Overview of hemolytic anemias in children

INTRODUCTION — Anemia is among the most frequent laboratory abnormalities encountered by a practicing pediatrician. Anemia is caused by one of three broad mechanisms: decreased production of red blood cells (RBCs), increased loss of RBCs, or premature destruction (hemolysis) of RBCs. A combination of these mechanisms can occur simultaneously in some conditions.

The approach to a child with hemolytic anemia is discussed here. A broader approach to the anemic child is discussed separately. (See "Approach to the child with anemia".)

THE HEMOLYTIC PROCESS — After release from the bone marrow, mature, non-nucleated erythrocytes (red blood cells [RBCs]) generally survive for 100 to 120 days in the circulation [1]. In the steady state, approximately 1 percent of the circulating erythrocytes are destroyed daily and are replaced by an equal number of new erythrocytes (reticulocytes) released from the bone marrow (picture 1 and picture 2). The basic pathophysiology of the hemolytic anemias is a reduced erythrocyte lifespan due to premature destruction, ranging from nearly normal to remarkably shortened. (See "Red blood cell survival: Normal values and measurement".)

In compensation for reduced RBC lifespan, the bone marrow increases production of erythrocytes, a response mediated by increased erythropoietin. As an example, in adults with hereditary spherocytosis (HS), the bone marrow can increase output of erythrocytes six- to eightfold. With this maximal erythropoietic response, affected patients may not manifest anemia despite the substantially reduced RBC lifespan (fully compensated hemolysis). The limits of erythrocyte production in other hemolytic states have not been determined, particularly in infants and children, but they are probably lower in infants than in adults. (See "Regulation of erythropoiesis".)

As a result of increased RBC production in response to hemolysis, the reticulocyte count often exceeds 2 percent, with an absolute reticulocyte count usually >100,000/microL [2]. When a chronic hemolytic process is present, hyperplasia of the erythropoietic marrow elements often occurs, with reversal of the myeloid-to-erythroid ratio from the normal 3:1 to 1:1 or less (picture 3 and picture 4). In the severe chronic hemolytic processes of childhood (eg, thalassemia major, congenital spherocytosis, sickle cell disease [SCD]), hypertrophy of the marrow may expand the medullary spaces, producing bony changes, particularly in the skull (ie, frontal bossing) and hands [3]. (See "Overview of the clinical manifestations of sickle cell disease", section on 'Skeletal complications' and "Diagnosis of hemolytic anemia in adults", section on 'High reticulocyte count' and "Diagnosis of thalassemia (adults and children)", section on 'Skeletal changes'.)

DIAGNOSTIC PRINCIPLES

Overview — A hemolytic process can be detected by the presence of increased levels of the metabolic products of hemolysis, including indirect bilirubin, lactate dehydrogenase (LDH), and plasma free hemoglobin. In older children, a reduced haptoglobin level provides evidence of hemolysis; however, this is not a reliable marker in children <18 months. In addition, a hemolytic process may be inferred, in a nonbleeding patient, by the presence of reticulocytosis. However, reticulocytosis also occurs in some nonhemolytic conditions such as the recovery phase after an aplastic event (eg, parvovirus infection or transient erythroblastopenia of childhood) or in a child with anemia caused by iron or vitamin B12 deficiency who is responding to treatment. The degree of hemolysis can also be measured directly via isotopic or non-isotopic erythrocyte survival studies; however, these tests are generally not necessary in clinical practice. (See 'Diagnostic approach' below and "Red blood cell survival: Normal values and measurement".)

Classification — The hemolytic disorders are generally classified as either intrinsic abnormalities of the erythrocyte or extrinsic processes acting on a normal erythrocyte (table 1). These two categories are not mutually exclusive, and some hemolytic disorders are caused by a combination of intrinsic and extrinsic mechanisms (eg, paroxysmal nocturnal hemoglobinuria [PNH]):

●Intrinsic hemolytic anemias generally result from inherited or congenital hemoglobinopathies (eg, sickle cell disease [SCD], thalassemia), erythrocyte membrane defects (hereditary spherocytosis [HS], elliptocytosis, pyropoikilocytosis, or stomatocytosis) or enzyme deficiencies (eg, glucose-6-phosphate dehydrogenase [G6PD], pyruvate kinase [PK]). (See 'Intrinsic hemolytic anemias' below.)

●Extrinsic hemolytic anemias are typically acquired and result from forces or agents that immunologically, chemically, or physically damage the erythrocyte. These include autoimmune hemolytic anemias (AIHAs), hypersplenism, thrombotic microangiopathies, and many drugs including oxidant agents such as dapsone and nitrites. (See 'Extrinsic hemolytic anemias' below.)

Time-course of hemolysis — Chronic hemolytic anemia is generally at least partially compensated by a chronic reticulocytosis as well as other adaptive mechanisms. Thus, patients are often relatively asymptomatic despite significant anemia.

