Clinical manifestations and diagnosis of paroxysmal nocturnal hemoglobinuria

INTRODUCTION — Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired disorder in which hematopoietic stem cells and their cellular progeny have reduced or absent glycosylphosphatidylinositol (GPI)-anchored proteins on the cell surface. Loss of the GPI-linked complement inhibitors, CD55 and CD59, on red blood cells (RBCs) leads to chronic and/or paroxysmal intravascular hemolysis and a propensity for thrombosis, organ dysfunction, and hypocellular or dysplastic bone marrow. A subset of patients with PNH have clinically significant aplastic anemia or myelodysplastic syndrome.

Diagnosis of PNH may be delayed because of its nonspecific clinical features, variable clinical presentation, and rarity. Prompt and accurate diagnosis is particularly important since effective complement inhibitors have become available.

This topic discusses the clinical manifestations and diagnosis of PNH.

Pathogenesis, management, and prognosis of PNH are presented separately. (See "Pathogenesis of paroxysmal nocturnal hemoglobinuria" and "Treatment and prognosis of paroxysmal nocturnal hemoglobinuria".)

EPIDEMIOLOGY — PNH primarily affects adults, with no clear predilections for geography, race, ethnicity, or sex.

The incidence of clinically significant PNH is estimated to be at least 1 to 10 cases per million population, but this may be an underestimate as some individuals remain undiagnosed [1,2]. PNH is mostly a disease of adults, with a median age of onset in the 30s, but childhood cases have been reported [3-7]. There is no known racial or ethnic association, and PNH has been reported worldwide [6-8].

PNH affects males and females equally [8]. The vast majority of cases of PNH are caused by an acquired mutation in hematopoietic stem cells (ie, not a germline mutation) of PIGA, a gene on the X chromosome that is involved with glycosylphosphatidylinositol (GPI) synthesis. The equal sex distribution of PNH reflects the fact that somatic cells use only one X chromosome; in females, the other X chromosome is inactivated ("lyonized"). (See "Pathogenesis of paroxysmal nocturnal hemoglobinuria", section on 'PIGA gene mutation'.)

CLINICAL MANIFESTATIONS

Clinical presentation — The clinical presentation of PNH varies between individuals [9]. PNH-associated hemolysis is associated with a range of symptoms, clinical findings, and complications, including anemia-related findings (eg, dyspnea, fatigue), pain, organ dysfunction, and increased risk for thrombosis. PNH is also associated with aplastic anemia (AA) and, less commonly, with myelodysplastic syndromes (MDS) and can have manifestations of thrombocytopenia and/or neutropenia.

Many patients present with unexplained hemolytic anemia, fatigue, jaundice, or red/pink/dark urine, while others experience headache, dysphagia, scleral icterus, or confusion [6]. Some patients present with venous thrombosis (especially at atypical sites) or have only nonspecific symptoms associated with smooth muscle dystonia, which can cause abdominal pain, erectile dysfunction, renal insufficiency, or pulmonary hypertension.

An international registry reported clinical findings in 1610 patients with PNH [6]. Nearly all (>93 percent) were symptomatic and many experienced a poor quality of life, had been hospitalized (23 percent), or were unable to work (17 percent):

●Fatigue – 80 percent

●Dyspnea – 64 percent

●Hemoglobinuria – 62 percent

●Abdominal pain – 44 percent

●Bone marrow suppression – 44 percent

●Erectile dysfunction – 38 percent

●Chest pain – 33 percent

●Thrombosis – 16 percent

●Renal insufficiency – 14 percent

A retrospective review of 220 patients with PNH reported that half had neutropenia and/or thrombocytopenia, with a cumulative incidence of pancytopenia in 15 percent at eight years [7].

Hematologic — PNH is typically associated with anemia-related findings caused by hemolysis of red blood cells (RBCs); bone marrow hypoplasia or dysplasia and folate or iron deficiency may also contribute.

Hemolysis — The degree of hemolysis and associated manifestations generally correlate with the size (ie, percentage) of the RBC PNH population.

