Paroxysmal Nocturnal Hemoglobinuria

Background

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, chronic, debilitating disorder that most frequently presents in early adulthood and usually continues throughout the life of the patient. PNH results in the death of approximately 50% of affected individuals due to thrombotic complications and, until recently, had no specific therapy.

The name of the disorder is a descriptive term for the clinical consequence of red blood cell (RBC) breakdown with release of hemoglobin into the urine, which manifests most prominently as dark-colored urine in the morning (see image below).

View Image

This series of containers holds urine of a patient with paroxysmal nocturnal hemoglobinuria, showing the episodic nature of the dark urine (hemoglobin....

The term "nocturnal" refers to the belief that hemolysis is triggered by acidosis during sleep and activates complement to hemolyze an unprotected and abnormal RBC membrane. However, this supposition was later disproved. Hemolysis has been shown to occur throughout the day and is not actually paroxysmal, but the concentration of urine overnight produces the dramatic change in color.

PNH has been referred to as "the great impersonator" because of the variety of symptoms observed during its initial manifestation and course. This variety reflects the contributions of the following three underlying pathophysiologic events^{[1, 2, 3, 4]} :

• An acquired intracorpuscular hemolytic anemia due to the abnormal susceptibility of the RBC membrane to the hemolytic activity of complement
• Thromboses in large vessels, such as hepatic, abdominal, cerebral, and subdermal veins
• A deficiency in hematopoiesis that may be mild or severe, such as pancytopenia in an aplastic anemia state

The triad of hemolytic anemia, pancytopenia, and thrombosis makes PNH a unique clinical syndrome.

For patient education information, see Anemia.

• References

Pathophysiology

Paroxysmal nocturnal hemoglobinuria (PNH) was previously classified as purely an acquired hemolytic anemia due to a hematopoietic stem cell mutation defect. This classification was abandoned because of the observation that surface proteins were missing not only in the RBC membrane but also in all blood cells, including the platelet and white cells.

The common denominator in the disease, a biochemical defect, appears to be a genetic mutation leading to the inability to synthesize the glycosyl-phosphatidylinositol (GPI) anchor that binds these proteins to cell membranes.^{[4, 5, 6]} The corresponding gene PIGA (phosphatidylinositol glycan class A) in the X chromosome can have several mutations, from deletions to point mutations.^[7]

Due to its location on the X chromosome, and X inactivation in female somatic cells, only one mutation is required in either males or females to abolish the expression of GPI-linked proteins. Most type II PNH cells (total lack of GPI-linked protein) are due to a frame shift mutation occurring in the early hematopoietic progenitor cells, resulting in the same mutation in all blood cell lines.

The essential group of membrane proteins that are lacking in all hematopoietic cells are called complement-regulating surface proteins, including the decay-accelerating factor (DAF), or CD55^[8] ; homologous restriction factor (HRF), or C8 binding protein; and membrane inhibitor of reactive lysis (MIRL), or CD59.^[9] All of these proteins interact with complement proteins, particularly C3b and C4b, dissociate the convertase complexes of the classic and alternative pathways, and halt the amplification of the activation process.

The absence of these regulating proteins results in uncontrolled amplification of the complement system. This leads to intravascular destruction of the RBC membrane, to varying degrees

Breakdown of RBC membranes by complement leads to the release of hemoglobin into the circulation. Hemoglobin is bound to haptoglobin for efficient clearance from the circulation. After saturating the haptoglobin, free hemoglobin circulates and binds irreversibly with nitric oxide (NO), depleting NO levels in peripheral blood.

Because NO regulates smooth muscle tone, depletion of NO levels leads to smooth muscle contraction with consequent vasoconstriction, constriction of the gut, and pulmonary hypertension. Resultant symptoms may include the following:

• Abdominal pain
• Bloating
• Back pain
• Headaches
• Esophageal spasms
• Erectile dysfunction
• Fatigue

Progressive chronic renal failure can occur after several years of hemoglobinuria from the acute tubulonecrosis effects of heme and iron (pigment nephropathy), decreased renal perfusion from renal vein thrombosis, and tubular obstruction with pigment casts. Patients with PNH experience a high incidence (40%) of thrombotic events (mostly venous) in large vessels (cerebral, hepatic, portal, mesenteric, splenic, and renal veins) and, most recently recognized, arterial thrombosis.

The pathophysiology of thrombophilia in PNH is not fully understood, but the increased incidence during hemolytic episodes suggest a direct relationship with the hemolytic process. Increased procoagulant and fibrinolytic activity, suggesting increased fibrin generation and turnover, increased plasma levels of urokinase-type plasminogen activator and platelets deficient in GPI-linked proteins activated by complement, have been implicated. However, none of these identified platelet and coagulation abnormalities can fully explain the hypercoagulable state in PNH.

Bone marrow failure is present in all patients with PNH, even when peripheral blood counts are normal and the bone marrow is hypercellular. The degree of marrow failure may vary from severe aplastic anemia to a decrease in the number of hematopoietic stem cells. Bone marrow failure is possibly due to similar destruction by complement, but the cause or causes are still poorly understood.

Venous thrombosis usually manifests as a sudden catastrophic complication, with severe abdominal pain, a rapidly enlarging liver, and ascites (Budd-Chiari syndrome). This thrombosis may be due to a lack of CD59 on platelet membranes, which induces platelet aggregation and is highly thrombogenic, particularly in the venous system.

