Chapter 34. Thrombotic Microangiopathies

The thrombotic microangiopathies (TMAs) are classified together due to their overlapping clinical and morphologic findings. Their major clinical manifestations include hemolytic anemia, microvascular thrombosis, and thrombocytopenia. Most TMAs have renal involvement with similar morphologic changes in the kidney reflecting the vasculopathy these lesions share. There are a number of underlying etiologic factors inducing TMA including systemic diseases, infection, and medications, which are summarized in Table 34–1.

TMAs originally were separated into the distinct disease entities of hemolytic uremic syndrome (HUS), thrombotic thrombocytopenic purpura (TTP), and scleroderma renal crisis according to their clinical features. In fact, HUS and TTP were suggested to be varying manifestations of a single disease. However, as the underlying pathogenetic factors are being elucidated, it is clear that the TMAs are distinct entities with specific etiologies and associated laboratory and clinical findings.

Essentials of Diagnosis

• Microangiopathic hemolytic anemia.
• Thrombocytopenia.
• Renal involvement.

General Considerations

Historically, HUS and TTP were considered to be variable expressions of the same disease process. With the identification of etiologic roles for factor H in HUS and for ADAMTS 13 in TTP, these are now thought to be distinct disorders with overlapping clinical features. HUS may present in a diarrheal (D+) or a nondiarrheal (D−) form. Most cases of D+ occur in summer and autumn, and result from Shiga-like toxin producing bacteria, primarily Escherichia coli 0157:H7, although other organisms may be involved. Transmission has occurred via undercooked ground beef, contaminated water, and unpasteurized milk or apple cider, and epidemics may ensue. The Shiga-like toxin binds to colonic epithelium inducing inflammation and tissue injury thereby allowing the toxin to enter the circulation. The toxin then binds to vascular and renal tubular epithelial cell receptors resulting in endothelial injury, inflammation, thrombosis, and renal failure.

The D− form of HUS is less well understood. It is associated with a number of underlying clinical predisposing factors including genetic predisposition, medication use, nongastrointestinal infections, pregnancy, neoplasms, collagen-vascular diseases, systemic vasculitis, and bone marrow transplantation. Deficiency of prostacyclin PGI2 has been implicated in some cases of D− HUS, while other cases likely result from an abnormality of the coagulation cascade or the endothelial cell membrane creating a prothrombotic milieu. A more common atypical subset of HUS is characterized by complement dysregulation due to factor H or membrane cofactor abnormalities. Factor H regulates the alternative complement pathway and membrane cofactor is a transmembrane complement regulatory protein. Mutations leading to deficiency or loss of activity of these regulatory elements allow activation of complement with deposition on and injury to glomerular and vascular endothelial cells and platelet aggregation resulting in thrombosis. Factor H deficiency is inherited in an autosomal recessive manner and is associated with low C3 levels and earlier disease onset. Factor H functional abnormalities are inherited in an autosomal dominant mode and these patients have normal serum complement levels, a later disease onset, and often other initiating events or stimuli.

Clinical Findings

Symptoms and Signs

HUS classically presents as a triad consisting of microangiopathic hemolytic anemia, thrombocytopenia, and renal disease. The signs and symptoms overlap those of TTP, but in HUS there is a predominance of renal and hematologic features. D+ HUS is associated with watery to hemorrhagic diarrhea, and epidemics associated with ingestion of undercooked hamburger or unpasteurized milk or cider occur sporadically. The other systemic manifestations of the diarrheal and nondiarrheal forms are similar and include fever, severe hypertension, pulmonary edema, cerebral edema and seizures, congestive heart failure, and cardiac arrhythmias.

Laboratory Findings

The classical features are microangiopathic hemolytic anemia and renal dysfunction, frequently presenting as acute renal failure in adults, and less commonly in children. D+ HUS is associated with infection, and the offending organism such as E coli 0157:H7, Salmonella, or Shigella may be identifiable in stool culture. Leukocytosis and fluid and electrolyte disturbances may be found.