●Anemia-related symptoms – Many patients present with persistent or episodic fatigue (which may seem out of proportion to the degree of anemia), weakness, jaundice, or hemoglobinuria (ie, red, pink, or "cola-colored" urine) [9]. Hemolysis typically occurs at a low level throughout the day and increases at night, as the name of the disease implies. Some individuals have paroxysms of hemolysis that are triggered by infections, inflammatory stimuli, surgery, strenuous physical activity, blood transfusion, or alcohol use [10]. Iron repletion in an iron-deficient patient can increase hemolysis by facilitating production of a large population of PNH RBCs that are highly susceptible to complement lysis.

●Vasospasm – Hemoglobin release can cause vasospasm by depleting circulating nitric oxide (NO), leading to smooth muscle dystonia, abdominal or muscle pain, pulmonary hypertension, or renal insufficiency.

●Thrombosis – Hemolysis contributes to the hypercoagulable state associated with PNH, as described below. (See 'Thrombosis' below.)

Bone marrow dysfunction — Some patients with PNH have an overlap syndrome with AA or, less commonly, MDS, which can cause other cytopenias and exacerbate hemolysis-associated anemia.

Most patients with PNH have some degree of bone marrow dysfunction, which can range from mild, asymptomatic cytopenias to severe bone marrow hypoplasia with recurrent infections or bleeding/bruising [11]. Compared with classical hemolytic PNH, patients with bone marrow failure often have less intravascular hemolysis and a smaller population of PNH blood cells. Over time, bone marrow dysfunction may improve while the PNH clone expands, leading to increased findings associated with hemolysis. Rarely, acute myeloid leukemia can emerge from PNH associated with MDS.

●Frequency of bone marrow failure in patients with PNH – The frequency of bone marrow failure in patients with PNH varies with the population. In one series, 15 percent of 220 patients had pancytopenia at eight years [7]. PNH-associated bone marrow failure may be more common in Asia, the Pacific Islands, and Latin America, but reasons for this geographic variation are unknown [12].

●Frequency of PNH in patients with bone marrow failure – The frequency of PNH in patients with bone marrow failure also varies with the population [10,13-17]. A series that included more than 800 patients reported PNH-type cells in 57 percent of patients with AA and 20 percent of those with MDS; however, most PNH clones were small (80 percent of patients had a clonal population <1 percent of blood cells) [13]. Other studies reported PNH in up to 70 percent of patients with AA or MDS [14,15]. Over time, the size of the PNH population can increase, decrease, or remain stable. In one series, the PNH clones expanded, persisted, or disappeared in 17, 59, and 24 percent of patients, respectively [13]. Spontaneous remissions of PNH in patients with AA have been observed [18-26].

PNH clones are almost never found in patients with inherited bone marrow failure syndromes [27].  

Other causes of cytopenias — Other factors besides hemolysis and bone marrow dysfunction may contribute to cytopenias in patients with PNH:

●Chronic, severe hemolysis can lead to iron deficiency due to hemoglobinuria and/or deposition of urinary hemosiderin.

●Hypersplenism caused by splenomegaly from portal hypertension or thrombosis of intra-abdominal veins can exacerbate cytopenias.

Thrombosis — Thrombosis is the leading cause of death in patients with PNH. Thromboses often involve unusual sites, such as mesenteric, cerebral, or dermal veins; arterial events are less common. The frequency of thrombosis has diminished substantially since effective complement inhibitors have become available. (See "Treatment and prognosis of paroxysmal nocturnal hemoglobinuria".)

●Clinical presentation – The presentation of PNH-associated thrombosis varies with the site of involvement. The onset of clots can be insidious or abrupt and some are discovered incidentally. Thromboses occurred in up to 40 percent of patients prior to the availability of complement inhibitors, but the risk in treated patients now appears to be similar to age-matched controls [10].

