Megaloblastic Anemias

Published on 22/03/2015 by admin

Filed under Pediatrics

Last modified 22/04/2025

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1566 times

Chapter 448 Megaloblastic Anemias

Norma B. Lerner

Megaloblastic anemia is a macrocytic anemia characterized by ineffective erythropoiesis, a kinetic term that describes active erythropoiesis associated with premature cell death and decreased red blood cell (RBC) output from the bone marrow. The RBCs are larger than normal at every developmental stage, and maturational asynchrony between the nucleus and cytoplasm of erythrocytes is present. The delayed nuclear development becomes increasingly evident as cell divisions proceed. Myeloid and platelet precursors are also affected, and giant metamyelocytes and neutrophil bands are often present in the bone marrow. There is usually an associated thrombocytopenia and leukopenia. The peripheral blood smear is notable for large, often oval, RBCs, with increased mean corpuscular volume (MCV). Neutrophils are characteristically hypersegmented, with many having >5 lobes. Almost all cases of childhood megaloblastic anemia result from folic acid or vitamin B12 deficiency; rarely, they may be caused by inborn errors of metabolism. Because folate and vitamin B12 are both required for the manufacture of nucleoproteins, deficiencies result in defective DNA and, to a lesser extent, RNA and protein synthesis. Megaloblastic anemias resulting from malnutrition are relatively uncommon in the USA but are important worldwide (Chapters 1 and 43).

448.1 Folic Acid Deficiency

Norma B. Lerner

Folic acid, or pteroylglutamic acid, consists of pteroic acid conjugated to glutamic acid. Biologically active folates are derived from folic acid and serve as one-carbon donors and acceptors in many biosynthetic pathways. As such, they are essential for DNA replication and cellular proliferation. Like other mammals, humans cannot synthesize folate and depend on dietary sources, including green vegetables, fruits, and animal organs (e.g., liver, kidney). Folates are heat labile and water soluble, and consequently boiling or heating folate sources leads to decreased amounts of vitamin. Naturally occurring folates are in a polyglutamated form that is less-efficiently absorbed than the monoglutamate species (i.e., folic acid). Dietary folate polyglutamates are hydrolyzed to simple folates that are absorbed primarily in the proximal small intestine by a specific carrier-mediated system. There is an active enterohepatic circulation. Folic acid is not biologically active and is reduced by dihydrofolate reductase to tetrahydrofolate, which is transported into tissue cells and polyglutamated. Because body stores of folate are limited, megaloblastic anemia will occur after 2-3 mo on a folate-free diet.

Etiology

Folic acid deficiency can occur as a consequence of inadequate folate intake, decreased folate absorption, or acquired and congenital disorders of folate metabolism.

Inadequate Folate Intake

Anemia due to decreased folate intake usually becomes manifest in the context of clinical conditions that are associated with increased vitamin requirements (e.g., pregnancy, growth in infancy and childhood, chronic hemolysis). Folate requirements increase markedly during pregnancy, in part to meet fetal needs, and deficiencies are common in mothers, particularly those who are poor or malnourished. Folate supplementation is recommended from the start of pregnancy to prevent neural tube defects and to meet the needs of the developing fetus. Fortunately, folate-deficient mothers generally do not give birth to infants with clinical folate deficiency because there is selective transfer of folate to the fetus via placental folate receptors.

Rapid growth after birth increases demands for folic acid. Human breast milk, infant formulas, and pasteurized cow’s milk provide adequate amounts of folic acid. Goat’s milk is deficient, and supplementation must be given when it is the child’s main food. Unless supplemented, powdered milk may also be a poor source of folic acid. Infants who are premature or ill and those with certain hemolytic disorders will have higher folate requirements.

Malnutrition is the most common cause of folate deficiency in older children. Those with hemoglobinopathies, infections, and/or malabsorption are at increased risk.

Decreased Folate Absorption

Malabsorption due to chronic diarrheal states or diffuse inflammatory disease can lead to folate deficiency. In both situations, some of the decreased folate absorption may be caused by impaired folate conjugase activity. Chronic diarrhea also interferes with the enterohepatic circulation of folate, thereby enhancing folate losses because of rapid intestinal passage. Megaloblastic anemia due to folic acid deficiency can occur in celiac disease or chronic infectious enteritis and in association with enteroenteric fistulas. Previous intestinal surgery is another potential cause of decreased folate absorption.

