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Hemolytic Anemias

Chapter 105 | Part 4: Oncology and Hematology · Part 4 – Oncology: Hematologic Malignancies

Detailed clinical reference synthesised from Harrison's Principles of Internal Medicine, 22nd Edition

🔑 Key Clinical Points

  1. Reticulocyte count is the definitive parameter for hemolysis; percentage and absolute count both increase.
  2. Hemolytic anemias (HAs) are classified as inherited or acquired, intracorpuscular or extracorpuscular.
  3. Hereditary spherocytosis (HS) prevalence is 1:2000–5000 in European ancestry; splenectomy is indicated for severe cases at age 4–6 years.
  4. Glucose-6-phosphate dehydrogenase (G6PD) deficiency is X-linked; heterozygous females show variable expression due to X-chromosome inactivation mosaicism.
  5. Pyruvate kinase (PK) deficiency is autosomal recessive; mitapivat is an allosteric activator of PK.
  6. Chronic extravascular hemolysis leads to iron overload (secondary hemochromatosis) and organ damage (liver, heart).
  7. Osmotic fragility testing is the main diagnostic test for HS; EMA-binding test is also used.
  8. Splenectomy is contraindicated in stomatocytosis due to severe thromboembolic complications.
  9. Parvovirus B19 infection can cause aplastic crisis in patients with chronic hemolysis.
  10. Compensated hemolysis may present without anemia; decompensation occurs in pregnancy, folate deficiency, or renal failure.

📑 Table of Contents

📋 Figures in This Chapter

# Type Description
1 🔀 Flowchart Red blood cell (RBC) metabolism
1 🖼 Figure Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary stomatocytosis (HSt) are three...
2 🖼 Figure The red cell membrane and cytoskeleton schematic diagram
3 🖼 Figure The role of glucose-6-phosphate dehydrogenase (G6PD) in 6-phosphogluconate dehydrogenase—two of the enzymes...
4 🖼 Figure Basic mechanisms involved in warm antibody– and cold opsonized red cells are...
5 🖼 Figure Impact and implications of anticomplement therapy in paroxysmal composition in examples of...
6 🖼 Figure The complement cascade and the fate of red cells
7 🖼 Figure Different phenotypes of heterozygotes for red cell enzymopathies
8 🖼 Figure Peripheral blood smear from patients with membrane-cytoskeleton abnormalities
9 🖼 Figure Peripheral blood smear from patients with membrane-cytoskeleton abnormalities
10 🖼 Figure Peripheral blood smear from patients with membrane-cytoskeleton abnormalities
11 🖼 Figure Hemolytic Anemias FIGURE 105-7 Epidemiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency throughout the...

1. DEFINITION & OVERVIEW

📖 Harrison's defines this as:

'Hemolytic anemias (HAs) are anemias resulting from increased destruction of red cells.'

  • Hemolytic anemias (HAs) are defined as anemias resulting from increased destruction of red cells.
  • All patients who are anemic as a result of either increased destruction of red cells or acute blood loss have one important element in common: the anemia results from overconsumption of red cells from the peripheral blood, whereas the supply of cells from the bone marrow is normal (indeed, it is usually increased).
  • However, with blood loss, as in acute hemorrhage, the red cells are physically lost from the body itself; this is fundamentally different from destruction of red cells within the body, as in hemolytic anemias (HAs).
  • With respect to primary etiology, HAs may be inherited or acquired; from a clinical point of view, they may be more acute or more chronic, and they may vary from mild to very severe; the site of hemolysis may be predominantly intravascular or extravascular.
  • With respect to mechanisms, HAs may be due to intracorpuscular causes or to extracorpuscular causes.

Table 1 — Table 105-1 Classification of Hemolytic Anemias

Classification Intracorpuscular Defects Extracorpuscular Factors
Inherited Hemoglobinopathies Paroxysmal nocturnal hemoglobinuria (PNH) Enzymopathies Familial (atypical) hemolytic-uremic syndrome Membrane-cytoskeletal defects Mechanical destruction Acquired Toxic agents Microangiopathic Drugs Infectious Autoimmune

1.1 General Clinical and Laboratory Features

  • The clinical presentation of a patient with anemia is greatly influenced in the first place by whether the onset is abrupt or gradual.
  • A patient with autoimmune HA or with favism may be a medical emergency, whereas a patient with mild hereditary spherocytosis (HS) or with cold agglutinin disease (CAD) may be diagnosed after years.
  • What differentiates HAs from other anemias is that the patient has signs and symptoms arising directly from hemolysis (Table 105-2).
  • At the clinical level, the main sign is jaundice; in addition, the patient may report discoloration of the urine.
  • In many cases of HA, the spleen is enlarged because it is a preferential site of hemolysis; in some cases, the liver may be enlarged as well; and gallstones are common.
  • In all severe congenital forms of HA, there may also be skeletal changes due to overactivity of the bone marrow: they are never as severe as in thalassemia.
  • The laboratory features of HA are related to (1) hemolysis per se and (2) the erythropoietic response of the bone marrow.
  • In most cases, hemolysis is largely extravascular, and it produces an increase in unconjugated bilirubin and aspartate aminotransferase (AST) in the serum; urobilinogen will be increased in both urine and stool.
  • If hemolysis is mainly intravascular, the telltale sign is hemoglobinuria (often associated with hemosiderinuria); in the serum, there is free hemoglobin, lactate dehydrogenase (LDH) is increased, and haptoglobin is reduced.
  • In contrast, the serum bilirubin level may be normal or only mildly elevated.
  • The main sign of the erythropoietic response by the bone marrow is an increase in reticulocytes (a test all too often neglected in the initial workup of a patient with anemia).
  • Usually the increase will be reflected in both the percentage of reticulocytes (the more commonly quoted figure) and in the absolute reticulocyte count (the more definitive parameter).
  • The increased number of reticulocytes is associated with an increased mean corpuscular volume (MCV) in the blood count.
  • On the blood smear, this is reflected in the presence of macrocytes; polychromasia is also present, and sometimes one sees nucleated red cells.
  • In most cases, a bone marrow aspirate is not necessary in the diagnostic workup; if it is done, it will show erythroid hyperplasia.

Table 2 — Table 105-2 Features Common to Most Patients with a Hemolytic Disorder

Feature General Examination Other Physical Findings Hemoglobin Level MCV, MCH Reticulocytes Bilirubin LDH Haptoglobin
Jaundice, Pallor Spleen may be enlarged; bossing of skull in severe congenital cases From normal to severely reduced Usually increased Usually increased Almost always increased (mostly unconjugated) Increased (up to 10× normal with intravascular hemolysis) Reduced to absent if hemolysis is at least in part intravascular

2. EPIDEMIOLOGY

  • Hereditary spherocytosis (HS) is the most common among this group of HAs, with an estimated prevalence of 1:2000–1:5000 in populations of European ancestry.
  • The global incidence of hereditary elliptocytosis (HE) is 1:2000–4000 subjects.
  • Pyruvate kinase (PK) deficiency has an estimated prevalence in most populations of 1:10,000.
  • G6PD deficiency is common in certain populations (e.g., malaria-endemic regions).
  • Southeast Asia ovalocytosis (SAO) has a frequency of up to 5–7% in certain populations (e.g., Papua New Guinea, Indonesia, Malaysia, Philippines).

