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INTRODUCTION

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The renal circulation is complex and is characterized by a highly perfused arteriolar network, reaching cortical glomerular structures adjacent to lower-flow vasa recta that descend into medullary segments. Disorders of the larger vessels, including renal artery stenosis and atheroembolic disease, are discussed elsewhere (Chap. 354). This chapter examines primary disorders of the renal microvessels, many of which are associated with thrombosis and hemolysis.

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THROMBOTIC MICROANGIOPATHY

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Thrombotic microangiopathy (TMA) is characterized by injured endothelial cells that are thickened, swollen, or detached mainly from arterioles and capillaries. Platelet and hyaline thrombi causing partial or complete occlusion are integral to the histopathology of TMA. TMA is usually the result of microangiopathic hemolytic anemia (MAHA), with its typical features of thrombocytopenia and schistocytes. In the kidney, TMA is characterized by swollen endocapillary cells (endotheliosis), fibrin thrombi, platelet plugs, arterial intimal fibrosis, and a membranoproliferative pattern. Fibrin thrombi may extend into the arteriolar vascular pole, producing glomerular collapse and at times cortical necrosis. In kidneys that recover from acute TMA, secondary focal segmental glomerulosclerosis may be seen. Diseases associated with this lesion include thrombotic thrombocytopenic purpura (TTP), hemolytic-uremic syndrome (HUS), malignant hypertension, scleroderma renal crisis, antiphospholipid syndrome, preeclampsia/HELLP (hemolysis, elevated liver enzymes, low platelet count) syndrome, HIV infection, and radiation nephropathy.

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HEMOLYTIC-UREMIC SYNDROME/THROMBOTIC THROMBOCYTOPENIC PURPURA

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HUS and TTP are the prototypes for MAHA. Historically, HUS and TTP were distinguished mainly by their clinical and epidemiologic differences. TTP develops more commonly in adults and was thought to include neurologic involvement more often. HUS occurs more commonly in children, particularly when associated with hemorrhagic diarrhea. However, atypical HUS (aHUS) can first appear in adulthood, and better testing has revealed that neurologic involvement is as common in HUS as in TTP. Accordingly, HUS and TTP now should be differentiated and treated according to their specific pathophysiologic features.

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Hemolytic-Uremic Syndrome
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HUS is loosely defined by the presence of MAHA and renal impairment. At least four variants are recognized. The most common is Shiga toxin–producing Escherichia coli (STEC) HUS, which is also known as D+HUS or enterohemorrhagic E. coli (EHEC) HUS. Most cases involve children <5 years of age, but adults also are susceptible, as evidenced by a 2011 outbreak in northern Europe. Diarrhea, often bloody, precedes MAHA within 1 week in >80% of cases. Abdominal pain, cramping, and vomiting are frequent, whereas fever is typically absent. Neurologic symptoms, including dysphasia, hyperreflexia, blurred vision, memory deficits, encephalopathy, perseveration, and agraphia, often develop, especially in adults. Seizures and cerebral infarction can occur in severe cases. STEC HUS is caused by the Shiga toxins (Stx1 and Stx2), which are also referred to as verotoxins. These toxins are produced by certain strains of E. coli and Shigella dysenteriae. In the United States and Europe, the most common STEC strain is O157:H7, but HUS due to other strains (O157/H, O111:H, O26:H11/H, O145:H28, and O104:H4) has occurred. After entry into the circulation, Shiga toxin binds to the glycolipid surface receptor globotriaosylceramide ...

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