Intra-abdominal sites of thrombosis (eg, hepatic, portal, mesenteric veins) account for two-thirds of clots in patients with PNH, followed by intracerebral sites (10 to 20 percent), and other locations (eg, skin, lower extremity) [28,29]. Clinical manifestations include the following:

•Hepatic vein (Budd-Chiari syndrome) – Hepatic vein thrombosis can develop insidiously or suddenly, the latter often during an episode of brisk hemolysis [30-32]. Hepatic vein thromboses tend to recur, causing cirrhosis and rerouting blood from the portal circulation, which can be exacerbated by portal vein thrombosis [33]. (See "Budd-Chiari syndrome: Epidemiology, clinical manifestations, and diagnosis".)

•Inferior vena cava, portal and splenic veins – Thrombosis of the inferior vena cava, portal vein, and/or splenic vein can cause splenic congestion and hypersplenism [34]. Microvascular thrombosis of splanchnic vessels can also occur and produce bouts of abdominal pain and/or mucosal ulceration [35,36]. (See "Acute portal vein thrombosis in adults: Clinical manifestations, diagnosis, and management" and "Chronic portal vein thrombosis in adults: Clinical manifestations, diagnosis, and management".)

•Cerebral veins – Cerebral vein thrombosis can occur as a catastrophic event or with an insidious onset that may be confused with other causes of headache [37-40]. The major venous sinuses (eg, superior sagittal, lateral, cavernous, sigmoid) are most often involved, but thrombosis may also occur in the veins covering the cerebrum (particularly the parietal lobe).

In a series of 15 patients with PNH and cerebral vein thrombosis, 12 were women and 6 had hormonal risk factors (eg, pregnancy, oral contraceptives) [40]. Cerebral vein thrombosis was the presenting finding of PNH in four of these patients, three had concomitant hepatic vein thrombosis, and one died of the cerebral vein thrombosis; all others were discharged without neurologic disability. (See "Cerebral venous thrombosis: Etiology, clinical features, and diagnosis".)

•Dermal veins – Dermal vein thrombosis can present as discrete areas of erythema, swelling, and pain or as a syndrome resembling purpura fulminans [41-43]. It may occur in areas of trauma or in sites of previous inflammation or allergic reaction. (See "Approach to the patient with retiform (angulated) purpura".)

●Arterial thrombosis – The incidence of arterial thrombosis (eg, cerebral or coronary arteries) is increased in patients with PNH, but it is much less common than venous thrombosis [41,44-47]. Arterial thromboses tend to occur at an earlier age compared with an age-matched population.

●Risk factors for thrombosis – The major risk factor for thrombosis appears to be the size of the PNH clone and/or the degree of intravascular hemolysis [10]. Individuals with >60 percent PNH granulocyte clones appear to be at highest risk.

Thrombotic risk may vary with geography or ethnicity. In one study, thrombosis was reported in 19 percent of 176 patients from the United States (US) but only 6 percent of 209 patients from Japan [48]. Other studies have also reported higher rates of thrombosis in the US and Europe (eg, 30 to 40 percent) than in Asia (<10 percent) [7,49-51]. A single institution study of 64 patients in the US reported thrombosis in 73 percent of Black patients, 50 percent of Hispanic patients, and 36 percent of White patients or Asian patients [29]. It is unclear if these differences are related to larger clonal populations of PNH cells or more severe hemolysis [28,48].

Factors that contribute to hypercoagulability in patients with PNH include depletion of NO from circulating free hemoglobin, procoagulant microparticles released from platelets, deficiency of glycosylphosphatidylinositol (GPI)-anchored anticoagulant and fibrinolytic factors, and increased levels of complement component, C5, which generates proinflammatory and prothrombotic cytokines. (See "Pathogenesis of paroxysmal nocturnal hemoglobinuria", section on 'Thrombosis'.)

Smooth muscle dystonia — Smooth muscle dystonia is associated with dysphagia/odynophagia, abdominal pain, and erectile dysfunction in patients with PNH. NO relaxes smooth muscle, but in PNH, intravascular hemolysis releases free hemoglobin into the circulation and the resulting depletion of NO is thought to cause excessive smooth muscle contraction and dystonia [10].

