Charles T. Quinn
Hemoglobin (Hb) is the oxygen-carrying protein within red blood cells (RBCs). It is composed of four globular protein subunits, called globins, and four oxygen-binding heme groups, which are attached to each globin. The two main types of globins are the α-globins and the β-globins, which are made in essentially equivalent amount in precursors of RBCs. Normal adult Hb (Hb A) has two α-globins and two β-globins (α2 β2). Genes on chromosomes 16 and 11 encode the α- and β-globins, respectively. There are also distinct embryonic, fetal, and minor adult analogs of the α- and β-globins, all of which are encoded by separate genes. See Chapter 429 for a discussion of the developmental changes in Hb production.
Disorders of Hb can be classified as qualitative or quantitative disorders. Qualitative abnormalities of Hb arise from mutations that change the amino acid sequence of the globin, thereby producing structural and functional changes in the Hb. There are four ways in which Hb can be qualitatively abnormal: (1) decreased solubility, (2) instability, (3) altered oxygen affinity, and (4) altered oxidation state of the heme-coordinated iron. Qualitative Hb disorders are often referred to as hemoglobinopathies, even though the term can technically apply to both qualitative and quantitative disorders. Quantitative Hb disorders result from the decreased and imbalanced production of generally structurally normal globins. For example, if β-globin production is diminished by a mutation, there will be a relative excess of α-globins. Such imbalanced production of α- and β-globins damages RBCs and their precursors in the bone marrow. These quantitative Hb disorders are called thalassemias. Both qualitative and quantitative disorders of Hb can be subdivided by the particular globin that is affected; for example, there can be α-thalassemias and β-hemoglobinopathies. We begin this chapter with a review of several of the common qualitative Hb disorders and end with a discussion of the thalassemias.
Table 434-1. Comparison of the Common Sickle Cell Diseases and Sickle Cell Trait
SICKLE CELL DISEASE
Sickle cell disease (SCD) is the name for a group of related disorders caused by sickle Hb (Hb S).1 Hb S is a qualitatively abnormal Hb caused by a point mutation of the β-globin gene. This change decreases the solubility of Hb S in the deoxygenated state. Thus, as sickle red blood cells (RBCs) traverse the circulation, cycling through oxygenated and deoxygenated states, Hb S repeatedly forms rigid polymers that damage the RBC membrane, causing a hemolytic anemia and, ultimately, the manifestations of SCD.
EPIDEMIOLOGY
Sickle cell trait, the heterozygous or carrier state for the Hb S mutation, protects against severe malarial infections, so it arose and became a balanced polymorphism in historically malarious regions of the world. As such, sickle cell disease (SCD) occurs most commonly among individuals of African, Mediterranean, Middle Eastern, or Asian Indian ancestry. However, SCD is now found throughout the world in diverse populations, and it can occur in people of any skin color. In the United States, approximately 1 in 2500 newborns and 1 in 400 African American newborns has SCD. It is estimated that there are 100,000 individuals living with SCD in the United States.
GENETICS AND PATHOPHYSIOLOGY
The prototypical, most common, and most severe form of sickle cell disease (SCD) is the homozygous state for the Hb S mutation (βS), called sickle cell anemia (Hb SS). Less common forms of SCD are compound heterozygous states that result from the coinheritance of the βS gene with other qualitatively or quantitatively abnormal β-globin genes. These include, in order of decreasing frequency, sickle-hemoglobin C disease (Hb SC), sickle-β+-thalassemia (Hb Sβ+), and sickle-β0-thalassemia (Hb Sβ0). Hb SS and Hb Sβ0 are clinically indistinguishable. Hb SC and Hb Sβ+ are, on average, milder forms of SCD. Sickle cell trait, the carrier state, is not a form of SCD. See Table 434-1to compare and contrast the different forms of SCD and sickle trait.
As they traverse the circulation, red blood cells (RBCs) that contain mostly Hb S go through cycles of sickling (polymerization of Hb S) and unsickling (depolymerization of Hb S) due to deoxygenation of Hb S in the tissues and reoxygenation in the lungs. The tendency of individual RBCs to sickle is influenced by several factors. These include the presence of other Hbs that inhibit the polymerization of deoxy-Hb S, the hydration of the RBC, and the degree to which the Hb S is deoxygenated. When a solution of Hb S is deoxygenated, there is a characteristic delay time during which no polymerization occurs, followed by a phase of rapid polymerization. In vivo, this delay time allows most Hb S-containing RBCs to traverse the capillary beds before polymerization and sickling occur. A small number of RBCs remain permanently sickled due to membrane damage, even when fully oxygenated. These are called irreversibly sickled cells.
The two main pathophysiologic consequences of polymerization of Hb S, or sickling, are hemolysis and vasoocclusion (Fig. 434-1). Hemolysis, or destruction of RBCs, in SCD occurs predominantly in the extravascular compartment. Cycles of sickling damage the RBC, especially its membrane. These damaged RBCs are recognized as abnormal and removed from circulation by the reticuloendothelial system. Some intravascular hemolysis occurs as well, by microvascular trapping and destruction of adhesive and rigid sickle RBCs. The mean RBC life span in Hb SS is dramatically shortened to 10 to 20 days from the normal RBC life span of 120 days. The rate of hemolysis in SCD usually exceeds the rate at which new RBCs can be produced by the bone marrow. Therefore, SCD is characterized by a partially compensated hemolytic anemia.
In addition to their shortened life span, sickle erythrocytes are also abnormally adhesive and have decreased flexibility. Consequently, they can adhere to and damage the endothelium of blood vessels and block the flow of blood. This microvascular obstruction, called vasoocclusion, leads to ischemia and infarction of different tissues. Vasoocclusion is believed to be the main cause of acute episodes of pain, which are characteristic of SCD, as well as chronic organ damage.
DIAGNOSTIC EVALUATION
In the United States and some other countries, universal newborn screening programs for hemoglobinopathies now identify individuals with sickle cell disease (SCD) shortly after birth. Such early diagnosis is key to preventing early mortality from sepsis and acute splenic sequestration (see later in the chapter). Results of newborn screening for hemoglobinopathies are usually reported as patterns, where all detected Hbs and their relative abundance are reported. All newborns, unless transfused, will have more Hb F than other Hbs. The remaining Hbs are reported in order of decreasing abundance. These patterns, in combination with confirmatory testing, permit the accurate, early diagnosis of SCD (Table 434-2).
FIGURE 434-1. Overview of the pathophysiology of sickle cell disease (SCD) showing the hemolytic and vasoocclusive components and their overlap. ACS, acute chest syndrome.
Beyond the immediate newborn period, the laboratory evaluation of suspected SCD should include a complete blood count, reticulocyte count, and examination of the peripheral blood smear (Fig. 434-2). Table 434-3 lists the usual hematologic findings in the common forms of SCD. To confirm a diagnosis of SCD, however, some analysis of Hb types must be performed. Abnormal Hbs must be identified using at least two methods because they can be difficult to differentiate. Contemporary Hb separation methods include IEF and HPLC. DNA-based diagnostic methods, which are increasingly available, and family testing may be needed in occasional diagnostic challenges. It is important to know that a “sickle prep” or the Sickledex does not differentiate between SCD and sickle cell trait. These tests only confirm the presence of Hb S, which is found in both SCD and trait, so it is not helpful for the diagnosis of SCD.
