Macrocytic Anaemia

Table Of Contents
  1. Overview
  2. Folate Deficiency
  3. B12 deficiency

Overview

Macrocytic anemia is characterized by Low Hb and High MCV (> 100). B12 and folate are required for the maturation of RBCs. Immature RBCs are larger, hence the increase in MCV in B12/Folate deficiency. A false normal MCV can occur in severely malnourished patients with both B12/Folate deficiency and IDA.

  • Causes of macrocytic anemia
    • Megaloblastic anemia
      • B12 deficiency
      • Folate deficiency
      • Cytotoxic drugs
        • Phenytoin
        • Sulfa drugs
        • Trimethoprim
        • Hydroxyurea
        • Methotrexate
        • 6-MP
    • Non-megaloblastic anemia
      • Chronic alcoholism
      • Liver disease
      • Hypothyroidism
      • Diabetes Mellitus
  • Characteristic pathologic finding in macrocytic anemia
    • Large, immature RBCs (macro-ovalocytes)
    • Hypersegmented neutrophils (more than six lobes)
  • Investigations
    • Complete Blood count
      • Decreased Hb
      • Elevated MCV (macrocytic > 100fL)
      • Decreased HCT
      • Increased MCH
      • Normal MCHC (Hb count is proportionally increased)
      • WBC variable
      • PLT variable
    • PBF
      • RBC – Macro-ovalocytes in megaloblastic, Rounded-macrocytes in non-megaloblastic
      • Anisopoikilocytosis
      • Hyperchromic red cells
      • RBC inclusions: Basophilic stippling and Howell-jolly bodies
      • Leukocytes: Polysegmented PMNs (Right-shift neutrophils) and **Leukopenia (**variable)
      • Decreased platelets
      • Low reticulocytes
    • Bone Marrow
      • Diserythropoiesis: Nuclear borders, Nuclear bridging, Abnormal mitosis – abnormal erythropoiesis with bizarre bone marrow morphology and ineffective erythropoiesis
      • Marrow is markedly hypercellular (mainly due to nuclear proliferating erythroid precursors)
      • Reversed M:E ratio (from 1:1 → 1:6; normal 2:1 → 4:1) – marked erythroid hyperplasia
      • Intramedullary haemolysis (Ineffective erythropoiesis) – death of developing erythroid cells at the site of production and or production of non-viable red cells
      • Megaloblastic changes: asynchrony of nuclear and cytoplasmic development
      • Abnormalities in Granulopoiesis: Giant metamyelocytes
    • Biochemical tests
      • Elevated homocysteine
      • Elevated serum bilirubin
      • Elevated serum LDH
      • Decreased Haptoglobin
    • Diagnostic test for B12 deficiency
      • Decreased serum B12
      • Elevated serum methylmalonic acid
      • Elevated urine methylmalonic acid
      • Schilling test for B12 deficiency: diff IF Deficiency from other caused of B12 Deficiency
    • Diagnostic test for Folic acid deficiency
      • Decreased serum folic acid
      • Normal methyl malonic acid levels
Macro-ovalocytes
Hypersegmented neutrophil
Hypersegmented neutrophil
Increased MCV, characteristic of megaloblastic anemia
Increased MCV, characteristic of megaloblastic anemia

Differences in clinical presentation of Folate and B12 deficiency

Folate deficiencyB12 deficiency
Peripheral neuropathy absentPeripheral neuropathy present
Ataxia absentAtaxia present
Subacute combined degeneration absentSubacute combined degeneration present
Normal serum and urine methylmalonic acidElevated serum and urine methylmalonic acid
Elevated serum homocysteineElevated serum homocysteine

Folate Deficiency

Folate deficiency is classically seen in chronic alcohol intake, malnutrition, *and *people who do not eat vegetables. Folate stores last shorten than B12 stores (about 3 months), so it is more common than B12 deficiency

