Megaloblastic anemia

Definition

Megaloblastic anemias, a group of disorders characterized by a distinct morphologic pattern in hematopoietic cells, are commonly due to a deficiency of vitamin B₁₂ (cobalamin) or folates. These anemias are globally prevalent and carry a significant burden of morbidity. Folate and cobalamin are both required to sustain one-carbon metabolism, which involves the transfer of one-carbon groups such as methyl-, formyl-, methylene-, methenyl-, and formimino- in enzyme reactions essential for pyrimidine and purine biosynthesis, including the synthesis of three of the four nucleotides of DNA. Thus, a deficiency in cobalamin or folate results in the common biochemical feature of a defect in DNA synthesis, along with lesser alterations in RNA and protein synthesis, leading to a state of unbalanced cell growth and impaired cell division. The majority of megaloblastic cells have DNA values between 2 and 4 N because of delayed cell division. This is morphologically expressed as larger-than-normal “immature” nuclei with finely particulate chromatin, whereas the relatively unimpaired RNA and protein synthesis results in large cells with greater “mature” cytoplasm and cell volume. The microscopic appearance of this nuclear-cytoplasmic asynchrony (or dissociation) is morphologically described as megaloblastic. Megaloblastic hematopoiesis commonly presents with anemia, the most easily recognized manifestation of a global defect in DNA synthesis in all proliferating cells (especially of the gastrointestinal and reproductive tracts). Because vitamin replacement is curative, precise identification of the deficient vitamin is essential. In the case of cobalamin, the cause of the deficiency (Table 167-1) dictates the dose and duration of replacement therapy.

Publisher Summary

In more than 95% of cases, megaloblastic anemia is as a result of folate and vitamin B₁₂ deficiency. Megaloblastic anemias are characterized by the presence of megaloblasts in the bone marrow and macrocytes in the blood. Megaloblastic anemia may also result from rare inborn errors of metabolism of folate or vitamin B12. In addition, deficiencies of ascorbic acid, tocopherol and thiamine may be related to megaloblastic anemia. The causes of megaloblastosis are also highlighted in this chapter. The age of presentation helps to focus on the most likely diagnosis. This chapter lists out disorders that give rise to megaloblastic anemia in early life and their likely age at presentation. Later, the chapter also illustrates treatment strategies for both vitamin B₁₂ and folate deficiency.

Background

Megaloblastic anemia is characterized by morphological abnormalities of hemopoietic cells that include the formation of abnormally large erythrocyte precursors (megaloblasts) and giant metamyelocytes in the bone marrow, and abnormally large (macrocytic) erythrocytes and hypersegmented neutrophils in the blood. Megaloblastic anemia is caused by a reduction in the rate of DNA biosynthesis, resulting in abnormal nuclear maturation and ineffective erythropoiesis. Folate and cobalamin deficiencies are the most common causes of the impaired DNA synthesis that leads to megaloblastic hemopoiesis; other less common factors are the use of drugs that interrupt DNA biosynthesis and inherited conditions presenting defective enzymes of DNA biosynthesis. Megaloblastic anemia caused by folate and/or cobalamin deficiencies present indistinguishable morphological abnormalities, and will be the main focus in this chapter.

Megaloblastic Anemia

Megaloblastic anemias are a group of disorders characterized by peripheral blood cytopenia due to ineffective hematopoiesis in the bone marrow. The most common cause is folate or vitamin B₁₂ deficiency or both. Vitamin B₁₂ or folate deficiency may result due to poor nutrition, malabsorption, and drugs (e.g., methotrexate and hydroxyurea). Pernicious anemia is an important cause of vitamin B₁₂ deficiency and is due to autoimmune destruction of the parietal cells. The parietal cells normally secrete intrinsic factor, which is required for the absorption of vitamin B₁₂. Antiparietal cell antibody and anti-intrinsic factor antibodies are found in patients with pernicious anemia. In megaloblastic anemia, macrocytic red cells are observed in the peripheral blood, and these are classically oval macrocytes. Hypersegmented polymorphonuclear leukocytes may be seen. Megaloblastic anemia is a cause of pancytopenia. The bone marrow shows erythroid hyperplasia with large erythroid precursors. This is known as megaloblastoid change. The myeloid precursors may also be large. Giant myelocytes and giant metamyelocytes may also be observed. Nuclear cytoplasmic dyssynchrony may also be seen. Overt features of dysplasia are seen in megaloblastic anemia. As such, this condition remains a differential diagnosis of myelodysplasia. Both conditions are associated with peripheral blood cytopenias, bone marrow hyperplasia, and dysplasia.