3. ETIOLOGY & PATHOPHYSIOLOGY

  • The essential pathophysiologic process common to all HAs is an increased red cell turnover; in many HAs, this is due at least in part to an acceleration of the senescence process described above.
  • The mature red cell is the product of a developmental pathway that brings the phenomenon of differentiation to an extreme.
  • An orderly sequence of events produces synchronous changes, whereby the cytoplasmic body, instead of disintegrating, is now able to provide oxygen to all cells in the human organism for some remaining 120 days of the red cell life span.
  • As a result of this unique process of differentiation and maturation, intermediary metabolism is drastically curtailed in mature red cells.
  • Cytochrome-mediated oxidative phosphorylation has been lost with the loss of mitochondria (through a process of physiologic autophagy); therefore, there is no backup to anaerobic glycolysis, which in the red cell is the only provider of adenosine triphosphate (ATP).
  • Also, the capacity of making protein has been lost with the loss of ribosomes.
  • This places the cell’s limited metabolic apparatus at risk, because if any protein component deteriorates, it cannot be replaced, as it would be in most other cells; and in fact, the activity of most enzymes gradually decreases as red cells age.
  • During their long time in circulation, various red cell components inevitably accumulate damage and become physically denser.
  • The anion exchanger known as band 3 is the most abundant protein in the red cell membrane, with about 1.2 million molecules per red cell.
  • As red cells age and become denser, probability is increased that a region of the band 3 molecule becomes exposed on the cell surface and contributes to creating an antigenic site recognizable by low-avidity naturally occurring anti–band 3 IgG antibodies.
  • Senescent red cells thus become opsonized, and this is the signal for phagocytosis by macrophages in the spleen, in the liver, and elsewhere.
  • This process may become accelerated in various ways in HA.
  • The red cell has three essential components: (1) hemoglobin, (2) the membrane-cytoskeleton complex, and (3) the metabolic machinery necessary to keep hemoglobin and the membrane-cytoskeleton complex in working order.
  • Diseases caused by inherited abnormalities of hemoglobin, or hemoglobinopathies, are covered in Chap. 103. Here we will deal with diseases of the other two components.
  • The membrane-cytoskeleton complex has essentially three functions: it is an envelope for the red cell cytoplasm; it maintains the normal red cell shape; and it provides cross-membrane transport of electrolytes and of metabolites such as glucose and amino acids.
  • In the membrane-cytoskeleton complex, the individual components are so intimately associated with each other that an abnormality of almost any of them will be disturbing or disruptive, causing mechanical instability of the membrane and/or reduced red cell deformability, ultimately causing hemolysis.
  • These abnormalities are almost invariably inherited mutations; thus, diseases of the membrane-cytoskeleton complex belong to the category of inherited HAs.
  • Before the red cells lyse, they often exhibit more or less specific changes that alter the normal biconcave disk shape.
  • Thus, the majority of the diseases in this group have been known for over a century as hereditary spherocytosis (HS) and hereditary elliptocytosis (HE).
  • More recently, a third morphologic entity, whereby on a blood smear the round-shaped central pallor of a red cell is replaced by a linear-shaped central pale area, has earned the name stomatocytosis; because this abnormal shape is related to abnormalities of channel molecules, the underlying disorders are also referred to as channelopathies.
  • The ankyrin complex provides mainly radial (also called vertical) connections; the junctional complex provides mainly tangential (also called horizontal) connections.
  • Pathogenic changes in the former can cause spherocytosis, whereas pathogenic changes in the latter can cause elliptocytosis; pathogenic changes in spectrin can cause either.
  • The variability in clinical manifestations that is observed among patients with HS is largely due to the different underlying molecular lesions.
  • Not only are mutations of several genes involved, but also different mutations of the same gene can give very different clinical manifestations.
  • In milder cases, hemolysis is often compensated (see above), but changes in clinical expression may be seen even in the same patient because intercurrent conditions (e.g., pregnancy, infection) may cause decompensation.
  • The anemia is usually normocytic with the characteristic morphology that gives the disease its name.
  • An increased mean corpuscular hemoglobin concentration (MCHC >34 g/dL) and increased red cell distribution width (RDW >14%) associated with normal or slightly decreased MCV on an ordinary blood count report should raise the suspicion of HS.
  • The spleen plays a key role in HS through a dual mechanism. On one hand, because HS red cells are less deformable, transit through the splenic circulation makes them more prone to vesiculate; on the other hand, like in many other HAs, the spleen itself is a major site of destruction through phagocytosis by macrophages.
  • When there is a family history, it is usually easy to make a diagnosis based on features of HA and typical red cell morphology.
  • However, family history may be negative for at least two reasons. First, the patient may have a de novo mutation, i.e., a mutation that has taken place in a germ cell of one of the patient’s parents or early after zygote formation. Second, the patient may have a recessive form of HS.
  • In such cases, more extensive laboratory investigations are required, including osmotic fragility, the acid glycerol lysis test, the eosin-5′-maleimide (EMA)–binding test, sodium dodecyl sulfate (SDS)-gel electrophoresis of membrane proteins, and ektacytometry (testing red cell deformability as a function of shear stress at different osmolality).
  • Sometimes a definitive diagnosis can be obtained only by molecular studies demonstrating a mutation in one of the genes underlying HS.
  • Enzyme Abnormalities: When an important defect in a component of the membrane-cytoskeleton complex is present, hemolysis is a direct consequence of the fact that the very structure of the red cell is compromised.
  • Instead, when one of the enzymes is defective, the consequences will depend on the precise role of that enzyme in the metabolic machinery of the red cell.
  • This machinery has two main functions: (1) to provide energy in the form of ATP, and (2) to prevent oxidative damage to hemoglobin and to other proteins by providing sufficient reductive potential; the key molecule for this is NADPH, required for regeneration of glutathione (GSH) and for degradation of HO.
  • Because red cells, in the course of their differentiation, have sacrificed not only their nucleus and their ribosomes but also their mitochondria, they rely exclusively on the anaerobic portion of the glycolytic pathway for producing ATP, most of which is required by the red cell for cation transport against a concentration gradient across the membrane.
  • If this fails due to a defect of any of the enzymes of the glycolytic pathway, the result will be hemolytic disease.
  • Abnormalities of the glycolytic pathway are all inherited and all rare.
  • Among them, deficiency of pyruvate kinase (PK) is the least rare, with an estimated prevalence in most populations of 1:10,000.
  • However, recently, a polymorphic PK mutation (E277K) was found in some African populations with heterozygote frequencies of 1–7%, suggesting that this may be another malaria-related polymorphism.
  • HA secondary to PK deficiency is an autosomal recessive disease.
  • The clinical picture of homozygous (or biallelic) PK deficiency is that of an HA that often presents in the newborn with severe neonatal jaundice, requiring nearly always phototherapy and frequently exchange transfusion; the jaundice often persists, and it is often associated with reticulocytosis.
  • The anemia is of variable severity; sometimes it is so severe as to require regular blood transfusion treatment, whereas sometimes it is mild, bordering on a nearly compensated hemolytic disorder.
  • As a result, the diagnosis may be delayed: in some cases, it is made, for instance, in a young woman during her first pregnancy, when the anemia may get worse.
  • The delay in diagnosis may be caused in part by the fact that the anemia is often remarkably well tolerated because the metabolic block at the last step in glycolysis causes an increase in 2,3-bisphosphoglycerate (or DPG; Fig. 105-1), a major effector of the hemoglobin-oxygen dissociation curve; thus, for a certain level of hemoglobin, the oxygen delivery to the tissues is enhanced, a remarkable compensatory feat.
  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.
  • G6PD deficiency–related HA is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.
  • Iron Handling: In normal subjects, the iron from effete red cells is very efficiently recycled by the body; however, with chronic intravascular hemolysis, the persistent hemoglobinuria will cause considerable iron loss, needing replacement.
  • With chronic extravascular hemolysis, the opposite problem, iron overload, is more common, especially if the patient needs frequent blood transfusions.
  • Even without blood transfusion, when erythropoiesis is massively increased, the release of erythroferrone from erythroid cells suppresses hepcidin, causing increased iron absorption.
  • In the long run, in the absence of iron-chelation therapy, iron overload will cause secondary hemochromatosis; this will cause damage particularly to the liver, eventually leading to cirrhosis, and to the heart muscle, eventually causing heart failure.