Certain anticonvulsant drugs (e.g., phenytoin, primidone, phenobarbital) can impair folic acid absorption, and many patients treated with these drugs have low serum levels of folic acid. Frank megaloblastic anemia is rare and readily responds to folic acid therapy, even when administration of the offending drug is continued. Alcohol overuse has also been associated with folate malabsorption.

Congenital Abnormalities in Folate Transport and Metabolism

Inborn errors of folate transport or metabolism are rare but can be life threatening. Those associated with megaloblastic anemia include hereditary folate malabsorption and certain extremely uncommon enzyme deficiencies. Hereditary folate malabsorption (HFM) has been associated with an inability to absorb folic acid, 5-tetrahydrofolate, 5-methyltetrahydrofolate, or 5-formyltetrahydrofolate (folinic acid). It can become apparent at 2-6 mo of age with megaloblastic anemia and other deficits resulting from folate deficiency. Serum and cerebrospinal fluid (CSF) folate levels are very low, with a loss of the normal 3 : 1 ratio of CSF to serum folate. Several loss-of-function mutations have been identified in the protein-coupled folate transporter (PCFT/SLC46A1) gene in patients with HFM. The megaloblastic anemia in HFM can be reversed with relatively low levels of serum folate, but folate sufficiency should be maintained in both the blood and the CSF to avoid other important complications. Adequate CSF levels may be particularly difficult to achieve, and very large oral doses, parenteral folate, or intrathecal folate may be needed. Though methylenetetrahydrofolate (MTHFR) deficiency is the most common inborn error of folate metabolism, severe cases produce a number of neurologic and vascular complications, but these are not associated with megaloblastic anemia.

Drug-Induced Abnormalities in Folate Metabolism

A number of drugs have anti–folic acid activity as their primary pharmacologic effect and regularly produce megaloblastic anemia. Methotrexate binds to dihydrofolate reductase and prevents formation of tetrahydrofolate, the active form of folate. Pyrimethamine, used in the therapy of toxoplasmosis, and trimethoprim, used for treatment of various infections, can induce folic acid deficiency and, occasionally, megaloblastic anemia. Therapy with folinic acid (5-formyltetrahydrofolate) usually is beneficial.

Clinical Manifestations

Mild megaloblastic anemia has been reported in very-low-birthweight infants, and folic acid supplementation is advised. Although rare nowadays, megaloblastic anemia due to folate deficiency has its peak incidence at 4-7 mo of age, somewhat earlier than iron-deficiency anemia, although both conditions may be present concomitantly in infants with poor nutrition. Besides the usual clinical features of anemia, folate-deficient infants can have irritability, chronic diarrhea, and poor weight gain. Hemorrhages from thrombocytopenia can occur in advanced cases. Congenital folate malabsorption and other rare etiologies of folate deficiency may be further associated with hypogammaglobulinemia, severe infections, failure to thrive, neurologic abnormalities, and cognitive delays.

Laboratory Findings

The anemia is macrocytic (mean corpuscular volume >100 fL). Variations in RBC shape and size are common (see Fig. 441-2). The reticulocyte count is low, and nucleated RBCs demonstrating megaloblastic morphology often are seen in the blood. Neutropenia and thrombocytopenia may rarely be present, particularly in patients with long-standing and severe deficiencies. The neutrophils are large, some with hypersegmented nuclei. Normal serum folic acid levels are 5-20 ng/mL; with deficiency, levels are <3 ng/mL. Levels of RBC folate are a better indicator of chronic deficiency. The normal RBC folate level is 150-600 ng/mL of packed cells. Levels of iron and vitamin B12 in serum usually are normal or elevated. Serum activity of lactate dehydrogenase (LDH), a marker of ineffective erythropoiesis, is markedly elevated. The bone marrow is hypercellular because of erythroid hyperplasia, and megaloblastic changes are prominent. Large, abnormal neutrophilic forms (giant metamyelocytes) with cytoplasmic vacuolation also are seen.

