Chapter 433. Hemolytic Anemia

Red blood cells (RBCs) have a normal life span of approximately 120 days. An operational definition of hemolysis, therefore, is accelerated RBC destruction with premature removal from the circulation, which usually causes anemia. Hemolytic anemia can occur in a wide range of clinical settings featuring various etiologies; signs and symptoms reflect the location and severity of hemolysis. Intravascular hemolysis occurs when erythrocytes are destroyed in the blood vessel itself, whereas extravascular hemolysis occurs in the hepatic and splenic macrophages within the reticuloendothelial system. Intravascular hemolysis is often dramatic, with free hemoglobin released into the plasma leading to hemoglobinuria (positive blood on urine dipstick but few erythrocytes on microscopic examination). Examples of intravascular hemolysis include enzyme defects such as glucose-6-phosphate dehydrogenase (G6PD) deficiency or certain immune-mediated processes. Extravascular hemolysis usually results from more subtle RBC destruction, typically with chronic splenic enlargement and jaundice. Extravascular hemolysis is more common with RBC membrane disorders such as hereditary spherocytosis. Some forms of hemolytic anemia feature both intravascular and extravascular hemolysis. The degree of anemia depends on how rapidly the erythrocytes are being removed from circulation and how well the bone marrow compensates with increased reticulocytosis.

The clinical presentation of hemolytic anemia is highly variable, ranging from mild anemia with asymptomatic splenomegaly, to jaundice with splenomegaly and dark urine, to acute severe anemia as a consequence of a parvovirus-induced aplastic crisis. Symptoms of hemolysis may include pallor, fatigue, abdominal pain, dark urine, and “yellow” eyes. On physical examination, patients may have a cardiac flow murmur, splenomegaly, and scleral icterus. Some children have a relatively benign physical examination and medical history.

The classic laboratory finding of hemolysis is anemia with an elevated reticulocyte count. The reticulocytosis reflects normal bone marrow function and occurs in response to the premature RBC destruction; reticulocytes are larger than older erythrocytes and have a blue-purple color known as polychromasia (Fig. 433-1). Reticulocytosis generally occurs 3 to 5 days after a sudden drop in hemoglobin concentration but is relatively constant in children with congenital hemolytic anemia. Making the diagnosis of hemolytic anemia begins with recognizing the constellation of signs and symptoms, then obtaining a complete blood count with reticulocyte count, and finally examining the peripheral blood smear. Additional laboratory findings supporting the diagnosis of hemolysis include elevated total serum bilirubin and lactate dehydrogenase (LDH; LDH being released from RBCs during hemolysis). Intravascular hemolysis also causes decreased or undetectable levels of haptoglobin, but this test is not specific so is not generally helpful. Dipstick urinalysis may reveal bilirubin, protein, or blood; the presence of blood without erythrocytes on microscopic analysis indicates hemoglobinuria reflecting intravascular hemolysis.

Hemolytic anemia in children most commonly results from intrinsic defects within the erythrocytes. They are typically inherited genetic abnormalities and thus congenital in origin, although most are not clinically apparent in the newborn period. Intrinsic RBC defects vary widely and reflect abnormalities in the outer cell membrane, in the various cellular enzymes, or in the abundant hemoglobin molecules. In all cases, there is hemolysis, as reflected by anemia and increased number of reticulocytes.

Erythrocyte Membrane Defects

The RBC membrane consists of a lipid bilayer and membrane proteins forming a cytoskeleton, a multiprotein complex comprised of α- and β-spectrin, ankyrin, band 3, protein 4.2, and others. Disruption in the synthesis of the individual proteins or abnormal assembly of the complex results in a defective RBC membrane.1 The erythrocytes are initially formed with the normal biconcave disc shape, but then change to a spherical shape (spherocyte), elliptical shape (elliptocyte), or other unusual forms.

Spherocytosis

Pathophysiology and Genetics

Spherocytosisis an inherited disorder of the RBC membrane characterized by the presence of spherocytes on the peripheral blood smear (Fig. 433-2A). Defects in the erythrocyte membrane lead to loss of membrane surface area and eventual morphologic change. Spherocytes have a decreased ability to deform and quickly become trapped in the sinusoids of the spleen; the majority of hemolysis in spherocytosis thus occurs in the extravascular compartment.2,3 Spherocytosis is usually inherited in an autosomal-dominant fashion with an affected parent and often multiple affected family members. About one third of children with spherocytosis will have a negative family history, usually resulting from a new (spontaneous) mutation that will be subsequently transmitted as an autosomal-dominant trait. Spherocytosis is most prevalent among persons of Northern European descent but can occur in any ethnic or racial group.