In contrast, acute hemolytic anemia, as may occur in the setting of infection or in AIHA, can cause significant symptoms despite relatively mild anemia, due to the abruptness of the drop in hemoglobin.

INTRINSIC HEMOLYTIC ANEMIAS — Intrinsic hemolytic anemias include hemoglobinopathies, erythrocyte membrane defects, and enzyme deficiencies.

Hemoglobinopathies — Hemoglobinopathies are common causes of hemolytic disease and include sickle cell disease (SCD), the thalassemias, and unstable hemoglobins:

●SCD – SCD refers to any one of the syndromes in which the sickle mutation in the beta globin gene is coinherited with a mutation at the other beta globin allele that reduces or eliminates normal beta globin production. These include sickle cell anemia (homozygous hemoglobin S mutation), the sickle-beta thalassemias, hemoglobin SC disease, and others. In the United States, most children with SCD are diagnosed through newborn screening. The clinical manifestations of SCD are protean. The major features are related to hemolytic anemia, inflammation, and vaso-occlusion, which can lead to acute and chronic pain as well as tissue ischemia or infarction. Splenic infarction generally leads to functional hyposplenism early in life, which, in turn, increases the risk of infection. These complications have a major impact on morbidity and mortality. SCD is discussed in greater detail separately. (See "Overview of the clinical manifestations of sickle cell disease".)

●The thalassemias – The thalassemias are a group of disorders characterized by reduced or absent production of one or more globin chains, resulting in disruption of the tightly regulated ratio of alpha and beta globin chains. The thalassemia syndromes are remarkable for their heterogeneity, particularly in terms of clinical severity. Beta thalassemia major is a severe form of thalassemia due to homozygous mutations associated with absent (or very severely reduced) production of beta chains. The diagnosis is usually made through newborn screening or at around 6 to 12 months of age due to the presence of pallor, irritability, growth retardation, abdominal swelling due to hepatosplenomegaly, and jaundice. The laboratory examination shows severe hemolytic anemia with markedly abnormal hypochromic, microcytic red cells (picture 5). The diagnosis is generally confirmed on hemoglobin analysis (table 2), though DNA testing may be necessary for confirmation of the exact type in certain situations. The thalassemias are discussed in greater detail separately. (See "Diagnosis of thalassemia (adults and children)".)

●Unstable hemoglobins – Unstable hemoglobin variants are rare inherited mutations affecting globin genes. Examples include hemoglobin Hasharon and hemoglobin Poole. Denaturation of hemoglobin leads to precipitation in the red blood cell (RBC), which causes Heinz body formation and both intravascular and extravascular hemolysis (picture 6). Although some can be identified in the newborn screen, most patients present in childhood or adolescence with varying degrees of hemolytic anemia and other manifestations of hemolysis such as jaundice, gallstones, splenomegaly, and/or hemoglobinuria. Initial testing includes routine laboratory studies for hemolytic anemia, review of the peripheral blood smear, and Heinz body preparation. Hemoglobin analysis may identify an unstable hemoglobin, but some variants are too unstable to be detected by this method. Genetic testing is the most definitive means of establishing the diagnosis and characterizing the variant. Unstable hemoglobin variants are discussed in greater detail separately. (See "Unstable hemoglobin variants".)

Erythrocyte membrane defects — Membrane defects include hereditary spherocytosis (HS), elliptocytosis, and other less common disorders:

●HS – HS is the most common erythrocyte membrane defect. It presents with hemolytic anemia of variable severity and spherical erythrocytes (spherocytes) on the blood smear (picture 7). HS results from heterogeneous alterations in one of six genes (most often the ankyrin gene) that encode for proteins involved in vertical associations that tie the membrane skeleton to the lipid bilayer. HS can present at any age and with any severity. Most affected individuals have mild or moderate hemolytic anemia. Neonates with HS often present with jaundice and hyperbilirubinemia; the serum bilirubin level may not peak until several days after birth. In older children and adults, the presentation may be that of an incidental finding of hemolytic anemia or spherocytes on the blood smear (picture 7) or the individual may be symptomatic from anemia, splenomegaly, or pigment gallstones. Exacerbations of anemia may occur in certain settings such as parvovirus infection, mononucleosis, or pregnancy. The diagnosis of HS requires specialized confirmatory testing. Management is generally supportive. Splenectomy may be performed in severely affected patients. The diagnosis and management of HS are discussed in greater detail separately. (See "Hereditary spherocytosis".)

●Hereditary elliptocytosis (HE) – HE (also called hereditary ovalocytosis) is a heterogeneous group of inherited erythrocyte disorders, most of which are autosomal dominant, that have in common the presence of elongated, oval, or elliptically shaped RBCs on the peripheral blood smear (picture 8). The morphology of the erythrocytes is the most important diagnostic feature of HE. Hemolytic anemia in these disorders ranges from absent to life-threatening. The most severe form of HE is hereditary pyropoikilocytosis. HE is discussed in greater detail separately. (See "Hereditary elliptocytosis and related disorders".)