Table 434-2. Common Patterns in Newborn Screening for Hemoglobinopathies
Because patients with SCD have a chronic hemolytic anemia (Table 434-3), they also have unconjugated hyperbilirubinemia and variable elevations of lactate dehydrogenase (LDH) and aspartate transaminase (AST). It is generally not necessary to measure these substances. After the first year of life, the peripheral blood smear in Hb SS and Hb Sβ0 shows variable numbers of pathognomonic irreversibly sickled cells (Fig. 434-2), as well as polychromatophilic cells, Howell-Jolly bodies, poikilocytes, and target cells. Howell-Jolly bodies are nuclear remnants, and their presence on the blood smear outside the immediate neonatal period is indicative of hyposplenism. Patients with Hb SC and Hb Sβ+ usually do not have any irreversibly sickled cells, but they instead have a larger number of target cells. The mean cell volume (MCV) is normal in Hb SS and Hb SC, unless there is coinheritance of α-thalassemia. The MCV is low in Hb Sβ0 and Hb Sβ+. Leukocyte and platelet counts are usually moderately increased in SCD in the absence of infection.
FIGURE 434-2. Peripheral blood film in sickle cell anemia (Hb SS).
CLINICAL COURSE
A patient with SCD has at baseline a chronic hemolytic anemia to which he or she becomes physiologically adapted. This baseline anemic but relatively healthy state is called the steady state. This steady state is punctuated by intermittent, acute episodes of illness, called vasoocclusive episodes, events, or crises. Recurrent vasoocclusion and chronic anemia also produce chronic organ damage. There are a number of different types of acute episodes and chronic organ damage that are described in this chapter. Patients with any form of SCD can experience these complications, but they tend to occur earlier and more frequently in Hb SS and Hb Sβ0.
At birth, newborns with Hb SS disease have normal birth weight and are not anemic. Anemia and reticulocytosis usually appear between 2 and 6 months of age. Along with the anemia come jaundice and a cardiac flow murmur. Jaundice and flow murmurs are expected findings that should not cause concern. Splenic infarction and hyposplenism may begin to occur by 3 months of age. Therefore, it is necessary to prescribe prophylactic penicillin before this time to prevent pneumococcal sepsis. Before splenic infarction is complete, the spleen may be palpably enlarged. Nevertheless, it is still poorly or nonfunctional. Acute vasoocclusive events are unusual before 6 months of age. The first painful event is often dactylitis, which is a painful swelling of the hands and feet. Dactylitis is rare beyond 3 years of age. Although birth weight is normal, growth retardation is commonly observed during childhood. On average, individuals with Hb SS tend to be thinner and shorter than their peers, but a normal adult height can often be attained. The onset of puberty and development of secondary sex characteristics are usually delayed by 2 to 3 years.
The course of the disease does not improve with age, but certain complications occur more commonly in adolescents and adults, such as retinal disease, leg ulcers, and pulmonary hypertension. The median life expectancy for patients with Hb SS and Hb Sβ0 in developed nations is now estimated to be 53 years for men and 58.5 years for women. Patients with Hb SC and Hb Sβ+ have a near normal life expectancy. In many Third World countries, 90% of children with Hb SS disease do not survive beyond 5 years of age, and falciparum malaria is the leading cause of death. Although sickle cell trait offers protection against malaria, the combination of Hb SS and malaria is usually fatal.
The common acute complications of sickle cell disease in childhood and some of the forms of chronic organ damage and dysfunction that occur with time are presented.
Acute Episodes: Infections
Until the end of the 20th century, infections were the major cause of death in young children with Hb SS and Hb Sβ0 in the United States. Fatal Streptococcus pneumoniae sepsis occurred in 15% to 20% of children in the first 5 years of life. These infections were typically fulminant, with death occurring within 24 hours of the onset of fever. Children with sickle cell disease (SCD) have an unusual vulnerability to severe pneumococcal sepsis due to their early loss of splenic reticuloendothelial function (functional hyposplenism) and their lack of circulating antibodies against polysaccharide-encapsulated bacteria. In early life, the spleen, although often palpably enlarged, loses reticuloendothelial function because of continuous vasoocclusive infarction. The enlarged spleen gradually becomes small and fibrotic, and it is rarely palpable after 6 years of age. As a normal developmental phenomenon, young children cannot efficiently produce antibodies against polysaccharide antigens, such as those on the capsule of the pneumococcus. When circulating antipneumococcal antibodies are lacking, the spleen is almost exclusively responsible for the clearance of pneumococci in the circulation. The combination of hyposplenism and lack of antibodies against polysaccharide antigens accounts for the high susceptibility and the historically high frequency and mortality of pneumococcal infections in young children with Hb SS and Hb Sβ0.
Table 434-3. Typical Laboratory Findings in the Common Forms of Sickle Cell Disease
Fatal pneumococcal sepsis is now rare in children with SCD because of universal newborn screening for hemoglobinopathies, prophylactic penicillin, and use of the conjugated pneumococcal vaccine. Newborn screening programs identify children with SCD before they develop hyposplenism. This allows time for physicians to teach parents to seek medical attention for any high fever (generally 101.5°F or more) and to initiate prophylactic penicillin. A national placebo-controlled study (PROPS) showed that oral penicillin prophylaxis reduced the incidence of invasive pneumococcal infections by 84% in Hb SS children younger than 5 years. A follow-up study (PROPS-II) did not show effectiveness of continuing penicillin prophylaxis after 5 years of age, so most centers discontinue prophylaxis at this age. Compared to Hb SS and Sβ0, individuals with Hb SC and Hb Sβ+ have a much lower risk of pneumococcal sepsis because their splenic infarction is delayed and incomplete, so prophylactic penicillin is generally not necessary for these patients. The newer heptavalent conjugated pneumococcal vaccine (Prevnar) has further decreased the incidence of pneumococcal infection, and it is recommended for all children with SCD and given according to guidelines for all children. The older, 23-valent polysaccharide vaccine (eg, Pneumovax) should be given at age 2 and 5 years. Although meningococcal infections have been reported only rarely in SCD, it might also be prudent to administer the newer conjugated meningococcal vaccine (Menactra) to patients with SCD.
Patients with SCD also have a predilection for osteomyelitis . Salmonella species cause about half the cases of osteomyelitis in SCD and staphylococci most of the rest. It can be difficult to differentiate osteomyelitis from an acute painful episode that results in infarction of cortical bone. Both can cause bony tenderness, effusions, and lucencies on roentgenograms. Even bone scans and magnetic resonance imaging fail to distinguish between the two. Clinical features are more helpful than imaging studies in this situation. The occurrence of fever, a single focus of pain, and a positive blood culture are more consistent with a diagnosis of osteomyelitis than an acute painful episode.
Acute Episodes: Splenic Sequestration
In young children in whom splenic autoinfarction is not yet complete, the spleen may become acutely enlarged and engorged with blood sequestered from the systemic circulation, with consequent severe anemia, hypovolemia, and marked splenic enlargement. Thrombocytopenia usually occurs, as well, due to hypersplenism. The recognition of acute splenic enlargement and the signs and symptoms of acutely severe anemia by both parents and health care professionals is important to prevent a fatal outcome. Parents can be taught how to palpate the spleen size and to seek medical attention for any sudden enlargement. Rapid transfusion of blood is the most important intervention for severe anemia and hypovolemic shock due to acute splenic sequestration. Not all sequestration is severe. Some episodes are characterized by only mild decreases in hemoglobin concentration (1–2 g/dL) and modest increases in spleen size. These may be associated with viral infections and require only careful observation. Splenic sequestration is often recurrent, and splenectomy is often advocated after recovery from a severe episode or recurrent episodes of lesser severity. Because the spleen is usually not functional, splenectomy does not appear to further increase the risk of infection.