Deficient hematopoiesis may occur due to diminished blood cell production by a hypoplastic bone marrow; thus, patients have a 10-20% chance of developing aplastic anemia in their course. In turn, PNH eventually develops in 5% of patients with aplastic anemia.^{[7, 10]} The nature of the pathogenetic link between those two diseases is still unknown.

Pathogenesis

When intravascular hemolysis occurs, RBCs release hemoglobin (Hb) into the plasma. This free Hb is rapidly dimerized and bound by serum protein haptoglobin and rapidly removed by macrophages, which then degrades it after endocytosis. Since haptoglobin is not recycled, large amounts of free Hb can deplete the body's supply, leaving the excess Hb free in the plasma.

When the capacity to manage and degrade free Hb during acute or chronic hemolysis is reached, levels of Hb and heme increase in the plasma and urine. Plasma Hb has the ability to scavenge nitric oxide (NO), resulting in rapid consumption of NO and clinical sequelae of NO depletion. NO plays a major role in vascular homeostasis and has been shown to be a critical regulator of basal and stress-mediated smooth muscle relaxation and vasomotor tone, endothelial adhesion, and platelet activation and aggregation.

Thus, the clinical consequences of excessive cell-free plasma Hb levels during intravascular hemolysis or the administration of Hb preparations include dystonias involving the gastrointestinal, cardiovascular, pulmonary, and urogenital systems, as well as clotting disorders. Many of the clinical sequelae of intravascular hemolysis in a prototypic hemolytic disease, PNH, are readily explained by Hb-mediated NO scavenging.

Paroxysms or episodes of symptoms occur during sudden and marked increases in the rate of intravascular hemolysis. These episodes can be precipitated by infections, drugs, or trauma or they can occur spontaneously. During paroxysms, PNH patients exhibit symptoms consistent with smooth muscle perturbation through the release of Hb and NO scavenging, including abdominal pain, esophageal spasms, and erectile dysfunction.

NO also plays an important role in the maintenance of normal platelet functions through the down-regulation of platelet aggregation and adhesion and the regulation of molecules in the coagulation cascade. Accordingly, the long-term consumption of NO by plasma Hb has been implicated in the formation of clots in PNH patients.^[11]

Thrombosis is the most common cause of death in persons with PNH, accounting for 50% of the mortality from the disease. The most frequent sites of thrombosis include the hepatic, pulmonary, cerebral, and deep and superficial veins, as well as the inferior vena cava. Approximately 15%-20% of thrombosis in PNH is arterial.^[12] Interestingly, there is a close correlation between thrombosis and a large PNH clone, and clone size correlates with hemolytic rates.

The reason for the propensity for thrombosis is not entirely clear. Intravascular hemolysis may provide altered membrane surfaces upon which coagulation may be initiated.

More likely, it is the effect of the activation of complement on platelets and perhaps endothelial cells. In platelets, the deposition of C9 complexes on the surface stimulates their removal by vesiculation; these vesicles are very thrombogenic. Because PNH platelets lack the mechanism for down-regulating C9 deposition (ie, CD59), even a minimal stimulus from activated complement results in a greatly increased production of these vesicles.^[13]

• References

Etiology

For some time, paroxysmal nocturnal hemoglobinuria (PNH) has been known to result from somatic mutations in the PIGA gene, which encodes phosphatidylinositol glycan class A (PIGA). These mutations result in hematopoietic stem cells that are deficient in glycosyl-phosphatidylinositol anchor protein (GPI-AP). Nonmalignant clonal expansion of one or several of these stem cells leads to clinical PNH.

Shen et al have identified additional somatic mutations associated with PNH. These mutations were in genes known to be involved in myeloid neoplasm pathogenesis, including TET2, SUZ12, U2AF1, and JAK2. Clonal analysis indicated that these additional mutations arose either as a subclone within the PIGA-mutant population or had occurred prior to PIGA mutation.^[14]

The clinical pathology in PNH may actually be an epiphenomenon resulting from an adaptive response to injury, such as an immune attack on hematopoietic stem cells.

In PNH, the peripheral blood and bone marrow is a mosaic composed of GPI-AP+ and GPI-AP– cells; with GPI-AP–, cells can be derived from multiple mutant stem cells. The GPI-AP– mutant cells may appear to dominate hematopoiesis in PNH by providing a proliferative advantage under some pathologic conditions. For example, if damage to stem cells causing bone marrow failure is mediated through a GPI-linked surface molecule, the PNH cells lacking these molecules will survive. The close association of PNH with aplastic anemia and myelodysplastic syndrome (MDS) suggests that the selection process arises as a consequence of this specific type of bone marrow injury.

• References

Epidemiology

Paroxysmal nocturnal hemoglobinuria (PNH) is an uncommon disorder of unknown frequency both in the United States and worldwide. There is little information on the incidence of PNH, but the rate is estimated to be 5-10 times less than that of aplastic anemia; thus, PNH is a rare disease.

Attempts to determine the incidence more accurately and to learn more about the natural course of the disease are currently in progress under the auspices of the PNH Registry—"a worldwide collection of data aiming at improving and sharing the understanding of PNH for a better management of patients with PNH".

It has been suggested that, like aplastic anemia, PNH may be more frequent in Southeast Asia and in the Far East.