●Hereditary stomatocytosis and hereditary xerocytosis – Hereditary stomatocytosis and hereditary xerocytosis are rare inherited disorders with altered RBC volume caused by mutations affecting ion channels. Patients may be completely asymptomatic or they can present with chronic hemolytic anemia of varying severity. Hereditary stomatocytosis is associated with stomatocytes on the peripheral blood smear (picture 9), and hereditary xerocytosis is associated with xerocytes and eccentrocytes (picture 10). Splenectomy in patients with these disorders has been associated with severe complications (eg, pulmonary hypertension and thromboembolic events), and it is therefore reserved as a last resort for patients with severe hemolysis and other complications. Hereditary stomatocytosis and hereditary xerocytosis are discussed in greater detail separately. (See "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)".)

Enzyme deficiencies — Hemolytic anemias caused by inherited enzyme deficiencies include those in the hexose monophosphate shunt and glutathione pathways (eg, glucose-6-phosphate dehydrogenase [G6PD], glutathione synthase) and those in the glycolytic pathway (eg, pyruvate kinase [PK], hexokinase, glucose phosphate isomerase, and others). Most are inherited as autosomal recessive disorders. With the exception of G6PD deficiency, these are rare disorders. Inherited enzyme deficiencies are reviewed briefly here, and more detail is provided in linked topic reviews.

This group of congenital hemolytic anemias are classified as "nonspherocytic" because spherocytic erythrocytes are generally absent from the peripheral blood smear. A diagnosis is established by demonstrating reduction of the enzyme as well as decreased levels of glycolytic metabolites downstream of the deficient enzyme. However, testing can be challenging in the setting of acute hemolysis due to the higher enzyme levels of surviving erythrocytes. Deficiencies of glycolytic enzymes compromise adenosine triphosphate (ATP) generation. The metabolic energy requirements of the erythrocytes cannot be met, and the erythrocyte lifespan is shortened. Deficiencies of the enzymes in the glutathione pathway result in inadequate levels of glutathione in erythrocytes, which impair the ability to handle oxidant stress. In this setting, hemoglobin becomes denatured and precipitates into erythrocyte inclusions called Heinz bodies (picture 6). These bodies damage the erythrocyte membrane, resulting in acute intravascular and extravascular hemolysis [4].

●G6PD deficiency – G6PD deficiency is the most common erythrocyte enzyme disorder, affecting >400 million people worldwide. G6PD deficiency is an X-linked inherited condition that can result in a spectrum of clinical severity:

•Class II and III variants are quite common and typically mild. These variants lack chronic anemia and reticulocytosis but can lead to acute hemolytic episodes induced by infection or certain drugs (table 3).

•Class I variants are rare and result in a severe chronic, nonspherocytic hemolytic anemia with acute worsening after exposure to oxidant stress.

Clinical manifestations of G6PD deficiency depend upon the severity of the deficiency as well as the inciting event. G6PD deficiency is diagnosed by detection of decreased enzyme activity in red cells. A number of screening tests are available for this purpose; however, false-negative results can occur during or immediately following an episode of hemolysis and, therefore, retesting at a later date is advised if clinical suspicion persists. "Bite cells" and Heinz bodies (picture 6) may be seen after an acute episode of hemolysis. Transfusion may be necessary in cases of severe hemolysis and symptomatic anemia. There is no specific treatment for subjects with class I variants who have chronic hemolysis. Avoidance of known triggers including oxidant agents is an important aspect of management. G6PD deficiency is discussed in greater detail separately. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Genetics and pathophysiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency".)

●PK deficiency – An inherited deficiency of PK is the most common of the erythrocyte glycolytic enzyme deficiencies [5]. PK activity, measured in the erythrocytes, is reduced markedly, but the enzyme activity in other blood cells and tissues is normal. The severity of hemolysis is highly variable, ranging from a life-threatening, transfusion-dependent hemolytic anemia to a fully compensated hemolytic process [6]. Laboratory tests are consistent with the presence of extravascular hemolysis with a brisk reticulocyte response. Demonstration of reduced erythrocyte PK enzyme activity and/or genetic testing for two relevant PKLR gene mutations is required for making a specific diagnosis. Treatment of PK deficiency depends upon the time when the disorder becomes evident. Phototherapy and/or exchange transfusion may be needed for severe hyperbilirubinemia during the neonatal period. If the hemolysis is fully compensated (ie, minimal to no anemia), observation is the general management approach. In patients with severe symptomatic anemia, splenectomy is an option for reducing symptoms and the frequency of red cell transfusions. The diagnosis and management of PK deficiency are discussed in greater detail separately. (See "Pyruvate kinase deficiency".)