Treatment

When the diagnosis of folate deficiency is established, folic acid may be administered orally or parenterally at 0.5-1.0 mg/day. If the specific diagnosis is in doubt, smaller doses of folate (0.1 mg/day) may be used for 1 wk as a diagnostic test, because a hematologic response can be expected within 72 hr. Doses of folate >0.1 mg can correct the anemia of vitamin B12 deficiency but might aggravate any associated neurologic abnormalities. In most medical settings in developed countries, this therapeutic trial to distinguish the different causes of megaloblastic anemia is rarely necessary because vitamin B12 and folate blood levels usually are readily available. Folic acid therapy (0.5-1.0 mg/day) should be continued for 3-4 wk until a definite hematologic response has occurred. Maintenance therapy with a multivitamin (containing 0.2 mg of folate) is adequate. Very high doses of folate may be required in the setting of HFM. Transfusions are indicated only when the anemia is severe or the child is very ill.

Bibliography

Babior Bernard M. Folate, cobalamin, and megaloblastic anemias. In Lichtman MA, Beutler E, Kipps TJ, et al, editors: Williams hematology, ed 7, New York: McGraw-Hill, 2006.

Carmel R, Green R, Rosenblatt DS, et al. Update on cobalamin, folate and homocysteine. Hematology Am Soc Hematol Educ Program. 2003:62-81.

Rosenblatt DS, Whitehead VM. Cobalamin and folate deficiency: acquired and hereditary disorders in children. Semin Hematol. 1999;36:19-34.

Watkins D, Whitehead M, Rosenblatt DS. Nathan and Oski’s hematology of infancy and childhood, ed 7. Philadelphia: WB Saunders; 2009.

Whitehead VM. Acquired and inherited disorders of cobalamin and folate in children. Br J Haematol. 2006;124:125-136.

448.2 Vitamin B12 (Cobalamin) Deficiency

Norma B. Lerner

Because cobalamin is synthesized exclusively by certain microorganisms, animals must rely on dietary sources for their needs. Animal protein is the major source of vitamin B12 in nonvegetarians. Vitamin B12 serves as a cofactor in 2 essential metabolic reactions, namely methylation of homocysteine to methionine and conversion of methylmalonyl coenzyme A (CoA) to succinyl CoA. It is necessary for the production of tetrahydrofolate, which is important in DNA synthesis. In contrast to the situation with folate stores, older children and adults have sufficient vitamin B12 stores to last 3-5 yr. However, in young infants born to mothers with low vitamin B12 stores, clinical signs of cobalamin deficiency can become apparent in the first 6-18 mo of life.

Metabolism

Under normal circumstances, cobalamin is released from food protein in the stomach via peptic digestion at low pH. It then binds to R protein, a glycoprotein found in gastric juice and saliva. When this complex moves into the duodenum, the R binder is digested by pancreatic proteases and cobalamin is liberated. It is then taken up by intrinsic factor (IF), a protein produced by gastric parietal cells. The cobalamin-IF complex is subsequently absorbed by mucosal cells in the ileum, where cobalamin is ultimately released. It is then bound to the transport protein transcobalamin (TC)-II. It appears in the portal circulation after 3-5 hr, mostly bound to TC-II, which carries it to the liver, bone marrow, and other tissue storage sites. TC-II enters cells by receptor-mediated endocytosis, and cobalamin is converted to active forms (methylcobalamin and adenosylcobalamin) that are important in the transfer of methyl groups and in DNA synthesis. The plasma also contains two other vitamin B12–binding proteins: TC-I and TC-III. Although TC-III appears to have a transport role, TC-I does not. Both of these transcobalamins reflect vitamin B12 tissue stores, because almost all plasma vitamin B12 is bound to TC-I and TC-III.

Clinical Manifestations

Children with cobalamin deficiency often present with nonspecific manifestations such as weakness, fatigue, failure to thrive, and irritability. Other common findings include pallor, glossitis, vomiting, diarrhea, and icterus. Neurologic symptoms also occur and can include paresthesias, sensory deficits, hypotonia, seizures, developmental delay, developmental regression, and neuropsychiatric changes. Neurologic problems from vitamin B12 deficiency can occur in the absence of any hematologic abnormalities.