Clinical Features and Diagnosis

Typical manifestations of spherocytosis include partially compensated hemolysis with variable anemia but substantial reticulocytosis. Most affected children have intermittent jaundice and palpable splenomegaly, and are at risk for developing pigmented (bilirubin) gallstones due to chronic hemolysis. Because of the dependence on an active marrow output of reticulocytes, severe anemia requiring erythrocyte transfusion (a so-called aplastic crisis) can occur in association with acute parvovirus B19 infection, and may be the initial clinical presentation. In the newborn period, spherocytosis is often associated with nonphysiologic jaundice in the first 24 hours of life; anemia may be exaggerated during the first year of life and require periodic transfusions.4

Spherocytosis should be considered in any child with anemia, especially if there is a positive family history and splenomegaly. The complete blood count will reveal mild-to-moderate anemia with increased reticulocytes but normal leukocyte and platelet counts. The mean corpuscular hemoglobin concentration (MCHC) is often increased and virtually pathognomonic for spherocytosis when greater than 36 g/dL.2 Small dense spherocytes without central pallor can be identified on the peripheral smear, along with larger reticulocytes. A negative direct antiglobulin test excludes an autoimmune hemolytic process. The “gold standard” test to establish the diagnosis of spherocytosis is increased osmotic fragility.

Treatment

Treatment of children with spherocytosis is primarily supportive. Transfusions are not required except with severe hemolysis; most patients have active reticulocytosis and a relatively stable hemoglobin concentration. Pigmented gallstones may develop and lead to abdominal pain and increased jaundice. Laparoscopic cholecystectomy is recommended for children with symptomatic gallstones. Splenomegaly is common but usually asymptomatic; over time, the enlarged spleen can lead to abdominal pain, early satiety, and worsening anemia. Although spontaneous splenic rupture is extremely rare, a large spleen is theoretically at risk of injury from trauma or contact sports. The need for splenectomy in young patients with spherocytosis is controversial. Although removal of the spleen eliminates the erythrocyte trapping and destruction and thus improves the anemia, splenectomy is not a “cure” because spherocytes continue to be produced and circulate. Moreover, splenectomy leads to an increased risk of developing overwhelming infection and sepsis from encapsulated bacteria, and appears to be associated with long-term effects related to thrombosis and vasculopathy, including pulmonary hypertension.5 For these reasons, splenectomy is often postponed until the second decade of life or even longer. The subtotal (partial) splenectomy procedure leaves a portion of functioning splenic tissue and therefore provides an attractive alternative to total splenectomy; long-term outcomes with the partial splenectomy procedure are encouraging.6

Elliptocytosis

Hereditary elliptocytosis (HE) is similar to spherocytosis, but the erythrocyte shape is elliptical or cigar shaped secondary to structural protein defects (Fig. 433-2B). In this disease spectrin, heterodimers fail to self-associate into heterotetramers, leading to a functional spectrin deficiency.7 Elliptocytosis occurs most frequently in persons of African or Mediterranean descent with autosomal-dominant transmission.2 Most patients with elliptocytosis are asymptomatic, so the diagnosis is often made incidentally after routine laboratory work. Only approximately 10% have a shortened RBC life span leading to anemia, which is normochromic and normocytic. Hemolysis is mild and treatment is rarely needed, although splenectomy can be used in severely affected patients.2 However, occasional children have a more severe hemolytic anemia with both elliptocytes and spherocytes (Fig. 433-2C). These patients often have Southeast Asian ancestry and more substantial RBC protein defects resulting from autosomal-recessive or compound heterozygous transmission.

Pyropoikilocytosis

Additional RBC membrane disorders are much rarer but hereditary pyropoikilocytosis (HPP) is notable for hemolytic anemia in the newborn period or first year of life, which tends to lessen in severity during childhood. The morphologic feature of this disorder is small misshapened RBC fragments, including triangular and elliptical cells (Fig. 433-2D). So named for their odd shapes and sensitivity to heat, pyropoikilocytes are fragile and feature both intravascular and extravascular hemolysis, not unlike hemolytic anemia that occurs in patients with thermal burns. Children with HPP are often of African ancestry and have a parent with elliptocytosis.2

Enzyme Defects

Circulating erythrocytes have enzymes with a wide variety of functions, including metabolism of glucose, generation of adenosine triphosphate (ATP), protection against oxidation, and modification of the hemoglobin-oxygen dissociation curve. Among the dozens of RBC enzymes affected by inherited defects, two are relatively common and lead to hemolytic anemia: glucose-6-phosphate dehydrogenase (G6PD) deficiency and pyruvate kinase (PK) deficiency.