Abdominal pain/dysphagia — Smooth muscle in the gastrointestinal tract appears to be preferentially affected in PNH. The most common manifestation is esophageal spasm; esophageal manometry has noted intense peristaltic waves in association with symptoms. Many patients describe dysphagia, abdominal cramping, or pressure in the chest during episodes of hemoglobinuria.

Erectile dysfunction — Erectile dysfunction is associated with PNH, particularly during hemolytic episodes, because NO is required for vascular dilatation in the corpora cavernosa.

Pulmonary hypertension — Pulmonary hypertension can result from pulmonary emboli and from NO depletion in the pulmonary circulation; when clinically significant pulmonary hypertension occurs, it is usually associated with pulmonary emboli. Transthoracic Doppler echocardiography detected elevated pulmonary artery pressure in half of 29 patients with PNH [52] and elevated N-terminal pro-brain natriuretic peptide (a marker of right ventricular dysfunction) was reported in nearly half of 73 patients with severe PNH-associated hemolysis [53].

Renal insufficiency — PNH can be associated with acute and chronic renal disease. Chronic intravascular hemolysis can cause renal hemosiderosis (renal iron deposition), which interferes with proximal tubule function and causes interstitial scarring and cortical infarcts [54-57]. Severe acute hemolytic episodes can cause acute renal failure from direct toxicity of free heme in the kidney [58]. (See "Clinical features and diagnosis of heme pigment-induced acute kidney injury".)

Inflammatory manifestations — A rare variant of PNH caused by loss of PIGT (a gene on chromosome 20q involved with GPI synthesis) may be associated with inflammatory manifestations, including aseptic meningitis, recurrent urticaria, and arthralgias that can predate other PNH findings by years [59]. This syndrome is associated with a germline mutation in one PIGT allele and somatic mutation of the other allele.

EVALUATION — Evaluation for PNH should document complement-mediated hemolysis, assess organ function, and determine if there are cytopenias or dysplasia that indicate bone marrow failure.

Clinical

●History should include symptoms related to anemia (eg, fatigue, dyspnea, weakness), hematuria, thrombosis (including abdominal, cerebral, or dermal veins), unexplained abdominal pain, erectile dysfunction, dysphagia, bleeding/bruising, and recurrent infections.

●Physical examination should seek signs of anemia (eg, pallor, tachycardia, tachypnea), excessive bleeding/bruising or infection, and evidence of thrombosis (eg, redness or swelling of an extremity, splenomegaly).

Laboratory — Laboratory findings should document hemolytic anemia, exclude other causes of hemolysis (eg, immune, microangiopathic, and mechanical), and evaluate organ function.

●Hematology

•Complete blood count (CBC) with differential count

•Reticulocyte count

•Blood smear for red blood cell (RBC) morphology and other abnormalities

•Direct antiglobulin (Coombs) testing (DAT)

•Prothrombin time (PT), activated partial thromboplastin time (aPTT), fibrinogen, D-dimer

●Serum chemistries

•Electrolytes, blood urea nitrogen (BUN), creatinine, and liver function tests, including lactate dehydrogenase (LDH) and direct and indirect bilirubin

•Serum haptoglobin, free hemoglobin

•Iron, transferrin saturation, and ferritin should be tested, as clinically indicated

●Urine for hemoglobin and hemosiderin

Interpretation of hemolysis-related studies (table 1) is discussed separately. (See "Non-immune (Coombs-negative) hemolytic anemias in adults".)

Flow cytometry/FLAER — Flow cytometry is the preferred method for evaluating and diagnosing PNH. Flow cytometry documents reduction or loss of glycosylphosphatidylinositol (GPI)-anchored proteins on blood cells and defines the size of the PNH population of blood cells.

●Reagents – Flow cytometry is performed with fluorescently labeled monoclonal antibodies that bind to GPI-anchored proteins (eg, CD59, CD55). Most tests for PNH also incorporate FLuorescent AERolysin (FLAER), a reagent derived from the bacterial toxin aerolysin that directly binds the GPI anchor.

