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Megaloblastic Anemias

Chapter 104 | Part 4: Oncology and Hematology · Part 4 – Oncology: Hematologic Malignancies

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

🔑 Key Clinical Points

  1. Megaloblastic anemias are characterized by ineffective erythropoiesis and distinctive morphologic appearances of developing red cells in the bone marrow.
  2. The cause is usually a deficiency of either cobalamin (vitamin B12) or folate, but may occur due to genetic or acquired abnormalities affecting vitamin metabolism or DNA synthesis.
  3. Neurologic manifestations (neuropathy, dementia) are specific to cobalamin deficiency and can worsen with folic acid supplementation if cobalamin deficiency is untreated.
  4. Pernicious anemia is defined as a severe lack of intrinsic factor due to gastric atrophy and is the dominant cause of severe cobalamin deficiency in Western countries.
  5. Hypersegmented neutrophils (more than five nuclear lobes) and oval macrocytes are hallmark hematologic findings.
  6. Folic acid supplementation in pregnancy reduces neural tube defects by ~80%.
  7. MTHFR C677T polymorphism is associated with lower serum folate, higher homocysteine, and increased risk of neural tube defects and colorectal cancer.
  8. Schilling tests are no longer performed; diagnosis relies on serum cobalamin, methylmalonic acid (MMA), homocysteine, and intrinsic factor antibodies.
  9. Vegans are at risk of cobalamin deficiency due to lack of animal products, though deficiency usually does not progress to anemia without other factors.
  10. After total gastrectomy, cobalamin deficiency is inevitable and prophylactic therapy should commence immediately.

📑 Table of Contents

📋 Figures in This Chapter

# Type Description
1 🖼 Figure The role of folates in DNA synthesis and in formation of GSH,...
2 🖼 Figure A
3 🖼 Figure A

1. DEFINITION & OVERVIEW

  • Megaloblastic anemias are a group of disorders characterized by the presence of distinctive morphologic appearances of the developing red cells in the bone marrow.
  • The marrow is usually hypercellular, and the anemia is based on ineffective erythropoiesis.
  • The cause is usually a deficiency of either cobalamin (vitamin B12) or folate, but megaloblastic anemia may occur because of genetic or acquired abnormalities that affect the metabolism of these vitamins or because of defects in DNA synthesis not related to cobalamin or folate.
  • Conditions that give rise to megaloblastic changes have in common a disparity in the availability of the four immediate precursors of DNA or a block in their condensation to form DNA.
  • The four precursors are the deoxyribonucleoside triphosphates (dNTPs)—dA(adenine)TP and dG(guanine)TP (purines), dT(thymine)TP, and dC(cytosine)TP (pyrimidines).
  • In deficiencies of either folate or cobalamin, conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), the precursor of deoxythymidine triphosphate (dTTP), fails.
  • This occurs because folate is needed as the coenzyme 5,10-methylene-THF polyglutamate for conversion of dUMP to dTMP.
  • The availability of 5,10-methylene-THF is reduced in either cobalamin or folate deficiency.
  • An alternative and less likely theory for megaloblastic anemia in cobalamin or folate deficiency is misincorporation of uracil into DNA because of the accumulation of deoxyuridine triphosphate (dUTP) at the DNA replication fork as a consequence of the block in conversion of dUMP to dTMP.

1.1 Classification

  • Cobalamin deficiency or abnormalities of cobalamin metabolism.
  • Folate deficiency or abnormalities of folate metabolism.
  • Therapy with antifolate drugs (e.g., methotrexate).
  • Independent of either cobalamin or folate deficiency and refractory to cobalamin and folate therapy: Some cases of acute myeloid leukemia, myelodysplasia.
  • Therapy with drugs interfering with synthesis of DNA (e.g., cytosine arabinoside, hydroxyurea, 6-mercaptopurine, azidothymidine [AZT]).
  • Orotic aciduria (responds to uridine).
  • Thiamine-responsive.

