Lecture 3 - Megaloblastic Anaemia Flashcards
Microcytic anemia
- iron deficiency
- thalassaemia
- sideoblastic anaemia
Normocytic anemi
- acute bleeding
- hemolysis
- Chronic disease
- Metabolic, including renal failure
- endocrine dysfunction
- stem cell disorders
Macrocytic anemia
- megaloblastic anaemia
- alcohol + liver disease
- hypothyroidism
- reticulocytosis
- drugs
- bone marrow disorders
Macrocytic anemia
- MCV > 100fl, hb low
- megaloblastic or non-megaloblastic types
Non-megaloblastic macrocytic anemia
- MCV
Megaloblastic anaemia caused by impaired DNA synthesis
- deficiency of vit B12 or folate
- abnormalities of vit B12 or folate metabolism: transcobalamin deficiency or NO exposure, antifolate drugs
- other defects of DNA synthesis: congenital enzyme deficiency or drugs
Vitamin B12 : cobalamin
- water soluble vitamin
- only synthesized by bacteria
- only found in animal products
- not affected by cooking
- total body stores: 2-5mg (half in liver), sufficient for 3/4 years
- in plasma 20% bount to TCII - active form
- 80% bound to TCI and TCIII, TCI=R protein, produced by salivary glands
Functions of vit B12
- essential coenzyme for methionine synthase, which has a role in DNA synthesis
- essential coenzyme for MCM which catalyses the formation of succinyl coA from methylmalonic acid
Causes of Vit B12 deficiency
1)inadequate intake
2) digestion and absorption problems:
- inadequate release of B12 from food due to absent/reduced enzymes and/or acid
- inadequate production of functional IF: pernicious anaemia, gastrectomy
- terminal ileal disease: coeliac disease, ileal resection, Crohn’s disease
- competition for intestinal B12: bacterial overgrowth or fish tapeworm
3) Transport abnormalities: deficiency in TCII
Folate
- water soluble vitamin (B9),
- found in most foods, highest in liver and yeast, also in green vegetables and nuts
- easily destroyed by cooking
- body stores (mostly in liver) = 5=10 mg, lasts only 3-4 months
Folic acid fortification
- since 2009, folic acid must be added to wheat flour for bread making in Australia
- not if organic, unpackaged or non-wheat based bread
- also added to other foods
- prevalence of folate deficiency has fallen by 77%
Folate absorption
- folate in food travels to upper small intestine
- at the jejunal microvilli: enzymatic deconjugation to form methyl-THF
- absorption via active carrier mediated transport and concentration dependent diffusion
Folate transport
- in plasma, methyl THF is bound to albumin (weak) or soluble folate receptor (strong)
- rapidly cleared from plasma by entering liver and other cells via membrane transport proteins
- inside the cell, folate is polyglutamated and unable to diffuse back to plasma
Folate function
- coenzyme in transfer of carbon molecules between compounds
- methyl THF used in methylation of homocysteine to form methionine
- THF acts as intermediate acceptor of 1 carbon units for synthesis of purines and pyrimidines
Causes of folate deficiency
- inadequate intake
- malabsorption: small bowel disease, alcohol abuse, phenytoin
- increased requirements: pregnancy and lactation, infancy, haemolytic anaemia, hyperthyroidism, malignancy
- defective utilisation: inhibition of DHFR
- hereditary: folate malabsorption, metabolic defects
Concequences of B12 and folate deficiency
1) B12 deficiency results in
- increased homocysteine levels
- reduced methionine levels
- reduced formation of THF - also caused by folate deficiency
Reduced THF causes
1) impaired DNA syntehsis, affecting all rapidly growing tissues
- bone marrow: megaloblastic anaemia
- epithelial surfaces: GIT symptoms
- developing foetus: neural tube defects
How does B12 deficiency result in neurological damage
- MMA accumulation destabilises myelin
- role for methionine deficiency
- only B12 deficiency, not folate, results in neurological damage in adults
Summary: B12 vs folate