3.1 Inherited Hemolytic Anemias

  • The red cell has three essential components: (1) hemoglobin, (2) the membrane-cytoskeleton complex, and (3) the metabolic machinery necessary to keep hemoglobin and the membrane-cytoskeleton complex in working order.
  • Diseases caused by inherited abnormalities of hemoglobin, or hemoglobinopathies, are covered in Chap. 103. Here we will deal with diseases of the other two components.
  • Diseases of the membrane-cytoskeleton complex belong to the category of inherited HAs.
  • Diseases of the metabolic machinery are enzyme abnormalities.

Table 3 — Table 105-3 Inherited Diseases of the Red Cell Membrane-Cytoskeleton Complex

Gene Chromosomal Location Protein Produced Disease(s) with Certain Mutations (Inheritance) Comments
SPTA1 1q22-q23 α-Spectrin HS (recessive) Rare HE (dominant) Mutations of this gene account for about 65% of HE. More severe forms may be due to coexistence of an otherwise silent mutant allele.
SPTB 14q23-q24.1 β-Spectrin HS (dominant) Rare HE (dominant) Mutations of this gene account for about 30% of HE, including some severe forms.
ANK1 8p11.2 Ankyrin HS (dominant) May account for majority of HS.
SLC4A1 17q21 Band 3; also known as AE (anion exchanger) or AE1 HS (dominant) Mutations of this gene may account for about 25% of HS. Southeast Asia ovalocytosis (dominant) Polymorphic mutation (deletion of nine amino acids); in heterozygotes, clinically asymptomatic and protective against Plasmodium falciparum. Stomatocytosis (cryohydrocytosis) Certain specific missense mutations shift protein function from anion exchanger to cation conductance.
EPB41 1p33-p34.2 Band 4.1 HE (dominant) Mutations of this gene account for about 5% of HE, mostly with prominent morphology but little/no hemolysis in heterozygotes; severe hemolysis in homozygotes.
EPB42 15q15-q21 Band 4.2 HS (recessive) Mutations of this gene account for about 3% of HS.
RHAG 6p21.1-p11 Rhesus-associated glycoprotein Chronic nonspherocytic hemolytic anemia (recessive) Very rare; associated with total loss of all Rh antigens. One specific mutation in this gene entails loss of stomatin from the cell membrane, causing overhydrated stomatocytosis.
PIEZO1 16q23-q24 PIEZO1 (mechanosensitive ion channel component 1 channel) Dehydrated hereditary stomatocytosis (dominant) Also known as xerocytosis with pseudohyperkalemia. Patients may present with perinatal edema.
KCNN4 19q13.31 KCNN4 Intermediate conductance calcium-activated potassium channel protein 4 (Gardos channel) Dehydrated hereditary stomatocytosis (dominant) Clinical presentation similar to that of PIEZO1 mutants.
ABCB6 2q35-q36 ATP-binding cassette subfamily B member 6 Familial pseudohyperkalemia (dominant) Increased potassium leakage upon storage in blood bank condition: this can cause hyperkalemia in the recipient. ABCB6 mutation is present in 0.3% of blood donors.
SLC2A1 1p34.2 GLUT1 glucose transporter Overhydrated hereditary stomatocytosis Associated with serious neurologic manifestations.

3.2 Enzyme Abnormalities

  • When an important defect in a component of the membrane-cytoskeleton complex is present, hemolysis is a direct consequence of the fact that the very structure of the red cell is compromised.
  • Instead, when one of the enzymes is defective, the consequences will depend on the precise role of that enzyme in the metabolic machinery of the red cell.
  • This machinery has two main functions: (1) to provide energy in the form of ATP, and (2) to prevent oxidative damage to hemoglobin and to other proteins by providing sufficient reductive potential; the key molecule for this is NADPH, required for regeneration of glutathione (GSH) and for degradation of HO.
  • Because red cells, in the course of their differentiation, have sacrificed not only their nucleus and their ribosomes but also their mitochondria, they rely exclusively on the anaerobic portion of the glycolytic pathway for producing ATP, most of which is required by the red cell for cation transport against a concentration gradient across the membrane.
  • If this fails due to a defect of any of the enzymes of the glycolytic pathway, the result will be hemolytic disease.
  • Abnormalities of the glycolytic pathway are all inherited and all rare.
  • Among them, deficiency of pyruvate kinase (PK) is the least rare, with an estimated prevalence in most populations of 1:10,000.
  • However, recently, a polymorphic PK mutation (E277K) was found in some African populations with heterozygote frequencies of 1–7%, suggesting that this may be another malaria-related polymorphism.
  • HA secondary to PK deficiency is an autosomal recessive disease.
  • G6PD deficiency is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.

Table 4 — Table 105-4 Red Cell Enzyme Abnormalities Causing Hemolysis

Enzyme (Acronym) Gene Symbol; Chromosomal Location Prevalence of Enzyme Deficiency (Rank) Clinical Manifestations Extra-Red Cell Comments
Hexokinase (HK) HK1; 10q22 Very rare May benefit from splenectomy; BMT
Glucose-6-phosphate isomerase (G6PI) GPI; 19q31.1 Rare (4); at least 60 cases reported NM, CNS May benefit from splenectomy
Phosphofructokinase (PFK) PFKM; 12q13 Very rare Myopathy; myoglobinuria
Aldolase ALDOA; 16q22-24 Very rare Myopathy
Triose phosphate isomerase (TPI) TPI1; 12p13.31 Very rare CNS (severe), NM
Glyceraldehyde 3-phosphate dehydrogenase (GAPD) GAPDH; 12p13.31 Very rare Myopathy
Bisphosphoglycerate mutase (BPGM) BPGM; 7q33 Very rare Erythrocytosis rather than hemolysis; some of the rare mutations are in the enzyme active site
Phosphoglycerate kinase (PGK) PGK1; Xq21.1 Very rare CNS, NM May benefit from splenectomy; BMT
Pyruvate kinase (PK) PKLR; 1q22 Rare (2) May benefit from splenectomy; BMT
Glucose-6-phosphate dehydrogenase (G6PD) G6PD; Xq28 Common (1) In almost all cases, only AHA from exogenous trigger
Glutathione synthase (GSS) GSS; 20q11.22 Very rare CNS
Glutathione reductase (GSR) GSR; 8p12 Very rare Cataracts AHA from exogenous trigger (favism)
γ-Glutamylcysteine synthase (GCLC) GCLC; 6p12.1 Very rare CNS Mutations affect catalytic subunit
Cytochrome b5 reductase (CYB5R3) CYB5R3; 22q13.2 Rare CNS Methemoglobinemia rather than hemolysis
Adenylate kinase (AK) AK1; 9q34.11 Very rare CNS May benefit from splenectomy
Pyrimidine 5’ nucleotidase (P5N) NTSC3A; 7p14.3 Rare (3) May benefit from splenectomy