Glucose-6-Phosphate Dehydrogenase Deficiency

Pathophysiology and Genetics

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive disorder, so males as hemizygotes are affected and females as heterozygotes are usually unaffected carriers. The G6PD enzyme is a key part of the intracellular pathway that produces glutathione, which protects RBC proteins, including hemoglobin, against oxidative damage.8 G6PD activity decreases with the age of the cell, so older erythrocytes are more susceptible to oxidative injury. G6PD enzymopathy is very common and represents one of the most prevalent genetic abnormalities worldwide; approximately 10% of the African American male population has an abnormal G6PD gene.9

Clinical Features

The clinical presentation of G6PD deficiency varies widely because the genetic defect can range from mild to severe. Some affected children have exaggerated or prolonged neonatal jaundice, but most are asymptomatic until an acute infection or drug exposure causes oxidative stress.9 Such children typically have a mild G6PD genetic abnormality such as the A– variant found among African Americans. Many triggers have been implicated in causing G6PD hemolysis, including infections, sulfa antibiotics, naphthaline-containing mothballs, and fava beans. Hemolysis is typically intravascular and can be dramatic, with older erythrocytes undergoing hemolysis most easily, due to reduced protection against oxidative stress. Younger erythrocytes are relatively resistant to hemolysis, so the anemia is usually brief and self-limited, although a transfusion may be required acutely.8 Persons of Mediterranean ancestry with G6PD deficiency also have this phenotype, but hemolysis is more frequent and more severe than the A– variant. Rarely, some children have a severe form of G6PD deficiency with chronic hemolytic anemia and exacerbations of acute hemolysis. These patients generally have a mild-to-moderate anemia with reticulocytosis, and are at risk for developing gallstones and splenomegaly.9

Diagnosis and Treatment

The diagnosis of G6PD deficiency is based on clinical suspicion and measuring enzyme activity in erythrocytes, although acute hemolysis can remove the most enzyme-deficient cells and potentially give a “false-negative” result. Neonates who develop jaundice in the first 24 hours of life should be evaluated for G6PD deficiency and other forms of nonphysiologic jaundice. In G6PD-deficient erythrocytes, oxidized and denatured hemoglobin cross-links and precipitates as a Heinz body. In the acute phase, “blister” or “bite” cells may be seen (Fig. 433-3A), but these abnormal erythrocytes are quickly removed from the circulation by splenic filtration. Once the diagnosis of G6PD deficiency is established, education should be provided about avoidance of inciting agents that lead to hemolysis, especially drugs and foods.9 However, the common and clinically mild A– variant does not warrant this level of concern or intervention because normal childhood infections and even sulfa medications are typically well tolerated. Comprehensive newborn screening programs for G6PD deficiency are thus ill advised because thousands of affected children who need no specific counseling or treatment would be identified.

Pyruvate Kinase Deficiency

Pyruvate kinase (PK) deficiency is an autosomal-recessive disorder occurring most commonly in persons of Northern European descent. PK converts phosphoenol-pyruvate within the erythrocyte to pyruvate, resulting in the formation of ATP. Enzyme deficiency is usually moderate to severe and leads to reduced production of ATP, which is needed by the cell for critical functions such as maintenance of membrane stability, osmotic and salt gradients, and nucleotide transport. PK-deficient erythrocytes can undergo intravascular hemolysis, as well as develop abnormal physical characteristics, including unusual forms (Fig. 433-3B) that undergo extravascular clearance by the spleen. The clinical presentation of PK deficiency ranges from severe neonatal anemia to childhood jaundice to splenomegaly and symptomatic gallstones. Interestingly, PK deficiency leads to increased erythrocyte 2,3-DPG (2,3-diphosphoglycerate) levels that help unload oxygen from hemoglobin; thus, fatigue or exercise intolerance is uncommon, despite anemia. Children with PK deficiency almost always have partially compensated anemia with jaundice and splenomegaly. A definitive diagnosis requires quantitation of RBC enzyme activity. Splenectomy may reduce the need for transfusions, but does not lead to the overall improvement and normalization of blood counts observed in children with spherocytosis.10

Genetic abnormalities in hemoglobin are very common and include thalassemia and sickle cell disease. These hemoglobinopathies are discussed in Chapter 434. Occasionally, a mutation in the globin chains leads to an unstable hemoglobin molecule that precipitates within the erythrocyte and leads to accelerated hemolysis (Fig. 433-3C).