Renal biopsy findings are identical in the D+ and D± forms, as they are in all TMAs and are characterized by the accumulation of fibrin in the lumina and walls of arteries, arterioles, and glomerular capillaries (Figure 34–1). By light microscopy, fibrin and platelet thrombi are present in variable numbers of glomerular capillaries. Glomeruli may appear ischemic with wrinkled and partially collapsed capillaries or develop a lobular appearance with capillary wall double contours. Fibrin is in the walls and/or lumina of arterioles and, to a lesser extent, arteries, which also show muscular hypertrophy and mucoid intimal thickening, and endothelial cell swelling resulting in luminal narrowing. This may have a concentric or “onion skin” appearance. Immunofluorescence discloses fibrin in vascular walls and lumina, and glomerular capillaries without immune complexes. On ultrastructural examination, glomerular capillary walls have wide subendothelial zones containing flocculent electron-lucent and electron-dense material representing altered fibrin, which may contain trapped erythrocytes. Often there is a new layer of basement membrane material beneath the widened subendothelial zones accounting for the double contour appearance of capillaries. There are no electron-dense (immune complex) deposits. It has been suggested that patients with HUS may have more fibrin and erythrocytes with fewer platelets within intrarenal thrombi compared with TTP, and that HUS is more likely to have renal cortical necrosis than other TMAs. However, there are no definitive distinguishing features among any of the TMAs on renal biopsy.

Differential Diagnosis

The presence of thrombocytopenia, microangiopathic hemolytic anemia, and renal disease suggests the presence of a thrombotic microangiopathy. The renal biopsy most often cannot distinguish among the underlying causes. Therefore, the history, physical examination, and laboratory evaluation are important in distinguishing one form from another. HUS generally is associated with more severe renal involvement than TTP. The latter is sometimes also accompanied by the other features of the clinical pentad, including fever and central nervous system involvement. The finding of typical infectious agents on stool culture or a suspicious history in the D+ form indicates the likely diagnosis of HUS. Some laboratories are now performing clinical testing for ADAMTS13 (von Willebrand cleaving factor), which is currently the only definitive way to distinguish these syndromes. Markedly elevated blood pressure in the absence of other signs or symptoms of systemic disease suggests the presence of malignant hypertension. Systemic manifestations and/or laboratory findings of systemic lupus erythematosus (SLE) or scleroderma would implicate those as the underlying cause, since most often, the TMA is not the initial presentation of these disorders. A history of fetal wastage and/or arterial or venous thrombosis may suggest underlying antiphospholipid syndrome. A history of particular medication ingestion or a viral or bacterial infection prodrome also may be helpful in implicating those as causative. All TMAs are in the differential diagnosis whenever thrombocytopenia, microangiopathic hemolytic anemia, and renal clinical or biopsy features are present. These include HUS, TTP, scleroderma renal crisis, and antiphospholipid syndrome, regardless of the etiology; malignant hypertension may have a similar presentation and appearance on renal biopsy.

Treatment

Therapy for HUS is primarily supportive only. Factor H replacement using fresh frozen plasma infusion has not been effective even in patients from families with documented factor H deficiency. However, recombinant Factor H currently is being developed as a potential therapy in this setting. Antibiotics, antiplatelet agents, anticoagulants, fibrinolytics, intravenous immunoglobulin, plasma infusion, plasmapheresis, and prostacyclin infusion have not been proven efficacious in treating HUS.

Essentials of Diagnosis

• Microangiopathic hemolytic anemia.
• Thrombocytopenia.
• Fever.
• Central nervous system disorder, including mental status changes, seizures, focal neurologic abnormalities.
• Renal involvement.

General Considerations

TTP previously was considered to be pathogenetically linked to HUS with a somewhat different clinical presentation. However, the identification of an abnormal plasma protein in TTP has led to the understanding that TTP and HUS are pathogenetically distinct entities. Endothelial cells produce large multimers of von Willebrand factor (vWF) that normally are cleaved by a circulating zinc metalloprotease termed ADAMTS13 (a disintegrin and metalloprotease with eight thrombospondin-1-like domains). In TTP, ADAMTS13 is reduced in amount or function, allowing persistence of many unusually large vWF multimers; identification of these multimers is diagnostic for TTP. These multimers bind to extracellular matrix and platelets, inducing platelet aggregation and activation, thrombosis, and thrombocytopenia with organ ischemia. During times of remission, vWF multimers undergo normal cleaving. In the familial and chronic relapsing forms of TTP, gene mutations reduce functional ADAMTS13 to account for less than 5% observed in healthy individuals. The acquired form is due to the presence of an autoantibody interfering with the function of the cleaving protein and may be triggered by certain medications, of which quinine, mitomycin-C, cyclophilin inhibitors, and ticlopidine are the most common. The acquired forms of TTP also may complicate collagen-vascular disorders such as scleroderma and SLE. Neoplasms and infections (most notably HIV) have been associated with TTP.