Laboratory Findings

The hematologic manifestations of folate and cobalamin deficiency are identical. The anemia resulting from cobalamin deficiency is macrocytic, with prominent macro-ovalocytosis of the RBCs (see Fig. 441-2). The neutrophils may be large and hypersegmented. In advanced cases, neutropenia and thrombocytopenia can occur, simulating aplastic anemia or leukemia. Serum vitamin B12 levels are low, and the serum concentrations of methylmalonic acid and homocysteine usually are elevated. Concentrations of serum iron and serum folic acid are normal or elevated. Serum LDH activity is markedly increased, a reflection of the ineffective erythropoiesis. Moderate elevations of serum bilirubin levels (2-3 mg/dL) also may be found. Excessive excretion of methylmalonic acid in the urine (normal, 0-3.5 mg/24 hr) is a reliable and sensitive index of vitamin B12 deficiency.

Etiology

Vitamin B12 deficiency can result from inadequate dietary intake of cobalamin (Cbl), lack of IF, impaired intestinal absorption of IF-Cbl, or absence of vitamin B12 transport protein.

Inadequate Vitamin B12 Intake

Daily pediatric requirements range from 0.4 to 2.4 µg. Because vitamin B12 is present in many foods, dietary deficiency is rare. However, it does occur in cases of extreme restriction (e.g., strict vegetarians or vegans) wherein no animal products or vitamin B12 supplements are consumed. In children, megaloblastic anemia from inadequate vitamin B12 intake can appear in the 1st year of life when infants are breast-fed by mothers who are vegan, have pernicious anemia, or have short gut syndrome or previous gastric bypass surgery. Maternal pernicious anemia is manifested by reduced serum vitamin B12 with or without macrocytic anemia.

Lack of Intrinsic Factor

Congenital IF deficiency is a rare autosomal recessive disorder caused by either a lack of gastric IF or by the secretion of functionally abnormal IF. It differs from typical adult pernicious anemia in that gastric acid is secreted normally and the stomach is histologically normal. It is not associated with parietal cell antibodies or endocrine abnormalities. Symptoms become prominent at an early age (6-24 mo), consistent with exhaustion of vitamin B12 stores acquired in utero. As the anemia becomes severe, weakness, irritability, anorexia, and listlessness occur. The tongue is smooth, red, and painful. Neurologic manifestations include ataxia, paresthesias, hyporeflexia, Babinski responses, and clonus.

Classic pernicious anemia usually occurs in older adults but rarely can affect children (juvenile pernicious anemia). In such cases, the disorder is immunologic. There may be atrophy of the gastric mucosa, achlorhydria, and antibodies in serum against IF and parietal cells. These children can have additional immunologic abnormalities, cutaneous candidiasis, hypoparathyroidism, and other endocrine deficiencies. An abnormal Schilling test result (see Diagnosis) is corrected by addition of exogenous IF. Parenteral vitamin B12 should be administered regularly to these patients.

Gastric surgery can also lead to intrinsic factor deficiency, and children receiving medications that impair gastric acid secretion are also at risk. Pancreatic insufficiency can result in impaired cleavage and IF complex formation that can also lead to cobalamin deficiency.

Impaired Intestinal Absorption

Patients with inflammatory diseases such as regional enteritis, neonatal necrotizing enterocolitis, or celiac disease might have impaired absorption of vitamin B12. An overgrowth of intestinal bacteria within diverticula or duplications of the small intestine can also cause vitamin B12 deficiency by consumption of (or competition for) the vitamin or by splitting of its complex with IF. In these cases, hematologic response can follow appropriate antibiotic therapy. In endemic areas, when the fish tapeworm Diphyllobothrium latum infests the upper small intestine, similar mechanisms may be operative. When megaloblastic anemia occurs in these situations, the serum vitamin B12 level is low, the gastric juice contains intrinsic factor, and the abnormal Schilling test result is not corrected by addition of exogenous IF. When the terminal ileum has been surgically removed, lifelong parenteral administration should be used if there is evidence that vitamin B12 is not absorbed.