4. CLINICAL FEATURES

  • The clinical presentation of a patient with anemia is greatly influenced in the first place by whether the onset is abrupt or gradual, and HAs are no exception.
  • A patient with autoimmune HA or with favism may be a medical emergency, whereas a patient with mild hereditary spherocytosis (HS) or with cold agglutinin disease (CAD) may be diagnosed after years.
  • This is due in large measure to the remarkable ability of the body to adapt to anemia when it is slowly progressing.
  • What differentiates HAs from other anemias is that the patient has signs and symptoms arising directly from hemolysis.
  • At the clinical level, the main sign is jaundice; in addition, the patient may report discoloration of the urine.
  • In many cases of HA, the spleen is enlarged because it is a preferential site of hemolysis; in some cases, the liver may be enlarged as well; and gallstones are common.
  • In all severe congenital forms of HA, there may also be skeletal changes due to overactivity of the bone marrow: they are never as severe as in thalassemia.
  • The anemia is usually normocytic with the characteristic morphology that gives the disease its name.
  • An increased mean corpuscular hemoglobin concentration (MCHC >34 g/dL) and increased red cell distribution width (RDW >14%) associated with normal or slightly decreased MCV on an ordinary blood count report should raise the suspicion of HS.
  • The spleen plays a key role in HS through a dual mechanism. On one hand, because HS red cells are less deformable, transit through the splenic circulation makes them more prone to vesiculate; on the other hand, like in many other HAs, the spleen itself is a major site of destruction through phagocytosis by macrophages.
  • When there is a family history, it is usually easy to make a diagnosis based on features of HA and typical red cell morphology.
  • However, family history may be negative for at least two reasons. First, the patient may have a de novo mutation, i.e., a mutation that has taken place in a germ cell of one of the patient’s parents or early after zygote formation. Second, the patient may have a recessive form of HS.
  • In such cases, more extensive laboratory investigations are required, including osmotic fragility, the acid glycerol lysis test, the eosin-5′-maleimide (EMA)–binding test, sodium dodecyl sulfate (SDS)-gel electrophoresis of membrane proteins, and ektacytometry (testing red cell deformability as a function of shear stress at different osmolality).
  • Sometimes a definitive diagnosis can be obtained only by molecular studies demonstrating a mutation in one of the genes underlying HS.
  • The clinical picture of homozygous (or biallelic) PK deficiency is that of an HA that often presents in the newborn with severe neonatal jaundice, requiring nearly always phototherapy and frequently exchange transfusion; the jaundice often persists, and it is often associated with reticulocytosis.
  • The anemia is of variable severity; sometimes it is so severe as to require regular blood transfusion treatment, whereas sometimes it is mild, bordering on a nearly compensated hemolytic disorder.
  • As a result, the diagnosis may be delayed: in some cases, it is made, for instance, in a young woman during her first pregnancy, when the anemia may get worse.
  • The delay in diagnosis may be caused in part by the fact that the anemia is often remarkably well tolerated because the metabolic block at the last step in glycolysis causes an increase in 2,3-bisphosphoglycerate (or DPG; Fig. 105-1), a major effector of the hemoglobin-oxygen dissociation curve; thus, for a certain level of hemoglobin, the oxygen delivery to the tissues is enhanced, a remarkable compensatory feat.
  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.
  • G6PD deficiency–related HA is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.
  • Channelopathies are rare conditions characterized by abnormalities in red cell ion content and alteration of erythrocyte volume.
  • Cation leak can cause hyperkalemia; in some cases, this leak is accelerated in the cold (the resulting spuriously high serum K+ is then referred to as pseudo-hyperkalemia).
  • The less rare form, dehydrated stomatocytosis (DHS; also referred to as xerocytosis), is a (usually compensated) macrocytic hemolytic disorder, with increased MCHC (generally >36 g/dL) associated with mild jaundice.
  • Mutations in either PIEZO1, encoding an ion channel activated by pressure (mechanoreceptor), or in KCCN4, encoding the Ca2+ activated K+ channel (Gardos channel) have been recognized to cause DHS.
  • Another form is overhydrated stomatocytosis (OHS). OHS is also macrocytic (MCV >110 fL), but the MCHC is low (<30 g/dL).
  • The underlying mutation is in the Rhesus gene RHAG, which encodes an ammonia channel.
  • Yet other patients with stomatocytosis have mutations in SLC4A1 (encoding band 3) and SLC2A1 (encoding the glucose transporter GLUT1).
  • Mutations of the latter are responsible for cryohydrocytosis, a channelopathy in which the red cells swell and burst when they are cooled.
  • In vivo hemolysis can vary from relatively mild to quite severe.
  • Familial hyperkalemia has been recently linked to mutations in ABCB6, resulting in abnormal cation leak with extracellular release of a large amount of K+ (hyperkalemia).
  • Mutations in ABCB6 have been identified in almost 0.3% of blood donors.
  • However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in splenectomized DHS patients.

4.1 Compensated Hemolysis versus Hemolytic Anemia

  • Red cell destruction is a potent stimulus for erythropoiesis, which is mediated by erythropoietin (EPO) produced by the kidney.
  • This mechanism is so effective that in many cases the increased output of red cells from the bone marrow can fully balance an increased destruction of red cells.
  • In such cases, we say that hemolysis is compensated.
  • The pathophysiology of compensated hemolysis is similar to what we have just described, except there is no anemia.
  • This notion is important from the diagnostic point of view, because a patient with a hemolytic condition, even an inherited one, may present without anemia.
  • It is also important from the point of view of management because compensated hemolysis may become decompensated, i.e., anemia may suddenly appear in certain circumstances, for instance in pregnancy, folate deficiency, or renal failure interfering with adequate EPO production.
  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.

5. DIFFERENTIAL DIAGNOSIS

  • Other anemias must be distinguished from hemolytic anemias.
  • The key differentiating feature is the reticulocyte count.
  • In other anemias (e.g., iron deficiency, thalassemia), the reticulocyte count is low or normal.
  • In hemolytic anemias, the reticulocyte count is increased.
  • Other causes of jaundice (e.g., liver disease, Gilbert syndrome) must be distinguished from hemolysis.
  • In liver disease, bilirubin is often conjugated; in hemolysis, bilirubin is mostly unconjugated.
  • In Gilbert syndrome, bilirubin is unconjugated, but reticulocyte count is normal.
  • Acute blood loss must be distinguished from hemolysis.
  • In acute blood loss, the red cells are physically lost from the body itself; this is fundamentally different from destruction of red cells within the body, as in hemolytic anemias.
  • In acute blood loss, the reticulocyte count may be increased due to marrow response, but the hemoglobin level drops rapidly.
  • In hemolysis, the hemoglobin level may be stable or drop slowly, depending on the rate of destruction vs production.

6. INVESTIGATIONS & DIAGNOSIS

  • Once an HA is suspected, specific tests will usually be required for a definitive diagnosis of a specific type of HA.
  • In most cases, a bone marrow aspirate is not necessary in the diagnostic workup; if it is done, it will show erythroid hyperplasia.
  • When there is a family history, it is usually easy to make a diagnosis based on features of HA and typical red cell morphology.
  • However, family history may be negative for at least two reasons. First, the patient may have a de novo mutation, i.e., a mutation that has taken place in a germ cell of one of the patient’s parents or early after zygote formation. Second, the patient may have a recessive form of HS.
  • In such cases, more extensive laboratory investigations are required, including osmotic fragility, the acid glycerol lysis test, the eosin-5′-maleimide (EMA)–binding test, sodium dodecyl sulfate (SDS)-gel electrophoresis of membrane proteins, and ektacytometry (testing red cell deformability as a function of shear stress at different osmolality).
  • Sometimes a definitive diagnosis can be obtained only by molecular studies demonstrating a mutation in one of the genes underlying HS.
  • Laser diffraction analysis, or ektacytometry, can measure the deformability of red blood cells subjected to either increasing shear stress or to an osmotic stress.
  • This technique has been used extensively to investigate membrane-cytoskeleton abnormalities, and it can differentiate stomatocytosis from spherocytosis.
  • Osmotic fragility testing is the main diagnostic test for HS.
  • The increase in osmotic fragility became the main diagnostic test for HS.
  • Today we know that HS, thus defined, is genetically heterogeneous; i.e., it can arise from a variety of mutations in one of several genes.
  • It has been also recognized that the inheritance of HS is not always autosomal dominant (with the patient being heterozygous); indeed, some of the most severe forms are instead autosomal recessive (with the patient being homozygous).
  • The variability in clinical manifestations that is observed among patients with HS is largely due to the different underlying molecular lesions.
  • Not only are mutations of several genes involved, but also different mutations of the same gene can give very different clinical manifestations.
  • In milder cases, hemolysis is often compensated (see above), but changes in clinical expression may be seen even in the same patient because intercurrent conditions (e.g., pregnancy, infection) may cause decompensation.
  • The anemia is usually normocytic with the characteristic morphology that gives the disease its name.
  • An increased mean corpuscular hemoglobin concentration (MCHC >34 g/dL) and increased red cell distribution width (RDW >14%) associated with normal or slightly decreased MCV on an ordinary blood count report should raise the suspicion of HS.
  • The spleen plays a key role in HS through a dual mechanism. On one hand, because HS red cells are less deformable, transit through the splenic circulation makes them more prone to vesiculate; on the other hand, like in many other HAs, the spleen itself is a major site of destruction through phagocytosis by macrophages.
  • When there is a family history, it is usually easy to make a diagnosis based on features of HA and typical red cell morphology.
  • However, family history may be negative for at least two reasons. First, the patient may have a de novo mutation, i.e., a mutation that has taken place in a germ cell of one of the patient’s parents or early after zygote formation. Second, the patient may have a recessive form of HS.
  • In such cases, more extensive laboratory investigations are required, including osmotic fragility, the acid glycerol lysis test, the eosin-5′-maleimide (EMA)–binding test, sodium dodecyl sulfate (SDS)-gel electrophoresis of membrane proteins, and ektacytometry (testing red cell deformability as a function of shear stress at different osmolality).
  • Sometimes a definitive diagnosis can be obtained only by molecular studies demonstrating a mutation in one of the genes underlying HS.
  • Laser diffraction analysis, or ektacytometry, can measure the deformability of red blood cells subjected to either increasing shear stress or to an osmotic stress.
  • This technique has been used extensively to investigate membrane-cytoskeleton abnormalities, and it can differentiate stomatocytosis from spherocytosis.
  • G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.

