosis is often difficult and often represents a diagnosis of exclusion. Once made, the patient's condition has usually dramatically worsened. Corticosteroids are often used in conjunction with plasma exchange, a dramatic procedure performed over a 3- to 5-day period in which the patient's plasma is removed and replaced by ABO matched fresh frozen plasma that is cryoprecipitate poor (lacking fibrinogen and vWF). The use of plasma exchange has dramatically improved the survival rate from a low of 3% before 1960
to 82% presently.11 Few hospital facilities provide plasma exchange capabilities. Specialty teams of medical professionals using equipment designed for plasma-pheresis are usually called upon. Timing is essential to the patient's welfare and long-term recovery. Recovery for TTP patients has improved in the past decade, with more than 80% surviving.11
Hemolytic uremic syndrome (HUS) frequently occurs in children between the ages of 6 months and 4 years. This clinical condition mimics TTP, with the exception that the renal damage is more severe. The kidney is the primary site of damage by the toxin Escherichia coli O157:H7 or the Shigella toxin.11 The endotoxin produced by this particular strain of E. coli inevitably leads to cell death, particularly in the renal environment, where platelet thrombi predominate.12 A child may initially present with bloody diarrhea and vomiting; however, hemolytic anemia, thrombocytopenia, and renal failure soon follow. Patients may also experience fever and abdominal pain, and the hemolytic anemia is microangiopathic with schistocytes present. The illness in children is usually self-limiting once the toxin is eliminated from the body; however, there have been reports of patients relapsing. Renal dialysis may be needed for those children suffering from acute renal failure. Most children make a complete recovery, but some have residual kidney problems into adulthood. HUS may occur in adults, but it is more similar to TTP in course of disease (Table 16.2).
Disorders such as heparin-induced thrombocy-topenia and disseminated intravascular coagulation also lead to thrombocytopenia. These will be covered in subsequent chapters.
Table 16.2 O Hemolytic Uremic Syndrome Versus Thrombotic Thrombocytopenic Purpura intrinsic defect of platelets. Many of these disorders (with the exception of vWD) are rare, and in most cases the bleeding time is prolonged. Often, the qualitative defects are separated into disorders of adhesion and platelet release or storage pool defects.
Table 16.2 O Hemolytic Uremic Syndrome Versus Thrombotic Thrombocytopenic Purpura
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