Mortality/Morbidity

The disease process of PNH is insidious and has a chronic course, with a median survival of about 10.3 years. Morbidity depends on the variable expressions of hemolysis, bone marrow failure, and thrombophilia that define the severity and clinical course of the disease.

A study of the first 1610 patients enrolled in the International PNH Registry found that overall, 16% of patients had a history of thrombotic events and 14% a history of impaired renal function. Frequently reported symptoms included the following^[15] :

• Fatigue (80%)
• Dyspnea (64%)
• Hemoglobinuria (62%)
• Abdominal pain (44%)
• Chest pain (33%)

Patients also reported impairment in quality of life from their disease, with 17% stating that they were not working or were working less because of PNH.

In several large studies, the main cause of death in patients with PNH was venous thrombosis, followed by complications of bone marrow failure; however, spontaneous long-term remission or leukemic transformation of the PNH clone has been reported and well documented.

The median survival after diagnosis was 10 years in a series of 80 consecutive patients seen at the Hammersmith Hospital in London who were treated with supportive measures, such as oral anticoagulant therapy after an established thrombosis, and transfusions.^[16] Sixty patients died; of 48 patients whose cause of death was known, 28 died from venous thrombosis or hemorrhage. Thirty-one individuals (39%) had one or more episodes of venous thrombosis during their illness.^[16] No leukemic transformations occurred in this series.

Twenty-two of the 80 patients (28%) survived for 25 years.^[16] Of the 35 patients who survived for 10 years or more, 12 had spontaneous clinical recovery; no PNH-affected cells were found among the RBCs or neutrophils during their prolonged remission, but a few PNH-affected lymphocytes were detectable in 3 of 4 patients tested.^[16]

Race

The differences of PNH) among races were shown in a study that compared 176 American patients seen at Duke University and 209 patients from Japan.^[17] White American patients were younger and had significantly more classic symptoms of the disease, including thrombosis, hemoglobinuria, and infection, whereas Asian patients were older with more marrow aplasia and a smaller PNH clone. Survival analysis showed a similar death rate in each group, although the causes of death were different, with more thrombotic deaths seen in the American patients. Japanese patients had a longer mean survival time (32.1 vs 19.4 y), but Kaplan-Meier survival curves were not significantly different.^[17]

Other geographic ethnic differences were observed in the thrombosis incidence in 64 patients with classic PNH.^[18] The investigators found that African Americans (n = 11) and Latin Americans (n = 8) had a higher risk or rate of thrombosis by Cox regression analysis and that this had an impact on length of survival compared with other patients (n = 45).

Sex- and Age-related Demographics

Men and women are affected equally with PNH, and no familial tendencies exist.

PNH may occur at any age, from children (10%) as young as 2 years to adults as old as 83 years, but median age at the time of diagnosis was 42 years (range, 16-75 y) in an English series of 80 consecutive patients.^[16] In childhood through adolescence, patients with PNH presented with more of the primary features of aplastic anemia than the healthy adult population. Other complications, such as infections and thrombosis, occurred with equal frequency in all age groups.

• References

History

The classic manifestation of paroxysmal nocturnal hemoglobinuria (PNH) is dark urine during the night with partial clearing during the day (see image below). However, hemoglobinuria may occur every day in severe cases; more frequently, it occurs in episodes lasting 3-10 days; and in some cases, it does not occur at all.

View Image

This series of containers holds urine of a patient with paroxysmal nocturnal hemoglobinuria, showing the episodic nature of the dark urine (hemoglobin....

A working classification has been developed for PNH that includes all the variations in its presentation, clinical manifestations, and natural history. PNH can present as any of the following three syndromes or sets of symptoms:

• Classic PNH
• PNH in the setting of another specified bone marrow disorder (eg, PNH/aplastic anemia or PNH/refractory anemia-myelodysplastic syndrome [MDS])
• Subclinical PNH (PNH-sc)

Hemolytic anemia is usually in the form of intravascular hemolysis. The most common presentation is the presence of anemia associated with dark cola-colored urine that is a manifestation of hemoglobinuria. The latter may be confused with hematuria, and erroneous treatment could be given for urosepsis.

Thrombosis involves the venous system, and it typically occurs in unusual veins, namely the hepatic, abdominal, cerebral, and subdermal veins. The tendency of patients with PNH to suffer thrombosis has been recognized as a major part of the syndrome and interpreted as a very bad prognostic sign and the most common cause of death in PNH. About 30-40% of patients of European origin have serious thrombosis at some time; for unexplained reasons, only 5-10% of patients of East Asian (Chinese, Japanese, and Thai) or Mexican origin develop this complication.^{[19, 20]}

Hepatic vein thrombosis results in Budd-Chiari syndrome, which manifests as a sudden and catastrophic event characterized by jaundice, abdominal pain, a rapidly enlarging liver, and accumulation of ascitic fluid. This syndrome may be severe and lead to vascular collapse and death, or it can be slow and insidious, leading to hepatic failure.

Abdominal vein thrombosis presents as upper abdominal pain, or pain elsewhere in the abdomen, lasting 1-6 days. It can lead to bowel infarction in severe cases.

Cerebral vein thrombosis can range from the mildest form to a severe headache, depending on which veins are involved. The sagittal vein is commonly affected, which can give rise to papilledema and pseudotumorcerebri.