MEGALOBLASTIC ANEMIA

Megaloblastic anemias are associated with deficiencies of folic acid and vitamin B¹². These anemias are characterized by megaloblastic proliferation of actively growing cells, as is typically described in bone marrow aspirations, but also seen in the epithelial cells of the GI tract. Owing to impaired DNA synthesis, actively dividing cells in the gastric pits, small bowel, and colonic crypts typically show enlarged, immature-appearing nuclei (Fig. 6-5). The nucleus-to-cytoplasm ratio3 is decreased. The overall numbers of mitotic figures are also reduced. In addition, PAS-negative, Alcian blue-negative cytoplasmic vacuoles have been described in duodenal enterocytes.⁹¹ Megaloblastic anemia can be caused by pernicious anemia secondary to autoimmune gastritis; therefore, gastric findings of atrophic autoimmune gastritis may also be present.

Megaloblastic anaemia

Megaloblastic anaemia is commonly caused by deficiency of vitamin B₁₂ or folate, both of which are essential for DNA synthesis, or the administration of drugs that interfere with DNA synthesis (e.g. methotrexate). Defective DNA synthesis results in the nucleus maturing at a slower rate than the cytoplasm, thereby producing a red cell that is larger than normal (macrocyte). Teardrop cells and red cell fragments may also be seen, as a consequence of ineffective erythropoiesis (see Fig. 26.6). A common finding is the presence of hypersegmented neutrophils (defined as presence of a nucleus with more than five lobes).

Megaloblastic Anemias

Megaloblastic anemias are anemias with macrocytic, hyperchromatic erythrocyte indices. The two most common forms are vitamin B₁₂ deficiency and folic acid deficiency. Both vitamin B₁₂ and folic acid are important cofactors in the synthesis of DNA. A deficiency of either vitamin leads to an insufficient amount of DNA, resulting in the inability of bone marrow to produce an adequate amount of blood cells. This in turn results in large blood cells, each packed with an abnormally high amount of hemoglobin.^29,30

Vitamin B₁₂ deficiencyis most often caused by an autoimmune disease and results in pernicious anemia.²⁹ An autoantibody targeted toward the intrinsic factor leads to the inability to absorb vitamin B₁₂. Intrinsic factor is produced by gastric parietal cells and is required to absorb vitamin B₁₂ (extrinsic factor) in the terminal ileum. Other causes are rare and include strict vegetarian diet, malabsorption syndromes, stasis (blind loop) syndrome, and tapeworm (Diphyllobothrium latum) infection (Box 11-2).

Vitamin B₁₂ deficiency can also lead to neurologic and gastroenterologic symptoms. An atrophic tongue, known as Hunter's glossitis, is a typical sequela of vitamin B₁₂ deficiency. Degeneration of the lateral and posterior spinal cord leads toperipheral neuropathy and gait ataxia. Depression and psychotic symptoms are also seen. Clinically, the loss of sensation to vibration is an early warning sign. The diagnosis is obtained by measuring vitamin B₁₂ concentrations in plasma. At present, parenteral administration of vitamin B₁₂ is the only therapeutic option.

Folic acid deficiency is the third most common cause of anemia seen in pregnancy, resulting from increased requirements. Other risk factors for folic acid deficiency are alcoholism, abnormal dietary habits, and certain medications (methotrexate, phenytoin). Folic acid deficiency does not present with neurologic sequelae in the adult, although it has been linked to neural tube defects in early stages of pregnancy. The diagnosis is confirmed, as in vitamin B₁₂ deficiency, by measuring plasma concentrations. Folic acid, however, can be supplemented orally.