Acute Episodes: Aplastic Crisis
Red blood cell (RBC) life span is greatly shortened from the normal of 120 days to 10 to 20 days in Hb SS and Hb Sβ0. Consequently, patients must chronically maintain a marked increase in RBC production by the bone marrow, producing a chronic reticulocytosis, to maintain a stable Hb concentration compatible with life. If RBC production is impaired, for even a short time, the Hb concentration will fall rapidly. During many viral infections and inflammatory states, erythropoiesis may be modestly reduced, resulting in relative reticulocytopenia and transiently more severe anemia. Human parvovirus, the agent of fifth disease, has especially profound effects on patients with chronic hemolytic anemia. Parvovirus destroys early RBC precursors in the bone marrow and causes RBC aplasia for about a week. The consequent erythroblastopenia and reticulocytopenia cause a dramatic and potentially life-threatening anemia, with the Hb concentration falling as low as 1 to 2 g/dL without measurable reticulocytes. This episode of severe anemia is called the aplastic crisis. Provided the patient does not die, the acute RBC aplasia is transient and self-limited. Spontaneous recovery begins about 1 week after the onset of reticulocytopenia due to antibody-mediated clearance of the virus. Recovery is heralded by the appearance of nucleated RBCs in the circulation, followed shortly by a brisk reticulocytosis. Transfusion of blood is the most important intervention for symptomatic or severe anemia. In contrast to acute splenic sequestration, blood must be transfused relatively slowly to patients with aplastic crisis because their blood volume is typically normal or increased as a physiologic adaption to a more slowly progressive anemia. Lifelong immunity against parvovirus prevents recurrent episodes. Patients with Hb SC and Sβ+ can also have aplastic crisis, but their anemia tends to be less severe because their RBC life span is longer than Hb SS and Sβ0 patients.
Acute Episodes: Painful Episodes
The acute painful episode is the hallmark of sickle cell disease (SCD). It is the most common reason for medical consultation and hospitalization in this population. The pain seems to be caused by acute vasoocclusion, primarily in bones and bone marrow, with consequent ischemia and inflammation. Pain can occur anywhere in the body, but it is most commonly osteoarticular and juxtavertebral. The earliest physical manifestation of Hb SS is often a characteristic painful episode called dactylitis. Dactylitis is an often symmetric swelling of the hands and feet that occurs in about 30% of children in the first 3 years of life. As the child ages, painful episodes involve instead the long bones, vertebrae, sternum, ribs, lower back, and abdomen. Unlike dactylitis, most other painful episodes typically do not have accompanying physical signs. Infection, dehydration, and exposure to the cold may precipitate pain, but specific precipitating features are usually not identified. Although painful events severe enough to require hospitalization are infrequent (< 1/year) for most patients, less severe painful events that can be managed at home with oral analgesics occur more frequently.
The treatment of the painful episode is symptomatic—controlling the pain. Analgesia must be tailored to the degree of pain and the patient. Moderate pain without fever or other signs of concomitant illness can usually be managed at home with hydration, analgesics, and rest. However, some pain is so severe that hospitalization for intravenous hydration and parenteral opioids is necessary. Overhydration is not helpful, and it may actually precipitate acute chest syndrome. A combination of nonsteroidal anti-inflammatory drugs and opioid analgesics, titrated to effect, will usually achieve adequate pain relief. Patients with SCD-related pain, even severe, typically have no accompanying physical signs, such as edema or erythema, so it is important to believe the patient’s report of pain and its intensity. Transfusion does not relieve acute sickle cell pain, and there is no therapy to shorten the duration of the painful episode. The pain must be treated until the episode resolves spontaneously, which may take as long as a week. Once the pain begins to resolve, opioid therapy can be decreased relatively quickly.
Acute Episodes: Acute Chest Syndrome
Acute chest syndrome is a descriptive term for almost any acute pulmonary illness in a child with sickle cell disease (SCD). It is defined practically as a new pulmonary infiltrate with some combination of fever, cough, chest pain, tachypnea, dyspnea or hypoxemia. Acute chest syndrome has many inciting causes, including pulmonary infection, infarction, atelectasis, overhydration causing pulmonary edema, bronchospasm and airway inflammation, or fat embolism from bone marrow infarction. Acute chest syndrome often starts as a small infiltrate in one lobe, but it can progress rapidly to involve multiple lobes, resulting in respiratory distress severe enough to require intubation and ventilatory support. Acute chest syndrome is a leading cause of death in adolescents and adults. Management includes oxygen supplementation for hypoxemia, maintenance of hydration without overhydration, adequate but not excessive analgesia, and antibacterials. An infectious etiology is often not apparent; however, the use of empiric antibiotics active against pneumococcus, Mycoplasma, and Chlamydia is prudent. Simple RBC transfusions or exchange transfusions may be needed for moderate and especially severe cases of acute chest syndrome.
Acute Episodes: Stroke
Clinically overt stroke occurs in about 11% of patients with Hb SS by 18 years of age if they do not receive primary stroke prophylaxis. The reason for the high incidence in young children is unknown. There are three main clinical presentations of overt cerebrovascular disease in Hb SS: cerebral infarction, cerebral hemorrhage, and transient ischemic attacks (TIAs). Infarction and hemorrhage cause weakness, paralysis, and aphasia, and sometimes seizures and headache. TIAs are episodes of weakness, paralysis, or aphasia that last less than 24 hours. TIAs are likely to be overlooked in young children, but they are harbingers of overt stroke. All three presentations can occur at any age, but hemorrhagic stroke is more common in patients older than 20 years. In young children, stenosis of the large intracerebral blood vessels is the usual antecedent of stroke. Magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) are needed to visualize cerebral infarction and stenosis of intracranial arteries. Computerized tomography (CT) can be obtained more quickly than MRI, and it may be helpful when cerebral hemorrhage is strongly suspected. Acute management includes exchange transfusion, careful hydration, and control of any seizures. An additional 20% to 30% of patients experience covert or “silent” cerebral infarctions that are not accompanied by motor signs. Both overt and covert strokes can cause neurocognitive impairment.
Overt stroke has a 50% to 90% risk of recurrence, so secondary stroke prophylaxis is used to prevent further brain injury. Recurrence can be lowered to approximately 10% to 20% by chronic RBC transfusions. Chronic transfusions usually aim to maintain the percentage of Hb S in the blood at less than 30%. Although chronic transfusions can effectively prevent recurrent stroke in most children, complications include iron overload and the need for chelation therapy, alloimmunization, and transfusion-transmitted infections. An increasingly popular therapeutic strategy is to prevent first strokes. Transcranial Doppler (TCD) ultra-sonography is a noninvasive screening tool that can detect stenosis in the major cerebral arteries and thereby identify the children at highest risk of overt stroke. Children with abnormal TCD measurements can be offered chronic transfusions for primary stroke prophylaxis. It is not yet known when or if chronic transfusions can ever be stopped in primary or secondary stroke prophylaxis. It is also not yet known whether covert strokes can be prevented by chronic transfusions. These are areas of active clinical investigation.
Acute Episodes: Priapism
Priapism is a painful, prolonged erection of the penis. It may occur starting in early childhood, and the estimated prevalence in males with Hb SS is 30% to 40% by 18 years of age. Episodes occur in two patterns: prolonged, lasting more than 4 hours, and shorter “stuttering” episodes. Stuttering priapism may occur in clusters and sometimes daily. A prolonged episode of priapism requires rapid intervention to prevent ischemic damage to the penis and erectile dysfunction. Management includes hydration, pain control, and oral adrenergic medications such as pseudoephedrine. If an episode lasts longer than 4 hours, aspiration of the corpora cavernosa and irrigation with a dilute solution of phenylephrine or epinephrine is indicated. Chronic transfusion may prevent recurrent events, but the use of simple transfusion for an acute event is unproven.
