Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content + DEFINITION Download Section PDF Listen +++ ++ Disorders caused by impaired synthesis of DNA. Prototype is pernicious anemia, a deficiency of vitamin B12 (cobalamin) caused by lack of intrinsic factor, required for its absorption. Characteristics are megaloblastic cells, typically present in the erythroid series as large cells with immature-appearing nuclei but with increasing hemoglobinization of the cytoplasm—often referred to as nuclear-cytoplasmic asynchrony. Megaloblastic granulocytic cells have large size, giant band neutrophils are a feature in the marrow and hypersegmented neutrophils in the marrow and blood. Megakaryocytes may be abnormally large with nuclear abnormalities. Other rapidly dividing tissue cells, such as intestinal epithelium and uterine cervical epithelium may also show cytologic abnormalities. A uterine cervical smear when megaloblastic cytopoiesis is present may be strikingly dysplastic. + ETIOLOGY AND PATHOGENESIS Download Section PDF Listen +++ ++ Table 8–1 lists causes of megaloblastic anemia. Underlying defect is impaired DNA synthesis because of failure of conversion of dUMP to dTMP. Intramedullary destruction of red cell precursors (ineffective erythropoiesis) is a major feature of megaloblastic anemia. Ineffective granulopoiesis and thrombopoiesis are also present and can result in neutropenia and thrombocytopenia. Ineffective hematopoiesis is characterized by marked hyperplasia of precursor cells (hypercellular marrow) but exaggerated apoptosis of late precursors, which results in blood cytopenias. Mild hemolysis also occurs; the red cell life span is reduced by about 40 percent. ++Table Graphic Jump LocationTABLE 8–1CAUSES OF MEGALOBLASTIC ANEMIASView Table||Download (.pdf) TABLE 8–1 CAUSES OF MEGALOBLASTIC ANEMIAS Folate Deficiency Decreased intake Poor nutrition Old age, poverty, alcoholism Hyperalimentation Hemodialysis Premature infants Spinal cord injury Children on synthetic diets Goat's milk anemia Impaired absorption Nontropical sprue Tropical sprue Other disease of the small intestine Increased requirements Pregnancy Increased cell turnover Chronic hemolytic anemia Exfoliative dermatitis Cobalamin Deficiency Impaired absorption Gastric causes Pernicious anemia Gastrectomy Zollinger-Ellison syndrome Intestinal causes Ileal resection or disease Blind loop syndrome Fish tapeworm Pancreatic insufficiency Decreased intake Vegans Acute Megaloblastic Anemia Nitrous oxide exposure Severe illness with Extensive transfusion Dialysis Total parenteral nutrition Drugs Dihydrofolate reductase inhibitors Antimetabolites Inhibitors of deoxynucleotide synthesis Anticonvulsants Oral contraceptives Others Inborn errors Cobalamin deficiency Imerslund-Graesbeck disease Congenital deficiency of intrinsic factor Transcobalamin deficiency Errors of cobalamin metabolism "Cobalamin mutant" syndromes with homocystinuria and/or methylmalonic acidemia Errors of folate metabolism Congenital folate malabsorption Dihydrofolate reductase deficiency N 5-methyl FH4 homocysteine-methyltransferase deficiency Other errors Hereditary orotic aciduria Lesch-Nyhan syndrome Thiamine-responsive megaloblastic anemia Unexplained Congenital dyserythropoietic anemia Refractory megaloblastic anemia Erythroleukemia Source: Williams Hematology, 8th ed, Chap. 41, Table 41–5, p. 545. + CLINICAL FEATURES Download Section PDF Listen +++ ++ Anemia develops gradually and patients can adapt to very low hemoglobin levels. Eventually, as it progresses, the presenting symptoms are those of anemia with weakness, palpitation, fatigue, light-headedness, and shortness of breath. May present initially with neurologic manifestations without anemia. Folic acid deficiency and cobalamin deficiency have indistinguishable blood and marrow changes (megaloblastosis), but the former deficiency is not associated with neuropathology and the latter characteristically is (see below "Pernicious Anemia"). + LABORATORY FEATURES Download Section PDF Listen +++ ++ Erythrocytes show marked anisocytosis and poikilocytosis, with many oval macrocytes