Dermal vein thrombosis manifests as raised, painful, red nodules in the skin affecting large areas, such as the entire back, which subsides within a few weeks, usually without necrosis. In cases that do result in necrosis, skin grafting may be necessary.

Patients with deficient hematopoiesis usually present with anemia, despite the presence of an erythroid marrow with suboptimal reticulocytosis. In some cases, neutropenia and thrombocytopenia can occur in a hypoplastic bone marrow similar to aplastic anemia (aplastic episodes).

Other symptoms of PNH include esophageal spasms that occur in the morning and, like the dark-colored urine, clear up later in the day. In males, erectile dysfunction can occur concomitantly with hemoglobinuria; the cause of this is unknown.

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Physical

Most commonly, in patients with paroxysmal nocturnal hemoglobinuria (PNH), pallor suggests anemia; fever suggests infections; and bleeding, such as mucosal bleeding or skin ecchymoses, suggests thrombocytopenia similar to that in aplastic anemia. Other physical examination findings may include the following:

• Hepatomegaly and ascites in the presence of Budd-Chiari syndrome
• Splenomegaly in the presence of splenic vein thrombosis
• Absent bowel sounds in the presence of bowel necrosis
• Papilledema in the presence of cerebral vein thrombosis
• Skin nodules that are red and painful in the presence of dermal vein thrombosis

• References

Approach Considerations

In addition to a complete blood cell count, the principal studies used to establish the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) are flow cytometry of peripheral blood and bone marrow analysis. Flow cytometry measures the percentage of cells that are deficient in the glycosyl phosphatidylinositol–anchored proteins (GPI-APs) and identifies discrete populations with different degrees of deficiency. Because of the missing GPI-APs, red blood cells (RBCs) and other cells in patients with PNH lack DAF (CD55) and MIRL (CD59), which regulate complement.

Hemosiderin is nearly always present in the urine sediment and can accumulate in the kidneys; this is visible on magnetic resonance images (MRI) or computed tomography (CT) scans. An elevated reticulocyte count and serum lactate dehydrogenase (LDH) level with a low serum haptoglobin level in the absence of hepatosplenomegaly are the hallmarks of intravascular hemolysis.

Bone marrow examination will differentiate classic PNH from PNH that develops in the setting of other bone marrow disorders.^[23] In addition, bone marrow examination will identify an erythroid and hyperplastic bone marrow during the hemolytic phase or a hypoplastic bone marrow in the aplastic phase.

Imaging studies are indicated in patients with venous thrombosis.

• References

Laboratory Studies

The tests involved in establishing the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) demonstrate the presence of red blood cells (RBCs) that are exceptionally sensitive to the hemolytic action of complement. These tests include the following:

• Flow cytometry
• Acidified serum lysis and Ham test
• Complement lysis sensitivity test
• Sugar water or sucrose lysis test

Most laboratories no longer perform the Ham test or the sugar water test.

Flow cytometry

The state-of-the-art laboratory test is flow cytometry of the patient's blood to detect CD59 (MIRL), a glycoprotein, and CD55 (DAF) in regulation of complement action. Absence or reduced expression of both CD59 and CD55 on RBCs is diagnostic of PNH.

The use of flow cytometry in PNH differs from many applications in that the diagnosis depends upon demonstrating the absence of relevant antigens. In this context, it is important that at least two glycosyl-phosphatidylinositol (GPI)–linked antigens are studied to exclude rare congenital deficiencies of single antigens (CD55 and CD59) and polymorphism with individual antigens (CD16), which render them undetectable by some monoclonal antibody clones.

Standard and high-sensitivity flow cytometric procedures for detecting PNH cells are now available.^[24] For routine analysis and diagnosis of suspected PNH, the standard test is sufficient. This test can detect 1% or more PNH cells, bu; most laboratories report only 10% or more as a positive result. High-sensitivity analysis (in which as little as 0.01% PNH cells can be detected) may be helpful in aplastic anemia patients, who may eventually develop PNH, and possibly in those with hypoplastic myelodysplasia syndrome (MDS), to predict responses to immunosuppressive therapy.

Fluorescent aerolysin

A more accurate alternative reagent for PNH screening and PNH clone measurement is the bacterial toxin aerolysin, which binds to RBCs via GPI anchor and initiates hemolysis. A modified, nonhemolytic form of a fluorescently labeled molecule has been developed that can detect PNH cells to a level of 0.5% (fluorescently labeled inactive toxin aerolysin [FLAER] binding of peripheral blood granulocytes). The advantage of this assay is that it can detect the clone in all hematopoietic cell lineages in one assay.

This is the most specific test for PNH, as FLAER binds the GPI anchor specifically. Thus, the lack of FLAER binding to granulocytes (measured by flow cytometry) is sufficient for the diagnosis of PNH. The disadvantage of the test is in measuring binding in the absence of adequate granulocytes, such as in severe aplastic anemia, when the number of circulating granulocytes is extremely low.

Immmunotyping

Peripheral blood is the most suitable specimen for immunophenotyping for PNH, and it is important to screen both RBCs and granulocytes, because RBC transfusions are common among these patients and granulocytes may not be present in severe hypoplastic anemia patients.

Studies have shown that the size of the PNH clone correlates with the risk for venous thrombosis. Patients with less than 50% PNH granulocytes seldom develop thrombosis, whereas patients with larger clone sizes appear to be at great risk and will require anticoagulation.