Chronic Organ Dysfunction and Damage
In addition to early splenic dysfunction and involution, other forms of progressive organ dysfunction or damage occur with increasing age in the kidneys, bones, eyes, lungs, heart, and liver. Renal complications begin early in life with hyposthenuria, a defect in renal concentration. The consequent production of dilute urine causes enuresis and predisposes to dehydration. Other renal complications include papillary necrosis and hematuria, proteinuria, and renal failure. Renal failure is uncommon in children. Avascular necrosis of bone from vasoocclusion, especially of the heads of the femur and humerus, may develop in some patients and causes chronic bone pain. Occlusion of retinal vessels may lead to proliferative retinopathy, retinal detachment, and loss of vision in adolescents and adults. Chronic hemolytic anemia and, perhaps, pulmonary infarction may cause pulmonary hypertension, hypoxemia, and right-sided heart failure. Pulmonary hypertension is a risk factor for early death in adults with sickle cell disease (SCD). High-output, left-sided heart failure may occur in patients with more severe degrees of anemia. Cholelithiasis secondary to chronic hemolysis can occur as early as 3 to 4 years of age, and more than half of adults with Hb SS have gallstones. Cholecystectomy is one of the most common surgical procedures performed in patients with SCD. Hepatomegaly and hepatic dysfunction or failure may occur as a result of sequestration, infarction, hepatobiliary stasis, or infection.
TREATMENT
There are three main disease-modifying treatments that can reduce the overall severity of sickle cell disease (SCD) or cure it: hydroxyurea, chronic transfusions, and hematopoietic stem cell transplantation. Hydroxyurea is a cytostatic chemotherapeutic agent that has multiple beneficial effects in patients with SCD. Hydroxyurea increases the concentration of Hb F, decreases the leukocyte count, decreases the platelet count, and improves blood rheology. Clinically, hydroxyurea reduces the frequency of painful crises, acute chest syndrome, and transfusions by about 50% in adults. Smaller studies in children have shown similar effects. The long-term safety of hydroxyurea in patients with SCD of any age, and especially those younger than 5 years, has not been clearly established.
Chronic transfusion programs entail regular, usually monthly, transfusions of packed RBCs with the aim to maintain the percentage of Hb S in the blood less than 30%. Chronic transfusions are effective at preventing most complications of SCD, but the most common indications are primary and secondary stroke prophylaxis. Complications of transfusions include iron overload and the need for chelation therapy, alloimmunization, and transfusion-transmitted infections.
Stem cell (or bone marrow) transplantation is the only cure for SCD. Widespread use of transplantation is limited by the lack of donor availability and toxicities of the procedure. Transplantation is safest when hematopoietic stem cells are obtained from an human leucocyte antigen (HLA)-matched sibling (without SCD), but only 10% of patients actually have a potential donor. Transplantation is associated with mortality rate of about 5%, a graft rejection rate of about 5%, and the occurrence of significant graft-versus-host disease in about 5%. Therefore, the chances of a successful, curative transplant without major morbidity are around 85% to 90%. In North America, the most common indication has been stroke or cerebrovascular disease, but transplantation is also offered for other, especially recurrent, vasoocclusive complications.
SICKLE CELL TRAIT
Sickle cell (Hb S) trait is the carrier or heterozygous state for the βS gene; the other allele encodes a normal β-globin. Hb S trait is very common: 1 in 12 African Americans has it. Hb S trait is not a form of sickle cell disease (SCD); carriers have normal life expectancy, blood counts, RBC indices, reticulocyte counts, and peripheral smears. Hb electrophoresis shows more Hb A (60–70%) than Hb S (30–40%) in Hb S trait, in contrast to Hb Sβ+ where there is more Hb S than Hb A (Tables 434-1 and 434-2). The predominance of Hb A within Hb S trait RBCs prevents sickling under normal oxygen tensions. The hypertonicity and relative acidosis in the renal medulla can induce sickling in the kidney, however. Therefore, hyposthenuria and renal papillary necrosis with gross hematuria are known potential medical complications of Hb S trait.
It is important for individuals to know that they have Hb S trait because of the risk to their offspring. Hgb SS is an autosomal-recessive disease, so if both parents have Hb S trait, then each of their offspring will have a 25% chance of having Hb SS. Even if only one parent has Hb S trait, offspring are still at risk of having SCD if the parent without Hb S trait happens to have Hb C trait or β-thalassemia trait, for example. Nondirective genetic counseling should be offered to adolescents and adults with Hb S trait. Preimplantation and prenatal diagnosis are available.
HEMOGLOBIN C
Like Hb S, Hb C is a qualitative abnormality of the β-globin. Hb C originated in West Africa, and Hb C trait is found in about 2.5% of African Americans. Individuals with Hb C trait have no hematologic abnormalities, except for increased numbers of target cells on the peripheral blood smear and the presence of 30% to 40% the slowly migrating Hb C on electrophoresis. Homozygotes for Hb C (Hb CC disease) have a mild hemolytic anemia and splenomegaly, but do not have vasoocclusive symptoms, splenic infarction, or hyposplenism. Hb CC disease is not a form of sickle cell disease. The red blood cells are microcytic, and the blood smear shows many target cells, folded cells, and Hb C crystals (Fig. 434-3). The compound heterozygous state, Hb C-β-thalassemia, is similar to Hb CC disease.
FIGURE 434-3. Peripheral blood film in homozygous Hb C disease (Hb CC).
HEMOGLOBIN E
Hb E (α2 βE2) is one of the most common hemoglobinopathies in the world, occurring mainly among people of Southeast Asian ancestry. Individuals with Hb E trait are hematologically normal, except for approximately 40% Hb E on electrophoresis and target cells on the peripheral blood smear. Homozygous Hb E disease (Hb EE) and Hb E-β+-thalassemia (Hb Eβ+) are characterized by a mild, microcytic anemia with many target cells. Life expectancy is normal, and complications are few. Hb E-β0-thalassemia (Hb Eβ0), however, is associated with moderate-to-severe anemia and a thalassemia intermedia phenotype. Some patients with Hb Eβ0 are transfusion dependent. See below for a discussion of the thalassemias, including Hb Eβ0.
UNSTABLE HEMOGLOBINS
The unstable Hbs are qualitative abnormalities of Hb that are caused by mutations that affect its stability and solubility.2 These mutations often weaken the interactions between the heme group and the globin, leading to spontaneous loss of the heme group from the globins; however, other changes in secondary, tertiary, or quaternary structure can also produce unstable Hbs. These abnormal Hbs undergo spontaneous oxidative denaturation to form intraerythrocytic precipitates that bind to and damage the RBC membrane, resulting in hemolytic anemia. These precipitates, visible by supravital staining with new methylene blue or crystal violet, are called Heinz bodies. Heinz bodies are not specific features of unstable Hbs, however. They also occur in erythrocyte enzyme deficiencies, such as glucose-6-phosphate dehydrogenase (G6PD) deficiency (see Chapter 433), and acquired methemoglobinemia (see later in the chapter).
Unstable Hbs are typically inherited in an autosomal-dominant fashion. So, hemolytic anemia occurs in the heterozygote. More than 100 mutations are described, mostly single amino acid substitutions that affect the β-globin more often than the β-globin. The anemia in unstable β-chain hemoglobinopathies does not occur in the first few months of life when Hb F predominates. However, unstable α-chain hemoglobinopathies may cause hemolytic anemia in neonates and throughout life.
Clinical manifestations depend on the degree of instability of the Hb. The hemolytic rate is roughly proportional to the degree of instability, but concomitant alterations in the oxygen affinity of the unstable Hb may modify the degree of anemia. Physical findings vary and may include pallor, jaundice, and splenomegaly. Patients may have dark urine (pigmenturia) because of the excretion of dipyrrole methenes formed from the heme groups that are spontaneously lost from unstable Hbs. Fever, infections, and oxidative drugs, including those that induce hemolysis in patients with G6PD deficiency, can greatly enhance the oxidation and denaturation of unstable Hbs and cause hemolytic crises, or temporary exacerbations of the anemia.