Table 5 — Table 105-2 Features Common to Most Patients with a Hemolytic Disorder

Feature General Examination Other Physical Findings Hemoglobin Level MCV, MCH Reticulocytes Bilirubin LDH Haptoglobin
Jaundice, Pallor Spleen may be enlarged; bossing of skull in severe congenital cases From normal to severely reduced Usually increased Usually increased Almost always increased (mostly unconjugated) Increased (up to 10× normal with intravascular hemolysis) Reduced to absent if hemolysis is at least in part intravascular

6.1 Diagnostic Algorithms

  • Step 1: Suspect HA based on clinical features (jaundice, splenomegaly, pallor) and lab features (reticulocytosis, elevated bilirubin, elevated LDH, reduced haptoglobin).
  • Step 2: Perform peripheral blood smear to assess red cell morphology (spherocytes, elliptocytes, stomatocytes).
  • Step 3: If HS is suspected, perform osmotic fragility test or EMA-binding test.
  • Step 4: If HE is suspected, perform ektacytometry.
  • Step 5: If enzyme deficiency is suspected, perform specific enzyme assays (G6PD, PK).
  • Step 6: If molecular diagnosis is needed, perform genetic testing for relevant genes (ANK1, SPTA1, SPTB, SLC4A1, etc.).
  • Step 7: If stomatocytosis is suspected, assess for channelopathies (PIEZO1, KCNN4, RHAG, SLC2A1, ABCB6).
  • Step 8: If iron overload is present, consider chelation therapy.
  • Step 9: If splenectomy is indicated, vaccinate against encapsulated bacteria (Neisseria meningitidis and Streptococcus pneumoniae) before procedure.
  • Step 10: Monitor for complications (gallstones, aplastic crisis, iron overload).

7. MANAGEMENT & TREATMENT

  • We do not have a causal treatment for HS; i.e., no way has yet been found to correct the basic defect in the membrane-cytoskeleton structure.
  • Given the special role of the spleen in HS (see above), splenectomy is often beneficial.
  • Current recommendations are to proceed with splenectomy at the age of 4–6 years in severe cases, to delay splenectomy until puberty in moderate cases, and to avoid splenectomy in mild cases.
  • Partial splenectomy can be considered in certain cases, and it is helpful to know about the outcome of splenectomy in the patient’s affected relatives.
  • Before splenectomy, vaccination against encapsulated bacteria (Neisseria meningitidis and Streptococcus pneumoniae) is imperative; penicillin prophylaxis after splenectomy is controversial.
  • Along with splenectomy, cholecystectomy should not be carried out automatically, but it should be carried out, usually by the laparoscopic approach, whenever it is clinically indicated, mainly when gallstones are symptomatic.
  • The most severe cases of HS (estimated at 36 g/dL) associated with mild jaundice.
  • Mutations in either PIEZO1, encoding an ion channel activated by pressure (mechanoreceptor), or in KCCN4, encoding the Ca2+ activated K+ channel (Gardos channel) have been recognized to cause DHS.
  • Another form is overhydrated stomatocytosis (OHS). OHS is also macrocytic (MCV >110 fL), but the MCHC is low (<30 g/dL).
  • The underlying mutation is in the Rhesus gene RHAG, which encodes an ammonia channel.
  • Yet other patients with stomatocytosis have mutations in SLC4A1 (encoding band 3) and SLC2A1 (encoding the glucose transporter GLUT1).
  • Mutations of the latter are responsible for cryohydrocytosis, a channelopathy in which the red cells swell and burst when they are cooled.
  • In vivo hemolysis can vary from relatively mild to quite severe.
  • Familial hyperkalemia has been recently linked to mutations in ABCB6, resulting in abnormal cation leak with extracellular release of a large amount of K+ (hyperkalemia).
  • Mutations in ABCB6 have been identified in almost 0.3% of blood donors.
  • However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in splenectomized DHS patients.
  • Until recently, the management of PK deficiency has been supportive.
  • Folic acid supplements should be given constantly.
  • Blood transfusion should be used as necessary, and iron chelation may be required even in some patients who, though not receiving blood transfusion, may be developing iron overload (see General Pathophysiology above).
  • About one-half of patients sooner or later undergo splenectomy, which usually provides a modest but significant increase in hemoglobin (paradoxically, reticulocytes also often increase, because they were previously trapped in the spleen).
  • Cholecystectomy may also be required.
  • A major advance has been the introduction of mitapivat, an allosteric activator of PK.
  • Enzyme Abnormalities: When an important defect in a component of the membrane-cytoskeleton complex is present, hemolysis is a direct consequence of the fact that the very structure of the red cell is compromised.
  • Instead, when one of the enzymes is defective, the consequences will depend on the precise role of that enzyme in the metabolic machinery of the red cell.
  • This machinery has two main functions: (1) to provide energy in the form of ATP, and (2) to prevent oxidative damage to hemoglobin and to other proteins by providing sufficient reductive potential; the key molecule for this is NADPH, required for regeneration of glutathione (GSH) and for degradation of HO.
  • Because red cells, in the course of their differentiation, have sacrificed not only their nucleus and their ribosomes but also their mitochondria, they rely exclusively on the anaerobic portion of the glycolytic pathway for producing ATP, most of which is required by the red cell for cation transport against a concentration gradient across the membrane.
  • If this fails due to a defect of any of the enzymes of the glycolytic pathway, the result will be hemolytic disease.
  • Abnormalities of the glycolytic pathway are all inherited and all rare.
  • Among them, deficiency of pyruvate kinase (PK) is the least rare, with an estimated prevalence in most populations of 1:10,000.
  • However, recently, a polymorphic PK mutation (E277K) was found in some African populations with heterozygote frequencies of 1–7%, suggesting that this may be another malaria-related polymorphism.
  • HA secondary to PK deficiency is an autosomal recessive disease.
  • The clinical picture of homozygous (or biallelic) PK deficiency is that of an HA that often presents in the newborn with severe neonatal jaundice, requiring nearly always phototherapy and frequently exchange transfusion; the jaundice often persists, and it is often associated with reticulocytosis.
  • The anemia is of variable severity; sometimes it is so severe as to require regular blood transfusion treatment, whereas sometimes it is mild, bordering on a nearly compensated hemolytic disorder.
  • As a result, the diagnosis may be delayed: in some cases, it is made, for instance, in a young woman during her first pregnancy, when the anemia may get worse.
  • The delay in diagnosis may be caused in part by the fact that the anemia is often remarkably well tolerated because the metabolic block at the last step in glycolysis causes an increase in 2,3-bisphosphoglycerate (or DPG; Fig. 105-1), a major effector of the hemoglobin-oxygen dissociation curve; thus, for a certain level of hemoglobin, the oxygen delivery to the tissues is enhanced, a remarkable compensatory feat.
  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.
  • G6PD deficiency–related HA is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.