Acidified serum lysis and Ham test

If performed properly, acidified serum lysis and the Ham test (from Thomas Hale Ham) are reliable ways to diagnose PNH (see image below). Dr. Ham demonstrated that the RBCs in PNH were lysed by complement when normal serum was acidified or activated by alloantibodies.

View Image

The Ham test (acidified serum lysis) establishes the diagnosis of paroxysmal nocturnal hemoglobinuria (PNH), demonstrating a characteristic abnormalit....

The serum pH is lowered to about 6.2 and the magnesium level is adjusted to 0.005 mol/L to achieve maximum sensitivity. The cells that are hemolyzed are the sensitive cells, and those that remain intact are normal cells, indicating 2-3 subpopulations of RBCs in the circulation.

A false-positive test result is seen in congenital dyserythropoietic anemia, type II (hereditary erythroblastic multinuclearity with positive acidified serum tests [HEMPAS]). These patients have a negative sucrose hemolysis ("sugar water test") result. Some normal serum can give a false-negative Ham test result; thus, the sucrose water test is more sensitive but less specific for paroxysmal nocturnal hemoglobinuria (PNH).

Complement lysis sensitivity test

The complement lysis sensitivity test of Rosse and Dacie is a more precise method for diagnosing PNH. RBCs are sensitized with a potent lytic anti-I antigen and hemolyzed with limiting amounts of normal serum as a source of complement.^{[25, 26, 27]} This demonstrates threee groups of RBCs in patients with PNH, including the following:

• PNH I cells are normal in sensitivity to complement
• PNH II cells are moderately more sensitive to complement than normal cells
• PNH III cells are markedly sensitive to complement, requiring one fifteenth to one twentieth of the amount of complement for an equal degree of lysis; this group of cells is increased in patients with more severe PNH, and it is associated with a mean life span of 10-15 days

Sugar water or sucrose lysis test

The sugar water or sucrose lysis test uses the ionic strength of serum that is reduced by adding an iso-osmotic solution of sucrose, which then activates the classic complement pathway, and complement-sensitive cells are lysed. This test is less specific but more sensitive for PNH than the Ham test, because some RBCs hemolyze from autoimmune hemolytic anemias, leukemia, and aplastic anemia to a minor degree. Although the tests are inexpensive and simple to perform, they are more labor intensive and less sensitive due to the short half-life of circulating PNH RBCs.

Other tests for intravascular hemolysis

Other tests to demonstrate intravascular hemolysis include the following:

• Elevated serum lactate dehydrogenase (LDH)
• Elevated reticulocyte count
• Low-to-absent serum haptoglobin
• Hemoglobinuria and hemosiderinuria; however, hemolysis may occur intermittently and hence can be missed easily, depending on when the tests are performed

• References

Imaging Studies

Thromboses of major veins are best evaluated by radiographic means.

Investigate hepatic vein thrombosis with a routine technetium-99m (^99m Tc) colloid scan of the liver and spleen. This study often reveals diminished function in all portions of the liver except the caudate lobe, which is spared because it is drained by the inferior vena cava rather than the hepatic vein. A magnetic resonance imaging (MRI) study or ultrasonogram can demonstrate the cessation of flow through the hepatic vein or by injection or use of a dye to demonstrate a thrombus in the vein.

MRI with contrast may demonstrate sagittal vein thrombosis.

• References

Other Tests

PIG-A gene mutation analysis is still limited to research laboratories. In addition, although it is very specific, it is still not diagnostic for paroxysmal nocturnal hemoglobinuria (PNH).

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Approach Considerations

According to current understanding of paroxysmal nocturnal hemoglobinuria (PNH), the ideal treatment is to replace the defective hematopoietic stem cell with a normal equivalent by stem cell transplantation; however, this is not realistic for many patients, because stem cell transplantation requires a histocompatible donor and is associated with significant morbidity and mortality. This form of treatment is reserved for severe cases of PNH with aplastic anemia or transformation to leukemia, both of which are life-threatening complications.

In 2007, eculizumab (Soliris), an anti-complement antibody targeting the CD5 complement component, was approved by the US Food and Drug Administration (FDA). Eculizumab alleviates the hemolysis associated with PNH and its sequelae, dramatically improving symptoms, improving quality of life, and eliminating complications of PNH.^{[8, 28]} Eculizumab does not alter the underlying defect of the disease, however; thus, treatment needs to continue life-long or until spontaneous remission, which occurred only in a minority of patients (12 of 80 patients in one study^[16] ) before the advent of eculizumab.

Treatment of bone marrow hypoplasia

Bone marrow hypoplasia is a serious cause of morbidity and mortality. It is treated most effectively with bone marrow transplantation; however, if there is no suitable donor available, antithymocyte globulin (ATG) has been used in the treatment of aplastic anemia with considerable success.

Thromboembolism

Patients with PNH who develop acute thrombosis should immediately be started on eculizumab, if they are not already taking it, as this reduces the risk of thrombosis extension or recurrence.^[12] Otherwise, management of thrombotic complications follows standard principles, including using heparin emergently, then maintenance therapy with the use of an oral anticoagulant, such as warfarin. Sometimes, heparin can exacerbate the thrombotic problem, possibly by activating complement. This can be prevented using inhibitors of the cyclooxygenase system, such as aspirin, ibuprofen, and sulfinpyrazone.