Clinical Findings

Symptoms and Signs

The classic pentad of TTP consists of fever, microangiopathic hemolytic anemia, TTP, renal disease, and central nervous system symptoms ranging from lethargy, somnolence, and confusion to focal neurologic signs, seizures, and/or coma. The neurologic symptoms often dominate the overall clinical picture. Renal signs and symptoms are common but often mild. It has been estimated that renal involvement is present in as many as 88% of patients

Laboratory Findings

The hemolytic anemia is characterized by many circulating fragmented erythrocytes in the form of schistocytes and helmet cells, which are thought to be produced by shear stress injury as blood flows through vessels narrowed by platelet thrombi. High lactate dehydrogenase (LDH) levels correlate with the severity of the disease. Renal manifestations include microscopic hematuria, rarely gross hematuria, mild to moderate proteinuria, and azotemia with up to 10% of patients having acute renal failure. Renal biopsy may show thrombi with a higher number of platelets and fewer erythrocytes or fibrin.

Treatment

Replacement of the missing cleaving protein activity through plasma or cryosupernatant (cryoprecipitate-poor plasma) infusion is the mainstay of therapy. Plasma exchange may provide additional benefit by removing circulating autoantibodies against ADAMTS13 in those with the acquired form of TTP, and by facilitating the infusion of large amounts of fresh frozen plasma (FFP)(average course of FFP is ˜21 L). Steroids may be useful adjunctive therapy. Rituximab has been used with some success in a few refractory patients. Splenectomy and platelet inhibitors are occasionally used in patients refractory to standard therapy, but are not of proven value. Patients with renal failure may require supportive dialysis therapy. Aspirin and platelet transfusion are contraindicated. Response to therapy is best monitored by following serial serum LDH levels.

Essentials of Diagnosis

• Severe hypertension.
• Autoantibodies.

General Considerations

Scleroderma encompasses altered cell- and humoral-mediated immunity, abnormalities of the microvasculature, and aberrant production and accumulation of extracellular matrix resulting in vascular injury and fibrosis. The pathogenesis of scleroderma is not fully understood and incorporates complex genetic and environmental factors. Chemical, infectious, and physical agents all have been proposed as predisposing factors in the development of scleroderma, but the specific pathogenesis is unknown. It is possible that this heterogeneous disorder may have varying inciting events and mechanisms operative in different affected individuals.

Not until 1952 was renal disease recognized as a major cause of morbidity and mortality in systemic sclerosis. Scleroderma renal crisis likely is initiated by endothelial injury leading to vascular dysfunction. The endothelial damage may be prompted by effects of antiendothelial antibodies inducing upregulation of growth factors and cytokines, decreases in intrinsic complement regulatory proteins, cell-mediated immunity, and/or proteolytic activities in serum. This endothelial injury results in increased vascular permeability with intimal edema, myointimal proliferation, and increased extracellular matrix production, platelet aggregation and adhesion, and fibrin deposition. The consequent vascular luminal narrowing and reduced renal perfusion trigger increased renin production, thereby exacerbating hypertension. In the setting of scleroderma, fibroblasts express increased levels of tumor growth factor (TGF)-β receptors and become persistently activated by small amounts of TGF-β, overproducing extracellular matrix components. It is unclear whether this is an abnormal response to injury or a dysregulation of the relevant gene expression. When immune activation occurs due to one or more inciting events, fibroblasts produce excessive amounts of extracellular matrix material that accumulates in the target organs. Irrespective of the upstream pathogenetic mechanisms involved, fibroblast stimulation appears to be a final pathway in the sclerosis associated with scleroderma.

Numerous autoantibodies are associated with scleroderma, and there appears to be some specificity linked to the pattern of clinical disease presentation. Patients who develop scleroderma renal crisis often produce anti-RNA polymerase III and anti-Th/To RNP antibodies. These phenotype-specific antibodies may merely be markers of disease, or may have a pathogenetic role. The abnormal immune response may influence the onset or progression of scleroderma via several mechanisms. Autoantibodies may be directly pathogenic; between 25% and 85% of patients with scleroderma have antiendothelial cell antibodies, which could induce injury resulting in vascular damage as previously described. Antibody binding can change the sites of antigen proteolysis and/or promote the uptake of complexed proteins; this can spread the immune response to different antigenic components (epitope spreading) thereby enhancing the immune reaction. Involved organs often display T cell and macrophage infiltrates, possibly in response to environmental stimuli, with subsequent cytokine and growth factor release. In addition, altered B cells may upregulate complement receptor signaling further inducing target cell injury and augmenting T cell effector responses. These immune reactions may incite endothelial and fibroblast responses, leading to the vasculopathy and fibrosis of scleroderma. There is no firm evidence that cold temperatures, cardiac dysfunction, pregnancy, or specific drugs such as nonsteroidal anti-inflammatory drugs (NSAIDs) or calcium channel blockers induce this process, although high steroid doses over time have been linked to renal crisis.