The peripheral blood smear may be normal or show nonspecific findings such as polychromasia and basophilic stippling. Heinz bodies require supravital staining to be seen. Sometimes they are visible in stained samples of fresh blood, but usually 24-hour incubation with the supravital stain in the absence of glucose is needed to visualize them. Heinz bodies are harder to detect in patients with intact spleens because the spleen removes them from erythrocytes. The two main screening tests for unstable Hbs are the heat stability test and the isopropanol test. Some unstable Hbs are detectable by only one of these two (or other) methods; thus, if clinical suspicion is high, additional methods should be used despite a negative test. Hb electrophoresis is not the best screening test for unstable Hbs because the mutation may not alter the charge of the Hb. Less than half of unstable Hbs are demonstrable by electrophoresis, and sometimes the electrophoretic band appears to be a smeared or diffuse band.
The treatment of patients with unstable hemoglobins is primarily supportive. Occasionally, transfusion may be needed for symptomatic exacerbations of anemia due to drugs or illnesses. Parvovirus also causes the aplastic crisis, as it does in sickle cell disease (SCD), in patients with unstable Hbs. The greater the degree of instability of the Hb, hence, the shorter the life span of the RBC, the lower the Hb concentration will fall during the aplastic crisis. Transfusion of packed RBCs may be needed for symptomatic or severe anemia. Splenectomy, which may reduce but not eliminate hemolysis, is reserved for patients with symptomatic or severe hemolytic anemia.
HEMOGLOBINS WITH ABNORMAL OXYGEN AFFINITY
Hb is an oxygen-transport protein. It must have a high enough affinity to oxygen to bind it in the lungs, as well as a low enough affinity to oxygen to release it to the tissues. Hbs may be qualitatively abnormal due to alterations in this precise balance.2 Mutations that alter the oxygen affinity of Hb typically alter amino acids at either the contact points between the α- and β-globin chains or at the C-terminal end of the β-globin chain. These sites are critical for stabilization of Hb in either the oxygenated or deoxygenated state and for interaction with 2,3-bisphosphoglycerate (2,3-BPG). 2,3-BPG is a molecule in the RBC that physiologically modulates the oxygen affinity of Hb.
FIGURE 434-4. Oxyhemoglobin dissociation curve and its physiologic modulation and pathophysiologic perturbations. BPG, biphosphoglycerate.
If oxygen affinity of Hb is increased, then its oxygen delivery to tissues is decreased. The body compensates physiologically by increasing erythropoietin production, stimulating the bone marrow to increase erythropoiesis, thereby increasing the total Hb concentration and the oxygen-carrying capacity of the blood. Hence, patients with high oxygen affinity Hbs have erythrocytosis, but this is functionally appropriate because they maintain appropriate oxygen delivery to their tissues. Individuals with high oxygen affinity mutants usually have mild erythrocytosis, but some may have total Hb concentrations as high as 18 to 20 g/dL. Almost 200 qualitatively abnormal Hbs with high oxygen affinity have been discovered.
In contrast, if the oxygen affinity of Hb is decreased, its oxygen delivery to tissues is increased. The body compensates physiologically by decreasing erythropoietin production, thereby decreasing the total Hb concentration and the oxygen-carrying capacity of the blood. Hence, individuals with low oxygen affinity Hbs may have a total Hb concentration that is lower than normal. Despite a low Hb concentration, affected individuals are not functionally anemic because they maintain appropriate oxygen delivery to their tissues. The anemia is usually mild, but some may have total Hb concentrations as low as 9 to 10 g/dL. Individuals who have Hbs with greatly decreased oxygen affinity may also have cyanosis because a substantial fraction of Hb is deoxygenated. More than 50 qualitatively abnormal Hbs with low oxygen affinity have been discovered.
An abnormal Hb with altered oxygen affinity should be included in the differential diagnosis of erythrocytosis (high affinity) or anemia with or without cyanosis (low affinity). If the abnormal Hb is not also unstable or otherwise abnormal, then RBC morphology is normal. To determine whether a Hb has altered oxygen affinity, one must measure its p50 from the oxy-Hb dissociation curve (Fig. 434-4). The p50 is the partial pressure of oxygen at which Hb is 50% saturated with oxygen. Hb A is 50% saturated at an oxygen tension of about 26 mm Hg, which is the normal value of p50. The p50 of Hb is modulated physiologically by pH, temperature, and concentration of 2,3-BPG (Fig. 434-4). High-affinity Hbs have a low p50 value and a left-shifted curve, whereas low-affinity Hbs have a high p50 value and a right-shifted curve (Fig. 434-4). Cyanosis only occurs when the curve is markedly right shifted. Electrophoresis may identify oxygen affinity variants when the mutation changes the net charge of Hb. However, not all mutations alter net charge, so a normal electrophoretic pattern does not exclude a Hb with altered oxygen affinity. Most patients with these Hb variants, even those with cyanosis, require no treatment.
METHEMOGLOBIN
The final way in which Hb can be qualitatively abnormal is when the oxidation state of the heme-coordinated iron is abnormal.2,3 An oxygen-binding heme group is attached to each of the four globins in the Hb molecule. In the center of each heme group is an atom of iron that binds oxygen. This iron can exist in the ferrous (Fe2+) or ferric (Fe3+) state, but it must be in the ferrous state to bind oxygen. When the iron is in the ferric state, the Hb molecule is called methemoglobin (or ferrihemoglobin), and it cannot bind oxygen. Therefore, methemoglobin cannot deliver oxygen to tissues. Methemoglobin is normally and continually formed throughout the life of the RBC by oxidation, but the intracellular enzyme cytochrome-b5 reductase (methemoglobin reductase) converts methemoglobin back to Hb. Methemoglobins can accumulate abnormally in acquired or congenital states. Acquired (or toxic) methemoglobinemia occurs when the oxidative stress to the RBC and Hb exceeds the capacity of cytochrome-b5 reductase to reduce methemoglobin (Fe3+) to Hb (Fe2+). Congenital methemoglobinemia is caused by either a deficiency of cytochrome-b5 reductase or by qualitative abnormalities of the Hb molecule itself. The latter is a rare cause of congenital methemoglobinemia.
The differential diagnosis of cyanosis includes methemoglobinemia. Cyanosis is caused by the presence of deoxy-Hb. Individuals with methemoglobinemia may have a similar discoloration of the skin called pseudocyanosis, which is described as more brown than blue. Pseudocyanosis does not improve with supplementation with 100% oxygen, unlike many causes of cyanosis.
A number of medications and chemicals are associated with acquired methemoglobinemia, such as local anesthetics, primaquine, dapsone, nitrites and nitrates, aniline dyes, and others. Certain infections and septic shock may also increase methemoglobin production. Neonates are particularly susceptible to acquired methemoglobinemia because of their developmentally limited capacity to reduce methemoglobin. Signs and symptoms of acquired methemoglobinemia include the acute onset of pseudocyanosis and, depending on the degree of methemoglobinemia, tachypnea, dyspnea, altered mental status, and cardiorespiratory failure.
A diagnosis of congenital methemoglobinemia should be considered in individuals with chronic “cyanosis” in the absence of cardiopulmonary disease. Cytochrome-b5 reductase deficiency occurs in two forms: the red blood cell type (type I) and the generalized type (type II). Both types are autosomal recessive. Patients with type I disease (red blood cell type) are rarely sick, despite chronic pseudocyanosis, although they may have fatigue with exercise. The pseudocyanosis is mainly a cosmetic issue. However, patients may have exacerbation of symptoms when exposed to drugs, chemicals, or other oxidant stresses that increase methemoglobin formation. Type II disease is a progressive, devastating, and fatal disease that causes microcephaly, developmental impairment, and neurologic abnormalities, in addition to congenital pseudocyanosis. Enzymatic and genetic tests are available to diagnose cytochrome-b5reductase deficiency.