7.1 Treatment of Hereditary Spherocytosis

  • We do not have a causal treatment for HS; i.e., no way has yet been found to correct the basic defect in the membrane-cytoskeleton structure.
  • Given the special role of the spleen in HS (see above), splenectomy is often beneficial.
  • Current recommendations are to proceed with splenectomy at the age of 4–6 years in severe cases, to delay splenectomy until puberty in moderate cases, and to avoid splenectomy in mild cases.
  • Partial splenectomy can be considered in certain cases, and it is helpful to know about the outcome of splenectomy in the patient’s affected relatives.
  • Before splenectomy, vaccination against encapsulated bacteria (Neisseria meningitidis and Streptococcus pneumoniae) is imperative; penicillin prophylaxis after splenectomy is controversial.
  • Along with splenectomy, cholecystectomy should not be carried out automatically, but it should be carried out, usually by the laparoscopic approach, whenever it is clinically indicated, mainly when gallstones are symptomatic.
  • The most severe cases of HS (estimated at <10%) are transfusion dependent, and in infants with severe HS, erythropoietin may prove to be transfusion sparing.

7.2 Treatment of Pyruvate Kinase Deficiency

  • Until recently, the management of PK deficiency has been supportive.
  • Folic acid supplements should be given constantly.
  • Blood transfusion should be used as necessary, and iron chelation may be required even in some patients who, though not receiving blood transfusion, may be developing iron overload (see General Pathophysiology above).
  • About one-half of patients sooner or later undergo splenectomy, which usually provides a modest but significant increase in hemoglobin (paradoxically, reticulocytes also often increase, because they were previously trapped in the spleen).
  • Cholecystectomy may also be required.
  • A major advance has been the introduction of mitapivat, an allosteric activator of PK.

8. PROGNOSIS & COMPLICATIONS

  • In the long run, in the absence of iron-chelation therapy, iron overload will cause secondary hemochromatosis; this will cause damage particularly to the liver, eventually leading to cirrhosis, and to the heart muscle, eventually causing heart failure.
  • With chronic extravascular hemolysis, the opposite problem, iron overload, is more common, especially if the patient needs frequent blood transfusions.
  • Even without blood transfusion, when erythropoiesis is massively increased, the release of erythroferrone from erythroid cells suppresses hepcidin, causing increased iron absorption.
  • In the long run, in the absence of iron-chelation therapy, iron overload will cause secondary hemochromatosis; this will cause damage particularly to the liver, eventually leading to cirrhosis, and to the heart muscle, eventually causing heart failure.
  • The most severe cases of HS (estimated at 36 g/dL) associated with mild jaundice.
  • Mutations in either PIEZO1, encoding an ion channel activated by pressure (mechanoreceptor), or in KCCN4, encoding the Ca2+ activated K+ channel (Gardos channel) have been recognized to cause DHS.
  • Another form is overhydrated stomatocytosis (OHS). OHS is also macrocytic (MCV >110 fL), but the MCHC is low (<30 g/dL).
  • The underlying mutation is in the Rhesus gene RHAG, which encodes an ammonia channel.
  • Yet other patients with stomatocytosis have mutations in SLC4A1 (encoding band 3) and SLC2A1 (encoding the glucose transporter GLUT1).
  • Mutations of the latter are responsible for cryohydrocytosis, a channelopathy in which the red cells swell and burst when they are cooled.
  • In vivo hemolysis can vary from relatively mild to quite severe.
  • Familial hyperkalemia has been recently linked to mutations in ABCB6, resulting in abnormal cation leak with extracellular release of a large amount of K+ (hyperkalemia).
  • Mutations in ABCB6 have been identified in almost 0.3% of blood donors.
  • However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in splenectomized DHS patients.

9. SPECIAL CONSIDERATIONS

  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.
  • G6PD deficiency–related HA is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.
  • One particular in-frame deletion of nine amino acids in the SLC4A1 gene encoding band 3 underlies the so-called Southeast Asia ovalocytosis (SAO): it is not a disease, but rather a polymorphism with a frequency of up to 5–7% in certain populations (e.g., Papua New Guinea, Indonesia, Malaysia, Philippines), presumably as a result of malaria selection.
  • It is asymptomatic in heterozygotes and probably lethal in homozygotes.
  • The cases of HE with the most severe HA are those with biallelic mutations of one of the genes involved (see Fig. 105-3), and these are said to have hereditary pyropoikilocytosis (HPP): here the instability of the cytoskeleton protein network may result from decreased tetramerization of spectrin dimers.
  • The red cell volume is decreased (MCV: 50–60 fL), and all kinds of bizarre poikilocytes are seen on the blood smear (Fig. 105-4C).
  • HPP patients have splenomegaly and often benefit from splenectomy.
  • Channelopathies are rare conditions (see Fig. 105-3) characterized by abnormalities in red cell ion content and alteration of erythrocyte volume.
  • Cation leak can cause hyperkalemia; in some cases, this leak is accelerated in the cold (the resulting spuriously high serum K+ is then referred to as pseudo-hyperkalemia).
  • The less rare form, dehydrated stomatocytosis (DHS; also referred to as xerocytosis), is a (usually compensated) macrocytic hemolytic disorder, with increased MCHC (generally >36 g/dL) associated with mild jaundice.
  • Mutations in either PIEZO1, encoding an ion channel activated by pressure (mechanoreceptor), or in KCCN4, encoding the Ca2+ activated K+ channel (Gardos channel) have been recognized to cause DHS.
  • Another form is overhydrated stomatocytosis (OHS). OHS is also macrocytic (MCV >110 fL), but the MCHC is low (<30 g/dL).
  • The underlying mutation is in the Rhesus gene RHAG, which encodes an ammonia channel.
  • Yet other patients with stomatocytosis have mutations in SLC4A1 (encoding band 3) and SLC2A1 (encoding the glucose transporter GLUT1).
  • Mutations of the latter are responsible for cryohydrocytosis, a channelopathy in which the red cells swell and burst when they are cooled.
  • In vivo hemolysis can vary from relatively mild to quite severe.
  • Familial hyperkalemia has been recently linked to mutations in ABCB6, resulting in abnormal cation leak with extracellular release of a large amount of K+ (hyperkalemia).
  • Mutations in ABCB6 have been identified in almost 0.3% of blood donors.
  • However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in splenectomized DHS patients.