●Other glycolytic enzymes – Deficiencies of several other glycolytic erythrocyte enzymes have been described in patients with congenital nonspherocytic hemolytic anemia. They include deficiencies of hexokinase, glucose phosphate isomerase, phosphofructokinase, aldolase, triose phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, 2,3-bisphosphoglycerate mutase, monophosphoglycerate mutase, enolase, and lactate dehydrogenase (LDH). These diseases are extremely rare, and most are transmitted in an autosomal recessive pattern. Some of them, as listed below, are associated with neurologic disease, metabolic myopathy, and abnormal glycogen metabolism. Chronic hemolysis, often manifested in infancy, is a common feature. Specific erythrocyte morphologic abnormalities are not seen. Diagnosis depends on demonstration of reduction of the specific erythrocyte enzyme. No specific therapy exists; splenectomy may reduce the rate of hemolysis in some, but not all, of these disorders. (See "Rare RBC enzyme disorders", section on 'Disorders of glycolysis'.)

Specific disorders include:

•Phosphofructokinase deficiency – A metabolic myopathy with hemolytic anemia (see "Phosphofructokinase deficiency (glycogen storage disease VII, Tarui disease)" and "Rare RBC enzyme disorders", section on 'PK deficiency')

•Phosphoglycerate kinase deficiency – Central nervous system dysfunction with seizures and intellectual disability associated with nonspherocytic hemolytic anemia (see "Phosphoglycerate kinase deficiency and phosphoglycerate mutase deficiency", section on 'Phosphoglycerate kinase deficiency' and "Rare RBC enzyme disorders", section on 'Phosphoglycerate kinase (PGK) deficiency')

•Aldolase deficiency (see "Other disorders of glycogen metabolism: GLUT2 deficiency and aldolase A deficiency" and "Rare RBC enzyme disorders", section on 'Aldolase deficiency')

•Triosephosphate isomerase deficiency – Hemolytic anemia coupled with a progressive, severe neurologic disorder [7] (see "Rare RBC enzyme disorders", section on 'Aldolase deficiency')

●Other enzymes in the hexose monophosphate shunt and glutathione pathways – G6PD deficiency is by far the most common disorder in this category. Deficiencies of other enzymes in the hexose monophosphate shunt and glutathione pathways are rare and are discussed separately. (See "Rare RBC enzyme disorders", section on 'Disorders of the HMP shunt and glutathione metabolism'.)

EXTRINSIC HEMOLYTIC ANEMIAS — Hemolytic anemias caused by conditions that damage normal erythrocytes and lead to premature destruction are categorized as "extrinsic" (table 1). These include the autoimmune hemolytic anemias (AIHAs; including warm-reactive AIHA, cold agglutinin disease, and paroxysmal cold hemoglobinuria [PCH]); hypersplenism; systemic disease such as infection, liver disease, renal disease, drugs, and toxins; thrombotic microangiopathies (including hemolytic uremic syndrome [HUS], thrombotic thrombocytopenic purpura [TTP], and disseminated intravascular coagulation [DIC]); and Wilson disease.

Autoimmune hemolytic anemia — AIHA is a collection of disorders characterized by the presence of pathologic autoantibodies that bind to the patient's own erythrocytes, leading to hemolysis and, when the rate of hemolysis exceeds the ability of the bone marrow to replace the destroyed red cells, to anemia and its attendant signs and symptoms. The diagnosis of AIHA is established based on laboratory evidence of hemolysis (anemia with elevated lactate dehydrogenase [LDH], indirect bilirubin, and plasma free hemoglobin; reduced haptoglobin; and/or hemoglobinuria) and, typically, a positive direct antiglobulin test (DAT). The degree of anemia and the reticulocyte response are variable. (See "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis".)

AIHA can occur as a primary process or it may be secondary to an underlying condition, particularly autoimmune or immunodeficiency disorders (eg, systemic lupus erythematosus, autoimmune lymphoproliferative syndrome, common variable immune deficiency, human immunodeficiency virus, and others) (table 4). In addition, AIHA can occur in association with certain infections (eg, Mycoplasma pneumoniae, Epstein-Barr virus [EBV]). (See "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis", section on 'Primary versus secondary AIHA'.)

AIHA is categorized as "warm" or "cold" based on the thermal reactivity of the autoantibodies (table 5):

●Warm-reactive AIHA – Warm-reactive AIHA is the most common form of primary AIHA in children and is caused by warm-reactive autoantibodies, usually immunoglobulin G (IgG), that bind preferentially to endogenous red cells at 37°C. This process leads to extravascular hemolysis that occurs mainly in the spleen, with resulting anemia, jaundice, and, rarely, splenomegaly. Spherocytes are typically seen on review of peripheral blood smear (picture 11). Warm-reactive AIHA can be life-threatening and usually does not resolve without treatment. The diagnosis and treatment of warm-reactive AIHA are discussed separately. (See "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis" and "Autoimmune hemolytic anemia (AIHA) in children: Treatment and outcome".)