Qualitative Hb variants that are prone to spontaneous oxidation to methemoglobin are called the M Hbs. In most of the M Hbs, a tyrosine replaces a critical histidine residue that helps maintain the heme iron in the ferrous state. The M Hbs are inherited in an autosomal-dominant manner, unlike cytochrome-b5 reductase deficiency, which is autosomal recessive. Most M Hbs do not denature spontaneously and, therefore, do not result in hemolysis. Despite the pseudocyanosis, individuals with Hb M have no other signs or symptoms. They have no dyspnea or clubbing, and their life expectancy is normal. The oxy-Hb dissociation curve of the M Hbs is shifted to the right, enhancing oxygen delivery to tissues. This explains why affected individuals have no respiratory symptoms. Hb electrophoresis and isoelectric focusing can detect many but not all M Hbs, so sometimes other techniques such as chromatography and spectrophotometry may be necessary.
Methylene blue is given in acquired (toxic) methemoglobinemia when symptomatic or life-threatening levels of methemoglobin accumulate. Methylene blue reduces methemoglobin through the nicotinamide adenine dinucleotide phosphate (NADPH)-flavin pathway, bypassing cyto-chrome-b5 reductase. Methylene blue can also be given for cosmetic purposes, decreasing chronic pseudocyanosis, in type I cytochrome-b5 reductase deficiency. Methylene blue is ineffective and may cause hemolysis if there is concomitant glucose-6-phosphate dehydrogenase (G6PD) deficiency (see Chapter 433). Ascorbic acid can also reduce methemoglobin and reverse cyanosis to some extent, but it does not act as quickly as methylene blue. In contrast to patients with toxic methemoglobinemia and cytochrome-b5 reductase deficiency, methylene blue and ascorbic acid are ineffective for patients with Hbs M. However, treatment is not necessary. For patients with Hbs M, the avoidance of misdiagnosis and unnecessary medical tests, procedures, and treatments is key.
THALASSEMIA
The thalassemias are quantitative disorders of Hb, in contrast to the qualitative disorders discussed previously.4 Thalassemia occurs when there is decreased synthesis of generally structurally normal globin proteins. Like qualitative disorders of Hb, quantitative disorders of Hb can also be subdivided by the particular globin that is affected; for example, there can be α-thalassemias and β-thalassemias.
GENETICS AND PATHOPHYSIOLOGY
Recall that Hb is composed of four globular protein subunits, called globins. The two main types of globins are the α-globins and the β-globins, which are made in essentially equivalent amount in precursors of RBCs. Normal adult Hb (Hb A) has two α-globins and two β-globins (α2β2). Humans normally have four α-globin genes, two on each chromosome 16, and two β-globin genes, one on each chromosome 11. Mutations that decrease the synthesis of α-globins cause β-thalassemia; mutations that decrease the synthesis of β-globins cause β-thalassemia. In general, α-thalassemias are caused by deletions of DNA, whereas β-thalassemias are caused by point mutations.
If a mutation decreases the synthesis of one globin, α or β, it results in a relative excess of the other and an imbalance between the two. For example, if β-globin synthesis is diminished by a mutation, there will be a relative excess of α-globins. Such imbalanced production of α- and β-globins results in damage to precursors of RBCs in the bone marrow. This damage occurs largely because the excess unpaired globin is unstable, and it precipitates within early RBC precursors in the bone marrow and oxidatively damages the cellular membrane. If the α/β-globin imbalance is severe, there is consequent destruction of most of the RBC precursors in the bone marrow before they can be released into the circulation. A severe, microcytic anemia is the result. The body attempts to compensate for the anemia by increasing erythropoietic activity throughout the marrow and sometimes in extra-medullary spaces, although this effort is inadequate and compensation is incomplete. This pathophysiologic process is called ineffective erythropoiesis.
In the α-thalassemia syndromes, the excess β-globins form tetramers with themselves, producing Hb H (β4). The fetal counterpart of Hb H is Hb Barts, which is a tetramer of γ-globins (γ4). Hbs H and Barts can be detected by electrophoresis, although Hb Barts largely disappears during the first 6 months of life as γ-globin production is silenced. Hb H inclusions can be visualized in RBCs by supravital staining with brilliant cresyl blue (BCB stain). There is no analogous tetramer of α-globins that occurs in α-thalassemias because free α-globin is so unstable.
NOMENCLATURE OF THALASSEMIAS
The thalassemias can be described simply by two independent nomenclatures: genetic and clinical. The genetic nomenclature refers to the causative mutation, such as β-thalassemia or β-thalassemia, indicating mutations of the α- and β-globin genes, respectively. The clinical nomenclature divides the thalassemias into the asymptomatic, carrier or trait state (thalassemia minor), severe transfusion-dependent anemia (thalassemia major), and everything in between (thalassemia intermedia). The two systems can be used together, giving α-thalassemia major or β-thalassemia intermedia, for example.
THE α-THALASSEMIAS
The α-thalassemia syndromes usually result from deletion of one or more of the four α-globin genes on chromosome 16. Recall, there are normally four α-globin genes, two on each chromosome. In general, the severity of -thalassemia is proportional to the number of α-globin genes deleted (Table 434-4).
The α-thalassemia syndromes are especially prevalent among people of Southeast Asian ancestry, but they also occur in people from Africa, the Mediterranean, the Middle East, India, and Oceana. The high prevalence of thalassemia in tropical areas is believed to be the result of increased resistance of heterozygotes to severe malarial infections. That is, α-thalassemia trait is a balanced polymorphism.
α-Thalassemia Silent Carrier (αα/α-)
The deletion or inactivation of one of the four α-globin genes is called the silent carrier state. Individuals with the silent carrier state are clinically and hematologically normal, but they may be identified at birth by the presence of small amounts (1–3%) of the fast-migrating Hb Barts (γ4) by newborn screening for hemoglobinopathies (Table 434-3). In later life, because it is silent, the state can be established only by determining the number of α-globin genes by molecular genetic studies.
Table 434-4. Overview of the Prototypical Forms of α-Thalassemia
α-Thalassemia Trait (α-/α- or αα/- -)
Individuals in whom two of the four α-globin genes are deleted have a lifelong, mild microcytic anemia that neither requires nor responds to iron supplementation. This is called α-thalassemia trait or α-thalassemia minor. At birth, microcytosis (MCV < 95 fL) with 5% to 8% of Hb Barts is present. Hb Barts disappears by 3 to 6 months of age, and the hemoglobin electrophoresis then becomes normal or shows only a decreased proportion of Hb A2. Hb H is not detected by usual methods. The peripheral blood smear shows a mild, microcytic anemia and target cells. After the newborn period, when Hb Barts disappears, a definitive diagnosis may be impractical or unnecessary in this mild disorder. It is usually suspected when other causes of microcytic anemia, such as β-thalassemia trait or iron deficiency, are excluded. The α-thalassemia trait is very common. It occurs in 3% of African Americans and up to 20% of Southeast Asians.
The α-thalassemia trait can occur in two forms: a cis-deletion in which two α-genes are deleted on the same chromosome (αα/- -), and a trans-deletion in which one α-gene is deleted on each chromosome (α-/α-). Both forms are clinically indistinguishable. The cis- and trans-deletions occur in Southeast Asian populations, whereas only the trans-deletions occur in people of African ancestry. Thus, although α-thalassemia trait is common among people of African ancestry, a maximum of only two genes can be deleted in any individual because of the trans-configuration. Consequently, the more severe α-thalassemia syndromes caused by three and four α-gene deletions are not seen in people of African ancestry. This is a key point in genetic counseling (Fig. 434-5).
FIGURE 434-5. Genetic counseling for α-thalassemia trait depends on the ancestry of the parents. Severe forms of α-thalassemia do not occur in African Americans, but they do occur in Asians.