2. ETIOLOGY & PATHOPHYSIOLOGY

  • Cobalamin (vitamin B12) exists in a number of different chemical forms. All have a cobalt atom at the center of a corrin ring.
  • In nature, the vitamin is mainly in the 2-deoxyadenosyl (ado) form, which is located in mitochondria. It is the cofactor for the enzyme L-methylmalonyl coenzyme A (CoA) mutase.
  • The other major natural cobalamin is methylcobalamin, the form in human plasma and in cell cytoplasm. It is the cofactor for methionine synthase.
  • Minor amounts of hydroxocobalamin are also present to which methyl- and ado-cobalamin are converted rapidly by exposure to light.
  • Cobalamin is synthesized solely by microorganisms. Ruminants obtain cobalamin from the foregut, but the only source for humans is food of animal origin, for example, meat, fish, and dairy products.
  • Vegetables, fruits, and other foods of nonanimal origin are free from cobalamin unless they are contaminated by bacteria.
  • A normal Western diet contains 5–30 μg of cobalamin daily.
  • Adult daily losses (mainly in the urine and feces) are 1–3 μg (~0.1% of body stores).
  • Because the body does not have the ability to degrade cobalamin, daily requirements are also about 1–3 μg.
  • Body stores are of the order of 2–3 mg, sufficient for 3–4 years if supplies are completely cut off.
  • Folic (pteroylglutamic) acid is a yellow, crystalline, water-soluble substance.
  • It is the parent compound of a large family of natural folate compounds, which differ from it in three respects: (1) they are partly or completely reduced to dihydrofolate (DHF) or tetrahydrofolate (THF) derivatives, (2) they usually contain a single carbon unit, and (3) of natural folates are folate-polyglutamates. These usually have a chain of four to six glutamate moieties rather than one, as in the monoglutamate folic acid.
  • The whole family is known as folate or vitamin B9.
  • Most foods contain some folate. The highest concentrations are found in liver, yeast, spinach, other greens, and nuts (>100 μg/100 g).
  • The total folate content of an average Western diet is 400–500 μg daily, but the amount varies widely according to the type of food eaten and the method of cooking.
  • Folate is easily destroyed by heating, particularly in large volumes of water.
  • Total-body folate in the adult is ~10 mg, with the liver containing the largest store.
  • Daily adult requirements are 100–200 μg, and so stores are sufficient for only 3–4 months in normal adults, and severe folate deficiency may develop rapidly.
  • Two mechanisms exist for cobalamin absorption. One is passive, occurring equally through buccal, duodenal, and ileal mucosa; it is rapid but extremely inefficient, with 400 μg is absorbed largely unchanged and converted to natural folates in the liver.
  • Lower doses are converted to 5-MTHF during absorption through the intestine.
  • About 60–90 μg of folate enter the bile each day and are excreted into the small intestine.
  • Loss of this folate, together with the folate of sloughed intestinal cells, accelerates the speed with which folate deficiency develops in malabsorption conditions.
  • Folate is transported in plasma; about one-third is loosely bound to an albumin, and two-thirds are unbound.
  • In all body fluids (plasma, cerebrospinal fluid, milk, bile), folate is largely, if not entirely, 5-MTHF.
  • Three types of folate-binding protein are involved.
  • A reduced folate transporter (RFC, SLC19A1) is the major route of delivery of plasma folate (5-MTHF) to cells.
  • Two folate receptors, FR2 and FR3 embedded in the cell membrane by a glycosylphosphatidylinositol anchor, transport folate into the cell via receptor-mediated endocytosis.
  • The third protein, proton-coupled folate transporter (PCFT), transports folate at low pH from the vesicle to the cell cytoplasm.
  • The reduced folate transporter also mediates uptake of methotrexate by cells.
  • Only two reactions in the body are known to require cobalamin.
  • Methylmalonyl-CoA isomerization requires adocobalamin.
  • The methylation of homocysteine to methionine requires both methylcobalamin and 5-MTHF.
  • This reaction is the first step in the pathway by which 5-MTHF, which enters bone marrow and other cells from plasma, is converted into all the intracellular folate coenzymes.
  • The coenzymes are all polyglutamated (the larger size aiding retention in the cell), but the enzyme folate polyglutamate synthase can use only THF, not 5-MTHF, as substrate.
  • In cobalamin deficiency, 5-MTHF accumulates in plasma, and intracellular folate concentrations fall due to failure of formation of THF, the substrate on which folate polyglutamates are built.
  • This has been termed THF starvation or the methylfolate trap.
  • This trap also occurs at the polyglutamate level with accumulation of the methyl form at the expense of the other one-carbon forms.
  • This theory explains the abnormalities of folate metabolism that occur in cobalamin deficiency (high serum folate, low cell folate, positive purine precursor aminoimidazole carboxamide ribonucleotide [AICAR] excretion).
  • This theory also explains why the anemia of cobalamin deficiency responds to folic acid in large doses, which overcome the methylfolate trap.

2.1 Biochemical Reactions of Folate Coenzymes

  • Folates (as the intracellular polyglutamate derivatives) act as coenzymes in the transfer of single-carbon units.
  • Two of these reactions are involved in purine synthesis and one in pyrimidine synthesis necessary for DNA and RNA replication.
  • Folate is also a coenzyme for methionine synthesis, in which methylcobalamin is also involved and in which THF is regenerated.
  • THF is the acceptor of single carbon units newly entering the active pool via conversion of serine to glycine.
  • Methionine, the other product of the methionine synthase reaction, is the precursor for S-adenosylmethionine (SAM), the universal methyl donor involved in >100 methyltransferase reactions.
  • During thymidylate synthesis, 5,10-methylene-THF is oxidized to DHF.
  • The enzyme DHF reductase converts this to THF.
  • The drugs or the methylfolate trap occur at the polyglutamate level with accumulation of the methyl form at the expense of the other one-carbon forms.

Table 1 — Table 104-2 Biochemical Reactions of Folate Coenzymes

REACTION INVOLVED COENZYME FORM OF FOLATE SINGLE CARBON UNIT TRANSFERRED IMPORTANCE
Formate activation THF −CHO Generation of 10-formyl-THF
Purine synthesis 5,10-Methylene-THF −CHO Formation of purines needed for DNA, RNA synthesis, but probably not rate-limiting
Formylation of aminoimidazole carboxamide ribonucleotide (AICAR) 10-Formyl (CHO)THF −CHO Pyrimidine synthesis
Methylation of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (dTMP) 5,10-Methylene-THF −CH3 Rate limiting in DNA synthesis
Oxidizes THF to DHF THF Some breakdown of folate at the C-9–N-10 bond
Amino acid interconversion 5-Methyl(M)THF −CH3 Homocysteine to methionine
Serine-glycine interconversion THF =CH2 Entry of single carbon units into active pool
Forminoglutamic acid to glutamic acid in histidine catabolism THF −HN−CH=