Primary prophylaxis of thromboembolism for patients with PNH has been advocated. Whether this approach is safe and effective in all patients with PNH remains controversial, however.

Corticosteroids

Modulation of complement is controlled poorly by high doses of glucocorticoids. The usual adult dose of prednisone is 20-40 mg/d (0.3-0.6 mg/kg/d) given daily during hemolysis and changed to alternate days during remission. On this regimen, about 70% of adult patients experience improvement in hemoglobin levels, but long-term therapy is fraught with complications.

Investigational agents

A variety of agents that inhibit complement are under development for treatment of PNH. Novel anti-C5 agents include monoclonal antibodies and an anti-C5 small interfering RNA.^[29] Because clinically relevant C3-mediated extravascular hemolysis can occur in PNH,^[30]  the anti-C3 small peptide compstatin and its derivatives are being investigated, along with inhibitors of complement Factor D or B.^[29]  

Yuan et al reported that two novel small-molecule inhibitors of Factor D, which is a component of the alternative complement pathway, show potential as oral agents for treating PNH. In the Ham test, using cells from PNH patients, the Factor D inhibitors significantly reduced complement-mediated hemolysis at concentrations as low as 0.01 μM. In an animal model, the compound ACH-4471 blocked alternative pathway activity.^[31]

• References

Eculizumab

The anticomplement agent eculizumab is a humanized monoclonal antibody against terminal protein C5; it has been shown to be highly effective in reducing intravascular hemolysis.^{[32, 33]} Eleven transfusion-dependent patients with PNH were given intravenous (IV) eculizumab at 900 mg over 30 minutes every 2 weeks. The mean lactate dehydrogenase (LDH) level decreased, transfusion requirements fell from 2.1 units per patient per month to 0.0, and a global improvement in quality of life occurred.^[33] Long-term analysis showed that these improvements can be maintained over 3 years, and erythropoietin can overcome anemia due to bone marrow failure in patients on the drug.

The 5-year survival of patients with PNH prior to eculizumab therapy in a cohort followed at Leeds Hospital in the United Kingdom was 66.8%. With eculizumab therapy, 5-year survival improved to 95.5%, which is not statistically different from age-matched controls in the general population.^[34]

Treatment breakthrough from complement control can occur in small minority (10%) of patients due to an inadequate dosing schedule. The eculizumab level must remain above 35 μg/mL, but trough levels at 2 weeks may fall below this level and cause recurrence of hemolysis.

The recommended adjustment for patients whose eculizumab levels fall into this category is to increase the dose to 900 mg every 12 days or 1200 mg every 2 weeks. Withdrawal hemolysis can occur by stopping therapy for any reason, as accumulation of PNH RBC increases over time by protecting type II and III PNH cells from destruction due to therapy, which can potentially trigger a massive hemolysis.

Infection prophylaxis

Consequences of complement inhibition include an increased risk of infections from Neisseria meningitides, as seen in inherited terminal complement deficiency.^[8] Before the administration of eculizumab, all patients should be vaccinated with a serogroup B meningococcal vaccine.^[35] Alashkar et al suggest that serologic response testing after vaccination is warranted, because immunological response to vaccines varies, and that re-vaccination with a tetravalent conjugate vaccine every 3 years is essential, or should be based on response rates, in patients on eculizumab therapy.^[36]

Despite vaccination, patients may develop meningococcal septicemia (not meningitis). Although this is rare, occurring at a rate of 0.5 cases per 100-patient years, prophylactic antibiotics are recommended to prevent this complication. One study used penicillin V, 500 mg twice daily orally, or erythromycin 500 mg twice daily for patients intolerant to penicillin.^[37]

Effect on thromboembolic complications

Eculizumab treatment reduces the risk of clinical thromboembolism in patients with PNH (the leading cause of death in PNH) and is recommended for PNH patients with a history of prior thromboembolism.^[38] The rate of thrombotic complications prior to eculizumab was 5.6 per 100 patient years; after eculizumab, it dropped to 0.8 per 100 patient years.

In an international multi-institutional cooperative study involving 195 PNH patients, the thromboembolic (TE) event rate per 100 patient-years with eculizumab treatment was 1.07, compared with 7.37 events (P< 0.001) prior to eculizumab treatment, a relative absolute reduction of 85%. With equalization of duration of exposure before and during treatment for each patient, TE events were reduced from 39 before eculizumab to 3 during eculizumab (P< 0.001). The TE event rate in antithrombotic-treated patients (n = 103) was reduced from 10.61 to 0.62 events/100 patient-years with eculizumab treatment (P< 0.001).

One study has documented elevated D-dimer levels in PNH patients with a history of thrombosis. D-dimer levels decreased immediately after initiation of eculizumab therapy.^[39]

Continuation of anticoagulation in patients with PNH with a previous thrombosis while on eculizumab is recommended, as stopping therapy has not been studied. However, patients with no previous thrombosis have discontinued warfarin after starting eculizumab, with no thrombotic sequelae.^{[37, 40]}

Eculizumab and renal dysfunction

Chronic hemosiderosis and/or microvascular thrombosis from PNH causes kidney dysfunction at an incidence of 65% for renal dysfunction or damage, defined by stages of chronic kidney disease (CKD), in a large cohort of PNH patients. Eculizumab treatment was safe and well-tolerated in patients with renal dysfunction or damage and resulted in the likelihood of improvement as defined as categorical reduction in CKD stage (P< 0.001) compared with baseline and placebo (P = 0.04).