At least two independent flow cytometry reagents should be used on at least two cell lineages (eg, granulocytes and RBCs) because different lineages display various combinations of GPI-linked proteins and some proteins bind the cell surface via both GPI-linked and GPI-independent mechanisms [2,60]. In addition to CD55 and CD59, other GPI-anchored proteins include CD14, CD15, CD16, CD24, CD45, and CD64.

Many laboratories also report the percentage of type I PNH cells (normal expression of GPI-anchored proteins), type II (partial expression), and type III (absent expression).

●Blood cells – Both granulocytes and RBCs should be evaluated to assess the size of the PNH clonal population. Testing of RBCs alone may underestimate the PNH population because of the short lifespan of PNH red cells and/or their dilution by transfused blood from unaffected donors.

Bone marrow — All patients with significant leukopenia or thrombocytopenia should undergo bone marrow examination to evaluate bone marrow failure as a contributor to the cytopenias. Bone marrow microscopy for cellularity and morphology and Giemsa-stained chromosome banding analysis should be performed.

Patients with classical hemolytic PNH typically have normocellular or hypercellular bone marrow with erythroid hyperplasia; there may also be erythroid dysplasia due to robust RBC turnover. Stainable iron is often absent. PNH, itself, is not associated with chromosomal abnormalities, but an abnormal karyotype may be found in patients with PNH-associated MDS.

Imaging — Imaging studies are performed as clinically indicated, but they are not required to diagnose PNH.

Abdominal ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI) should be used to evaluate venous thrombosis in patients with abdominal pain/swelling or headache.

DIAGNOSIS AND CLASSIFICATION — PNH should be suspected in individuals with direct antiglobulin testing (DAT; Coombs test)-negative hemolytic anemia, thrombosis at an unusual site or early age, unexplained abdominal pain, and for unexplained cytopenias, aplastic anemia (AA), or myelodysplastic syndrome (MDS).  

The category of PNH should be defined in all patients who are diagnosed with PNH. (See 'Classification' below.)

Diagnostic criteria — The diagnosis of PNH is established by flow cytometry that demonstrates a population of granulocytes and red blood cells (RBCs) that are deficient in glycosylphosphatidylinositol (GPI)-linked proteins (eg, CD55, CD59) in an appropriate clinical setting, such as DAT-negative hemolytic anemia, thrombosis, unexplained abdominal pain, AA, or MDS [10].

Findings from the diagnostic evaluation may include:

●Hemolytic anemia – PNH is typically associated with anemia, reticulocytosis, negative DAT, and elevated increased serum lactate dehydrogenase (LDH) [9]. Hemoglobin and hematocrit can be nearly normal in some patients with an adequate reticulocyte response. Findings of iron deficiency (eg, low serum iron, increased transferrin, low ferritin, absent bone marrow iron) are present in some patients due to chronic hemoglobinuria and hemosiderinuria.

●Clonal population – The size (percentage) of the clonal population of PNH-affected blood cells is highly variable. Patients with hemolytic ("classic") PNH generally have 40 to 99 percent PNH granulocytes, while patients with PNH with associated AA or MDS usually have a smaller percentage. The percentage of PNH RBCs may underestimate the size of the PNH population because of their shortened lifespan or dilution with recently transfused, normal RBCs. The diagnosis of PNH does not require a specific threshold of type III PNH cells (absent cell surface GPI-anchored proteins) or type II PNH cells (partial expression of GPI-anchored proteins).

In an international registry with 1610 patients, the median PNH granulocyte clone size was 68 percent; 17 percent clone size <10 percent, while 52 percent had >50 percent granulocyte clone size [6].

●Bone marrow examination – Bone marrow examination is not required for the diagnosis of PNH, but it should be performed in patients with leukopenia and/or thrombocytopenia to evaluate possible AA or MDS.

Classification — PNH is classified [12] according to:

●Symptoms – Anemia-related symptoms, transfusion-dependence, thrombosis, pain, or organ dysfunction; the severity of symptoms varies between individuals and PNH clone size and levels of LDH do not correlate well with symptom severity.

