Paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH), sometimes referred to as Marchiafava-Micheli syndrome, is a rare, acquired, potentially life-threatening disease of the blood characterised by complement-induced intravascular hemolytic anemia (anemia due to destruction of red blood cells in the bloodstream), red urine (due to the appearance of hemoglobin in the urine) and thrombosis. PNH is the only hemolytic anemia caused by an acquired (rather than inherited) intrinsic defect in the cell membrane (deficiency of glycophosphatidylinositol leading to absence of protective proteins on the membrane). It may develop on its own ("primary PNH") or in the context of other bone marrow disorders such as aplastic anemia ("secondary PNH"). Only a minority have the telltale red urine in the morning.

Allogeneic bone marrow transplantation is the only curative therapy, although the monoclonal antibody eculizumab (Soliris) is effective at reducing the need for blood transfusions, improving quality of life, and reducing the risk of thrombosis.

Signs and symptoms
Most people with "primary PNH" have red urine at some point in their disease course. Many of them continue to have low-grade breakdown of red blood cells, leading to anemia. Typical symptoms of anemia are tiredness, shortness of breath, and palpitations. On laboratory examination of the urine, breakdown products of red blood cells (hemoglobin and hemosiderin) may be identified. A small proportion of patients report abdominal pain, dysphagia (difficulty swallowing) and odynophagia (pain during swallowing), as well as erectile dysfunction in men - this occurs mainly when the breakdown of red blood cells is rapid.

Forty percent of patients develop thrombosis (a blood clot) at some point in their illness. This is the main cause of severe complications and death in PNH. These may develop in common sites (deep vein thrombosis of the leg veins and resultant pulmonary embolism when these clots break off and enter the lungs), but, in PNH, blood clots may also form in more unusual sites: the hepatic vein (causing Budd-Chiari syndrome), the portal vein of the liver (causing portal vein thrombosis), the superior or inferior mesenteric vein (causing mesenteric ischemia), and veins of the skin. Cerebral venous thrombosis, an uncommon form of stroke, is more common in PNH.

Diagnosis
Blood tests in PNH show changes consistent with intravascular hemolytic anemia: low hemoglobin, raised lactate dehydrogenase, raised reticulocytes (immature red cells released by the bone marrow to replace the destroyed cells), raised bilirubin (a breakdown product of hemoglobin), and decreased levels of haptoglobin. The direct antiglobulin test (DAT, or direct Coombs' test) is negative, as the hemolysis of PNH is not caused by antibodies.

Historically, the sucrose lysis test, in which a patient's red blood cells are placed in low-ionic-strength solution and observed for hemolysis, was used for screening. If this was positive, the Ham's acid hemolysis (after Dr Thomas Ham, who described the test in 1937) test was performed for confirmation.

Today, many labs use flow cytometry for CD55 and CD59 on white and red blood cells. Based on the levels of these cell proteins, erythrocytes may be classified as type I, II, or III PNH cells. Type I cells have normal levels of CD55 and CD59; type II have reduced levels; and type III have absent levels. The fluorescein-labeled proaerolysin (FLAER) test is being used more frequently to diagnose PNH. FLAER binds selectively to the glycophosphatidylinositol anchor and is more accurate in demonstrating a deficit than flow cytometry for CD59 or CD55.

Classification
PNH is classified by the context under which it is diagnosed:
 * Classic PNH. Evidence of PNH in the absence of another bone marrow disorder.
 * PNH in the setting of another specified bone marrow disorder.
 * Subclinical PNH. PNH abnormalities on flow cytometry without signs of hemolysis.

Pathophysiology
All cells have proteins attached to their membranes that are responsible for performing a vast array of functions. There are several ways for proteins to be attached to a cell membrane. PNH occurs as a result of a defect in one of these mechanisms.