and, in severe cases, basophilic stippling, Howell-Jolly bodies, and Cabot rings. Erythrocytes with megaloblastic nuclei may be present in the blood (Fig. 8–1). Absolute reticulocyte count is low. Anemia is usually macrocytic, with MCV of 100 to 150 fL or more, but coexisting iron deficiency, thalassemia trait, or inflammation may prevent macrocytosis. Leukopenia and thrombocytopenia are frequently present. Hypersegmented neutrophils are an early sign of megaloblastosis. Typically, the nuclei of more than 5 percent of the cells have more than five lobes. Normal blood has less than 1 percent of five-lobed neutrophils. Platelets are smaller than usual and vary more widely in size. Platelets are functionally abnormal in severe megaloblastic anemia. Marrow cells show erythroid hyperplasia with striking megaloblastic changes. Promegaloblasts with mitotic figures are abundant in severe cases. The number of sideroblasts is increased, and macrophage iron content may also be increased. Coexisting iron deficiency may reduce the megaloblastic erythroid morphologic changes, but hypersegmented neutrophils are still present in the blood, and giant metamyelocytes and bands persist in the marrow. Treatment of a patient with folic acid or cobalamin more than 12 hours before marrow biopsy may mask the megaloblastic changes. Serum bilirubin, iron, and ferritin levels are somewhat elevated. Serum lactic dehydrogenase-1 and -2 and muramidase activities are markedly elevated. See below, "Laboratory Diagnosis of Tissue Cobalamin Deficiency" and "Folic Acid Deficiency," for measurement of B12 and folate tissue deficiency. ++ FIGURE 8–1 A. Pernicious anemia. Blood film. Note the striking oval macrocytes, wide variation in red cell size, and poikilocytes. Despite the anisocytosis and microcytes, the mean red cell volume is usually elevated, as in this case (MCV = 121 fL). B. Marrow precursors in pernicious anemia. Note very large size of erythroblasts (megaloblasts) and asynchronous maturation. Cell on right is a polychromatophilic megaloblast with an immature nucleus for that stage of maturation. Cell on left is an orthochromatic megaloblast with a lobulated immature nucleus. An orthochromatic megaloblast with a condensed nucleus is between and above those two cells. C and D. Two examples of hypersegmented neutrophils characteristic of megaloblastic anemia. The morphology of blood and marrow cells in folate-deficient and vitamin B12-deficient patients is identical. The extent of the morphologic changes in each case is related to the severity of the vitamin deficiency. (Reproduced with permission from Lichtman's Atlas of Hematology, www.accessmedicine.com.) (Source: Williams Hematology, 8th ed, Chapter 41, Figure 41–12, p. 543.) Graphic Jump LocationView Full Size||Download Slide (.ppt) + DIFFERENTIAL DIAGNOSIS Download Section PDF Listen +++ ++ Macrocytosis occurs without megaloblastic anemia in patients with liver disease, hypothyroidism, aplastic anemia, myelodysplasia, pregnancy, and anemias with reticulocytosis, but in these settings, the MCV rarely exceeds 110 fL. Pancytopenia with reticulocytopenia, which is often present in severe megaloblastic anemia, should be distinguished from aplastic anemia (markedly hypocellular marrow without megaloblastic morphologic changes), myelodysplastic syndrome (often blasts in blood or marrow, dysmorphic neutrophils (e.g., acquired Pelger-Huet cells, hypogranular cells) and platelets (e.g., abnormal size and granulation)), and acute myelogenous leukemia (evident leukemic myeloblasts in marrow and usually blood). Certain chemotherapeutic drugs, especially folate antagonists (e.g., methotrexate), hydroxyurea, and antiretroviral agents, may induce megaloblastic marrow and blood cell changes. + SPECIFIC FORMS OF MEGALOBLASTIC ANEMIA Download Section PDF Listen +++ +++ Cobalamin Deficiency ++ Table 8–1 presents the causes of cobalamin deficiency. Cobalamin deficiency usually results from impaired absorption, most often as a consequence of a deficiency in gastric intrinsic factor (pernicious anemia). +++ Pernicious Anemia ++ Disease of later life, usually