Nitrous oxide (N₂O) can irreversibly oxidize the cobalt ion found in vitamin B₁₂. Therefore, use of N₂O should be avoided in patients with megaloblastic anemia, to avoid a synergistic effect. Otherwise, the same principles apply as in treating any other form of anemia.

Megaloblastic anemia

Causes of folate deficiency (Box 12.1)

Megaloblastic anemia due to a dietary folate deficiency tends to occur in the poor, the neglected elderly, the mentally disturbed, chronic alcoholics, and infants fed almost exclusively on goat's milk (which only contains 12% of the folate in cow's milk). ‘Goat's milk anemia’ has been reported in various countries including Germany, Italy, New Zealand and the USA. Inadequate folate intake contributes to the development of folate deficiency after gastric surgery, in patients with prolonged severe illnesses and in patients with epilepsy receiving anti-convulsant drugs. In countries where folic acid fortification has been implemented, the prevalence of low plasma folate has dropped from 22% in the population to 1.7%.⁷⁵

Malabsorption. Diseases such as gluten-sensitive enteropathy and tropical sprue, which affect the upper part of the small intestine, often cause anemia due to malabsorption of folate. Reduced absorption of folate is also seen after partial gastrectomy or jejunal resection and when Crohn's disease affects the upper small intestine. In addition, it has been reported in patients taking salazopyrine (asulphidine) for inflammatory bowel disease.

Increased requirements or loss. An increased requirement of folate due to increased nucleic acid turnover may lead to folate deficiency, particularly in those taking suboptimal quantities of folate in their diet. An increased requirement occurs in pregnancy because of the needs of the growing fetus,⁷⁶ in chronic hemolytic anemias due to compensatory erythroid hyperplasia, and in premature infants because of the rapid growth during the first 2–3 months. There is also an increased folate requirement in various malignant diseases (leukemia, lymphoma, myeloproliferative neoplasms, myeloma, carcinoma), presumably due to increased proliferation of neoplastic cells. The folate requirement of the newborn on a weight for weight basis is ten-fold that of an adult and premature babies may develop megaloblastic anemia at 4–6 weeks of age.

Before the use of folate supplements during pregnancy, megaloblastic anemia was found in the latter part of pregnancy in only 2.8% of women in the UK.⁷⁷ However, examination of the bone marrow revealed that megaloblastic hemopoiesis was much more common, being present in 25% and in over 50%, respectively, of women in the UK and in South India. With the increasing awareness of the importance of adequate folate intake pre-conceptually and during pregnancy, the incidence of megaloblastic anemia of pregnancy in the developed world is now quite low. Megaloblastic anemia is particularly common in twin pregnancies and is most likely to present after the 36th week of gestation, around the time of delivery or early in the postpartum period. Folic acid fortification has mitigated folate deficiency in pregnancy where this practice has been instituted.

Patients with chronic inflammation such as those with tuberculosis or severe rheumatoid arthritis tend to become folate-deficient, probably because of a combination of: 1) inadequate intake (as the result of a poor appetite) and, 2) an increased requirement to support the increased formation of chronic inflammatory cells. In psoriasis and exfoliative dermatitis there may also be increased loss of folate via desquamation of skin cells.

Some folate is lost during long-term hemodialysis or peritoneal dialysis as folates are only loosely bound to plasma proteins. This loss is modest but may aggravate negative folate balance caused by other mechanisms. There is a substantial increase in the urinary loss of folate (to >100 µg/day) in some patients with congestive heart failure or liver disease that has been attributed to hepatocellular damage.