2.2 Causes of Cobalamin Deficiency

  • Cobalamin deficiency is usually due to malabsorption.
  • The only other cause is inadequate dietary intake.
  • Dietary cobalamin deficiency arises in vegans who omit meat, fish, eggs, cheese, and other animal products from their diet.
  • The largest group in the world consists of Hindus, and it is likely that many millions of Indians are at risk of deficiency of cobalamin on a nutritional basis.
  • Subnormal serum cobalamin levels are found in up to 50% of randomly selected, young, adult Indian vegans, but the deficiency usually does not progress to megaloblastic anemia since the diet of most vegans is not totally lacking in cobalamin and the enterohepatic circulation of cobalamin is intact.
  • Dietary cobalamin deficiency may also arise rarely in nonvegetarian individuals who exist on grossly inadequate diets because of poverty or psychiatric disturbance.
  • Malabsorption causes include: Pernicious anemia, Gastric causes (Total or partial gastrectomy, Simple atrophic gastritis, Zollinger-Ellison syndrome, Gastric bypass or bariatric surgery, Use of proton pump inhibitors), Intestinal causes (Intestinal stagnant loop syndrome: jejunal diverticulosis, ileocolic fistula, anatomic blind loop, intestinal stricture, etc., Ileal resection and Crohn's disease, Selective malabsorption with proteinuria, Tropical sprue, Transcobalamin II deficiency, Fish tapeworm).
  • Gastric causes of cobalamin malabsorption that are not usually sufficiently severe and prolonged to cause megaloblastic anemia include: Simple atrophic gastritis (food cobalamin malabsorption), Zollinger-Ellison syndrome, Gastric bypass or bariatric surgery, Use of proton pump inhibitors.
  • Intestinal causes of cobalamin malabsorption that are not usually sufficiently severe and prolonged to cause megaloblastic anemia include: Gluten-induced enteropathy, Severe pancreatitis, HIV infection, Radiotherapy, Graft-versus-host disease.
  • Therapy with drugs interfering with synthesis of DNA (e.g., cytosine arabinoside, hydroxyurea, 6-mercaptopurine, azidothymidine [AZT]) can cause megaloblastic anemia independent of either cobalamin or folate deficiency.
  • Orotic aciduria responds to uridine.
  • Thiamine-responsive megaloblastic anemia exists.

Table 2 — Table 104-3 Causes of Cobalamin Deficiency Sufficiently Severe to Cause Megaloblastic Anemia

Cause Megaloblastic Anemia
NUTRITIONAL VEGANS
Malabsorption Pernicious anemia
Gastric causes Congenital absence of intrinsic factor or functional abnormality
Intestinal causes Intestinal stagnant loop syndrome: jejunal diverticulosis, ileocolic fistula, anatomic blind loop, intestinal stricture, etc.
Ileal resection and Crohn's disease
Selective malabsorption with proteinuria
Tropical sprue
Transcobalamin II deficiency
Fish tapeworm

Table 3 — Table 104-4 Malabsorption of Cobalamin May Occur in the Following Conditions but Is Not Usually Sufficiently Severe and Prolonged to Cause Megaloblastic Anemia

Cause Megaloblastic Anemia
Gastric causes Simple atrophic gastritis (food cobalamin malabsorption)
Zollinger-Ellison syndrome
Gastric bypass or bariatric surgery
Use of proton pump inhibitors
Intestinal causes Gluten-induced enteropathy
Severe pancreatitis
HIV infection
Radiotherapy
Graft-versus-host disease
Deficiencies of cobalamin, folate, protein,? riboflavin,? nicotinic acid
Therapy with colchicine, para-aminosalicylate, neomycin, slow-release potassium chloride, anticonvulsant drugs, metformin,a cytotoxic drugs
Alcohol