Rare cases of abnormalities of the receptor for IF-Cbl in the ileum have also been linked to vitamin B12 deficiency and megaloblastic anemia. In some instances there is an association with abnormality of renal tubular protein reabsorption (Imerslund-Grasbeck syndrome). Imerslund-Grasbeck syndrome is an autosomal recessive disorder that is caused by defects in amnionless (AMN) or cubilin (CUBN) genes. AMN and CUBN proteins combine to form the cubam receptor complex that functions as the receptor for the IF-Cbl receptor in the ileum and for certain urinary proteins in the kidney. Monthly treatment with parenteral vitamin B12 corrects the anemia.

Absence of Vitamin B12 Transport Protein

TC-II deficiency is a rare cause of megaloblastic anemia. The role of TC-II in B12 transport is similar to that of transferrin (Tf) for iron; specific receptors for TC-II and Tf exist on cells needing vitamin B12 or iron. A congenital deficiency is inherited as an autosomal recessive condition resulting in a failure to absorb and transport vitamin B12. Most patients lack TC-II, but some have functionally defective forms. Serum vitamin B12 levels are normal because the storage forms of cobalamin, TC-I and TC-III, are not affected. This disorder usually manifests in the first weeks of life. Characteristically, there is failure to thrive, diarrhea, vomiting, glossitis, neurologic abnormalities, and megaloblastic anemia. The diagnosis of this disorder is suggested by the presence of severe megaloblastic anemia with normal serum vitamin B12 and folate levels and no evidence of any other inborn errors of metabolism. The diagnosis is made by specific tests for TC-II. The serum vitamin B12 levels must be kept high to use cobalamin. Hence, the therapy for this disorder is large parenteral doses of vitamin B12 given twice a week for life.

Diagnosis

The specific cause of vitamin B12 deficiency often is apparent from the clinical history. In cases where there is a reasonable explanation for decreased vitamin B12 absorption (previous gastric or ileal surgery), it may be reasonable to start appropriate therapy without further evaluation. In very young children in whom dietary insufficiency may be a factor, evaluation of the mother for anemia and serum vitamin B12 often is rewarding. If there is no obvious cause for decreased serum vitamin B12, absorption of vitamin B12 can be assessed by the Schilling test.

Although it is considered the gold standard for testing vitamin B12 absorption, the Schilling test has become less widely available. When a normal person ingests a small amount of vitamin B12 into which cobalt-57 has been incorporated, the radioactive vitamin combines with the IF in stomach secretions and passes to the terminal ileum, where absorption occurs. Because the absorbed vitamin is bound to TC-II and incorporated into tissues, little or none is normally excreted in the urine. If a large dose (1 mg) of nonradioactive vitamin B12 is injected parenterally after 2 hr (flushing dose), 10-30% of the previously absorbed radioactive vitamin appears in the urine in 24 hr. Children with pernicious anemia usually excrete ≤2% under these conditions.

To confirm that absence of IF is the basis of the vitamin B12 malabsorption, IF is given with a second dose of radioactive vitamin B12. Normal amounts of radioactive vitamin should now be absorbed and flushed out in the urine. However, when vitamin B12 malabsorption results from absence of ileal receptor sites or other intestinal causes, no improvement in absorption occurs with IF. The Schilling test result remains abnormal in patients with pernicious anemia, even when therapy has completely reversed the hematologic and neurologic manifestations of the disease.

Treatment

Treatment regimens in children have not been well studied. The cause of vitamin B12 deficiency should ultimately dictate treatment dosage as well as the duration of therapy. Dose adjustments should be made in response to clinical status and laboratory values. The physiologic requirement for vitamin B12 is about 1-3 µg/day. Hematologic responses have been observed with small doses, indicating that administration of a minidose may be used as a therapeutic test when the diagnosis of vitamin B12 deficiency is in doubt or in circumstances where the anemia is severe and higher initial doses might result in severe metabolic disturbances.