●Cold agglutinin disease – Cold agglutinin disease is relatively uncommon in children. Most cases occur after M. pneumoniae or EBV infection. In this disorder, immunoglobulin M (IgM) autoantibodies bind erythrocyte I/i antigens at colder temperatures and fix complement, which leads to either complement-mediated intravascular hemolysis or immune-mediated extravascular clearance, mainly by hepatic macrophages. The anemia tends to be mild, and pharmacologic therapy is not necessary in most cases. Clumping is commonly seen on peripheral blood smear (picture 12), which can cause the reported mean corpuscular volume to be falsely elevated. Management involves keeping the patient warm and avoiding exposure to cold fluids. Severely affected children may require transfusion, pharmacologic therapy, and/or plasma exchange. Treatment of the underlying cause (eg, antibiotics in the case of Mycoplasma infection) is also an important aspect of management. The diagnosis and management of cold agglutinin disease are discussed in greater detail separately. (See "Cold agglutinin disease".)

●Paroxysmal cold hemoglobinuria (PCH) – PCH is an AIHA seen almost exclusively in children, most commonly after a viral illness. PCH is characterized by IgG autoantibodies that bind preferentially at colder temperatures; fix complement efficiently; and cause intravascular hemolysis with anemia, hemoglobinemia, and hemoglobinuria. Children with PCH usually have an abrupt but self-limited hemolytic process. Keeping the patient warm and avoiding exposure to cold fluids are the mainstays of management. Transfusion may be required at presentation, particularly in patients with severe and symptomatic anemia. PCH is discussed in detail separately. (See "Paroxysmal cold hemoglobinuria".)

Hypersplenism — When sufficiently enlarged, the spleen causes anemia because of both sequestration of red blood cells (RBCs) and increased destruction and clearance. The degree of hemolysis and anemia is proportional to the magnitude of the splenomegaly. Variable degrees of thrombocytopenia and neutropenia may also occur. Anemia due to hypersplenism is discussed separately. (See "Approach to the child with an enlarged spleen".)

Systemic disease — Broadly, systemic disorders associated with nonimmune extrinsic hemolytic anemia include infection and liver and renal diseases. Infections more commonly cause immune-mediated hemolysis; however, nonimmune hemolysis occurs with certain parasitic (eg, malaria) and bacterial (eg, Clostridium perfringens) infections. Liver disease causes hemolytic anemia due to hypersplenism and acquired alterations in the red cell membrane. Hemolysis can occur in patients with renal disease by fragmentation hemolysis associated with microvascular disease, the effects of uremic plasma, and/or and the effects of hemodialysis. System disorders associated with nonimmune hemolysis are discussed in greater detail separately. (See "Non-immune (Coombs-negative) hemolytic anemias in adults".)

Drugs and toxins — A number of drugs and toxins can cause hemolysis via several different pathways. Oxidative damage is most commonly seen in patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency taking drugs or toxins associated with induction of oxidative stress (table 3). Immune hemolysis can be seen after intravenous immune globulin (IVIG) and anti-D immune globulin administration. Drug-related AIHA can also occur with a number of agents (table 6), though these are not common in children. Direct red cell destruction can also occur. Hemolytic anemia due to drugs and toxins is discussed in greater detail separately. (See "Drug-induced hemolytic anemia".)

Microangiopathies — Microangiopathic hemolytic anemias result from intravascular RBC fragmentation that produces schistocytes on the peripheral blood smear (picture 13). (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)".)

●HUS – HUS is defined by the simultaneous occurrence of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. It is one of the main causes of acute kidney injury in children. Most cases are due to infectious etiologies (eg, Shiga toxin-producing Escherichia coli and Pneumococcus). Atypical HUS can be caused by a variety of other causes. HUS is discussed in greater detail separately. (See "Overview of hemolytic uremic syndrome in children" and "Clinical manifestations and diagnosis of Shiga toxin-producing Escherichia coli (STEC) hemolytic uremic syndrome in children" and "Treatment and prognosis of Shiga toxin-producing Escherichia coli (STEC) hemolytic uremic syndrome in children".)

●TTP – TTP is a thrombotic microangiopathy caused by severely reduced activity of the von Willebrand factor-cleaving protease ADAMTS13. TTP is most commonly acquired due to an autoantibody inhibitor of ADAMTS13. Rarely, genetic mutations result in congenital deficiency of ADAMTS13. Acquired TTP can be life-threatening without prompt treatment. It is characterized by severe microangiopathic hemolytic anemia and thrombocytopenia. Neurologic and renal abnormalities may be seen but are not always present. Timely initiation of plasma exchange can be lifesaving in TTP, so a high index of suspicion should be maintained in patients with anemia and thrombocytopenia, especially if schistocytes are present on the peripheral blood smear (picture 13). TTP is discussed in greater detail separately. (See "Diagnosis of immune TTP" and "Hereditary thrombotic thrombocytopenic purpura (hTTP)".)