9.1 Pregnancy and Infection

  • Another general feature of chronic HAs is seen when any intercurrent condition, such as an acute infection, depresses erythropoiesis.
  • When this happens, in view of the increased rate of red cell turnover, the effect will be predictably much more marked than in a person who does not have hemolysis.
  • The most dramatic example is infection by parvovirus B19, which may cause a rather precipitous fall in hemoglobin—an occurrence sometimes referred to as aplastic crisis.
  • G6PD deficiency–related HA is a prime example of an HA due to interaction between an intracorpuscular cause and an extracorpuscular cause; indeed, in the vast majority of cases, hemolysis is triggered by an exogenous agent.
  • Although the G6PD activity is decreased in most tissues of G6PD-deficient subjects, in other cells, the decrease is much less pronounced than in red cells, and it does not seem to impact on clinical expression.
  • The G6PD gene is X-linked, and this has important implications.
  • First, because males have only one G6PD gene (i.e., they are hemizygous for this gene), they must be either normal or G6PD deficient.
  • By contrast, females, who have two G6PD genes, can be either normal or deficient (homozygous) or intermediate (heterozygous).
  • Second, as a result of the phenomenon of X chromosome inactivation, heterozygous females are genetic mosaics, with a highly variable ratio of G6PD-normal to G6PD-deficient cells and an equally variable degree of clinical expression; some heterozygotes can be just as affected as hemizygous males.
  • The enzymatically active form of G6PD is either a dimer or a tetramer of a single protein subunit of 514 amino acids.
  • G6PD-deficient subjects have been found critical in the redox metabolism of all aerobic cells.
  • In red cells, its role is even more critical because it is the only source of NADPH, which directly and via GSH defends these cells against normal oxidative stress.
  • One particular in-frame deletion of nine amino acids in the SLC4A1 gene encoding band 3 underlies the so-called Southeast Asia ovalocytosis (SAO): it is not a disease, but rather a polymorphism with a frequency of up to 5–7% in certain populations (e.g., Papua New Guinea, Indonesia, Malaysia, Philippines), presumably as a result of malaria selection.
  • It is asymptomatic in heterozygotes and probably lethal in homozygotes.
  • The cases of HE with the most severe HA are those with biallelic mutations of one of the genes involved (see Fig. 105-3), and these are said to have hereditary pyropoikilocytosis (HPP): here the instability of the cytoskeleton protein network may result from decreased tetramerization of spectrin dimers.
  • The red cell volume is decreased (MCV: 50–60 fL), and all kinds of bizarre poikilocytes are seen on the blood smear (Fig. 105-4C).
  • HPP patients have splenomegaly and often benefit from splenectomy.
  • Channelopathies are rare conditions (see Fig. 105-3) characterized by abnormalities in red cell ion content and alteration of erythrocyte volume.
  • Cation leak can cause hyperkalemia; in some cases, this leak is accelerated in the cold (the resulting spuriously high serum K+ is then referred to as pseudo-hyperkalemia).
  • The less rare form, dehydrated stomatocytosis (DHS; also referred to as xerocytosis), is a (usually compensated) macrocytic hemolytic disorder, with increased MCHC (generally >36 g/dL) associated with mild jaundice.
  • Mutations in either PIEZO1, encoding an ion channel activated by pressure (mechanoreceptor), or in KCCN4, encoding the Ca2+ activated K+ channel (Gardos channel) have been recognized to cause DHS.
  • Another form is overhydrated stomatocytosis (OHS). OHS is also macrocytic (MCV >110 fL), but the MCHC is low (<30 g/dL).
  • The underlying mutation is in the Rhesus gene RHAG, which encodes an ammonia channel.
  • Yet other patients with stomatocytosis have mutations in SLC4A1 (encoding band 3) and SLC2A1 (encoding the glucose transporter GLUT1).
  • Mutations of the latter are responsible for cryohydrocytosis, a channelopathy in which the red cells swell and burst when they are cooled.
  • In vivo hemolysis can vary from relatively mild to quite severe.
  • Familial hyperkalemia has been recently linked to mutations in ABCB6, resulting in abnormal cation leak with extracellular release of a large amount of K+ (hyperkalemia).
  • Mutations in ABCB6 have been identified in almost 0.3% of blood donors.
  • However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in splenectomized DHS patients.

10. KEY PEARLS & CLINICAL TRAPS

  • Reticulocyte count is the definitive parameter for hemolysis; percentage and absolute count both increase.
  • Hemolytic anemias (HAs) are classified as inherited or acquired, intracorpuscular or extracorpuscular.
  • Hereditary spherocytosis (HS) prevalence is 1:2000-5000.
  • Glucose-6-phosphate dehydrogenase (G6PD) deficiency is X-linked, heterozygotes variable.
  • Pyruvate kinase (PK) deficiency is autosomal recessive; mitapivat is an allosteric activator of PK.
  • Chronic extravascular hemolysis leads to iron overload (secondary hemochromatosis) and organ damage (liver, heart).
  • Osmotic fragility testing is the main diagnostic test for HS.
  • Splenectomy is contraindicated in stomatocytosis due to severe thromboembolic complications.
  • Parvovirus B19 infection can cause aplastic crisis in patients with chronic hemolysis.
  • Compensated hemolysis may present without anemia; decompensation occurs in pregnancy, folate deficiency, or renal failure.

Flowcharts & Algorithms

Reproduced from Harrison's 22nd Edition.

Flowchart 1

Red blood cell (RBC) metabolism

Caption: FIGURE 105-1 Red blood cell (RBC) metabolism. The Embden-Meyerhof pathway (glycolysis) generates ATP required for cation transport and for membrane of maintenance. The generation of NADH maintains hemoglobin iron in a reduced state. The hexose monophosphate shunt generates NADPH that is used to of reduce glutathione, which protects the red cell against oxidant stress; the 6-phosphogluconate, after decarboxylation, can be recycled via pentose sugars to glycolysis. Regulation of the 2,3-bisphosphoglycerate level is a critical determinant of oxygen affinity of hemoglobin. Enzyme deficiency states in order of prevalence: glucose-6-phosphate dehydrogenase (G6PD) > pyruvate kinase > glucose-6- phosphate isomerase > rare deficiencies of other enzymes in the pathway. The more common enzyme deficiencies are encircled. of

Figures & Illustrations

Reproduced from Harrison's 22nd Edition.

Figure 1

Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary stomatocytosis (HSt)...

Caption: FIGURE 105-3 Hereditary spherocytosis (HS), hereditary elliptocytosis (HE), and hereditary stomatocytosis (HSt) are three morphologically distinct forms of congenital hemolytic anemia. It has emerged that each one can arise from mutation of of one of several genes and that different mutations of the same gene can give one in or another form. (See also Table 105-3.) Genes encoding membrane proteins are in black; genes encoding cytoskeleton proteins are in green; genes encoding proteins in the junctional and ankyrin complexes are in purple. — Figure 105-1: Red blood cell (RBC) metabolism diagram showing the Embden-Meyerhof pathway (glycolysis), hexose monophosphate shunt, and key enzymes like G6PD and pyruvate kinase.

Figure 2

The red cell membrane and cytoskeleton schematic diagram

Caption: FIGURE 105-2 The red cell membrane and cytoskeleton schematic diagram. Within the integral membrane proteins are shown (see inset). Other proteins, e.g., complement-regulatory proteins CD59 and CD55, are tethered to the membrane through the (GPI) anchor: in these cases, the entire polypeptide chain is extracellular. Many of polypeptide and/or carbohydrate red cell antigens. Underneath the membrane, the associate head-to-head into tetramers, together with actin and other proteins, form ankyrin complex, which also involves the band 4.2 protein, and the junctional complex, protein and dematin, connect the membrane to the cytoskeleton. The ankyrin complex called vertical) connections; the junctional complex provides mainly tangential (also Pathogenic changes in the former can cause spherocytosis, whereas pathogenic elliptocytosis; pathogenic changes in spectrin can cause either. Branched lines — Figure 105-2: Schematic diagram of the red cell membrane and cytoskeleton, illustrating integral membrane proteins (Band 3, RhAG), GPI-anchored proteins, and cytoskeletal connections (ankyrin, spectrin).

Figure 3

The role of glucose-6-phosphate dehydrogenase (G6PD) in 6-phosphogluconate dehydrogenase—two of...