3. CLINICAL FEATURES

  • Many symptomless patients are detected through the finding of a raised mean corpuscular volume (MCV) on a routine blood count.
  • The main clinical features in more severe cases are those of anemia.
  • Anorexia is usually marked, and weight loss, diarrhea, or constipation may be present.
  • Glossitis, angular cheilosis, a mild fever in more severely anemic patients, jaundice (unconjugated), and reversible melanin skin hyperpigmentation also may occur with a deficiency of either folate or cobalamin.
  • Thrombocytopenia sometimes leads to bruising, and this may be aggravated by vitamin C deficiency or alcohol in malnourished patients.
  • The anemia and low leukocyte count may predispose to infections, particularly of the respiratory and urinary tracts.
  • Cobalamin deficiency has also been associated in a few studies with impaired bactericidal function of phagocytes and with osteoporosis.
  • An important clinical problem is the nonanemic patient with neurologic or psychiatric abnormalities and a low or borderline serum cobalamin level.
  • In such patients, it is necessary to try to establish whether significant cobalamin deficiency is present, for example, by careful examination of the blood film for macrocytosis or hypersegmented neutrophils (see below), tests for pernicious anemia (PA) by serum gastrin level and antibodies to IF or parietal cells, and serum methylmalonic acid (MMA) measurement.
  • A trial of cobalamin therapy for at least 3 months will usually also be needed to determine whether the symptoms improve.
  • The biochemical basis for cobalamin neuropathy remains obscure.
  • Its occurrence in the absence of methylmalonic aciduria in TC II deficiency suggests that the neuropathy is related to the defect in homocysteine-methionine conversion.
  • Accumulation of S-adenosylhomocysteine in the brain, resulting in inhibition of transmethylation reactions, has been suggested.
  • Folate deficiency has been suggested to cause organic neurologic disease, but this is uncertain, although methotrexate injected into the cerebrospinal fluid may cause brain or spinal cord damage.
  • Psychiatric disturbance, as discussed above, is common in both folate and cobalamin deficiencies.
  • This, like the neuropathy, has been attributed to a failure of the synthesis of SAM, which is needed in methylation of biogenic amines (e.g., dopamine) as well as that of proteins, phospholipids, and neurotransmitters in the brain.
  • Epithelial Surfaces: After the marrow, the next most frequently affected tissues are the epithelial cell surfaces of the mouth (with glossitis), stomach, small intestine, and respiratory, urinary, and female genital tracts.
  • The cells show macrocytosis with increased numbers of multinucleate and dying cells.
  • The deficiencies may cause cervical smear abnormalities.
  • Complications of Pregnancy: The gonads are also affected, and infertility is common in both men and women with severe deficiency of either vitamin.
  • Maternal folate deficiency has been implicated as a cause of prematurity, and both folate and cobalamin deficiencies have been implicated in recurrent fetal loss and neural tube defects.
  • Neural Tube Defects: Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.
  • Cardiovascular Disease: Children with severe homocystinuria (blood levels ≥100 μmol/L) due to deficiency of one of three enzymes (methionine synthase, MTHFR, or cystathionine synthase) have vascular disease, for example, ischemic heart disease, cerebrovascular disease, or pulmonary embolus, as teenagers or in young adulthood.
  • Lesser degrees of raised serum homocysteine and low levels of serum folate and homozygous inherited mutations of MTHFR have been found to be associated with cerebrovascular, peripheral vascular, and coronary heart disease and with deep vein thrombosis.
  • Prospective randomized trials of lowering homocysteine levels with supplements of folic acid, vitamin B6, and vitamin B12 against placebo over a 5-year period in patients with vascular disease or diabetes have not, however, shown a reduction of first event fatal or nonfatal myocardial infarction, nor have these supplements reduced the risk of recurrent cardiovascular disease after an acute myocardial infarct.
  • Meta-analysis showed an 18% reduction in strokes.
  • The benefit for stroke prevention has been confirmed by a large (>20,000 subjects) randomized prospective study in hypertensive subjects in China. This showed a significant reduction in the first incidence of stroke in subjects receiving enalapril and folic acid compared to enalapril alone.
  • The effect was especially marked in the subjects commencing the prospective trial with the lowest serum folate levels.
  • Cognitive Decline: Association between low serum folate or cobalamin levels and higher homocysteine levels with the development of decreased cognitive function and of dementia in Alzheimer's disease has been reported.
  • A meta-analysis of randomized, placebo-controlled trials of homocysteine-lowering B-vitamin supplementation of individuals with and without cognitive impairment, however, showed that supplementation with vitamin B12, vitamin B6, and folic acid alone or in combination did not improve cognitive function or slow cognitive decline.
  • It is unknown whether prolonged treatment with these B vitamins can reduce the risk of dementia in later life.
  • Malignancy: Prophylactic folic acid in pregnancy has been found in some but not all studies to reduce the subsequent incidence of acute lymphoblastic leukemia (ALL) in childhood.
  • A significant negative association has also been found with the MTHFR C677T polymorphism and leukemias with mixed lineage leukemia (MLL) translocations, but a positive association was found with hyperdiploidy in infants with ALL or acute myeloid leukemia or with childhood ALL.
  • A second polymorphism in the MTHFR gene, A1298C, is also strongly associated with hyperdiploid leukemia.
  • Various positive and negative associations are noted between polymorphisms in folate-dependent enzymes and the incidence of adult ALL.
  • The C677T polymorphism is thought to lead to increased thymidine pools and "better quality" of DNA synthesis by shunting one-carbon groups toward thymidine and purine synthesis.
  • This may explain its reported association with a lower risk for colorectal cancer.
  • Most but not all studies suggest that prophylactic folic acid also protects against colon adenomas.
  • Other tumors that have been associated with folate polymorphisms or status include follicular lymphoma, breast cancer, and gastric cancer.
  • A meta-analysis of 50,000 individuals given folic acid (0.5–40 mg daily) or placebo in cardiovascular (n = 10) or colon adenoma prevention (n = 3) trials found that folic acid supplementation did not significantly increase or decrease the overall incidence of cancer or of any site-specific cancer during a weighted average scheduled treatment duration of 5.7 years.
  • Because folic acid may "feed" tumors, it probably should be avoided in those with established tumors unless severe megaloblastic anemia due to folate deficiency is present.
  • Venous thrombosis has been reported to be more frequent in folate-deficient or cobalamin-deficient subjects than in controls and to occur at unusual sites such as cerebral venous sinuses.
  • This tendency was ascribed to raised plasma homocysteine levels in folate or cobalamin deficiency, but no evidence exists that folic acid or cobalamin supplements reduce the prevalence of venous thrombosis.

3.1 Neurologic Manifestations

  • Cobalamin is needed for the myelination of the central nervous system.
  • Its deficiency may cause a bilateral peripheral neuropathy or degeneration (demyelination) of the cervical and thoracic posterior and lateral (pyramidal) tracts of the spinal cord and, less frequently, of the cranial nerves and of the white matter of the brain.
  • Optic atrophy and cerebral symptoms including dementia, depression, psychotic symptoms, and cognitive impairment may be prominent.
  • Anosmia and loss of taste may occur.
  • Magnetic resonance imaging (MRI) may show the "spongy" degeneration of the cord.
  • The patient, more frequently male, typically presents with paresthesias, muscle weakness, or difficulty in walking but sometimes may present with dementia, psychotic disturbances, or visual impairment.
  • Loss of proprioception and vibration sensation is usually present with positive Romberg and Lhermitte signs.
  • Gait may be ataxic with spasticity (hyperreflexia).
  • Autonomic nervous dysfunction can result in postural hypotension, impotence, and incontinence.
  • Long-term nutritional cobalamin deficiency in infancy leads to poor brain development and impaired intellectual development.
  • In infancy, folate feeding difficulties, lethargy, and coma may be noted.
  • Convulsions and myoclonus have been described.

3.2 Epithelial Surfaces

  • After the marrow, the next most frequently affected tissues are the epithelial cell surfaces of the mouth (with glossitis), stomach, small intestine, and respiratory, urinary, and female genital tracts.
  • The cells show macrocytosis with increased numbers of multinucleate and dying cells.
  • The deficiencies may cause cervical smear abnormalities.

3.3 Complications of Pregnancy

  • The gonads are also affected, and infertility is common in both men and women with severe deficiency of either vitamin.
  • Maternal folate deficiency has been implicated as a cause of prematurity, and both folate and cobalamin deficiencies have been implicated in recurrent fetal loss and neural tube defects.

3.4 Neural Tube Defects

  • Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.

3.5 Cardiovascular Disease

  • Children with severe homocystinuria (blood levels ≥100 μmol/L) due to deficiency of one of three enzymes (methionine synthase, MTHFR, or cystathionine synthase) have vascular disease, for example, ischemic heart disease, cerebrovascular disease, or pulmonary embolus, as teenagers or in young adulthood.
  • Lesser degrees of raised serum homocysteine and low levels of serum folate and homozygous inherited mutations of MTHFR have been found to be associated with cerebrovascular, peripheral vascular, and coronary heart disease and with deep vein thrombosis.
  • Prospective randomized trials of lowering homocysteine levels with supplements of folic acid, vitamin B6, and vitamin B12 against placebo over a 5-year period in patients with vascular disease or diabetes have not, however, shown a reduction of first event fatal or nonfatal myocardial infarction, nor have these supplements reduced the risk of recurrent cardiovascular disease after an acute myocardial infarct.
  • Meta-analysis showed an 18% reduction in strokes.
  • The benefit for stroke prevention has been confirmed by a large (>20,000 subjects) randomized prospective study in hypertensive subjects in China. This showed a significant reduction in the first incidence of stroke in subjects receiving enalapril and folic acid compared to enalapril alone.
  • The effect was especially marked in the subjects commencing the prospective trial with the lowest serum folate levels.