after age 40 years, caused by failure of secretion of intrinsic factor by the gastric mucosa. Autoimmune disease in which there is immune destruction of the acid- and pepsin-secreting cells of the stomach. Concordance with several other autoimmune diseases (e.g., immune thyroid diseases, type 1 diabetes mellitus, Addison disease, and others). A family history is common, and dominant inheritance with low penetrance has been proposed. More common in persons of Northern European or African descent. Less common in those of Asian descent. Gastric atrophy and achlorhydria occur in all patients. Absence of achlorhydria is incompatible with diagnosis of pernicious anemia. The skin often assumes a lemon-yellow hue because of pallor combined with slight hyperbilirubinemia. Lingual papillary atrophy (smooth, beefy red tongue) is seen in advanced disease. The clinical features of cobalamin deficiency are those of megaloblastic anemia generally, plus neurologic abnormalities specifically caused by cobalamin deficiency. Neurologic abnormalities may occur before the onset of anemia and may be irreversible. The neurologic disorder usually begins with paresthesias of the fingers and toes and disturbances of vibration and position sense. The earliest signs may be loss of position sense in the second toe and loss of vibration sense to 256 Hz but not to 128 Hz. If untreated, the disorder progresses to spastic ataxia because of demyelination of the posterior and lateral columns of the spinal cord, referred to as combined system disease. Cobalamin deficiency also affects the brain, and patients may develop somnolence and perversion of taste, smell, and vision, sometimes with optic atrophy. Dementia or frank psychosis may occur, the latter sometimes referred to as "megaloblastic madness." Magnetic resonance imaging can confirm cobalamin deficiency affecting the brain by detecting demyelinization as T2-weighted hyperintensity of the white matter. There is a twofold increase in the incidence of gastric cancer. ++ Diagnostic laboratory findings include: ++ Evidence of serum and tissue cobalamin deficiency (see below "Laboratory Diagnosis of Tissue Cobalamin Deficiency"). Serum parietal cell antibodies (present in 90% of patients but not specific). Serum intrinsic factor antibodies are present in 70 percent of patients and specific for pernicious anemia. Their elevation is definitive evidence for pernicious anemia in patients with megaloblastic anemia and is an underutilized test. Elevated serum gastrin levels. +++ Gastrectomy and Ileal Resection Syndromes ++ Cobalamin deficiency develops within 5 to 6 years of total gastrectomy or resection of the terminal ileum, as a result of loss of secretion of intrinsic factor from the stomach or failure to absorb cobalamin-intrinsic factor complexes in the ileum. The delay in onset of the anemia reflects the time required to exhaust cobalamin stores after absorption ceases. Diseases or injury to the terminal ileum may also lead to impaired cobalamin absorption and megaloblastic anemia (e.g., regional ileitis, radiation, sprue). Cobalamin absorption may also be impaired after subtotal gastrectomy. +++ Zollinger-Ellison Syndrome ++ Gastrin-secreting tumor, usually in the pancreas, stimulates gastric mucosa to elaborate immense amounts of hydrochloric acid. Sufficient acid may be secreted to inactivate pancreatic proteases and to prevent release of cobalamin from its binder, preventing its attachment to intrinsic factor; both are necessary for cobalamin absorption. +++ "Blind Loop" Syndrome ++ Intestinal stasis from anatomic lesions or impaired motility may lead to intestinal colonization with bacteria that bind cobalamin before it can be absorbed. +++ Diphyllobothrium latum Infestation ++ These intestinal parasites, usually ingested in raw fish, bind cobalamin and prevent absorption. Only about 3 percent of people infested with the parasites become anemic. It is most prevalent in the Baltic Sea region, Canada, and Alaska where raw or undercooked fish is consumed. Diagnosis is made by identification of tape worm ova in the feces. +++ Pancreatic Disease ++ Pancreatic exocrine insufficiency leads to deficiency of pancreatic proteases necessary for cobalamin absorption. Clinically significant deficiency of cobalamin is rare. The disorder can be ameliorated by oral trypsin therapy. +++ Dietary Cobalamin Deficiency ++ Occurs rarely, usually in strict vegetarians who also avoid dairy products and eggs ("vegans"). Symptomatic cobalamin deficiency can take decades to appear because of enterohepatic reabsorption of cobalamin conserving body stores. Breast-fed infants of vegan mothers may also develop cobalamin deficiency. +++ Acute Megaloblastic Anemia ++ Acute megaloblastic anemia refers to a syndrome of rapidly developing thrombocytopenia and/or leukopenia, with very little change in the hemoglobin level. The marrow is floridly megaloblastic. The most common cause is nitrous oxide anesthesia. Nitrous oxide destroys methylcobalamin, inducing cobalamin deficiency. The marrow becomes megaloblastic within 12 to 24 hours. Hypersegmented neutrophils appear in the blood after 5 days. Serum cobalamin levels are low in most affected patients but may be normal in cobalamin deficiency because of nitrous oxide inhalation and, in some of the inherited abnormalities, of cobalamin metabolism (see below). The effects of nitrous oxide disappear in a few days. Administration of folinic acid or vitamin B12 accelerates recovery. Fatal megaloblastic anemia has occurred in patients with tetanus who were treated with nitrous oxide for weeks. Acute megaloblastic anemia may occur in seriously ill patients in intensive care units, patients transfused extensively, patients on dialysis or total parenteral nutrition, or patients receiving weak folic acid antagonists. The diagnosis is made from finding a megaloblastic marrow. Treatment is with parenteral folic acid (5 mg) and cobalamin (1 mg). +++ Megaloblastic Anemia Caused by Drugs ++ Table 8–2 presents a partial list of drugs that cause megaloblastic anemia. Methotrexate acts by inhibiting dihydrofolate reductase, the enzyme which reduces folic acid to the active, tetrahydro form. Methotrexate toxicity is treated with folinic acid, which is already fully reduced, and therefore can bypass the inhibited dihydrofolate reductase. ++Table Graphic Jump LocationTABLE 8–2DRUGS THAT CAUSE MEGALOBLASTIC ANEMIAView Table||Download (.pdf) TABLE 8–2 DRUGS THAT CAUSE MEGALOBLASTIC ANEMIA Agents Comments Antifolates Methotrexate Very potent inhibitor of dihydrofolate reductase Aminopterin Treat overdose with folinic acid Pyrimethamine Much weaker than methotrexate and aminopterin Trimethoprim Treat with folinic acid or by withdrawing the drug Sulfasalazine Can cause acute megaloblastic anemia in susceptible patients, especially those with low folate stores Chlorguanide (-Proguanil) Triamterene Use of folate and cobalamin during pemetrexed treatment reduces toxicity Pemetrexed (Alimta) Purine analogues 6-Mercaptopurine Megaloblastosis precedes hypoplasia, usually mild 6-Thioguanine Responds to folinic acid but not folate Azathioprine Acyclovir Megaloblastosis at high doses Pyrimidine analogues 5-Fluorouracil Mild megaloblastosis Floxuridine (5-fluorodeoxyuridine) 6-Azauridine Blocks uridine monophosphate production by inhibiting orotidyl decarboxylase; occasional megaloblastosis with orotic acid and orotidine in urine Zidovudine (AZT) Severe megaloblastic anemia is the major side effect Ribonucleotide reductase inhibitors Hydroxyurea Marked megaloblastosis within 1–2 days of starting therapy; quickly reversed by withdrawing drug Cytarabine (cytosine arabinoside) Early megaloblastosis is routine Anticonvulsants Phenytoin (diphenylhydantoin) Occasional megaloblastosis, associated with low folate levels; responds to high-dose folate (1–5 mg/day); how anticonvulsants cause low folate is not understood, but may be related to a drug-induced rise in cytochrome P450 Phenobarbital Primidone Carbamazepine Other drugs that depress folates Oral contraceptives Occasional megaloblastosis; sometimes dysplasia of uterine cervix, corrected with folate Glutethimide Cycloserine H+/K+-ATPase inhibitors Omeprazole Long-term use causes decreased