Caption: FIGURE 105-6 The role of glucose-6-phosphate dehydrogenase (G6PD) in 6-phosphogluconate dehydrogenase—two of the enzymes of the pentose phosphate (GSH) when this is oxidized by reactive oxygen species (e.g., O– and HO). Thus when 2 2 2 produced by pro-oxidant compounds such as primaquine, or the glucosides in fava beans similarly, when rasburicase administered to degrade uric acid produces an equimolar glutathione peroxidase, catalase, and Prx2 (peroxiredoxin-2; all three mechanisms are reduced, NADPH production is limited, and it may not be sufficient to cope with the peroxide. This diagram also explains why a defect in GSH reductase has very similar — Figure 105-3: Peripheral blood smear abnormalities showing Hereditary Spherocytosis (HS), Hereditary Elliptocytosis (HE), and Hereditary Stomatocytosis (HSt).

Figure 4

Basic mechanisms involved in warm antibody– and cold opsonized red...

Caption: FIGURE 105-9 Basic mechanisms involved in warm antibody– and cold opsonized red cells are removed by Fc receptor–bearing macrophages, largely in the role, with the spleen again being the main site. Bottom. In cold agglutinin disease (CAD), through the classic pathway, with consequent intravascular hemolysis. In addition, with consequent extravascular hemolysis. The inset on the left illustrates the B cells autoimmune hemolytic anemia and monoclonal in the case of CAD. The new therapeutic shown in red. The inset highlights drugs targeting immune cells involved in generation of — Figure 105-4: Peripheral blood smear from patients with membrane-cytoskeleton abnormalities, including channelopathies and stomatocytosis.

Figure 5

Impact and implications of anticomplement therapy in paroxysmal composition in...

Caption: FIGURE 105-11 Impact and implications of anticomplement therapy in paroxysmal composition in examples of PNH patients. In the untreated patient, severe anemia is protected from intravascular hemolysis, although at the expense of iatrogenic severe. In the patient on pegcetacoplan, both intravascular and extravascular hemolysis red cells in the treated patients consists entirely of PNH red cells. B. Schematics of complex (MAC) produces intravascular hemolysis. On eculizumab (middle), C5 blockade consequent breakthrough hemolysis. On pegcetacoplan (right), C5 convertase cannot be is an enzymatic cascade upstream of C5 cleavage, if blockade is incomplete, MAC potentially more massive as the PNH red cells are greatly increased in both relative and — Figure 105-5: Phenotypes of heterozygotes for red cell enzymopathies, illustrating G6PD mosaicism in females.

Figure 6

The complement cascade and the fate of red cells

Caption: FIGURE 105-10 The complement cascade and the fate of red cells. A. In normal blood, primarily by the two glycosylphosphatidylinositol (GPI)-linked surface proteins CD55 [MAC] from inserting into the membrane). B. Paroxysmal nocturnal hemoglobinuria (PNH) (GPI) biosynthetic pathway is blocked as a result of a PIGA mutation; therefore, C3 action of the MAC. C. With drugs (monoclonal antibodies) that bind to C5 and prevent it MAC is not formed, and intravascular hemolysis (IVH) is abrogated. However, red cells extravascular hemolysis (EVH) varies in severity between patients. The Coombs test, spectrum” or an anticomplement reagent is used). D. With a drug that targets C3, C3b no MAC is formed (abrogating IVH), and at the same time, opsonization of red cells by target factor B or factor D, although C3b can still be formed through the classical with PNH. Blood 138:1908, 2021.) — Figure 105-6: Mechanism of G6PD deficiency in redox metabolism, showing defense against oxidative stress via NADPH and glutathione.

Figure 7

Different phenotypes of heterozygotes for red cell enzymopathies

Caption: FIGURE 105-5 Different phenotypes of heterozygotes for red cell enzymopathies. In a heterozygote for deficiency of pyruvate kinase (PK), encoded by an autosomal gene (see Table 105-4), the level of enzyme is about one-half of normal in all red cells. Because this level of enzyme is sufficient, there are no clinical consequences, i.e., PK deficiency is recessive. In a heterozygote for deficiency of glucose-6-phosphate dehydrogenase (G6PD), encoded by an X-linked gene, the situation is quite different: X-chromosome inactivation generates red cell mosaicism, whereby some red cells are entirely normal and others are G6PD deficient. Therefore, G6PD deficiency is expressed in heterozygotes; it is not recessive. — Table 105-1: Classification of Hemolytic Anemias into Intracorpuscular Defects (Inherited Hemoglobinopathies, Enzymopathies, Membrane-cytoskeletal defects) and Extracorpuscular Factors (Acquired PNH, Drugs, Infectious, Autoimmune).

Figure 8

Peripheral blood smear from patients with membrane-cytoskeleton abnormalities

Caption: FIGURE 105-4 Peripheral blood smear from patients with membrane-cytoskeleton abnormalities. A. Hereditary spherocytosis. B. Hereditary elliptocytosis, heterozygote. C. Pyropoikilocytosis, with both alleles of the α-spectrin gene mutated. ABCB6 have been identified in almost 0.3% of blood donors. However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in sple- in nectomized DHS patients. — Table 105-2: Features Common to Most Patients with a Hemolytic Disorder, including General Examination (Jaundice, Pallor), Physical Findings (Splenomegaly, Skull Bossing), and Lab Features (Reticulocytes, Bilirubin, LDH, Haptoglobin).

Figure 9

Peripheral blood smear from patients with membrane-cytoskeleton abnormalities

Caption: FIGURE 105-4 Peripheral blood smear from patients with membrane-cytoskeleton abnormalities. A. Hereditary spherocytosis. B. Hereditary elliptocytosis, heterozygote. C. Pyropoikilocytosis, with both alleles of the α-spectrin gene mutated. ABCB6 have been identified in almost 0.3% of blood donors. However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in sple- in nectomized DHS patients. — Table 105-3: Inherited Diseases of the Red Cell Membrane-Cytoskeleton Complex, listing Gene, Chromosomal Location, Protein, Disease, Inheritance, and Comments.

Figure 10

Peripheral blood smear from patients with membrane-cytoskeleton abnormalities

Caption: FIGURE 105-4 Peripheral blood smear from patients with membrane-cytoskeleton abnormalities. A. Hereditary spherocytosis. B. Hereditary elliptocytosis, heterozygote. C. Pyropoikilocytosis, with both alleles of the α-spectrin gene mutated. ABCB6 have been identified in almost 0.3% of blood donors. However, splenectomy is contraindicated in stomatocytosis due to the significant proportion of severe thromboembolic complications observed in sple- in nectomized DHS patients. — Table 105-4 Part 1: Red Cell Enzyme Abnormalities Causing Hemolysis (Glycolytic Pathway), listing Enzyme, Gene, Prevalence, and Clinical Manifestations.

Figure 11

Hemolytic Anemias FIGURE 105-7 Epidemiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency...

Caption: Hemolytic Anemias FIGURE 105-7 Epidemiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency throughout the world. Each country on the map is shaded in a color based on the best estimate of the mean frequency of G6PD deficiency allele(s) in that country (this is the same as the frequency of G6PD-deficient males). The small panel on the left gives the key to color shadings corresponding to each country. The larger panel gives a color-coded list of 10 common G6PD variants associated with G6PD deficiency: asterisk- shaped symbols in the corresponding colors are shown in the countries where these variants have been observed (for graphic reasons, symbols could not be inserted in all countries). (Reproduced with permission from L Luzzatto, M Ally, R Notaro. Glucose-6-phosphate dehydrogenase deficiency. 136:1225, 2020.) also seen with unstable hemoglobins). Since there is also a substantial absence of comorbidity, full recovery from AHA associated with G6PD component of extravascular hemolysis, unconjugated bilirubin is high deficiency is the rule. and there is often clinical icterus. The most serious threat from AHA It was primaquine (PQ)-induced AHA that led to the discovery of — Table 105-4 Part 2: Red Cell Enzyme Abnormalities Causing Hemolysis (Redox and Nucleotide Metabolism), listing Enzyme, Gene, Prevalence, and Clinical Manifestations.

Generated from Harrison's Principles of Internal Medicine, 22nd Edition.