Improvement in renal function was more commonly seen in those with less severe impairment. Improvements occurred quickly and were sustained for at least 18 months of treatment. Administration of eculizumab to patients with renal dysfunction or damage was well tolerated and was usually associated with clinical improvement.^[41]

Iron

Monitoring iron even if the patients no longer require transfusions is recommended, because hemosiderinuria—a protective mechanism in PNH to excrete iron—no longer occurs with eculizumab. Measuring serum ferritin is recommended and chelation therapy may be necessary in patients with high levels.

• References

Treatment of Anemia

The anemia of PNH may have three components: intravascular hemolysis, inadequate erythropoiesis, and superimposed iron deficiency (massive iron loss through hemoglobinuria). In view of increased rate of erythropoiesis, give 5 mg/d of folic acid orally. Assess iron stores with the use of the transferrin saturation index (TSI) and give oral ferrous sulfate if the result is < 20%. (Ferritin levels should not be used for this purpose, as ferritin is an acute-phase reactant and levels can be misleading.)

Determine steady-state hemoglobin levels after correction for iron deficiency. When appropriate, transfuse packed red blood cells (RBCs) with leukocytes depleted by filter. Washing RBCs is no longer necessary, and use of irradiated blood products is recommended for future stem cell transplantation.

Supportive care for severe anemia includes blood transfusion using leuko-depleted packed RBCs to prevent alloimmunization. Development of alloantibodies can be a problem with future transfusions because of activation of complement and delayed hemolysis of transfused blood.

Iron

Replacement of nutritional iron, because of increased loss of iron from the hemolysis and the 200-fold increase in iron urinary excretion, is necessary to prevent development of iron deficiency. Iron replacement can stimulate reticulocytosis that can trigger hemolysis by releasing a new cohort of complement-sensitive cells. This process can be prevented by adding prednisone during replacement therapy.

Stimulation of erythropoiesis using androgenic hormones has been successful in patients with a moderate decrease in RBC production. This has been replaced mainly by using recombinant erythropoietin therapy.

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Treatment in Pregnancy

Pregnancy in patients with PNH poses very significant risks. There is a very high risk of thrombotic complications for the expectant mother, as well a risk of developing hypoplastic anemia. Maternal mortality in these patients is approximately 20%, mostly from thrombosis and infections, and risk of fetal loss is increased. Consequently, full anticoagulation with low-molecular weight heparin (LMWH) is recommended for pregnant women with PNH. Warfarin may be substituted after the first trimester.

The use of eculizumab in pregnancy has proved beneficial.^[42] In a review of 75 pregnancies in 61 women with PNH, Kelly and colleagues reported a high rate of fetal survival and a low rate of maternal complications. No maternal deaths occurred. There were three fetal deaths (4%) and six first-trimester miscarriages (8%). During pregnancy, patients demonstrated increased requirement of red blood cell transfusions, and approximately half required an increase in the eculizumab dosage. Ten hemorrhagic events occurred, and two postpartum thrombotic events. Eculizumab was detected in some infants' cord blood, but not in breast milk.^[43]

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Hematopoietic Stem Cell Transplantation

Hematopoietic stem cell transplantation (HSCT) using allogeneic donors is the only curative therapy for PNH. With the advent of eculizumab, the indications for HSCT have changed. Clinical results from HSCT from various programs in a rare disease are limited to small numbers of patients. A retrospective analysis of the Italian BM transplantation group in 26 patients with a median age of 32 years (22-60 y, range) with 23 HLA-identical donors (22 siblings, one unrelated) shows a transplant-related mortality of 42%, 8% graft failure, and a 10-year survival (disease-free) of 57% for all patients.^[44] The mortality rate remains high, so this form of therapy is reserved for those who are severely hypoplastic and refractory to other forms of therapy.

The International Bone Marrow Transplant Registry (IBMTR) reported a 2-year survival probability of 56% in 48 recipients of HLA-identical sibling transplants between 1978 and 1995.^[45] Data using nonmyeloablative conditioning and haploidentical donors was similar to the identical donors, indicating some form of graft-versus-PNH effects. Now that an effective, nontransplant therapy is available, the use of allogeneic HSCT to treat PNH has decreased.

Before the introduction of eculizumab, PNH patients with severe symptoms from classic PNH and patients with AA/PNH with peripheral cytopenias meeting criteria for severe aplastic anemia were considered good candidates for allogeneic bone marrow transplantation, especially if a matched sibling donor was available.