3.6 Cognitive Decline

  • Association between low serum folate or cobalamin levels and higher homocysteine levels with the development of decreased cognitive function and of dementia in Alzheimer's disease has been reported.
  • A meta-analysis of randomized, placebo-controlled trials of homocysteine-lowering B-vitamin supplementation of individuals with and without cognitive impairment, however, showed that supplementation with vitamin B12, vitamin B6, and folic acid alone or in combination did not improve cognitive function or slow cognitive decline.
  • It is unknown whether prolonged treatment with these B vitamins can reduce the risk of dementia in later life.

3.7 Malignancy

  • Prophylactic folic acid in pregnancy has been found in some but not all studies to reduce the subsequent incidence of acute lymphoblastic leukemia (ALL) in childhood.
  • A significant negative association has also been found with the MTHFR C677T polymorphism and leukemias with mixed lineage leukemia (MLL) translocations, but a positive association was found with hyperdiploidy in infants with ALL or acute myeloid leukemia or with childhood ALL.
  • A second polymorphism in the MTHFR gene, A1298C, is also strongly associated with hyperdiploid leukemia.
  • Various positive and negative associations are noted between polymorphisms in folate-dependent enzymes and the incidence of adult ALL.
  • The C677T polymorphism is thought to lead to increased thymidine pools and "better quality" of DNA synthesis by shunting one-carbon groups toward thymidine and purine synthesis.
  • This may explain its reported association with a lower risk for colorectal cancer.
  • Most but not all studies suggest that prophylactic folic acid also protects against colon adenomas.
  • Other tumors that have been associated with folate polymorphisms or status include follicular lymphoma, breast cancer, and gastric cancer.
  • A meta-analysis of 50,000 individuals given folic acid (0.5–40 mg daily) or placebo in cardiovascular (n = 10) or colon adenoma prevention (n = 3) trials found that folic acid supplementation did not significantly increase or decrease the overall incidence of cancer or of any site-specific cancer during a weighted average scheduled treatment duration of 5.7 years.
  • Because folic acid may "feed" tumors, it probably should be avoided in those with established tumors unless severe megaloblastic anemia due to folate deficiency is present.

3.8 Venous Thrombosis

  • Venous thrombosis has been reported to be more frequent in folate-deficient or cobalamin-deficient subjects than in controls and to occur at unusual sites such as cerebral venous sinuses.
  • This tendency was ascribed to raised plasma homocysteine levels in folate or cobalamin deficiency, but no evidence exists that folic acid or cobalamin supplements reduce the prevalence of venous thrombosis.

4. HEMATOLOGIC FINDINGS

  • Oval macrocytes, usually with considerable anisocytosis and poikilocytosis, are the main feature.
  • The MCV is usually >100 fL unless a cause of microcytosis (e.g., iron deficiency or thalassemia trait) is present.
  • Some of the neutrophils are hypersegmented (more than five nuclear lobes).
  • There may be leukopenia due to a reduction in granulocytes and lymphocytes, but this is usually >1.5 × 10^9/L.
  • The platelet count may be moderately reduced, rarely to <40 × 10^9/L.
  • The severity of all these changes parallels the degree of anemia.
  • In a nonanemic patient, the presence of a few macrocytes and hypersegmented neutrophils in the peripheral blood may be the only indication of the underlying disorder.
  • In a severely anemic patient, the marrow is hypercellular with an accumulation of primitive cells due to selective death by apoptosis of more mature forms.
  • The erythroblast nucleus maintains a primitive, fine chromatin appearance despite maturation and hemoglobinization of the cytoplasm.
  • The cells are larger than normoblasts, and an increased number of cells with eccentric lobulated nuclei or nuclear fragments may be present.
  • Giant and abnormally shaped metamyelocytes and enlarged hyperpolyploid megakaryocytes are characteristic.
  • In severe cases, the accumulation of primitive cells ("blasts") may mimic acute myeloid leukemia, whereas in less anemic patients, the changes in the marrow may be difficult to recognize.
  • The terms intermediate, mild, and early have been used.
  • The term megaloblastoid is best avoided. It has been used to describe cells with both immature-appearing nuclei and defective hemoglobinization refractory to folic acid or cobalamin therapy, especially seen in myelodysplasia.
  • Bone marrow cells, transformed lymphocytes, and other proliferating cells in the body show a variety of chromosomal changes, including random breaks, reduced contraction, spreading of the centromere, and exaggeration of secondary chromosomal constrictions and overprominent satellites.
  • Similar abnormalities may be produced by antimetabolite drugs (e.g., cytarabine, hydroxyurea, thioguanine, and methotrexate) that interfere with either DNA replication or folate metabolism and that also cause megaloblastic appearances.
  • Unconjugated bilirubin accumulates in plasma due to the death of nucleated red cells in the marrow (ineffective erythropoiesis).
  • Other evidence for this includes raised urine urobilinogen, reduced haptoglobins, positive urine hemosiderin, and raised serum lactate dehydrogenase.
  • A weakly positive direct antiglobulin test due to complement only can lead to a false diagnosis of autoimmune hemolytic anemia.