And

●Bone marrow failure – Findings that meet criteria for AA or MDS on bone marrow examination (if performed for leukopenia or thrombocytopenia).

PNH is a dynamic condition and the category of PNH may evolve over time.

Hemolytic (classical) PNH — Patients with classical hemolytic PNH have findings of intravascular hemolysis, but mild or no evidence of bone marrow failure:

●Hemolysis – Prominent symptoms related to hemolysis (eg, fatigue, dyspnea, RBC transfusion-dependence, episodic hemoglobinuria, thrombosis, pain, and/or organ dysfunction); LDH is typically >1.5x upper limit of normal (ULN).

●Leukopenia and/or thrombocytopenia – Normal white blood cell (WBC) count and platelet count or modest, asymptomatic leukopenia and/or thrombocytopenia.

●Clone size – Granulocyte PNH clone is typically >50 percent; RBC clone size is variable, as described above. (See 'Flow cytometry/FLAER' above.)

●Bone marrow – Bone marrow is cellular with erythroid hyperplasia and no significant dysplasia.

●Other findings – Thrombosis, pain that requires hospital admission or opioid analgesia, and organ dysfunction (eg, renal insufficiency, pulmonary insufficiency or pulmonary hypertension) may be present.

Subclinical PNH — PNH is detected, but there are no substantial clinical findings and no bone marrow abnormalities:

●Hemolysis – No substantial anemia-related symptoms, thrombosis, pain, organ dysfunction, or ongoing transfusion-dependence; LDH is typically ≤1.5x ULN.

●Leukopenia and/or thrombocytopenia – Normal WBC count and platelet count or modest, asymptomatic leukopenia/thrombocytopenia.

●Clone size – PNH granulocytes are typically ≤20 percent; RBC clone size is small, but variable.

●Bone marrow – Bone marrow has normal or near-normal cellularity and morphology.

●Other findings – There is no thrombosis, pain, or organ dysfunction.

PNH with bone marrow failure — Bone marrow examination reveals hypoplasia or myelodysplasia that meets criteria for AA or MDS, with variable levels of hemolysis-associated findings:

●Hemolysis – Variable anemia-related symptoms, thrombosis, pain, or organ dysfunction and variable level of serum LDH.

●Leukopenia and/or thrombocytopenia – Prominent severe, symptomatic leukopenia and/or thrombocytopenia.

●Clone size – Variable size of granulocyte and RBC PNH clones.

●Bone marrow – Bone marrow cellularity and/or morphology meet criteria for severe AA or high-risk MDS.

Diagnostic criteria for severe AA and MDS prognostic category (table 2) (calculator 1) are detailed separately. (See "Aplastic anemia: Pathogenesis, clinical manifestations, and diagnosis", section on 'Diagnostic criteria' and "Overview of the treatment of myelodysplastic syndromes", section on 'Prognostic category'.)

●Other findings – Variable, but may include cytopenias, thrombosis, pain, or organ dysfunction.

DIFFERENTIAL DIAGNOSIS — The differential diagnosis of PNH includes other causes for direct antiglobulin test (DAT; Coombs test)-negative hemolytic anemia, bone marrow failure, venous thrombosis in atypical sites, and hematuria or myoglobinuria.

Hemolytic anemias — Other DAT-negative (ie, nonimmune) hemolytic anemias include hereditary red blood cell (RBC) membrane or enzymatic defects, drug-induced hemolysis, paroxysmal cold hemoglobinuria, and microangiopathic hemolytic anemias. These conditions are also associated with clinical and laboratory findings of hemolysis, but unlike PNH, flow cytometry does not identify a population of CD55/59 deficient blood cells. Examples include:

●Hereditary anemias – Hereditary anemias, such as hereditary spherocytosis (HS), hemoglobinopathies (eg, sickle cell disease, thalassemia), or enzyme variants (eg, glucose-6-phosphate dehydrogenase deficiency, pyruvate kinase deficiency) are identified by characteristic features, including a positive family history, findings on blood smear, and/or abnormal hemoglobin electrophoresis or enzymatic activity. (See "Hereditary spherocytosis" and "Diagnosis of sickle cell disorders" and "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Pyruvate kinase deficiency".)