With eculizumab for PNH, the indications for allogeneic HSCT in this setting have changed. First, HSCT should not be offered as initial therapy for most patients with classic PNH, given the high transplant-related mortality, especially when using unrelated or mismatched donors. Exceptions are PNH patients in countries where eculizumab is not available. HSCT is also a reasonable option for patients who do not have a good response to eculizumab therapy. Second, aplastic anemia/PNH patients continue to be reasonable candidates for HSCT if they have life-threatening cytopenias.^[46]

An analysis by the International PNH Interest Group reviewed data from 67 patients from single centers and from teo registry studies, with special emphasis in eliminating duplication in patient reporting.^[21] Results included the following:

• Of the seven patients transplanted from a twin syngeneic donor, the four who had no conditioning therapy either failed to engraft or relapsed after transplantation, indicating that a marrow ablative conditioning is necessary before syngeneic transplantation.
• In 47 of 67 patients, a human leukocyte antigen (HLA)-identical sibling was used as the donor, 1 from a haploidentical family member and 12 from an unrelated donor (matched unrelated donor [MUD]).
• In the only single-center study providing a Kaplan-Meier analysis, overall survival at 5 years was 58 +/- 13%. This is less favorable than the survival estimate of approximately 75% generated by combining the data from the other reports.
• Investigation is currently in progress regarding whether reduced-intensity conditioning can improve the outcome.

A retrospective study of 21 PNH patients who underwent HSCT after previous treatment with eculizumab found that HSCT was associated with almost 30% mortality, mainly due to infections and acute graft-versus-host disease (GvHD). Syngeneic HSCT transplants were well tolerated. The authors suggested that these results may call into question the role of HSCT for patients with classic PNH who continue to require transfusions despite eculizumab, in absence of a syngeneic donor.^[47]

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Medication Summary

The drugs used in treatment of paroxysmal nocturnal hemoglobinuria (PNH) include the following:

• Eculizumab, to stop hemolysis
• Recombinant erythropoietin or androgens, to stimulate erythropoiesis
• Anticoagulants, to treat thrombotic complications
• Immunosuppressive agents, to stimulate hematopoiesis in the aplastic phase

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Oxymetholone (Anadrol-50)

• Dosing, Interactions, etc.

Clinical Context:  Anabolic and androgenic derivative of testosterone in an oral formulation.

Used to stimulate erythropoiesis by increasing endogenous levels of erythropoietin and by enhancing the response of precursor cells to the growth factor.

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Stanozolol (Winstrol)

Clinical Context:  Anabolic and androgenic derivative of testosterone in an oral formulation.

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Danazol (Danocrine)

• Dosing, Interactions, etc.

Clinical Context:  Synthetic steroid analogue, derived from ethisterone, with strong antigonadotropic activity (inhibits LH and FSH) and weak androgenic action without adverse virilizing and masculinizing effects. Increases levels of C4 component of the complement. May push the resting hematopoietic stem cells into cycle, making them more responsive to differentiation by hematopoietic growth factors. May also stimulate endogenous secretion of erythropoietin.

May impair clearance of immunoglobulin-coated platelets and decreases autoantibody production.

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Class Summary

Androgens are used to stimulate erythropoiesis by increasing endogenous levels of erythropoietin and by enhancing the response of precursor cells to the growth factor.

Attenuated androgens, such as danazol, are recommended for use in women, as the attenuated androgen has fewer adverse virilizing and masculinizing effects.

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Antithymocyte globulin; rabbit (Atgam)

• Dosing, Interactions, etc.

Clinical Context:  Purified preparation of pasteurized polyclonal IgG obtained from rabbits immunized against human thymocytes (T cells) for IV use. This preparation has replaced the Upjohn preparation Atgam (horse serum) and is considered an equivalent.

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Class Summary

Antithymocyte globulin is an antiserum to human T cells prepared from horses or rabbits.^[48] The mechanism of action of polyclonal antilymphocyte preparations to suppress immune responses is not fully understood.

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Eculizumab (Soliris)

• Dosing, Interactions, etc.

Clinical Context:  Orphan drug indicated for treatment of paroxysmal nocturnal hemoglobinuria (PNH) to reduce hemolysis. Blocks complement-mediated destruction of PNH red blood cells. Inhibits C-5 component of complement system, thereby preventing final stages of complement activation.

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Class Summary

The C-5 inhibitor eculizumab has been designated as an orphan drug for the treatment paroxysmal nocturnal hemoglobinuria (PNH).

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Further Outpatient Care

In many patients with paroxysmal nocturnal hemoglobinuria (PNH), the abnormal clone may eventually disappear. This usually takes at least 5 years, and often as long as 15-20 years. Reactivation of PNH in these patients has been observed with acute infections. Patients with chronic anemia alone, without thrombotic complications, can live relatively normal lives for many years.

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Further Inpatient Care

In severe cases with of paroxysmal nocturnal hemoglobinuria (PNH) with an aplastic phase, referral to a bone marrow transplantation center is indicated for possible allogeneic bone marrow transplantation. Umbilical cord stem cell transplantation from the patient's own cord blood, from a related donor, or from the registry for HLA–matched unrelated donors may be an option in pediatric patients.

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Inpatient & Outpatient Medications

Outpatient RBC transfusion often is necessary to keep a patient with chronic anemia and paroxysmal nocturnal hemoglobinuria (PNH) able to function and live a normal life.

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Complications

Primary care physicians should be aware of the thrombotic complications of paroxysmal nocturnal hemoglobinuria (PNH) and how to diagnose them when they occur.

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Prognosis

The prognosis in patients with paroxysmal nocturnal hemoglobinuria (PNH) is variable, depending on the severity of symptoms and the presence of complications. An aplastic phase is a serious prognostic factor, because the resulting pancytopenia and thrombosis of hepatic, abdominal, and cerebral veins can have life-threatening consequences. Prophylactic anticoagulation has not been shown to be of benefit because of a lack of data from a clinical trial setting.

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