Clinical Findings

Symptoms and Signs

Classical scleroderma renal crisis occurs in approximately 10% of all patients with scleroderma, and is defined by the presence of new-onset often severe hypertension and/or rapidly progressive acute renal failure in this setting. However, this varies depending on the form of scleroderma. Patients with CREST (calcinosis, Raynaud's phenomenon, esophagitis, sclerodactyly, telangiectasias) syndrome and limited or localized systemic sclerosis are much less likely to develop renal crisis (˜1%), although occasionally renal crisis may occur in those with minimal symptoms of scleroderma. In contrast, those with diffuse systemic sclerosis have the largest risk, with up to 25% of patients developing this complication. Renal crisis is more likely to develop in those with diffuse disease, rapid skin thickening on the trunk or proximal limbs, fatigue, weight loss, and polyarthritis; carpal tunnel syndrome; onset of scleroderma within the prior 5 years, especially the prior 1 year; antitopoisomerase III (Scl-70) as opposed to anticentromere antibodies; African-American ethnicity; male gender; edema; and tendon friction rubs.

Marked blood pressure elevation is the most common presenting manifestation, occurring in 90% of patients. Hypertension is severe with 30% having diastolic blood pressure in excess of 120 mm Hg, and may emerge abruptly; there are cases in which blood pressure was normal within days of an acute presentation. The hypertension may be manifest as a significant increase in blood pressure for that individual within the normal range, in the minority of patients with normal blood pressures. Extrarenal manifestations occasionally precede the onset of renal crisis, and include pericardial effusions, congestive heart failure, and ventricular arrhythmias. Seizures rarely occur in those with renal crisis. Up to 50–60% of patients with systemic sclerosis who do not have renal crisis may have mild hypertension and up to 80% have abnormal morphologic findings in the kidney on renal biopsy.

Laboratory Findings

Plasma renin levels are invariably high in renal crisis, but whether this is the cause of the hypertension and renal ischemia or a reflection of it is unclear. The presence of microangiopathic hemolytic anemia and thrombocytopenia is a clinical clue to the presence of scleroderma renal crisis; the anemia is found in 43% of patients with renal crisis and thrombocytopenia occurs, but rarely is less than 50,000/mm3. However, scleroderma, hemolytic anemia, thrombocytopenia, and acute renal failure occasionally may arise in the absence of severe hypertension, suggesting that thrombotic microangiopathy may occur in scleroderma via a mechanism independent of and/or in addition to malignant hypertension. Other laboratory features on presentation include nonnephrotic proteinuria, dysmorphic usually microscopic hematuria, and an elevated serum creatinine. These are not specific as microscopic hematuria, proteinuria, and diminished glomerular filtration rate (GFR) frequently accompany accelerated or malignant hypertension at presentation, and may occur in up to 50–60% of patients without renal crisis.

Treatment

The standard treatment for patients with renal crisis is angiotensin-converting enzyme (ACE) inhibition, which has dramatically reduced the mortality associated with this disease. The mechanism of action of these agents likely is multifactorial as they are effective in patients with and without hypertension. When ACE inhibitors alone cannot adequately control hypertension, other antihypertensive agents should be prescribed to achieve a goal blood pressure of 125/75 mm Hg. It is recommended that ACE inhibitor therapy be continued in the setting of scleroderma renal crisis regardless of concerns that it may diminish renal perfusion pressure and exacerbate renal functional decline. Up to 50% of the patients who require dialysis during the course of renal crisis will become dialysis independent by 2 years after the initiation of therapy; therefore, ACE inhibition should be continued even if the patient requires dialysis. Anecdotal case reports suggest that combining ACE inhibition with angiotensin receptor blockers may worsen the renal outcome.

Essentials of Diagnosis

• Thrombosis.
• Recurrent spontaneous abortions.
• Antiphospholipid, anticardiolipin, or anti-β2 glycoprotein I antibodies.

General Considerations

The antiphospholipid syndrome is an autoimmune disorder characterized by recurrent venous, arterial, or microvascular thrombosis or recurrent pregnancy loss. These events are induced by the actions of a family of autoantibodies with broad reactivity to phospholipid epitopes and/or to the phospholipid binding protein β2-glycoprotein I. The syndrome may be present as a primary disorder or as a secondary disorder, the latter usually in association with SLE. The majority of the autoimmune anticardiolipin antibodies are directed against the phospholipid-binding protein rather than the phospholipid itself, and requires the binding protein to produce the effects.