●DIC – DIC is an acquired syndrome caused by increased activation of the coagulation system due to a variety of underlying disorders (eg, sepsis, trauma, malignancy). Clinical findings include hemorrhage and microthrombi; hemolysis is not a major feature of the disorder. DIC is discussed in greater detail separately. (See "Disseminated intravascular coagulation in infants and children".)

Mechanical damage — Hemolysis due to mechanical damage to erythrocytes can occur in various settings, including artificial heart valves and Kasabach-Merritt phenomenon (a syndrome of consumptive coagulopathy, thrombocytopenia, and erythrocyte fragmentation associated with giant hemangiomas). These are uncommon causes of hemolysis in children and are discussed in greater detail separately. (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Fragmentation'.)

Wilson disease — Hemolytic anemia may occur in patients with Wilson disease and is occasionally a presenting manifestation [8,9]. Prompt diagnosis of the underlying Wilson disease is important because treatment can prevent the progressive hepatic and neuropsychiatric consequences. Evaluation for Wilson disease should be considered in patients with unexplained hemolytic anemia and is essential in any patient with concomitant hepatic or neurologic symptoms. The mechanism for the hemolysis has not been established. (See "Wilson disease: Clinical manifestations, diagnosis, and natural history".)

COMBINED MECHANISM

Paroxysmal nocturnal hemoglobinuria — Paroxysmal nocturnal hemoglobinuria (PNH) is a rare chronic anemia with prominent intravascular hemolysis that may have its onset in late childhood. Hemolysis is characteristically worse during sleep, and morning hemoglobinuria usually is present. The mechanism of hemolysis in PNH involves both intrinsic erythrocyte abnormalities and external stimuli. Hemolysis occurs because glycosylphosphatidylinositol (GPI)-anchored complement inhibitors that normally protect erythrocytes from intravascular and extravascular hemolysis are missing. The degree of hemolysis depends on several factors, including the number of GPI-deficient cells, the degree of GPI deficiency, and the presence of additional precipitating events (eg, infection or inflammation). PNH is discussed in greater detail separately. (See "Clinical manifestations and diagnosis of paroxysmal nocturnal hemoglobinuria" and "Pathogenesis of paroxysmal nocturnal hemoglobinuria".)

DIAGNOSTIC APPROACH

Overview — A systematic approach, starting with an accurate history and physical examination, is the cornerstone of the evaluation. (See "Approach to the child with anemia", section on 'Evaluation'.)

Recognizing hemolysis is not difficult in the classic patient, who may have many of the following clinical findings:

●Pallor and anemia (rapid onset for acute forms)

●Jaundice

●Dark urine

●Splenomegaly

●Abnormal red blood cell (RBC) morphology on peripheral smear (eg, spherocytes in autoimmune hemolytic anemia [AIHA] and hereditary spherocytosis [HS]) (see 'Peripheral smear' below)

●Reticulocytosis

●Abnormal serum markers of hemolysis (eg, elevated indirect bilirubin, elevated lactate dehydrogenase [LDH], elevated plasma free hemoglobin, low or absent haptoglobin)

The diagnostic evaluation in children with a new onset of hemolysis should be carried out in consultation with a pediatric hematologist since sudden and life-threatening worsening of anemia may occur. Hemolysis may also be the first sign of an underlying systemic disorder (eg, leukemia, systemic lupus erythematosus, thrombotic thrombocytopenic purpura [TTP]) and may require urgent intervention to prevent disease-related complications.

Laboratory testing — Laboratory findings, including an examination of the peripheral smear, are used to confirm the presence of hemolysis and, if possible, to determine the underlying cause (table 1). Appropriate initial studies for such patients include the following (their interpretation is discussed in depth in the sections below):

●Complete blood count along with red cell indices, platelet count

●Reticulocyte count

●Examination of the peripheral blood smear

●Serum markers of hemolysis (including total and indirect bilirubin, LDH, plasma free hemoglobin and/or, in children >18 months, haptoglobin)

Reticulocyte count — The reticulocyte is reported as a percentage of the RBC population. After the first few months of life, the normal reticulocyte percentage is the same as that of the adult, approximately 1.5 percent. The normal reticulocyte percentage ranges from 1 to 2 percent. A high reticulocyte count (>3 percent) reflects an increased erythropoietic response to blood loss or hemolysis.

If the reticulocyte response is adequate and the hemolysis is not severe, patients may not become anemic. However, when the bone marrow is compromised (eg, infection, iron deficiency), the reticulocyte response may be blunted or ablated. (See "Approach to the child with anemia", section on 'Reticulocyte response'.)