4.1 Peripheral Blood

  • Oval macrocytes, usually with considerable anisocytosis and poikilocytosis, are the main feature.
  • The MCV is usually >100 fL unless a cause of microcytosis (e.g., iron deficiency or thalassemia trait) is present.
  • Some of the neutrophils are hypersegmented (more than five nuclear lobes).
  • There may be leukopenia due to a reduction in granulocytes and lymphocytes, but this is usually >1.5 × 10^9/L.
  • The platelet count may be moderately reduced, rarely to <40 × 10^9/L.
  • The severity of all these changes parallels the degree of anemia.
  • In a nonanemic patient, the presence of a few macrocytes and hypersegmented neutrophils in the peripheral blood may be the only indication of the underlying disorder.

4.2 Bone Marrow

  • In a severely anemic patient, the marrow is hypercellular with an accumulation of primitive cells due to selective death by apoptosis of more mature forms.
  • The erythroblast nucleus maintains a primitive, fine chromatin appearance despite maturation and hemoglobinization of the cytoplasm.
  • The cells are larger than normoblasts, and an increased number of cells with eccentric lobulated nuclei or nuclear fragments may be present.
  • Giant and abnormally shaped metamyelocytes and enlarged hyperpolyploid megakaryocytes are characteristic.
  • In severe cases, the accumulation of primitive cells ("blasts") may mimic acute myeloid leukemia, whereas in less anemic patients, the changes in the marrow may be difficult to recognize.
  • The terms intermediate, mild, and early have been used.
  • The term megaloblastoid is best avoided. It has been used to describe cells with both immature-appearing nuclei and defective hemoglobinization refractory to folic acid or cobalamin therapy, especially seen in myelodysplasia.

4.3 Chromosomes

  • Bone marrow cells, transformed lymphocytes, and other proliferating cells in the body show a variety of chromosomal changes, including random breaks, reduced contraction, spreading of the centromere, and exaggeration of secondary chromosomal constrictions and overprominent satellites.
  • Similar abnormalities may be produced by antimetabolite drugs (e.g., cytarabine, hydroxyurea, thioguanine, and methotrexate) that interfere with either DNA replication or folate metabolism and that also cause megaloblastic appearances.

4.4 Ineffective Hematopoiesis

  • Unconjugated bilirubin accumulates in plasma due to the death of nucleated red cells in the marrow (ineffective erythropoiesis).
  • Other evidence for this includes raised urine urobilinogen, reduced haptoglobins, positive urine hemosiderin, and raised serum lactate dehydrogenase.
  • A weakly positive direct antiglobulin test due to complement only can lead to a false diagnosis of autoimmune hemolytic anemia.

5. INVESTIGATIONS & DIAGNOSIS

  • Cobalamin deficiency or abnormalities of cobalamin metabolism (see Tables 104-3, 104-4).
  • Folate deficiency or abnormalities of folate metabolism (see Table 104-5).
  • Therapy with antifolate drugs (e.g., methotrexate).
  • Independent of either cobalamin or folate deficiency and refractory to cobalamin and folate therapy: Some cases of acute myeloid leukemia, myelodysplasia.
  • Therapy with drugs interfering with synthesis of DNA (e.g., cytosine arabinoside, hydroxyurea, 6-mercaptopurine, azidothymidine [AZT]).
  • Orotic aciduria (responds to uridine).
  • Thiamine-responsive.
  • Formerly, the pathogenesis of cobalamin malabsorption was distinguishable based on the results of a Schilling test in which a radioactive form of cobalamin was administered orally and its appearance in the urine was a sign of absorption.
  • Radioactive cobalamin is no longer available, and Schilling tests are no longer performed.
  • Other approaches to the differential diagnosis of cobalamin malabsorption are now employed.
  • Pernicious Anemia (PA), the dominant cause of severe cobalamin deficiency in Western countries, may be defined as a severe lack of IF due to gastric atrophy.
  • It is a common disease in northern Europeans but occurs in all countries and ethnic groups.
  • It is more frequent in people of African than Asian ancestry.
  • The overall incidence is about 120 per 100,000 population in the United Kingdom (UK).
  • The ratio of incidence in men and women among whites is ~1:1.6, and the median age of onset is 70–80 years, with only 10% of patients being 60 years.
  • The parietal cell antibody is directed against the α and β subunits of the gastric proton pump (H+, K+-ATPase).
  • Juvenile Pernicious Anemia: This usually occurs in older children and resembles PA of adults.
  • Gastric atrophy, achlorhydria, and serum IF antibodies are all present, although parietal cell antibodies are usually absent.
  • About one-half of these patients show an associated endocrinopathy such as autoimmune thyroiditis, Addison's disease, or hypoparathyroidism; in some, mucocutaneous candidiasis occurs.
  • Congenital Intrinsic Factor Deficiency or Functional Abnormality: An affected child usually presents with megaloblastic anemia in the first to third year of life; a few have presented as late as the second decade.
  • The child usually has no demonstrable IF but has a normal gastric mucosa and normal secretion of acid.
  • The inheritance is autosomal recessive.
  • Parietal cell and IF antibodies are absent.
  • Variants have been described in which the child is born with IF that can be detected immunologically but is unstable or functionally inactive, unable to bind cobalamin or to facilitate its uptake by ileal receptors.
  • Gastrectomy: After total gastrectomy, cobalamin deficiency is inevitable, and prophylactic cobalamin therapy should be commenced immediately after the operation.
  • After partial gastrectomy, 10–15% of patients also develop this deficiency.
  • The exact incidence and time of onset are most influenced by the size of the resection and the preexisting size of cobalamin body stores.
  • Food Cobalamin Malabsorption: Failure of release of cobalamin from binding proteins in food is responsible for this condition, which is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.

5.1 Diagnostic Criteria for Pernicious Anemia

  • Pernicious Anemia (PA), the dominant cause of severe cobalamin deficiency in Western countries, may be defined as a severe lack of IF due to gastric atrophy.
  • It is a common disease in northern Europeans but occurs in all countries and ethnic groups.
  • It is more frequent in people of African than Asian ancestry.
  • The overall incidence is about 120 per 100,000 population in the United Kingdom (UK).
  • The ratio of incidence in men and women among whites is ~1:1.6, and the median age of onset is 70–80 years, with only 10% of patients being 60 years.
  • The parietal cell antibody is directed against the α and β subunits of the gastric proton pump (H+, K+-ATPase).