The enzyme phosphatidylinositol glycan A (PIGA) is needed to make glycosylphosphatidylinositol (GPI), a molecule that anchors proteins to the cell membrane. The gene that codes for PIGA is located on the X chromosome, which means that only one active copy of the gene for PIGA is present in each cell (initially, females have two copies, but one is silenced through X-inactivation). If a mutation occurs in this gene then PIGA may be defective, which leads to a defect in the GPI anchor. When this mutation occurs in a bone marrow stem cell (which are used to make red blood cells as well as white blood cells and platelets), all of the cells it produces will also have the defect. Several of the proteins that anchor to GPI on the cell membrane are used to protect the cell from destruction by the complement system, and, without these anchors, the cells are more easily targeted by the complement proteins. The complement system is part of the immune system and helps to destroy invading microorganisms. Without the proteins that protect them from complement, red blood cells are destroyed. The main proteins that carry out this function are the decay-accelerating factor (DAF) (CD55), which disrupts formation of C3 convertase, and protectin (CD59), which binds the membrane attack complex and prevents C9 from binding to the cell.

The increased destruction of red blood cells results in anemia. The increased rate of thrombosis is due to dysfunction of platelets due to binding by complement, or possibly due to low nitric oxide levels.

The symptoms of esophageal spasm, erectile dysfunction, and abdominal pain are attributed to the fact that hemoglobin released during hemolysis binds with circulating nitric oxide, a substance that is needed to relax smooth muscle. This theory is supported by the fact that these symptoms improve on administration of nitrates or sildenafil (Viagra), which improves the effect of nitric oxide on muscle cells. There is a suspicion that chronic hemolysis causing chronically depleted nitric oxide may lead to the development of pulmonary hypertension (increased pressure in the blood vessels supplying the lung), which in turn puts strain on the heart and causes heart failure.

Long-term
PNH is a chronic condition. In patients with only a small clone and few problems, monitoring of the flow cytometry every six months gives information on the severity and risk of potential complications. Given the high risk of thrombosis in PNH, preventative treatment with warfarin decreases the risk of thrombosis in those with a large clone (50% of white blood cells type III).

Episodes of thrombosis are treated as they would in other patients, but, given that PNH is a persisting underlying cause, it is likely that treatment with warfarin or similar drugs needs to be continued long-term after an episode of thrombosis.

Acute attacks
There is disagreement as to whether steroids (such as prednisolone) can decrease the severity of hemolytic crises. Transfusion therapy may be needed; in addition to correcting significant anemia, this suppresses the production of PNH cells by the bone marrow, and indirectly the severity of the hemolysis. Iron deficiency develops with time, due to losses in urine, and may have to be treated if present. Iron therapy can result in more hemolysis as more PNH cells are produced.

A new monoclonal antibody, eculizumab, protects blood cells against immune destruction by inhibiting the complement system. It has been shown to reduce the need for blood transfusion in patients with significant hemolysis.

Screening
Screening for PNH is performed every year in people with previous aplastic anemia, and may be performed in people with myelodysplastic syndrome of the "refractory anemia" type.

Epidemiology
PNH is rare, with an annual rate of 1-2 cases per million. Many cases develop in people who have previously been diagnosed with aplastic anemia or myelodysplastic syndrome. The fact that PNH develops in MDS also explains why there appears to be a higher rate of leukemia in PNH, as MDS can sometimes transform into leukemia.

25% of female cases of PNH are discovered during pregnancy. This group has a high rate of thrombosis, and the risk of death of both mother and child are significantly increased (20% and 8% respectively).

History
The first description of paroxysmal hemoglobinuria was by the German physician Paul Strübing (Greifswald, 1852–1915) in 1882. A more detailed description was made by Dr Ettore Marchiafava and Dr Alessio Nazari in 1911, with further elaborations by Marchiafava in 1928 and Dr Ferdinando Micheli in 1931. The Dutch physician Enneking coined the term "paroxysmal nocturnal hemoglobinuria" (or haemoglobinuria paroxysmalis nocturia in Latin) in 1928.