●Drug- or toxin-induced anemias – Drug- or toxin-induced anemias are typically associated with introduction of a new medication or exposure to a toxin (eg, snake bite, insect sting, copper poisoning) and they typically resolve with removal of the implicated agent. (See "Drug-induced hemolytic anemia".)

●Paroxysmal cold hemoglobinuria – Paroxysmal cold hemoglobinuria is often associated with cold temperature-induced hemolysis and history of a recent infection or connective tissue disorder. DAT is negative for antibodies, but positive for complement, Donath-Landsteiner type antibody is detected, and flow cytometry for PNH is negative. (See "Paroxysmal cold hemoglobinuria".)

●Microangiopathic hemolytic anemia/disseminated intravascular coagulation (DIC) – Microangiopathic hemolytic anemia/DIC are associated with characteristic appearance of the blood smear (eg, schistocytes, nucleated RBCs) and an appropriate clinical setting; flow cytometry is negative for a population of PNH cells. (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)" and "Evaluation and management of disseminated intravascular coagulation (DIC) in adults".)

●Autoimmune hemolysis – Rarely, a patient with autoimmune hemolytic anemia (AIHA) may appear to have a negative DAT due to excessive hemolysis of antibody-coated cells. Unlike PNH, patients with AIHA generally have extravascular, rather than intravascular, hemolysis; as a result, free hemoglobin is not present in serum or urine. (See "Diagnosis of hemolytic anemia in adults".)

Abdominal or cerebral vein thrombosis — Thrombosis of abdominal (eg, portal, hepatic, renal, mesenteric) veins or cerebral veins can be associated with myeloproliferative disorders, inherited or acquired hypercoagulable conditions, or extrinsic compression of vessels. Unlike PNH, these other conditions generally are not associated with intravascular hemolysis, and flow cytometry for PNH cells is negative. (See "Budd-Chiari syndrome: Epidemiology, clinical manifestations, and diagnosis" and "Epidemiology and pathogenesis of portal vein thrombosis in adults" and "Cerebral venous thrombosis: Etiology, clinical features, and diagnosis" and "Clinical manifestations of antiphospholipid syndrome" and "Diagnosis of antiphospholipid syndrome".)

Bone marrow disorders — Aplastic anemia (AA) and myelodysplastic syndrome (MDS) cause cytopenias and can coexist with PNH or they can be independent of PNH; the distinction is made according to the presence or absence of a population of PNH cells. (See "Aplastic anemia: Pathogenesis, clinical manifestations, and diagnosis" and "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)".)

Acute myeloid leukemia (AML) and primary myelofibrosis (PMF) can cause cytopenias, but they are distinguished by characteristic findings on blood smear (eg, circulating blasts, nucleated RBCs), bone marrow biopsy, and molecular or cytogenetic findings, and they are negative for PNH cells. (See "Clinical manifestations and diagnosis of primary myelofibrosis".)

Hematuria or myoglobinuria — Hemoglobinuria (free hemoglobin in urine), hematuria (RBCs in urine), and myoglobinuria (free myoglobin in urine) can cause pink/red urine and a positive urine dipstick for heme. Examination of the urine sediment distinguishes hematuria from hemoglobinuria, while myoglobinuria generally occurs in the setting of rhabdomyolysis, which is associated with muscle pain, weakness, and elevated serum creatine kinase (CK). Unlike PNH, hematuria and myoglobinuria typically are not associated with hemolytic anemia. (See "Etiology and evaluation of hematuria in adults" and "Rhabdomyolysis: Clinical manifestations and diagnosis".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anticoagulation in pregnancy" and "Society guideline links: Bone marrow failure syndromes".)

SUMMARY

●Description – Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, complement-mediated hemolytic anemia that can be associated with aplastic anemia (AA) and/or myelodysplastic syndromes (MDS). PNH is caused by an acquired (ie, not inherited) gene mutation in hematopoietic stem cells that causes reduced/absent production of glycosylphosphatidylinositol (GPI), which links the complement inhibitor proteins, CD55 and CD59, to blood cell membranes.

