Paroxysmal Nocturnal Hemoglobinuria

Nocturnal 2
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Diagnosis and reasoning

This young lady has presented with dark urine, jaundice, and abdominal pain, while examination reveals the presence of pallor; this constellation of symptoms is strongly suggestive of an episode of hemolysis. This is supported by the presence of severe anemia in the complete blood count (CBC), and the marked indirect hyperbilirubinemia in the liver profile. Note also the presence of a positive urine dipstick test for heme, but the absence of erythrocytes in the centrifuged sediment; in this clinical context, this is most suggestive of hemoglobinuria, hinting that this is very likely an intravascular hemolysis. The elevated lactate dehydrogenase and decreased haptoglobin levels subsequently confirm this assertion. Thus, a hemolytic anemia is the probable diagnosis; the question lies as to which of the many such types it might be. At a high level, hemolytic anemias can be classified into congenital and acquired anemias; the terms intrinsic and extrinsic anemias can be used interchangeably instead. The relatively late age of presentation, absence of previous episodes, and lack of organomegaly makes most of the common congenital hemolytic anemias clinically less likely. A notable exception is G6PD deficiency; this can present (for the first time) as an acute hemolytic crisis at almost any age, following exposure to a stressor. Important acquired anemias include immune mediated anemias and nonimmune mediated anemias; the former are excluded by the negative direct antiglobulin test. Key non immune mediated hemolytic anemias which give rise to severe episodic intravascular hemolysis include paroxysmal nocturnal hemoglobinuria (PNH) and paroxysmal cold hemoglobinuria. Note that the absence of a history of episodic dark colored urine in cold weather excludes the latter. Thus, in conclusion, PNH is the most probable clinical diagnosis; confirmation requires flow cytometry. In the current (acute) setting, the main focus of the management should be stabilization of the patient; identification of the etiology of the hemolysis is secondary. Given the severe anemia, a blood transfusion should be considered to optimize the patient's condition; it is vital to obtain all necessary samples for hematological investigations beforehand. Prophylactic folic acid is important to prevent megaloblastosis secondary to rapid consumption of folate in acute hemolysis. Corticosteroids would have been indicated if an autoimmune hemolytic anemia was detected. In general, iron therapy is contraindicated in most hemolytic anemias, as this may aggravate the acute presentation; however, iron supplementation is indicated in intravascular hemolysis and severe anemic states due to hemoglobinuria causing substantial iron loss. Note that it would be best to confirm iron levels prior to initiation of such therapy. Following stabilization, flow cytometry was performed on a pre-transfusion blood sample of this patient; this showed absent expression of CD55 and CD59 on 78% of erythrocytes, thus confirming the presence of PNH.