Hemolytic anemia in children can also result from disease processes or mechanical forces that arise external to the erythrocyte. These extrinsic RBC defects are typically acquired disorders, although some have a genetic component. Extrinsic RBC defects vary widely and can be most easily classified into immune-mediated and non–immune-mediated etiologies.

Immune-Mediated Hemolytic Anemia

Pathophysiology

Both IgG and IgM antibodies can bind to the RBC surface and lead to hemolysis. IgG-sensitized RBCs are usually eliminated in the extravascular compartment (spleen and liver) by interactions with specific FcR or complement receptors on phagocytes within the reticuloendothelial system (RES). In contrast, IgM-sensitized RBCs often undergo hemolysis in the intravascular compartment via complement, although extravascular clearance also occurs in the liver.11

The related condition, alloimmune hemolytic anemia, occurs when antibodies develop in response to “foreign” erythrocytes. This occurs in hemolytic disease of the newborn (see Chapter 53), where maternal antibodies cross the placenta and destroy fetal erythrocytes, and in chronically transfused children who develop alloantibodies against transfused cells. In both conditions, immune-mediated hemolysis occurs as long as self–nonself interactions continue.

Clinical Features and Diagnosis

Autoimmune hemolytic anemia (AIHA) is, by definition, a condition in which antibodies arise with specificity against surface antigens present on the child’s own erythrocytes. The most common form is warm-reactive AIHA, characterized by IgG autoantibodies that bind optimally at warm (37�C) temperatures. Most cases are idiopathic, although some arise in association with immunodeficiency, lymphoproliferative diseases, or a broad autoimmune process such as systemic lupus erythematosus. Warm-reactive AIHA occurs in all ages and can be clinically severe; infants and teenagers have increased morbidity and occasionally mortality. The IgG antibodies bind circulating common antigens on the erythrocytes (eg, Rh proteins), and these opsonized (antibody-sensitized) cells are then cleared in the extravascular compartment, especially the spleen. Macrophages within the reticuloendothelial system (RES) often remove a portion of the IgG-coated RBC membrane, and the remaining cells reform into spherocytes that can return briefly to the circulation before removal by splenic filtration (Fig. 433-4A). Complement is not fixed to a substantial degree; hence, intravascular hemolysis is not common. The sine qua non of AIHA is a positive direct antiglobulin test (DAT), formerly known as the direct Coombs test. In warm-reactive AIHA, the DAT is positive for IgG and, occasionally, a small amount of C3.

A second form of childhood AIHA is paroxysmal cold hemoglobinuria (PCH), which is usually self-limited and typically follows a virallike illness. The autoantibodies are IgG, but bind best at cold (4�C) temperatures and fix complement avidly; hence, the clinical presentation includes intravascular hemolysis with dramatic hemoglobinuria and severe anemia. The DAT is positive for C3 but usually not for IgG. A third form of childhood AIHA is cold agglutinin disease (CAD), featuring IgM autoantibodies that bind circulating erythrocytes and fix complement well; the DAT is positive for C3, and both intravascular and extravascular hemolysis occurs. In children, CAD is usually self-limiting and develops in association with an infection such as Epstein-Barr virus (EBV) or mycoplasma; cold-induced agglutination of the erythrocytes can occasionally be observed on the peripheral blood smear (Fig. 433-4B).

Treatment

The treatment of AIHA in children depends on the type and the etiology. Warm-reactive AIHA responds well to corticosteroids and to splenectomy in severe or refractory cases. IVIG is not beneficial, although rituximab (anti-CD20 monoclonal antibody) that destroys circulating B-lymphocytes shows promise for this disorder.12 In cold-reactive disorders such as PCH or CAD, corticosteroids are of limited benefit, but treatment of the underlying infection can help. In all forms of AIHA, supportive care is usually sufficient; however, transfusions are occasionally needed for severe anemia. Although cross-matching of blood will not identify compatible units, blood should be administered when anemia is severe and life threatening because even incompatible erythrocytes will survive as well as endogenous cells.13

Paroxysmal Nocturnal Hemoglobinuria

This is a rare acquired clonal stem cell disorder characterized by chronic intravascular hemolysis. Paroxysmal nocturnal hemoglobinuria (PNH) most often develops in adults, although children and adolescents can be affected.14 Although not exclusively a disorder of erythrocytes, PNH features an increased sensitivity to complement-mediated intravascular hemolysis, which then leads to symptomatic hemoglobinuria. Additional clinical features of PNH include thrombosis and bone marrow failure. In some cases, PNH develops in patients with aplastic anemia who are successfully treated with antithymocyte globulin (ATG) and cyclosporin. Eculizumab is a novel C5-inhibitor monoclonal antibody that can reduce intravascular hemolysis in patients with PNH.15 Currently, the only available cure for PNH is bone marrow transplantation.16