Bibliography

Babior Bernard M. Folate, cobalamin, and megaloblastic anemias. In Lichtman MA, Beutler E, Kipps TJ, et al, editors: Williams hematology, ed 7, New York: McGraw-Hill, 2006.

Carmel R, Green R, Rosenblatt DS, et al. Update on cobalamin, folate and homocysteine. Hematology Am Soc Hematol Educ Program. 2003:62-81.

Grasbeck R. Imerslund-Grasbeck syndrome (selective vitamin B12 malabsorption with proteinuria). Orphanet J Rare Dis. 2006;19:1-17.

Monagle PT, Tauro GP. Infantile megaloblastosis secondary to maternal vitamin B12 deficiency. Clin Lab Haematol. 1997;19:23-25.

Rasmussen SA, Fernhoff PM, Scanlon KS. Vitamin B12 deficiency in children and adolescents. J Pediatr. 2001;138:10-17.

Rosenblatt DS, Whitehead VM. Cobalamin and folate deficiency: acquired and hereditary disorders in children. Semin Hematol. 1999;36:19-34.

Watkins D, Whitehead M, Rosenblatt DS. Nathan and Oski’s hematology of infancy and childhood, ed 7. Philadelphia: WB Saunders; 2009.

Whitehead VM. Acquired and inherited disorders of cobalamin and folate in children. Br J Haematol. 2006;124:125-136.

Xu D, Kozyraki R, Newman TC, et al. Genetic evidence of an accessory activity required specifically for cubilin brush-border expression and intrinsic factor-cobalamin absorption. Blood. 1999;94:3604-3606.

448.3 Other Rare Megaloblastic Anemias

Norma B. Lerner

Orotica ciduria is a rare autosomal recessive disorder that usually appears in the 1st year of life and is characterized by growth failure, developmental retardation, megaloblastic anemia, and increased urinary excretion of orotic acid (Chapter 83). This defect is the most common metabolic error in the de novo synthesis of pyrimidines and therefore affects nucleic acid synthesis. The usual form of hereditary orotic aciduria is caused by a deficiency (in all body tissues) of orotic phosphoribosyl transferase (OPT) and orotidine-5-phosphate decarboxylase (ODC), two sequential enzymatic steps in pyrimidine nucleotide synthesis. The diagnosis is suggested by the presence of severe megaloblastic anemia with normal serum B12 and folate levels and no evidence of TC-II deficiency. A presumptive diagnosis is made by finding increased urinary orotic acid. However, confirmation of the diagnosis requires assay of the transferase and decarboxylase enzymes in the patient’s erythrocytes. Physical and mental retardation often accompany this condition. The anemia is refractory to vitamin B12 or folic acid but responds promptly to administration of uridine.

Thiamine-responsive megaloblastic anemia (TRMA) is characterized by megaloblastic anemia, sensorineural deafness, and diabetes mellitus (Roger syndrome). Congenital heart defects, optic nerve atrophy, short stature, and strokes are also described. This disease is an autosomal recessive disorder that manifests in childhood and occurs in several ethnically distinct populations. The bone marrow is characterized not only by megaloblastic changes but also by ringed sideroblasts. The anemia usually improves with high doses of thiamine. The defect is due to mutations in the SCL192A gene on chromosome 1, which encodes a high-affinity thiamine transporter.

Megaloblastic anemia may also occur in certain inborn errors of cobalamin metabolism.

Bibliography

Bergmann AK, Sahai I, Falcone JF, et al. Thiamine responsive megaloblastic anemia: identification of novel compound heterozygotes and mutation update. J Pediatr. 2009;155:889-892.

Borgna-Pignatti C, Azzalli M, Pedretti S. Thiamine-responsive megaloblastic anemia syndrome: long term follow-up. J Pediatr. 2009;155:295-297.

Watkins D, Whitehead M, Rosenblatt DS. Nathan and Oski’s hematology of infancy and childhood, ed 7. Philadelphia: WB Saunders; 2009.

Related posts:

Anesthesia, Perioperative Care, and Sedation Goiter Default ThumbnailGynecologic Care for Girls with Special Needs Cardiac Development Hypothyroidism Cystic Diseases of the Biliary Tract and Liver