Hb H Disease (α-/- -)
Three α-globin gene deletions result in Hb H disease, which is characterized by a marked imbalance between α- and β-globin synthesis. As mentioned previously, the excess β-globins accumulate and combine to form Hb H (β4). Hb H is unstable and precipitates within RBC precursors, leading to ineffective erythropoiesis and a chronic, microcytic, hemolytic anemia. Laboratory findings include a moderately severe microcytic anemia (Hb 6–10 g/dL) with evidence of hemolysis. Precipitated Hb H can be demonstrated as inclusions within RBCs by supravital staining with brilliant cresyl blue. On Hb electrophoresis, Hb H migrates quickly (as a “fast band”) and accounts for ≈10% to 20% of the total Hb in typical Hb H disease.
Hb H disease is usually categorized as a thalassemia intermedia syndrome because it is more severe than the trait state (thalassemia minor), but regular blood transfusions are not needed for survival (thalassemia major). However, Hb H disease shows marked phenotypic variation. Some patients have a very mild, traitlike blood picture, whereas others might have severe anemia that requires frequent or regular transfusions. The reasons for this clinical variability are mostly unknown, but are the subject of ongoing investigation. In addition to anemia and jaundice, patients with Hb H disease may develop iron overload due to increased intestinal iron absorption, a consequence of ineffective erythropoiesis, and intermittent transfusions of blood. Hepatomegaly and splenomegaly may also develop as a consequence of extramedullary hematopoiesis. Marked splenomegaly may increase the severity of anemia and the need for blood transfusion, so splenectomy is sometimes a therapeutic consideration. Some patients may develop bony changes and thalassemic facies due to expansion of the bone marrow cavity.
Hydrops Fetalis: Homozygous α-Thalassemia Major (- -/- -)
Deletion of all four α-globin genes results in a syndrome of hydrops fetalis with stillbirth or immediate postnatal death. In the absence of α-globin synthesis, fetuses can only synthesize embryonic (eg, Hb Portland, δ2 γ2) and abnormal Hbs. At birth, Hb electrophoresis shows predominantly Hb Barts, smaller amounts of Hb Portland, and trace amounts of Hb H. The high oxygen affinity of Hb Barts makes it ineffective for oxygen transport, leading to severe hypoxemia out of proportion to the degree of anemia in utero. A number of infants with this syndrome who have been identified prenatally and treated with intrauterine and postnatal transfusions have survived. These rescued infants are transfusion dependent, but some have developed normally. The only cure is bone marrow transplantation. Termination of the pregnancy is often recommended because of a high frequency of severe maternal toxemia associated with a hydropic fetus.
THE β-THALASSEMIAS
The β-thalassemia syndromes are caused by mutations of the β-globin gene complex on chromosome 11. Most are point mutations within regulatory regions of the β-globin gene. β-Thalassemia genes can be classified generally by the amount of β-globin they produce: reduced amounts (β+) or none at all (β0). The genetics of the β-thalassemias are somewhat simpler than β-thalassemias because there are only two β-globin genes compared to the four β-globin genes (Table 434-5). However, there is greater phenotypic diversity of β-thalassemia than implied by Table 434-5 because of the variability in the amount of normal β-globin produced by different β+ mutations, the coinheritance of α-thalassemia, and other genetic modifiers.
The β-thalassemia genes are widely distributed throughout the world but are especially prevalent in southern Italy and Greece, with a gene frequency of 2% to 5% or more. A high prevalence of β-thalassemia also occurs in the Mideast, the Indian subcontinent, Pakistan, southern China, and Southeast Asia. It is found in about 0.5% of African Americans, but it is rare in persons of northern European ancestry. The high prevalence of thalassemia in tropical areas is believed to be the result of increased resistance of heterozygotes to severe malarial infections. That is, β-thalassemia trait is a balanced polymorphism.
Table 434-5. Overview of the Prototypical Forms of β-Thalassemia
β-Thalassemia Trait (β-Thalassemia Minor)
Heterozygosity for a β-thalassemia gene (β+ or β0) results in a mild reduction of β-globin synthesis and, therefore, a mild reduction in Hb A synthesis with consequent mild anemia. Hemoglobin levels are 1 to 2 g/dL lower than normal persons of the same age and sex. This mild anemia usually produces no symptoms, and longevity is normal. Thalassemia trait is almost always accompanied by microcytosis and hypochromia of the RBCs. Target cells, elliptocytes, and basophilic stippling are also seen on the peripheral blood smear (Fig. 434-6).
Almost all individuals with β-thalassemia trait have a mean cell volume (MCV) less than 75 fL, and the average MCV is 68 fL. In thalassemia trait, the MCV is disproportionately low for the degree of anemia because the RBC count is normal or increased. The red cell distribution width (RDW) tends to be normal in thalassemia trait, whereas it is typically increased in iron-deficiency anemia. Iron studies are normal. Unlike α-thalassemia trait, a diagnosis of β-thalassemia trait can be substantiated by Hb electrophoresis outside the neonatal period. β-Thalassemia trait typically produces an elevated Hb A2 (α2 δ2) level. The normal level of HbA2 is 1.5% to 3.5%, and Hb A2 > 3.5% is consistent with β-thalassemia trait. Levels of Hb F (α2 δ2) are normal (< 2.0%) in about half of individuals with classical thalassemia trait and moderately elevated (2.0–7.0%) in the rest.
The importance of establishing a diagnosis of β-thalassemia trait is to avoid unnecessary treatment with medicinal iron and to provide genetic counseling. Two individuals with β0- β-thalassemia trait face a 25% risk with each pregnancy of having a child with homozygous thalassemia major. Populations with a high prevalence of thalassemia trait can be screened to provide genetic counseling. In at-risk pregnancies, prenatal diagnosis can be performed as early as 10 to 12 weeks’ gestation using fetal DNA obtained by chorionic villus biopsy. Prenatal detection has greatly reduced the incidence of new cases of β-thalassemia major in Italy and Cyprus, as well in the Greek and Italian American communities of in the United States.
FIGURE 434-6. Peripheral blood film in thalassemia trait.
β-Thalassemia Intermedia
Some patients who inherit two β-thalassemia mutations are able to maintain a hemoglobin level between 6 and 9 g/dL without regular transfusions and are designated as having thalassemia intermedia. These patients are either homozygous for a mild thalassemia gene (eg, β+/+) or are compound heterozygotes for one severe and one mild thalassemia gene (eg, β0/β+). Some patients with thalassemia intermedia have mild anemia and few symptoms, whereas others have more severe anemia and the typical manifestations of thalassemia. The phenotype of β-thalassemia can be ameliorated by the coinheritance of β-thalassemia because this limits the imbalance between α- and β-globin synthesis (both α- and β-globin synthesis is reduced).
The phenotype of α- and β-thalassemia intermedia are similar. In addition to anemia and jaundice, patients with β-thalassemia intermedia disease may develop iron overload, hepatomegaly, and splenomegaly. Marked splenomegaly may increase the severity of anemia and the need for blood transfusion. Cholelithiasis (bilirubi-nate gallstones) can develop, and some patients have bony changes and thalassemic facies due to expansion of the bone marrow cavity.
Hb E-β0-Thalassemia
Hb E (α2 βE2) is the most common hemoglobinopathy in the world, occurring mainly among people of Southeast Asian ancestry. When the gene for Hb E (βE) is coinherited with a severe β-thalassemia allele (β0), this produces a form of β-thalassemia called Hb E-β0-thalassemia (Hb Eβ0). There is great clinical diversity among patients with Hb Eβ0, but affected individuals typically have a β-thalassemia intermedia phenotype with moderate-to-severe anemia and splenomegaly. Some are transfusion dependent.
