Paroxysmal Nocturnal Hemoglobinuria (PNH)


Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired, clonal hematologic condition that is due to somatic mutations in the phosphatidylinositol glycan class A (PIGA) gene in hematopoietic stem cells. Mutations in the PIGA gene result in the formation of blood cells in which expression of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) is reduced or absent. GPI-APs include the complement-regulatory proteins decay accelerating factor (DAF, CD55) and membrane inhibitor of reactive lysis (MIRL, CD59).1 The lack of these proteins on the surface of red blood cells allows complement-mediated intravascular hemolysis to take place.2 

PNH is usually diagnosed on the basis of clinical signs and symptoms and laboratory data. Flow cytometry is often used first, as it can clearly show a lack of GPI-APs in peripheral blood cells.3 It is also helpful to perform histological, cytological, and genetic studies of the bone marrow to identify the clinical form of PNH. If a patient with PNH is to receive effective treatment, an early and precise diagnosis is essential.4

Clinical Diagnosis of PNH

A patient’s description of dark urine either at night or first thing in the morning, along with partial clearing of urine during the day is characteristic of PNH. Hemoglobinuria may occur every day in severe cases, but more frequently, it occurs in episodes lasting 3 to 10 days. In some cases, however, it does not occur at all.5 Because of continual complement activation, hemolysis in PNH is chronic, and sudden paroxysms of hemolysis may be brought on by surgery, infection, and/or inflammation. Patients with classic PNH frequently pass dark urine during these episodes as a result of significant hemoglobinuria.1  

The presentation of PNH can vary. Manifestations include intravascular hemolysis, hemolytic anemia, neutropenia, thrombocytopenia, renal failure, arterial and pulmonary hypertension, smooth-muscle dystonia, infection, and thrombosis.1 Fatigue, asthenia, a diminished sense of well-being, abdominal pain, dyspnea, chest pain, odynophagia, impotence, and headache are some subjective signs and symptoms that can aid in the diagnosis. Hemoglobinuria, scleral icterus, jaundice, and thromboembolic events are the main objective signs.3 Pallor in a patient with PNH most frequently indicates anemia, fever indicates infection, and bleeding manifesting as mucosal hemorrhage or dermal ecchymosis indicates thrombocytopenia comparable with that seen in aplastic anemia (AA).5

Read more about PNH signs and symptoms

Laboratory Diagnosis of PHN

Laboratory testing can confirm a clinical diagnosis of PNH. Flow cytometry of peripheral blood is often used as first-line testing, but complete blood cell count and bone marrow analysis can support this diagnosis. 

Flow cytometry calculates the percentage of cells deficient in GPI-APs and detects discrete populations with varying degrees of deficiency. Because of the deficiency of GPI-APs, red blood cells and other cells in PNH are deficient in DAF (CD55) and MIRL (CD59), which regulate complement. Flow cytometry is a highly sensitive and specific test that can detect several GPI-APs, especially CD55 and CD59.6 Both red blood cells and leukocytes (granulocytes and monocytes) should be tested for a more precise diagnosis. Red blood cells are typically used in the test; however, previous red cell transfusions or severe hemolysis can compromise the quantification of GPI-negative (CD59) cells.4

Basic hematology testing and urinalysis can reveal hemolysis or tissue damage. Diagnostics should look for low hemoglobin values, low serum haptoglobin levels, increased reticulocyte counts, and increased serum lactate dehydrogenase (LDH).6

Bone marrow testing can help support a PNH diagnosis by ruling out other conditions. It can also evaluate how well the bone marrow is functioning.4

Read more about PNH testing

Classification of PNH 

The PNH classification system was developed by the International PNH Interest Group (IPIG), which brought together clinicians and researchers to consider the diagnosis and measurement of PNH clones. The following 3 subgroups were suggested:

  1. Classic PNH, which presents with intravascular hemolysis, large granulocyte clone, and majority type III PNH cells (complete GPI-AP deficiency) red blood cells;
  2. PNH in the setting of another bone marrow disease, such as aplastic anemia or myelodysplastic syndrome (MDS); 
  3. Subclinical PNH, characterized by a relatively small clone size, lack of overt PNH symptoms, and a comorbid bone marrow disorder.7

Read more about PNH guidelines

Laboratory Diagnosis of PNH Subtypes

Classic PNH

Classic PNH is characterized by florid intravascular hemolysis and indicated by a significantly increased serum lactate dehydrogenase level, lowered haptoglobin level, and unconjugated hyperbilirubinemia. It also may present with varying degrees of hemosiderinuria and hemoglobinuria and an elevated reticulocyte level due to a compensatory bone marrow response. However, leukocyte and platelet counts are often normal or nearly normal in individuals with classic PNH.1

PNH With a Bone Marrow Disorder

Bone marrow failure predominates in patients who have PNH comorbid with another bone marrow failure syndrome (AA-PNH or MDS-PNH), and hemolysis is typically a coincidental laboratory finding. However, leukopenia, thrombocytopenia, or both invariably accompany PNH in the context of another bone marrow failure disease.1

Subclinical PNH

Patients with subclinical PNH differ from those with AA-PNH and MDS-PNH in that clinical or laboratory evidence of hemolysis is absent. A highly sensitive flow cytometer assay is required for the diagnosis.1,7 

Read more about PNH differential diagnosis

References

  1. Lima M. Laboratory studies for paroxysmal nocturnal hemoglobinuria, with emphasis on flow cytometry. Pract Lab Med. 2020;20:e00158. doi:10.1016/j.plabm.2020.e00158
  2. Devos, T, Meers, S, Boeckx, N, et al. Diagnosis and management of PNH: review and recommendations from a Belgian expert panel. Eur J Haematol. 2018;101:737- 749. doi:10.1111/ejh.13166
  3. Parker CJ. Update on the diagnosis and management of paroxysmal nocturnal hemoglobinuria. Hematology Am Soc Hematol Educ Program 2016;2016(1):208-216. doi:10.1182/asheducation-2016.1.208 
  4. ​​Almomen AK, Al Bakistani AG, Alsaeed A, et al. Paroxysmal nocturnal hemoglobinuria: diagnosis and management protocol. J Appl Hematol. 2014;5:37-44. doi:10.4103/1658-5127.137081
  5. Besa EC. Paroxysmal nocturnal hemoglobinuria clinical presentation. Medscape. Updated May 20, 2021. Accessed November 15, 2022.
  6. Besa EC. Paroxysmal nocturnal hemoglobinuria workup. Medscape. Updated May 20, 2021. Accessed November 7, 2022.
  7. Manivannan P, Ahuja A, Pati HP. Diagnosis of paroxysmal nocturnal hemoglobinuria: recent advances. Indian J Hematol Blood Transfus. 2017;33(4):453-462. doi:10.1007/s12288-017-0868-y

Reviewed by Kyle Habet, MD, on 11/28/2022.

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