Discussion

Paroxysmal Nocturnal Hemoglobinuria (PNH) is a potentially life threatening, rare, acquired disease characterised by complement induced intravascular hemolytic anemia, hemoglobinuria and thrombosis. The condition affects 1 to 2 persons per million population; it is considered a disease of young adults with the mean age of diagnosis ranging from 35 to 40 years. Note that occasional cases have been diagnosed in childhood and adolescence. PNH is also the only hemolytic anemia caused by an acquired intrinsic defect of the cell membrane. In these patients, an acquired genetic mutation in chromosome X causes qualitative or quantitative defects in phosphatidylinositol glycan A (PIGA), an enzyme which contributes to the production of glycophosphatidylinositol (GPI). GPI acts an anchoring molecule for proteins, connecting them with cell membrane; in this respect, key proteins involved include CD55 and CD59, which prevent erythrocytes from complement mediated lysis. Thus, the low levels of GPI result in the absence of CD55 and CD59, resulting in increased vulnerability of erythrocytes to complement activation, and thus ultimately, increased intravascular hemolysis. Based on the etiology, PNH can be classified into 2 categories: primary and secondary; the latter is associated with bone marrow disorders such as aplastic anemia, and accounts for approximately 30% of cases. The condition can also be classified into 3 categories based on the presentation, clinical manifestations and natural history: - Classic PNH - PNH in the setting of another specified bone marrow disorder (e.g. aplastic anemia and myelodysplastic syndrome - MDS). - Subclinical PNH (an investigational diagnosis in the absence of clinical features) Reddish discolouration of urine is touted as the classical sign of PNH; this due to the presence of hemoglobin and hemosiderin following hemolysis. Note that despite the name of the disease, only a minority of individuals present with red coloured urine in the morning. It should also be appreciated that patients with PNH often manifest only vague symptoms such as abdominal pain, severe headache, back pain, excessive weakness, fatigue and recurrent infections; thus the diagnosis is often delayed. Multiple meta analyses have documented that 40% of patients with PNH develop thrombosis; this is the main cause of severe complications and death at some point of their lives. In this respect, deep vein thrombosis and pulmonary embolism are common presentations; clot formation in unusual sites such as the hepatic vein, portal vein, superior and inferior mesenteric vein may result in Budd-Chiari syndrome, portal vein thrombosis and mesenteric ischemia respectively. Cerebral venous thrombosis is also relatively more common. Preliminary laboratory testing of these patients should include a complete cell count (CBC), blood smear, and bone marrow examination. Changes consistent with an intravascular hemolytic anemia include low hemoglobin, raised lactate dehydrogenase (LDH), raised reticulocytes, raised bilirubin, and decreased levels of haptoglobin. The direct antiglobulin test (DAT or direct Coombs' test) is negative as antibodies are not encountered in the condition. The current gold standard for diagnosis is flow cytometry for CD55 and CD59 on red and white blood cells; the sucrose lysis screening test, followed by Ham's acid hemolysis test for confirmation is no longer recommended, unless flow cytometric facilities are unavailable. Note that the fluorescein-labeled proaerolysin (FLAER) test is also gaining acceptance as a technique to diagnose PNH. Radiography is of use in detection of thrombosis of major veins. A routine technetium-99m colloid scan of the liver and spleen should be conducted for detection of hepatic vein thrombosis, while an MRI or ultrasonography with dye insertion aids in demonstrating cessation of flow through the hepatic vein or thrombus formation in the lumen. Due to the chronicity of PNH, monitoring via flow cytometry every 6 months is helpful to assess the severity and risk of potential complications. Lifelong preventive anticoagulation with warfarin should be considered in patients with a large clone, as they are at high risk of thrombosis. Eculizumab, a monoclonal anti-complement antibody targeting the CD5 complement component, was approved by US Federal Drug Administration (FDA) in 2007, for the management of PNH. The drug has dramatically improved symptoms and quality of life in patients treated, while also eliminating complications and reducing the need of blood transfusion in individuals with significant hemolysis. Note that it does not alter the underlying enzymatic defect. Note that there are conflicting opinions on whether steroids should be used to decrease the severity of a hemolytic crisis. Transfusion therapy is recommended in acute attacks as it suppresses the production of PNH cells from the bone marrow in addition to correction of anemia. Due to prolonged loss in urine, iron deficiency can occur, necessitating treatment; this should be counterbalanced against the fact that iron therapy can result in further production of PNH cells, worsening the hemolysis. Stimulation of erythropoiesis via androgenic hormones has been successful in patients with mild to moderately decreased erythrocyte production; this has now been replaced with recombinant erythropoietin therapy. The management of thrombotic complications follows standard protocols, with the immediate usage of heparin followed by maintenance therapy with an oral anticoagulant. Hematopoietic stem cell transplantation (HSCT) using allogeneic donors is considered the only curative therapy. Bone marrow hyperplasia resulting in aplastic anemia is a serious cause of morbidity and mortality in these patients; it is treated effectively by bone marrow transplantation. In the absence of a suitable donor, antithymocyte globulin (ATG) therapy can be considered. Note that young patients with unexplained thrombosis, individuals with thrombosis at unusual sites, and persons with evidence of hemolysis or cytopenia of any kind including aplastic anemia and myelodysplastic syndrome should be screened annually for PNH. In most patients, PNH is a lifelong condition, requiring continuous therapy; however, spontaneous remission has been observed to occur in a small minority.


Take home messages

  1. PNH is a rare, chronic, debilitating disorder which frequently presents in early adulthood and continues throughout life.
  2. Reddish discoloration of urine is the classical sign of PNH; this is described as appearing in the morning and gradually reducing during the course of the day.
  3. Flow cytometry for CD55 and CD59 is the gold standard for the diagnosis of PNH.
  4. Therapy with Eculizumab dramatically improves symptoms, reduces complications and improves the quality of life of these patients.

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