β-Thalassemia Major (Cooley’s Anemia)
Homozygosity for two severe β-thalassemia genes (eg, β0/β0) causes a severe, transfusion-dependent anemia called β-thalassemia major or Cooley’s anemia. The fetus and the newborn infant with β-thalassemia major are clinically and hematologically normal, because they produce mainly Hb F (α2 γ2), which does not include β-globins. Signs and symptoms develop gradually over the first 6 to 12 months of life, when the normal postnatal silencing of γ-globin production causes a steady decline in Hb F concentration (see Chapter 429). Patients with β-thalassemia major do not have a concomitant increase in β-globin production, so no Hb A (α2 β2) can be formed as the absolute amount of Hb F falls. By the age of 6 to 12 months, most affected infants have pallor, irritability, growth retardation, jaundice, and hepatosplenomegaly as a result of extramedullary hematopoiesis. By 2 years of age, 90% of infants are symptomatic, and progressive changes in the facial and cranial bones develop without proper therapy. If thalassemia is unexpected, the Hb concentration might be as low as 3 to 5 g/dL at the time of diagnosis. Increasingly, homozygous β-thalassemia is being diagnosed in the United States and other developed countries by newborn screening for hemoglobinopathies. A newborn screen that shows only Hb F and no Hb A is consistent with β-thalassemia major (Table 434-2).
Without transfusions, patients with thalassemia major have symptomatic, severe, and life-threatening anemia with a marked increase in medullary and extramedullary ineffective hematopoiesis. Consequently, expansion of the marrow cavity in the maxilla and facial bones causes a severe cosmetic deformity with overgrowth and protrusion of the upper jaw and malocclusion. Expansion of the marrow in the cranium causes enlargement of the skull and frontoparietal bossing. This pattern of morphologic abnormalities is called thalassemic facies. Expansion of the marrow in the long bones results in marked thinning of the cortex that predisposes to pathologic fractures. Extra-medullary hematopoiesis in the liver and spleen causes abdominal enlargement from progressive hepatosplenomegaly. Paraspinous masses of extramedullary hematopoiesis may also occur. Without appropriate transfusion therapy, growth and sexual development are also greatly impaired.
The red blood cell (RBC) morphology is markedly abnormal, with marked hypochromia and microcytosis, bizarre poikilocytes, and target cells (Fig. 434-7). Nucleated RBCs are invariably present. The reticulocyte count is moderately elevated, as is the white blood cell count (corrected for nucleated RBCs). The platelet count is normal or increased but may be low if the spleen is enlarged. After splenectomy, thrombocytosis and increased numbers of nucleated RBC are seen. The unconjugated bilirubin concentration is increased. Hb electrophoresis shows predominantly Hb F. Hb A is absent in homozygous β0-thalassemia.
FIGURE 434-7. Peripheral blood film in thalassemia major.
THERAPY FOR THALASSEMIAS
Thalassemia trait (minor) requires no treatment whatsoever. The lifelong hypochromic, microcytic anemia neither requires nor responds to iron supplementation. Establishing a diagnosis of thalassemia trait will prevent repeated unnecessary diagnostic tests and inappropriate treatment with iron.
Patients with thalassemia intermedia, by definition, do not require regular or chronic transfusions of RBCs. However, intermittent simple transfusions may be needed in the event of illness or complications. For example, infection with parvovirus (the cause of erythema infectiosum or fifth disease) causes a transiently severe anemia called the aplastic crisis that may require transfusion. Other causes of worsening anemia include infection and drug-mediated hemolysis. Splenectomy may benefit patients who develop significant hypersplenism with severe anemia. Cholecystectomy may be necessary for symptomatic cholelithiasis (bilirubi-nate gallstones) or cholecystitis. Sometimes chelation therapy for iron overload is needed for older individuals, especially if they have received multiple transfusions.
Chronic Transfusion Therapy
By definition, patients with thalassemia major are transfusion dependent. Generally, modern chronic transfusion regimens are given to alleviate the severe anemia and to suppress and greatly diminish ineffective erythropoiesis. When started early in life and continued thereafter, chronic transfusions can prevent the bony and dental abnormalities, decrease extramedullary hematopoiesis, normalize growth and development, and prolong and improve quality of life. Transfusions are usually given monthly and sometimes more frequently to maintain a nadir Hb concentration in the 9.5 to 10 g/dL range. The risks of blood transfusions include alloimmunization, hemolytic and nonhemolytic transfusion reactions, transfusion-transmitted infections, and iron overload.
Chelation of Iron
Each milliliter of RBCs contains approximately 1 mg of iron, so each unit of packed RBCs (PRBCs) contains approximately 200 mg of iron. Loss of iron from the body is fixed, however, at approximately 1 mg/d, predominantly through the sloughing of gastrointestinal mucosa, and there is no physiological mechanism to excrete iron from the body. Therefore, iron overload (transfusional hemochromatosis) inevitably occurs in chronically transfused patients. Without iron chelation therapy, iron progressively accumulates in and damages the liver, pancreas, endocrine glands, and the heart. The result is hepatic fibrosis and cirrhosis, diabetes mellitus, hypothyroidism, hypoparathyroidism, dysrhythmias, and heart failure. Failure of sexual development occurs due to damage of the hypothalamic-pituitary-gonadal axis. Without appropriate chelation, myocardial siderofibrosis develops in the second decade of life and is manifested by tachydysrrhythmia and ultimately by refractory congestive heart failure, the usual cause of death.
Iron loading is monitored by serum tests of iron stores (ferritin), liver biopsy, and newer methods of iron quantitation by magnetic resonance imaging. After about 200 mL/kg of PRBCs have been transfused, chelation is usually necessary. Currently, there are two iron chelators that are approved for use in the United States: deferoxamine (Desferal) and deferasirox (Exjade). Deferoxamine is an established, effective drug that has been used for about half a century. Regular use of deferoxamine prevents endocrine, cardiac, and hepatic abnormalities and significantly prolongs life expectancy. Complications include painful swelling at injection sites, visual problems, hearing loss, and osteopenia. However, its main drawback is that it must be given by slow subcutaneous infusion over 8 to 12 hours using a battery-powered infusion pump 5 to 7 nights per week. This poses a substantial challenge to patients to adhere to the chelation regimen. Deferasirox is a newer, oral agent. It is also an effective iron chelator, but its long-term safety, efficacy, and effect of survival have yet to be established. Nevertheless, it is now widely used because of its convenient oral formulation. Its side effects include rash, nausea and vomiting, diarrhea, and renal and hepatic toxicities. Other oral iron chelators are being developed.
Splenectomy
In the past, minimally transfused patients developed splenomegaly, sometimes leading to splenectomy in early life. Massive splenomegaly is unusual in children who receive modern transfusion regimens. However, some do develop increasing transfusion requirements that exceed 200 mL of RBC/kg/year because of hypersplenism. Splenectomy is often considered for these patients to improve the survival of transfused RBCs and decrease transfusion requirements. There is an increased risk of bacterial sepsis following splenectomy, especially in young children, so antibacterial prophylaxis and immunization against the pneumococcus are necessary. There is also evidence that splenectomy increases the risk of thrombosis and, perhaps, pulmonary hypertension.
Stem Cell Transplantation
Stem cell (or bone marrow) transplantation is the only cure for thalassemia. More than 1000 transplants have been done worldwide, most in Italy. Transplantation is safest when hematopoietic stem cells are obtained from an HLA-identical sibling (without thalassemia), but only 25% of siblings will be HLA identical. Transplantation is associated with mortality rate of about 5%, a graft rejection rate of about 5%, and the occurrence of significant graft-versus-host disease in about 5%. For well-chelated children without liver disease, event-free survival at 5 years after transplantation is 85% to 95%, depending on the report. Results are considerably worse in adults and in patients who are poorly compliant with chelation and have hepatic disease. The relatively high immediate mortality associated with transplantation stands in contrast to the many years of reasonably normal life that can be obtained with appropriate transfusion and chelation therapies, which makes the decision to accept transplantation difficult for many families.