Numerous mechanisms have been proposed to account for the hypercoagulable state. Antiphospholipid antibodies may interfere with the function of phospholipid-binding proteins involved in the regulation of coagulation. Candidates for such interference include β2-glycoprotein I, prothrombin, protein C, and tissue factor. It has been postulated that through binding to cell receptors there is activation of endothelial cells with conversion from an anticoagulant to procoagulant phenotype with upregulation of adhesion molecules, the elaboration of cytokines, and alterations in the balance of prostacyclin and thromboxane. Antiphospholipid antibodies also bind to platelets via the apoER2 receptor. Oxidant-mediated endothelial injury may play a role, with autoantibodies to oxidized LDL occurring along with anticardiolipin antibodies. In fact, some anticardiolipin antibodies cross-react with oxidized LDL. Additionally, associated antibodies against tissue plasminogen activator with resulting reduced fibrinolysis may contribute to the clinical manifestations.

Clinical Findings

Symptoms and Signs

A diagnostic classification of antiphospholipid syndrome was adopted in 1999 and requires at least one laboratory and one clinical manifestation (Table 34–2). The clinical criteria include a vascular occlusion involving veins, arteries, or capillaries in any organ or pregnancy complications including at least three miscarriages before the tenth gestational week, loss of a normal fetus after 10 weeks, or prematurity of a normal fetus (earlier than 34 weeks). The antiphospholipid syndrome may involve one or more organs including the central nervous system, kidney, endocrine, gastrointestinal tract, lungs, skin, and cerebrovascular and cardiovascular systems. Renal manifestations are protean, and include acute renal failure, progressive chronic kidney disease sometimes culminating in kidney failure, cortical necrosis, mild to malignant hypertension, thrombotic microangiopathy, and thrombosis of renal allografts. The syndrome may occur in a “catastrophic” form, defined by involvement of at least three organ systems simultaneously. The kidney is the most frequently affected organ in the catastrophic form involved in 78% of cases, characterized by hypertension, which is often malignant. Dialysis is required in 25% of those with renal involvement. Other end-organ involvement includes pulmonary (66%), central nervous system such as cerebrovascular accidents (56%), cardiac including premature myocardial infarction (50%), and dermatologic (50%). Disseminated intravascular coagulation is uncommon.

Laboratory Findings

The diagnosis requires the presence of an antiphospholipid antibody, including immunoglobulin (Ig)G and/or IgM, anticardiolipin antibody, or lupus anticoagulant. Antibodies to phospholipid binding proteins frequently are present, but are not yet included in the diagnostic classification. Anticardiolipin antibodies are identified by immunoassays that measure pathologic reactivity to anionic phospholipids including the anticardiolipin antibody, antiphospholipid antibody, or false-positive VDRL test. Lupus anticoagulants are identified by abnormalities in coagulation assays including prolonged prothrombin time or partial thromboplastin time, particularly when the latter is not normalized when diluted with normal serum; abnormalities in the kaolin cephalin clotting time or the thromboplastin inhibition test; or abnormal Russel viper venom test. The anticardiolipin antibody or lupus anticoagulant must be detected twice at least 6 weeks apart according to the diagnostic criteria. Thrombocytopenia is frequent in antiphospholipid syndrome. Up to 5% of healthy individuals have circulating anticardiolipin antibodies, and it is unclear how many of these individuals eventually will develop antiphospholipid syndrome. It has been estimated that 12-30% of patients with SLE have anticardiolipin antibodies and 15–34% have evidence of a lupus anticoagulant. As many as 50–70% of these individuals may have an associated clinical event over the course of 20 years of follow-up.

Treatment

The mainstay of therapy is anticoagulation, which is required long term. Warfarin therapy is recommended for the primary syndrome and for those with associated systemic lupus. A recently published study suggested there were low rates of recurrent thrombosis in patients in whom the international normalized ratio (INR) was kept in the range of 2–3, instead of the 3 previously recommended. Mortality for patients with the catastrophic syndrome is high, approaching 50%.

Patients should avoid prothrombotic drugs such as oral contraceptives, calcineurin inhibitors, hydralazine, procainamide, and chlorpromazine. Aspirin should be prescribed for women with prior pregnancy losses. Although not evidence based, the addition of steroids, plasmapheresis, and intravenous immunoglobulin has been implemented as salvage therapy in patients with severe and/or multiple organ involvement. The role of hydroxychloroquine and/or chloroquine to prevent thrombosis in patients with SLE is controversial.