serum cobalamin levels Lansoprazole Miscellaneous N2O See "Acute Megaloblastic Anemia" p-Aminosalicylic acid Causes cobalamin malabsorption with occasional mild megaloblastic anemia Metformin Phenformin Causes cobalamin malabsorption but not anemia Colchicine Neomycin Arsenic Causes myelodysplastic hematopoiesis, sometimes with megaloblastic changes Source: Williams Hematology, 8th ed, Chap. 41, Table 41–7, p. 554. +++ Megaloblastic Anemia in Childhood ++ Cobalamin malabsorption occurs in the presence of normal intrinsic factor in an inherited disorder of childhood (selective malabsorption of cobalamin, or Imerslund-Graesbeck disease). There is associated albuminuria. Anemia usually develops before age 2 years. Treatment is with parenteral cobalamin. Congenital intrinsic factor deficiency is an autosomal recessive disorder in which parietal cells fail to produce intrinsic factor. The disease presents at 6 to 24 months of age. Treatment is with parenteral cobalamin. Transcobalamin II deficiency is an autosomal recessive disorder that leads to megaloblastic anemia in early infancy. Serum cobalamin levels are normal, but there is severe tissue cobalamin deficiency because transcobalamin II mediates transport of cobalamins into the tissues. The diagnosis is made by measuring serum transcobalamin II concentration. Treatment is with sufficiently large doses of cobalamin to override the deficient transport. True juvenile pernicious anemia is an extremely rare disorder that usually presents in adolescence. The diagnosis and treatment are the same as for the adult disease. +++ Other Megaloblastic Anemias and Changes ++ Megaloblastic anemia may occur in some patients with inborn errors of cobalamin metabolism, inborn errors of folate metabolism, hereditary orotic aciduria, and the Lesch-Nyhan syndrome. A thiamine responsive megaloblastic anemia has also been reported. Anemia with megaloblastic-like red cell morphology ("megaloblastoid") may occur in some patients with congenital dyserythropoietic anemias, myelodysplastic syndromes, and erythroleukemia. + LABORATORY DIAGNOSIS OF TISSUE COBALAMIN DEFICIENCY Download Section PDF Listen +++ ++ Measurement of serum cobalamin level (a low level alone is insufficient for diagnosis). Transcobalamin-bound cobalamin represents about 25 percent of the total plasma cobalamin and is the functionally important fraction. Assays permit measurement of this more relevant cobalamin level. Serum cobalamin levels may be low with normal tissue levels in vegetarians, older persons, the chronically ill, people taking megadoses of vitamin C, pregnancy (25%), transcobalamin I deficiency, or folate deficiency (30%). Serum cobalamin levels are low in most affected patients, but may be normal in cobalamin deficiency because of nitrous oxide inhalation and in some of the inherited abnormalities of cobalamin metabolism (see below). Methylmalonic aciduria and elevated serum levels of methylmalonic acid are reliable indicators of tissue cobalamin deficiency (except in the presence of severe renal insufficiency). They are the earliest changes and precede anemia or morphologic blood cell changes. If normal, they argue strongly against tissue deficiency even if serum levels of the vitamin are low. Elevated serum homocysteine can indicate cobalamin deficiency but, unlike abnormalities in methylmalonic acid noted above, it can also be elevated in folic acid deficiency, pyridoxine deficiency, and hypothyroidism. Some individuals are able to absorb free cobalamin but cannot release it from food and therefore may become cobalamin deficient. +++ Folic Acid Deficiency ++ Table 8–1 summarizes the causes of folic acid deficiency. Because of the possible development of neurologic complications in untreated patients with cobalamin deficiency, it is important to evaluate all patients with macrocytic anemia for both cobalamin and folic acid deficiency. Folic acid deficiency responds to physiologic doses of folic acid (200 μg/day), but cobalamin deficiency responds only to folic acid doses of 5 mg/day. Because neurologic complications may develop in patients with cobalamin