  • Absorption of folate
    • Folate is absorbed in the Jejunum and Ileum
    • Dietary folate is in the polyglutamate form while supplementary folate is in the monoglutamate form
    • Conjugase enzyme converts polyglutamate folate to its monoglutamate form
    • Absorption is by passive or active transport
    • Active transporters include: Reduced Folate carrier (RFC), Proton-coupled folate transporter (PCFT), and Folate receptor proteins (FRa, FRB)
  • Physiological role of folate
    • Synthesis of DNA (dTMP from dUMP)
    • Single carbon-unit transfer (conversion of homocysteine to methionine)
  • Role of B12 and Folate in hematopoiesis
    • Megaloblastic changes (due to nuclear-cytoplasm asynchrony/dysynchrony)
    • Ineffective erythropoiesis (defective DNA synthesis and block in cell division)
  • Causes of folate deficiency
    • Decreased intake
      • Chronic alcohol intake
      • Malnutrition
      • Overcooked vegetables and people who do not eat vegetables
      • Elderly
    • Decreased absorption
      • Coeliac disease
      • Tropical Sprue
      • Crohn’s disease
      • Short Bowel syndrome
    • Drugs
      • Phenytoin (inhibits intestinal conjugase needed for folate absorption – this is why we do not give Phenytoin to expectant mothers!)
      • Zidovudine
      • TMP-SMX
      • Methotrexate
      • Carbamazepine
      • Valproate
    • Increased demand
      • Pregnancy
      • Hemolytic anemia
      • Exfoliative skin disease
      • Hemodialysis
  • Treatment
    • PO Folate supplementation 5mg qd for 4 months
    • B12 supplementation (unless normal B12 levels)
    • Discontinue offending medication
    • Discourage alcohol use
  • Why should patients with severe deficiency receiving folate/B12 supplementation be admitted for observation (at least two weeks)?
    • Can precipitate severe Hypokalemia as RBCs are made

B12 deficiency

B12 deficiency is classically seen in chronic alcoholism, malnutrition, and strict vegetarians. Vitamin B12 stores in the liver last about 2-3 years so deficiency is rare. B12 is also required for myelination in addition to hematopoiesis, hence deficiency leads to subacute combined degeneration.

Absorption of B12
Absorption of B12
  • Absorption of B12
    • The salivary gland and fundus of the stomach secrete R-binders/ Transcobalamin-1/ Haptocorrin which binds to free B12
    • Dietary B12 bound to proteins is released by enzymes in the stomach and ileum
    • Pancreatic proteases release B12 from its R-binders
    • Parietal cells secrete Intrinsic Factor which binds to B12 in the intestines
    • B12-IF complex is absorbed by receptor- mediated endocytosis ATP-Binding Cassette Protein (ABCC1) or Multidrug Resistant Protein 1 (MDRP1)
  • Physiological role of B12
    • Folate metabolism (deficiency causes Folate trap and reduced DNA/RNA synthesis)
    • DNA/RNA synthesis (through DNA methylation by SAM)
    • Succinyl-CoA synthesis (for the synthesis of hemoglobin)
  • Causes of B12 deficiency
    • Decreased dietary intake (Vegans)
    • Pernicious Anemia
    • Atrophic Gastritis
    • Achlorhydria
    • Gastrectomy/ Bariatric surgery
    • Pancreatic insufficiency
    • Terminal ileal resection
    • Crohn’s disease
    • Diphyllobothrium latum infection
  • The Folate trap
    • In B12 deficiency, folate becomes trapped in its inactive 5-methyl-THF form
    • Therefore, it cannot be converted to it’s active form, 5,10-methylene TH4-folate, which is required for nucleic acid synthesis
Folate trap
Folate trap
  • Signs and symptoms
    • Anemia: fatigue, weakness, palpitations, pallor
    • Tongue: Red “beefy” tongue, glossitis, atrophy of papillae
    • Peripheral neuropathy
      • Glove and stocking distribution of pain and numbness (paraesthesia)
    • Subacute Combined Degeneration
      • Decreased deep tendon reflexes
      • Decreased vibration sense
      • Positive Babisnki
      • Paraesthesia
      • Psychiatric manifestations
    • Ataxia
    • Atherosclerosis: Due to increased homocysteine levels, also increases the risk for thrombosis and myocardial infaction
  • Treatment Nerve damage from B12 deficiency is permanent. Supplementation may improve peripheral neuropathy within the first 3-6 months. However, there is little effect on cord signs.
    • Monthly Parenteral Vitamin B12 replacement