A reticulocyte response can also be noted on examination of the peripheral blood smear since reticulocytes are slightly larger than normal RBCs, lack central pallor, and have irregular outlines along with a bluish tint (picture 1). Reticulocyte counts can be quantitated via a formal manual (picture 2) or automated reticulocyte count. (See "Automated complete blood count (CBC)", section on 'Reticulocytes'.)

Peripheral smear — Review of peripheral smear is a crucial component of the evaluation of children with hemolytic anemia. (See "Evaluation of the peripheral blood smear".)

RBC abnormalities suspicious for the presence of hemolysis include the following:

●Spherocytes (picture 7) in patients with AIHA or HS (see "Hereditary spherocytosis" and "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis")

●Fragmented RBC (schistocytes, helmet cells) indicating the presence of microangiopathic hemolytic anemia (picture 13) (see "Overview of hemolytic uremic syndrome in children" and "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)")

●Bite cells and Heinz bodies (picture 6) are seen in hemolytic anemia due to oxidant sensitivity, such as glucose-6-phosphate dehydrogenase (G6PD) deficiency (see "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Acute hemolytic anemia')

●Sickle cells, as seen in sickle cell disease (SCD) (picture 14) (see "Diagnosis of sickle cell disorders")

●Elliptocytes, as seen in congenital elliptocytosis (picture 8)

●Stomatocytes, as seen in hereditary or acquired stomatocytosis (picture 9) (see "Hereditary stomatocytosis (HSt) and hereditary xerocytosis (HX)")

●Target cells, as seen in the various hemoglobinopathies, including thalassemia, as well as in liver disease and post-splenectomy (picture 15 and picture 16) (see "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane")

●RBC agglutination (picture 12) is seen in cold agglutinin disease (see "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis", section on 'Cold AIHA')

●Acanthocytes (spur cells) in patients with liver disease (picture 17) (see "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Liver and kidney disease')

●RBCs with inclusions, as in malaria, babesiosis, or Bartonella infection (picture 18) (see "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Infections (RBC parasites and intracellular bacteria)')

●Red cell "ghosts" (picture 19), indicating the presence of intravascular hemolysis, most often associated with overwhelming bacterial infection (eg, C. perfringens) (see "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Infections (RBC parasites and intracellular bacteria)')

Bilirubin — Children with hemolytic anemia generally have elevated unconjugated bilirubin. However, overt clinical jaundice may be absent or may not be appreciated. Total bilirubin levels >5 mg/dL are unusual if hepatic function is normal. The increased excretion of bilirubin pigments that occurs in patients with chronic hemolysis also may lead to the production of pigmented gallstones that may develop in early childhood [10].

Serum LDH, haptoglobin, and plasma free hemoglobin — Tests used to diagnose the presence of hemolysis include LDH and plasma free hemoglobin, which are released from hemolyzed RBCs, and haptoglobin, which binds to hemoglobin released during hemolysis. Because the synthetic and binding capacities of haptoglobin are limited, serum levels of haptoglobin in hemolysis usually are either decreased (<20 mg/dL) or absent [11]. There are some caveats in assessing these tests, as follows:

●Elevated LDH level is a nonspecific marker of tissue damage and can be found in many different conditions other than hemolysis (table 7).

●Haptoglobin is not a reliable indicator of hemolysis in infants <18 months of age, because it is not synthesized well in young infants, it is an acute phase reactant, and its concentration may be elevated in the presence of infection or inflammation. Plasma free hemoglobin may be useful in these cases. (See "Acute phase reactants".)

Confirmatory testing — Hemolytic anemia is established on the basis of serum markers of hemolysis (increased indirect bilirubin, LDH, and plasma free hemoglobin; decreased haptoglobin), reticulocytosis, and abnormalities on the peripheral smear (eg, spherocytes, schistocytes, increased polychromasia). However, not all of these findings need to be present.

Further testing is guided by the clinical history, initial laboratory results, and peripheral smear findings. In most cases, consultation with a pediatric hematologist is appropriate.

Additional tests that may be helpful include:

●Direct antiglobulin test (DAT; formerly called Coombs test) if AIHA is suspected. (See "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis", section on 'Initial laboratory evaluation'.)

●G6PD testing if G6PD deficiency is suspected. Testing should be repeated after resolution of acute hemolysis if clinically indicated. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Diagnostic evaluation'.)

●Testing for other erythrocyte enzymopathies (eg, pyruvate kinase [PK] deficiency), if one of these disorders is suspected. (See "Pyruvate kinase deficiency", section on 'Diagnosis'.)

●Eosin-5-maleimide (EMA) binding/RBC band 3 protein testing if an erythrocyte membrane defect is suspected. (See "Hereditary spherocytosis", section on 'Confirmatory tests'.)

●Hemoglobin analysis if a hemoglobinopathy is suspected. (See "Methods for hemoglobin analysis and hemoglobinopathy testing".)