deficiency treated with folic acid, a trial with folic acid is not recommended as a diagnostic test. An inadequate diet is the principal cause of folic acid deficiency. Folic acid reserves are small, and deficiency can develop rapidly. Alcohol use can depress absorption and serum folate levels and can accelerate the appearance of megaloblastic anemia in people with early folate deficiency. In megaloblastic anemia with laboratory evidence of folic acid deficiency, a full response to physiologic doses of folic acid should occur. If a question of absorptive limitations is present, the folate should be administered intramuscularly. Serum folate levels are reduced, but a low level may merely reflect reduced oral intake in the few days preceding the test. The red cell folic acid level is a more accurate reflection of tissue folate because it is not affected by recent dietary intake or drugs. Both red cell and serum folate are decreased in folic acid deficiency. In cobalamin deficiency, red cell folate may be low but serum folate is normal or elevated. Thus, both measurements are required to assess tissue folate levels. + THERAPY, COURSE, AND PROGNOSIS Download Section PDF Listen +++ +++ Cobalamin Deficiency ++ Treatment consists of parenteral administration of cyanocobalamin (vitamin B12) or hydroxycobalamin in doses sufficient to replete tissue stores and provide daily requirements. Vitamin B12 has no toxicity per se, but parenteral cobalamin doses larger than 100 μg saturate the transport proteins and much is lost in the urine. A typical treatment schedule consists of 1000 μg of vitamin B12 intramuscularly daily for 2 weeks, then weekly until the hemoglobin level is normal, and then monthly for life. It has been recommended that after initial therapy, to return the hematocrit to normal, patients with neurologic abnormalities should receive 1000 μg every 2 weeks for 6 months. Infection can interfere with the response to vitamin B12 therapy. Transfusion may be required if the clinical picture requires prompt alleviation of anemia. Most patients, however, have adapted to severe anemia and can be treated with vitamin replacement therapy. Following initiation of cobalamin therapy, there is often a prompt improvement in the sense of well being. Marrow erythropoiesis converts from megaloblastic to normoblastic beginning about 12 hours after treatment is started. Reticulocytosis appears on days 3 to 5, and reaches a peak on days 4 to 10. The hemoglobin concentration should become normal within 1 to 2 months. Leukocyte and platelet counts normalize promptly, although neutrophil hypersegmentation persists for 10 to 14 days. Elevated serum bilirubin, serum iron, and lactic dehydrogenase levels fall to normal rapidly. Severe hypokalemia may develop after cobalamin therapy, and death from hypokalemia has occurred. Potassium levels must be monitored and appropriate replacement given. Cobalamin therapy should be administered to all patients after total gastrectomy or resection of the terminal ileum. After partial gastrectomy, patients should be monitored carefully for the development of anemia. The anemia of the blind loop syndrome will respond to parenteral cobalamin therapy, but it also responds to oral antibiotic therapy or successful correction of an anatomic abnormality. About 1 percent of an oral dose of vitamin B12 is absorbed even in the absence of intrinsic factor. Therefore, patients with pernicious anemia can be successfully treated with oral vitamin B12 in doses of 1000 μg/day. Patients receiving such therapy should be carefully monitored to ensure compliance and a response. +++ Folic Acid Deficiency ++ Folic acid is administered orally at a dose of 1 to 5 mg daily. At this dosage, patients with malabsorption usually respond. Pregnant women should receive 1 mg of folic acid daily. Women at risk for cobalamin deficiency, such as strict vegetarians, may also be given vitamin B12, 1 mg parenterally every 3 months, during the pregnancy. ++ For a more detailed discussion, see Ralph Green: Folate, Cobalamin, and Megaloblastic Anemias. Chapter 41, p. 533 in Williams Hematology, 8th ed.