5.2 Diagnostic Criteria for Juvenile Pernicious Anemia

  • This usually occurs in older children and resembles PA of adults.
  • Gastric atrophy, achlorhydria, and serum IF antibodies are all present, although parietal cell antibodies are usually absent.
  • About one-half of these patients show an associated endocrinopathy such as autoimmune thyroiditis, Addison's disease, or hypoparathyroidism; in some, mucocutaneous candidiasis occurs.

5.3 Diagnostic Criteria for Congenital Intrinsic Factor Deficiency

  • An affected child usually presents with megaloblastic anemia in the first to third year of life; a few have presented as late as the second decade.
  • The child usually has no demonstrable IF but has a normal gastric mucosa and normal secretion of acid.
  • The inheritance is autosomal recessive.
  • Parietal cell and IF antibodies are absent.
  • Variants have been described in which the child is born with IF that can be detected immunologically but is unstable or functionally inactive, unable to bind cobalamin or to facilitate its uptake by ileal receptors.

5.4 Diagnostic Criteria for Food Cobalamin Malabsorption

  • Failure of release of cobalamin from binding proteins in food is responsible for this condition, which is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.

6. MANAGEMENT & TREATMENT

  • Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.
  • After total gastrectomy, cobalamin deficiency is inevitable, and prophylactic cobalamin therapy should be commenced immediately after the operation.
  • After partial gastrectomy, 10–15% of patients also develop this deficiency.
  • The exact incidence and time of onset are most influenced by the size of the resection and the preexisting size of cobalamin body stores.
  • Food Cobalamin Malabsorption: Failure of release of cobalamin from binding proteins in food is responsible for this condition, which is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.

6.1 Treatment of Neural Tube Defects

  • Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.

6.2 Treatment of Gastrectomy

  • After total gastrectomy, cobalamin deficiency is inevitable, and prophylactic cobalamin therapy should be commenced immediately after the operation.
  • After partial gastrectomy, 10–15% of patients also develop this deficiency.
  • The exact incidence and time of onset are most influenced by the size of the resection and the preexisting size of cobalamin body stores.

6.3 Treatment of Food Cobalamin Malabsorption

  • Failure of release of cobalamin from binding proteins in food is responsible for this condition, which is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.

7. PROGNOSIS & COMPLICATIONS

  • Life expectancy is normal in women once regular treatment has begun.
  • Men had in earlier decades a slightly subnormal life expectancy as a result of a higher incidence of carcinoma of the stomach than in controls.
  • Venous thrombosis has been reported to be more frequent in folate-deficient or cobalamin-deficient subjects than in controls and to occur at unusual sites such as cerebral venous sinuses.
  • This tendency was ascribed to raised plasma homocysteine levels in folate or cobalamin deficiency, but no evidence exists that folic acid or cobalamin supplements reduce the prevalence of venous thrombosis.

7.1 Life Expectancy

  • Life expectancy is normal in women once regular treatment has begun.
  • Men had in earlier decades a slightly subnormal life expectancy as a result of a higher incidence of carcinoma of the stomach than in controls.

7.2 Venous Thrombosis

  • Venous thrombosis has been reported to be more frequent in folate-deficient or cobalamin-deficient subjects than in controls and to occur at unusual sites such as cerebral venous sinuses.
  • This tendency was ascribed to raised plasma homocysteine levels in folate or cobalamin deficiency, but no evidence exists that folic acid or cobalamin supplements reduce the prevalence of venous thrombosis.

8. SPECIAL CONSIDERATIONS

  • Vegans: Dietary cobalamin deficiency arises in vegans who omit meat, fish, eggs, cheese, and other animal products from their diet.
  • The largest group in the world consists of Hindus, and it is likely that many millions of Indians are at risk of deficiency of cobalamin on a nutritional basis.
  • Subnormal serum cobalamin levels are found in up to 50% of randomly selected, young, adult Indian vegans, but the deficiency usually does not progress to megaloblastic anemia since the diet of most vegans is not totally lacking in cobalamin and the enterohepatic circulation of cobalamin is intact.
  • Dietary cobalamin deficiency may also arise rarely in nonvegetarian individuals who exist on grossly inadequate diets because of poverty or psychiatric disturbance.
  • Infants: Cobalamin deficiency has been described in infants born to severely cobalamin-deficient mothers.
  • These infants develop megaloblastic anemia at about 3–6 months of age, presumably because they are born with low stores of cobalamin and because they are fed breast milk with low cobalamin content.
  • The babies have also shown growth retardation, impaired psychomotor development, and other neurologic sequelae.
  • MRI shows delayed myelination and brain atrophy.
  • Pregnancy: Maternal folate deficiency has been implicated as a cause of prematurity, and both folate and cobalamin deficiencies have been implicated in recurrent fetal loss and neural tube defects.
  • Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.
  • Elderly: Food cobalamin malabsorption is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.
  • Renal/Hepatic Impairment: Not explicitly detailed in the provided text.
  • Immunocompromised: HIV infection is listed as a cause of cobalamin malabsorption.

8.1 Vegans

  • Dietary cobalamin deficiency arises in vegans who omit meat, fish, eggs, cheese, and other animal products from their diet.
  • The largest group in the world consists of Hindus, and it is likely that many millions of Indians are at risk of deficiency of cobalamin on a nutritional basis.
  • Subnormal serum cobalamin levels are found in up to 50% of randomly selected, young, adult Indian vegans, but the deficiency usually does not progress to megaloblastic anemia since the diet of most vegans is not totally lacking in cobalamin and the enterohepatic circulation of cobalamin is intact.
  • Dietary cobalamin deficiency may also arise rarely in nonvegetarian individuals who exist on grossly inadequate diets because of poverty or psychiatric disturbance.