SUMMARY AND RECOMMENDATIONS

●Pathophysiology – The basic pathophysiology of the hemolytic anemias is a reduced erythrocyte lifespan. In compensation for a reduced red blood cell (RBC) lifespan, the bone marrow increases its output of erythrocytes. In the severe chronic hemolytic processes of childhood, hypertrophy of the marrow may expand the medullary spaces, producing bony changes. (See 'The hemolytic process' above.)

●Intrinsic versus extrinsic causes of hemolysis – Hemolysis can result from an intrinsic abnormality of the erythrocyte or an extrinsic abnormality acting on a normal erythrocyte (table 1). These two categories are not mutually exclusive. (See 'Classification' above.)

•Intrinsic hemolytic anemias include (see 'Intrinsic hemolytic anemias' above):

-Hemoglobinopathies (eg, sickle cell disease [SCD], thalassemia) (see "Overview of the clinical manifestations of sickle cell disease" and "Diagnosis of thalassemia (adults and children)")

-Erythrocyte membrane defects (eg, hereditary spherocytosis [HS], hereditary elliptocytosis [HE]) (see "Hereditary spherocytosis" and "Hereditary elliptocytosis and related disorders")

-Enzyme deficiencies (eg, glucose-6-phosphate dehydrogenase [G6PD] deficiency, pyruvate kinase [PK] deficiency) (see "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Pyruvate kinase deficiency")

•Extrinsic hemolytic anemias include (see 'Extrinsic hemolytic anemias' above):

-Autoimmune hemolytic anemias (AIHAs; including warm AIHA, cold agglutinins disease, and paroxysmal cold hemoglobinuria [PCH]) (see "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis" and "Paroxysmal cold hemoglobinuria")

-Hypersplenism (see "Approach to the child with an enlarged spleen")

-Systemic disease (eg, infection, liver disease, renal disease) (see "Non-immune (Coombs-negative) hemolytic anemias in adults")

-Drugs and toxins (table 6 and table 3) (see "Drug-induced hemolytic anemia")

-Microangiopathies (eg, hemolytic uremic syndrome [HUS], thrombotic thrombocytopenic purpura [TTP], disseminated intravascular coagulation [DIC]) (see "Overview of hemolytic uremic syndrome in children" and "Disseminated intravascular coagulation in infants and children" and "Diagnosis of immune TTP")

-Mechanical damage (eg, heart valves, Kasabach-Merritt phenomenon) (see "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Fragmentation')

-Wilson disease (see "Wilson disease: Clinical manifestations, diagnosis, and natural history")

•Combined mechanism (intrinsic and extrinsic):

-Paroxysmal nocturnal hemoglobinuria (PNH) (see "Clinical manifestations and diagnosis of paroxysmal nocturnal hemoglobinuria")

●Initial laboratory evaluation – Initial laboratory studies in children with suspected hemolytic anemia include (see 'Laboratory testing' above):

•Complete blood count along with red cell indices, platelet count

•Examination of the peripheral smear (see 'Peripheral smear' above)

•Reticulocyte count (see 'Reticulocyte count' above)

•Serum markers of hemolysis (including total and indirect bilirubin, lactate dehydrogenase (LDH), plasma free hemoglobin, and, in children >18 months, haptoglobin) (see 'Bilirubin' above and 'Serum LDH, haptoglobin, and plasma free hemoglobin' above)

●Confirming the presence of a hemolytic process – Hemolytic anemia is established based upon the following laboratory findings, though not all of these need to be present (see 'Diagnostic approach' above):

•Increased indirect bilirubin and LDH

•Decreased haptoglobin

•Reticulocytosis

•Abnormalities on the peripheral smear (eg, spherocytes, schistocytes, increased polychromasia)

●Further testing – Further testing is guided by the clinical history, initial laboratory results, and peripheral smear findings. In most cases, consultation with a pediatric hematologist is appropriate. Additional tests that may be helpful include:

•Direct antiglobulin test (DAT; formerly called Coombs test) if AIHA is suspected (see "Autoimmune hemolytic anemia (AIHA) in children: Classification, clinical features, and diagnosis", section on 'Initial laboratory evaluation')

•G6PD testing if G6PD deficiency is suspected (testing should be repeated after resolution of acute hemolysis if clinically indicated) (see "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency")

•Testing for other erythrocyte enzymopathies (eg, PK deficiency), if one of these disorders is suspected (see "Pyruvate kinase deficiency", section on 'Diagnosis')

•Eosin-5-maleimide (EMA) binding/RBC band 3 testing if an erythrocyte membrane defect is suspected (see "Hereditary spherocytosis", section on 'Confirmatory tests')

•Hemoglobin analysis if a hemoglobinopathy is suspected (see "Methods for hemoglobin analysis and hemoglobinopathy testing")

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Jenny Despotovic, DO, MS, who contributed to earlier versions of this topic review.