Non-Immune-Mediated Hemolytic Anemia

Occasionally, circulating erythrocytes are damaged by external forces or toxins that lead to hemolysis. When abnormal amounts of fibrin or high-molecular-weight von Willebrand factor (vWF) are present within the blood vessel, RBCs can be trapped and even sheared on these strands. Erythrocytes appear as schistocytes (torn cells) on the peripheral blood smear (Fig. 433-4C); platelets are often trapped, so there is concomitant thrombocytopenia. This process is known as microangiopathic hemolytic anemia because it often occurs in the smaller blood vessels.

In children, schistocytic anemia occurs most commonly as hemolytic uremic syndrome (HUS), defined as a triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. HUS usually affects children younger than 5 years; 90% of cases occur after the onset of a diarrheal illness, most often from the Shiga toxin-producing Escherichia coli O157:H7 (see Chapter 472). Children often present with abdominal pain and bloody diarrhea, sometimes with fever. RBC transfusions may be necessary to treat severe anemia, but platelet transfusions are not recommended. Treatment is primarily supportive and focuses on management of fluids, electrolytes, and hypertension. Antimicrobial therapy and antithrombotic agents are not helpful. Usually, the illness is self-limited, although some children will require dialysis during the acute illness, and a small number will subsequently develop chronic renal insufficiency.17

A related disorder is thrombotic thrombocytopenic purpura (TTP), which is a pentad that includes microangiopathic hemolytic anemia, fever, renal dysfunction, thrombocytopenia, and neurologic abnormalities.18 TTP develops when large vWF multimers are released from endothelial cells into the circulation, but the naturally occurring plasma vWF-cleaving protease (ADAMTS13) is not present to cut the large vWF molecules into smaller pieces. The large multimers cause a localized microangiopathy in multiple organs, especially the kidney and brain. TTP can occur in a congenital form with low amounts of vWF-cleaving protease due to genetic mutations in the ADAMTS13 gene; these children develop recurrent and relapsing microangiopathic hemolytic anemia. TTP also can occur as an acquired condition, usually in teenagers or adults who sometimes have a broad autoimmune disorder such as systemic lupus erythematosus. These patients develop a specific autoantibody inhibitor that neutralizes ADAMTS13 in plasma, rendering it functionally deficient. Treatment includes plasmapheresis, to remove the inhibitor and provide fresh plasma containing the vWF-cleaving protease, as well as corticosteroids and other immunomodulatory therapies.

Microangiopathic hemolytic anemia can occur in other clinical settings such as disseminated intravascular coagulation (DIC), pregnancy (preeclampsia, eclampsia, HELLP syndrome), drug exposure (ie, cocaine, cyclosporine, tacrolimus), mechanical heart valves, and malignant hypertension. The brown recluse spider (Loxosceles reclusa) produces a toxin that can lead to a painful local wound and occasionally severe hemolysis (Fig. 433-4D).

Complications of Hemolysis

Chronic hemolysis has substantial pathophysiologic consequences. Extravascular hemolysis leads to splenomegaly, jaundice, and increased risk of pigmented (bilirubin) gallstones. Intravascular hemolysis releases hemoglobin into the plasma; chronically, this can lead to renal damage. Plasma hemoglobin also binds nitric oxide (NO) produced by endothelial cells and monocytes for local smooth muscle relaxation and vasodilatation. Depletion of NO leads to relative vasoconstriction and eventual endothelial vasculopathy, culminating in an overall hypercoagulable state and occasionally pulmonary hypertension.19 The long-term consequences of intravascular hemolysis have only more recently been described, but pulmonary hypertension may ultimately prove to be a cause of substantial morbidity and even mortality among persons with chronic hemolytic anemia.

There are also long-term consequences to splenectomy. The most widely recognized complication is an increased risk of overwhelming sepsis, particularly from encapsulated bacteria. The development of a hypercoagulable state is less well known, but is increasingly recognized as a postsplenectomy complication. Most patients will have mild asymptomatic thrombocytosis after splenectomy. This elevation in platelet count and platelet activation may lead to thromboembolic complications with significant morbidity and mortality. A more recent study found that splenectomy greatly increased the likelihood of developing chronic thromboembolic pulmonary hypertension.5 Taken together, the available data suggest that patients who undergo splenectomy for hemolytic anemia during childhood may have late effects as adults. Additional research is needed to help clinicians determine the risks and benefits of splenectomy for individual patients with hemolytic anemia.