8.2 Infants

  • Cobalamin deficiency has been described in infants born to severely cobalamin-deficient mothers.
  • These infants develop megaloblastic anemia at about 3–6 months of age, presumably because they are born with low stores of cobalamin and because they are fed breast milk with low cobalamin content.
  • The babies have also shown growth retardation, impaired psychomotor development, and other neurologic sequelae.
  • MRI shows delayed myelination and brain atrophy.

8.3 Pregnancy

  • Maternal folate deficiency has been implicated as a cause of prematurity, and both folate and cobalamin deficiencies have been implicated in recurrent fetal loss and neural tube defects.
  • Folic acid supplements at the time of conception and in the first 12 weeks of pregnancy can reduce by ~80% the incidence of neural tube defects (NTDs) (anencephaly, meningomyelocele, encephalocele, and spina bifida) in the fetus.
  • Most of this protective effect can be achieved by taking folic acid, 0.4 mg daily, before and at the time of conception.
  • The incidence of cleft palate and harelip also can be reduced by prophylactic folic acid.
  • No clear simple relationship exists between maternal folate status and these fetal abnormalities, although for NTDs, it has been established that the lower the maternal folate, the greater is the risk to the fetus.
  • NTDs also can be caused by antifolate and antiepileptic drugs.
  • An underlying maternal folate metabolic abnormality has also been postulated.
  • One abnormality has been identified: reduced activity of the enzyme 5,10-methylene-THF reductase (MTHFR) caused by a common C677T polymorphism in the MTHFR gene.
  • In one study, the prevalence of this polymorphism was found to be higher than in controls in the parents of NTD fetuses and in the fetuses themselves: homozygosity for the TT mutation was found in 13% of cases compared with 5% of control subjects.
  • The polymorphism codes for a thermolabile form of MTHFR.
  • The homozygous state results in a lower mean serum and red cell folate level compared with control subjects, as well as significantly higher serum homocysteine levels.
  • Tests for mutations in other enzymes possibly associated with NTDs, for example, methionine synthase and serine–glycine hydroxymethylase, have been negative.
  • Serum cobalamin levels are also lower in the sera of mothers of NTD infants than in controls.
  • In addition, maternal TC II receptor polymorphisms are associated with increased risk of NTD births.
  • However, no studies have been undertaken that show that dietary fortification with cobalamin reduces the incidence of NTDs.

8.4 Elderly

  • Food cobalamin malabsorption is more common in the elderly.
  • It is associated with low serum cobalamin levels, with or without raised serum levels of MMA and homocysteine.
  • Typically, these patients have normal cobalamin absorption, as measured with crystalline cobalamin, but shunting of cobalamin to the liver occurs.

8.5 HIV Infection

  • HIV infection is listed as a cause of cobalamin malabsorption.

9. KEY PEARLS & CLINICAL TRAPS

  • Neurologic manifestations (neuropathy, dementia) are specific to cobalamin deficiency and can worsen with folic acid supplementation if cobalamin deficiency is untreated.
  • Hypersegmented neutrophils (more than five nuclear lobes) and oval macrocytes are hallmark hematologic findings.
  • Pernicious anemia is autoimmune destruction of parietal cells.
  • Schilling tests are no longer performed; diagnosis relies on serum cobalamin, methylmalonic acid (MMA), homocysteine, and intrinsic factor antibodies.
  • Folic acid supplementation in pregnancy reduces neural tube defects by ~80%.
  • MTHFR C677T polymorphism is associated with lower serum folate, higher homocysteine, and increased risk of neural tube defects and colorectal cancer.
  • After total gastrectomy, cobalamin deficiency is inevitable and prophylactic therapy should commence immediately.
  • Vegans are at risk of cobalamin deficiency due to lack of animal products, though deficiency usually does not progress to anemia without other factors.
  • Ineffective erythropoiesis leads to unconjugated bilirubin accumulation, raised urine urobilinogen, reduced haptoglobins, positive urine hemosiderin, and raised serum lactate dehydrogenase.
  • A weakly positive direct antiglobulin test due to complement only can lead to a false diagnosis of autoimmune hemolytic anemia.

Figures & Illustrations

Reproduced from Harrison's 22nd Edition.

Figure 1

The role of folates in DNA synthesis and in formation...

Caption: FIGURE 104-1 The role of folates in DNA synthesis and in formation of GSH, glutathione. (Reproduced with permission from AV Hoffbrand et al [eds]: — Figure 104-1: The role of folates in DNA synthesis and in formation of S-adenosylmethionine (SAM), which is involved in numerous methylation reactions. The diagram illustrates the biochemical pathways involving folate coenzymes (THF, 5-MTHF, 5,10-Methylene-THF, 10-Formyl-THF) in purine and pyrimidine synthesis, thymidylate synthesis, and methylation reactions. It also shows the methylation cycle involving homocysteine and methionine, and the role of cobalamin (methylcobalamin) in the conversion of homocysteine to methionine.

Figure 2

A

Caption: FIGURE 104-2 A. The peripheral blood in severe megaloblastic anemia. B. The bone Postgraduate Haematology, 5th ed. Oxford, UK, Blackwell Publishing, 2005; with — Figure 104-2A: The peripheral blood in severe megaloblastic anemia. The image shows oval macrocytes with considerable anisocytosis and poikilocytosis. Some neutrophils are hypersegmented (more than five nuclear lobes).

Figure 3

A

Caption: FIGURE 104-2 A. The peripheral blood in severe megaloblastic anemia. B. The bone Postgraduate Haematology, 5th ed. Oxford, UK, Blackwell Publishing, 2005; with — Figure 104-2B: The bone marrow in severe megaloblastic anemia. The image shows a hypercellular marrow with an accumulation of primitive cells due to selective death by apoptosis of more mature forms. The erythroblast nucleus maintains a primitive, fine chromatin appearance despite maturation and hemoglobinization of the cytoplasm. Giant and abnormally shaped metamyelocytes and enlarged hyperpolyploid megakaryocytes are characteristic.

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