Therapy with dihydrofolate reductase inhibitors.^78,79 The enzyme dihydrofolate reductase, which is present in most mammalian cells, catalyses the reduction of dihydrofolate to tetrahydrofolate as well as the reduction of pteroyl glutamic acid to dihydrofolate. The dihydrofolate is derived from the 5,10-methylenetetrahydrofolate-dependent methylation of deoxyuridylate to thymidylate in which the folate is oxidized to dihydrofolate. The administration of dihydrofolate reductase inhibitors (such as methotrexate, pyrimethamine and triamterene) appears to cause megaloblastic hemopoiesis by impairing the regeneration of 5,10-methylenetetrahydrofolate from dihydrofolate and thus reducing the rate of methylation of deoxyuridylate. Trimethoprim, which is present in co-trimoxazole (Septrin or Septra), is a weak inhibitor of mammalian dihydrofolate reductase: when used in conventional dosage it causes megaloblastic hemopoiesis only in patients with a preexisting impairment of the methylation of deoxyuridylate due, for example, to a mild degree of vitamin B₁₂ or folate deficiency. When necessary, as in the use of high dose or intrathecal methotrexate, the hematologic effects of dihydrofolate reductase inhibitors may be reversed by using folinic acid (5-formyl tetrahydrofolate)

Anti-convulsant therapy and ethanol abuse. Most patients with macrocytosis associated with anti-convulsant therapy^79,80 or chronic alcoholism^3,79,81–83 do not suffer from folate deficiency (see below). In those who do, the deficiency seems to be caused mainly by an inadequate diet. Although the data are conflicting, malabsorption of folate has been described both in treated epileptics and in chronic alcoholics, and may contribute to the development of folate deficiency. Differential susceptibility of individuals to these agents may reside in differences in polymorphisms of the enzymes involved in folate metabolism. This has been incompletely investigated.

Oral contraceptive drugs. Folate-responsive megaloblastic anemia has been reported in only a few women on the contraceptive pill in whom other causes of folate deficiency appeared to have been excluded. The evidence that the pill has a significant effect on folate status is weak and controversial. Some data suggest that the pill may cause impaired folate absorption and increased urinary folate loss.

A number of patients with hereditary folate malabsorption have been reported, in whom there appears to be an abnormality in a transport system specific for folic acid. The molecular basis of this disorder has recently been identified as a defect in the proton-coupled folate transporter.⁷⁰ The condition presents in the first few months of life with megaloblastic anemia (and other hematologic abnormalities such as macrocytosis, leukopenia and, occasionally, thrombocytopenia), vomiting, diarrhea, mouth ulcers, recurrent infections, failure to thrive and neurologic abnormalities. The neurologic abnormalities can be attributed to defective transport of folic acid across the choroid plexus. Neurologic abnormalities include hypotonia, seizures, mental retardation and ataxia. Folate levels in serum, red cells and cerebrospinal fluid (CSF) are very low. The hematologic abnormalities and gastrointestinal symptoms respond to high doses of folic acid, or preferably reduced folate such as folinic acid given orally or smaller doses parenterally, and in some cases seizures improve.

The most frequent inherited disorder of folate metabolism is methylene tetrahydrofolate reductase (MTHFR) deficiency. Patients may present at any time from infancy to childhood. Symptoms vary markedly in different cases and some infants are severely ill with seizures, abnormalities of gait, breathing disorders and coma. Megaloblastic anemia or other hematologic abnormalities are usually absent. Serum, red cell and CSF folate levels are reduced, plasma homocysteine levels are increased, plasma methionine levels are normal or reduced and there is homocystinuria. Arterial and venous thrombosis may occur and histopathologic features resembling subacute combined degeneration of the cord have been found at autopsy.

Rare cases of megaloblastic hemopoiesis with normal or high serum folate levels have been caused by glutamate formiminotransferase deficiency or cyclodeaminase deficiency (there is increased formiminoglutamic acid in blood and urine after histidine loading), dihydrofolate reductase deficiency, methionine synthase deficiency or deficiency of other enzymes involved in folate metabolism. Mental retardation has developed in some cases. In addition to the mutations in the above enzymes involved in folate metabolism, there is increasing recognition that SNP of the enzymes result in functional modification of those enzymes. The most interesting of these are the polymorphisms of MTHFR, some of which show high allelic frequency in some populations. These have been implicated as a risk factor for increased incidence of neural tube defects, hyperhomocysteinemia and possibly an increased risk of venous thrombosis.