●Epidemiology – The median age of onset is in the 30s with incidence of 1 to 10 cases per million. (See 'Epidemiology' above.)

●Clinical manifestations – Most patients have anemia-related fatigue and weakness; other common findings include venous thrombosis (especially at atypical sites), abdominal pain, and/or organ dysfunction. (See 'Clinical presentation' above.)

•Hemolysis-related – Fatigue, dyspnea, hemoglobinuria, and predisposition to venous thrombosis. (See 'Hemolysis' above.)

•Bone marrow dysfunction – Bone marrow failure (BMF) may exacerbate hemolysis-associated cytopenias. (See 'Bone marrow dysfunction' above.)

•Thrombosis – The leading cause of death and often involve atypical sites, such as abdominal veins, cerebral veins, or dermal veins. (See 'Thrombosis' above.)

•Smooth muscle dystonia – Depletion of intravascular nitric oxide (NO) due to free hemoglobin can cause smooth muscle dystonia with associated abdominal pain, erectile dysfunction, pulmonary hypertension, and/or renal insufficiency. (See 'Smooth muscle dystonia' above.)

●Evaluation – History, examination, and laboratory studies to investigate complement-mediated hemolysis evaluate for thrombosis, organ dysfunction, and BMF. (See 'Evaluation' above.)

•Flow cytometry – A population of PNH-affected granulocytes and erythrocytes is demonstrated by loss of GPI-anchored proteins (eg, CD55, CD59). (See 'Flow cytometry/FLAER' above.)

•Bone marrow examination – Microscopy and cytogenetics to evaluate for BMF and MDS. (See 'Bone marrow' above.)

●Diagnosis – PNH should be suspected in individuals with direct antiglobulin testing (DAT; Coombs test)-negative hemolysis, unexplained cytopenias, thrombosis at an early age or unusual site, unexplained abdominal pain, or AA or MDS. (See 'Diagnosis and classification' above.)

Diagnosis is based on flow cytometry that demonstrates a population of CD55/CD59 deficient granulocytes and erythrocytes in an appropriate clinical setting (eg, DAT-negative hemolytic anemia, thrombosis, unexplained abdominal pain, AA, or MDS). (See 'Diagnostic criteria' above.)

●Classification – Classification is based on anemia-related symptoms, thrombosis, pain, or organ dysfunction and findings from bone marrow examination:

•Hemolytic (classical) PNH – PNH with clinical findings of intravascular hemolysis, but no bone marrow failure. Typically, granulocyte PNH clone size is >50 percent, erythrocyte clone size is variable, elevated lactate dehydrogenase (LDH), and no bone marrow hypoplasia or dysplasia. (See 'Hemolytic (classical) PNH' above.)

•Subclinical PNH – PNH granulocytes and/or erythrocytes are present, but no substantial intravascular hemolysis or BMF; PNH granulocytes are typically ≤20 percent, RBC clone size is small, and modest elevation of LDH. (See 'Subclinical PNH' above.)

•PNH with bone marrow failure – PNH clone size and LDH are variable, but bone marrow cellularity and morphology meet criteria for severe AA or high-risk MDS. (See 'PNH with bone marrow failure' above.)

●Differential diagnosis – Other causes of DAT-negative hemolytic anemias, (ie, nonimmune hemolysis, inherited RBC membrane or enzymatic defects, drug-induced hemolysis, paroxysmal cold hemoglobinuria, and microangiopathic hemolytic anemias), atypical thromboses, and BMF should be excluded. (See 'Differential diagnosis' above.)

ACKNOWLEDGMENTS

The editors of UpToDate acknowledge the contributions of Stanley L Schrier, MD as Section Editor on this topic, his tenure as the founding Editor-in-Chief for UpToDate in Hematology, and his dedicated and longstanding involvement with the UpToDate program.

UpToDate acknowledges Wendell F Rosse, MD, who contributed to earlier versions of this topic review.