Megaloblastic Anemias
Disorders caused by impaired DNA synthesis.
Affected cells are mainly those with rapid turnover
such as hematopoietic precursors & GIT mucosae.
Vitamin B12 def & folic acid def
Folic Acid
It is pteroylmonoglutamic acid.
Synthesized by many plants (fruits &
vegetables) & bacteria.
Destroyed by excessive cooking.
Daily requirement 50 µg ( ↑ by metab demands
as preg)
Dietary polyglutamate converted into mono &
diglutamates then absorbed in jejunum.
Folate-binding protein in plasma, milk, & body
fluids.
Folate body stores = 5-20 mg (1/2 in liver)
Function of folate to transfer 1-carbon moiety such as
methyl & formyl groups to various organic
compounds.
They are usually transferred from serine that reacts
with tetrahydrofolate to produce glycine & methyl
tetrahydrofolate or from formiminoglutamic acid to
produce formiminotetrahydrofolate & glutamic acid.
This is essential for synthesis of purines, thymidine, &
methioninie. Dihydrofolate has to be reduced to
tetrahydrofolate by folate reductase. Inhibitors of
this enzyme cause folate def such as MTx,
pyrimethamine, triamterene, pentamidine,
trimethoprim.
Cobalamin
A complex organometallic vitamin with cobalt
atom situated in the middle of a corrin ring. It
cannot be synthesized in the human body &
must be taken in diet (only animal products)
Daily req = 2.5 mg
In stomach it is released from diet, then binds
R protein (glycoprotein found in saliva, milk,
gastric juice & bile).
Cobalamin-R in duodenum is digested
releasing Cobalamin that binds to intrinsic
factor (IF).
IF is a 50 kDalton glycoprotein produced by gastric
parietal cells (parallel to hydrochloric acid
secretion).
Cobalamin-IF not digested by proteolytic enzymes
reaches terminal ileum where specific receptors for
absorption.
Within intestinal mucosal cell IF destroyed &
Cobalamin transferred to TC-II (B12 binding transfer
protein)
CBL-TC complex enter circ taken up rapidly by liver,
BM & other cells. Liver store = 2 mg. Other stores =
2mg.
3-6 years needed to develop B12 deficiency in malabs
Most Cbl in circ bound to TC-I.
Methyl-Cbl is required for methionine synthase
in conversion of homocysteine to methionine
therefore DNA synthesis is impaired.
Plasma homocysteine ↑in both folate & vitB12
def
Adenosyl Cbl is required for conversion of
methylmalonyl CoA to succinyl CoA causing
non-physiological fatty acids are synthesized
& incorporated in neuronal lipids.
Etiology
I— Cobalamin deficiency
1- Inadequate intake (Vegetarian)
2-Malabsorption
A) Defective release from food
Gastric achlorhydria
Partial gastrectomy
Drugs blocking acid secretion
B) Inadequate IF production
Pernicious anemia
Total gastrectomy
Congenital absence or dysfunction of IF
C) Terminal ileum disease
Tropical & non-tropical sprue
Crohn's disease
Intestinal resection
Neoplasms & granulomatous disease e.g.
TB
Selective Cbl malabsorption (Immerslund's
syndrome)
D) Competition for Cbl
Fish tapeworm (Diphyllobothrium latum)
Bacteria (Blind loop syndrome)
3- Others
A) Nitrous oxide
B) TC II def
C) Congenital enzyme defects
II—Folate deficiency
1- Inadequate intake, unbalanced diet,
(alcoholics, teenagers & some infants)
2- Increased requirements
Pregnancy
Infancy
Malignancy
Increased hematopoiesis (chronic hemolytic
anemia)
Chronic exfoliative skin disorders
Hemodialysis
3- Malabsorption
Tropical & non-tropical sprue
Drugs (phenytoin, barbiturates & ethanol)
4- Impaired metabolism
Dihydrofolate reductase inhibitors (MTx,
pyrimethamine, triamterene, pentamidine &
trimethoprim)
Alcohol
Enzyme def (Dihydrofolate reductase &
others)
III— Other causes
1- Drugs impairing DNA metabolism
Purine antagonists (6-MP, azathioprine….etc.)
Pyrimidine antagonists (5-FU, cytosine arabinoside
…etc)
Others (procarbazine, hydroxyurea, acyclovir,
zidovudine)
2- Metabolic disorders
Hereditary Orotic aciduria
Lesch-Nyhan syndrome
3- Megaloblastic anemia of unknown etiology
Refractory Megaloblastic anemia
Di Guglielmo's syndrome
Congenital dyserythropoietic anemia
Clinical manifestations
B12 def
Involve blood, GIT, nervous system
Blood --Anemia ---- weakness, palpitation, light
headedness, vertigo & tinnitus, angina &
CHF
Pallor, lemon color complexion of
face, mild jaundice, tachycardia,
cardiomegaly.
GIT --- sore tongue, beefy red tongue,
anorexia, wt. loss, diarrhea
Neuropsychiatric
Fail to remit fully on ttt
Start with demyelination later axonal
degeneration & later neuronal death
Site- peripheral nerves, spinal cord, brain.
May precede clinical anemia
May be exaggerated or precipitated on folate
ttt
Numbness, paresthesia, weakness, ataxia, sphincter
disturbance, diminished reflexes, +ve Babinski sign,
+ve Rombergism, diminished position & vibration
sense, amnesia, dementia, irritability, psychosis.
Dx
Blood film --- Macrocytic normochromic
anemia ± pancytopenia
(macrocytic hypochromic RBCs seen in
Malabsorption syndrome
i.e. Dimorphic RBCs)
MCV ↑ > 100 fL
Retic count low
PMN --- nuclear hyper-segmentation
(> 6 lobes)
BM
--- Hypercellular,
↓ M: E ratio,
abundant stainable iron,
megaloblasts instead of normoblasts
with excessive hemoglobinization of
cytoplasm yet the nucleus not mature
(Nuclear/Cytoplasmic asynchrony),
ineffective erythropoiesis
S homocysteine level ↑ in both
S methylmalonic acid level ↑ in
B12 def
intramedullary hemolysis (↑UCB,
↑SLDH).
S Cbl ↓ ( N= 200-900 pg/mL) , STC-II ↓
S Folic acid ↓ (N = 6-20 ng/mL)
RBC folate level is more accurate not
affected by diet intake
Schilling test
Radioactive Cbl given orally, followed shortly by IM
inj of unlabelled Cbl.
Then 24 h urine collected & radioactivity measured
expressed as percentage of ingested radioactivity
(↓ in malabsorption)
2nd stage is to give labeled Cbl bound to IF
(corrected in PA)
3rd stage antibiotics given (corrected in bacterial
overgrowth)
Treatment
Cbl def ---1- Specific ttt of underlying cause e.g. AB
2- Cbl replacement usually IM 1000 µg / week for 8
weeks, then 1000 µg / month for life.
After several days improve wellbeing, retic response
start after 4-5 days peaks on day 7 & remission of
anemia over next few weeks. Hypokalemia & salt
retention, thrombocytosis may be seen
3- Bl T – for hemodynamically unstable patients
Folate def ---1- Replacement ttt 1-5 mg/d orally
Figure 3. Bone Marrow Aspirate showing megaloblastic erythropoeisis with very fine nuclear
chromatin and asynchrony of development of nucleus (immature) and cytoplasm (more
mature)
Figure 4. Nuclear cytoplasmic asynchrony is evident in these erythroid precursors