- source: animal product vs all food
- water soluble: yes/yes
- site of absorption: ileum/ duodenum, jejunum
- mechanism of absorption: uptake of If-CBl complex/ deconjugation of polyglutamate
- metabolic function: coenzymes
- body stores: 3-4 years / 3-4 months
- dietary deficiemncy: rare
- Megaloblastic anaemia: Yes
- neurological disease: Yes/no
Clinical presentation of B12 and folate deficiency
- symptoms of anaemia
- lemon-yellow skin
- shiny/beefy red tongue
- malabsorption
- vaginal atrophy
Neurological manifestations of B12 deficiency
- progressive neurological disease due to demyelination
- peripheral sensory nerves
- posterior and lateral columns of spinal cord affecting proprioception/vibration and lateral corticospinal tracts mediating motor function
- optic nerve atrophy
- CNS disease with cognitive disturbances
Blood results of megaloblastic anaemia
- low hb
- MCV 110-130 fl
- oval macrocytes
- aniso/poikilocytosis
- hypersegmented neutrophils
- WCC and platelets mostly normal but may be low
- signs of ineffective erythropoiesis: raised bilirubin, LDH
Bone marrow findings
- usually hypercellular
- erythroblasts: nuclear irregulatiry, increased N:C ratio and delayed nuclear maturation relative to that of the cytoplasm. RBC are so dysfunctional that they auto-hemolyze - raised bilirubin + LDH
- giant metamyelocytes
- dysplastic megakaryocytes
Think vitamin B12/folate deficiency if
- pancytopenia of uncertain cause
- oval macrocytes and hypersegmented neutrophils on blood film
- neurological: unexplained dementia, weakness, ataxia, paraesthesia
- elderly, alcoholics, malnourished, vegans, people with bariatric surgery
Vitamin B12 or folate deficiency diagnosis
- clinical
- serum B12 level + serum or RBC folate levels (serum for short term, RBC for long term)
- metabolic intermediates: raised homocysteine and raised MMA
Traps when diagnosing vit B12 deficiency
- falsely low levels of vit B12: drugs, reduced TCI lebels in pregnancy and neutropenic state/aplastic anaemia, injerited transcobalamin I deficiency
- falsely normal/high levels of vit B12 in hepatitis or cirrhosis, chronic renal failure, hypothyroidism, myeloproliferative diseases, TCII deficiency
Active B12 level: holotranscobalamin
- now widely available as additional test if low or equivocal serum vit B12 result
- measures only B12 bound to TCII
- normal range > 35pmol/L
Traps when diagnosing folate deficiency
1) serum folate
- can be corrected by one good meel
- unstable so can’t test on specimen older than 1-2 days
- high levels in heamolysed samples
- rises in Vit B12 deficiency
2) RBC folate
- low level doesnt distinguish between folate and B12 deficiency
- reticulocytes have high levels so high results in marked reticulocytosis
- transfusion invalidates results
Diagnosis of pernicious anaemia
- severe lack of IF due to autoimmune disease affecting gastric parietal cells
- peaks in 60s, more common in female
- associated with other organ specific autoimmune disorders (hypoparathyroidism, addison’s disease, thyroid disease)
- investigations: serum B12 levels +/- metabolic intermediates plus IF antibodies, gastric parietal cell antibodies and endoscopy
Management of megaloblastic anaemia
- treat cause
- give replacement therapy: initially both: never folic acid alone
- rarely necessary to transfuse
Replacement therapy
- in folate deficiency: folic acid 5 mg daily orally, for 3-4 months
- in vit B12 deficiency: 1000 mcg IMI daily x 4 doses, then weekly x 4 doses then every 1-3 months
- continue for life for patients with pernicious anaemia or ileal resection
Response to therapy
- reticulocyte response within 2-3 days with a peak at 6-7 days
- Hb should rise by 10g/L per week
- bone marrow normoblastic in 48 hours
- if inadequate response - further investigation
Which one of the following does not cause folate deficiency A) antiepileptic drugs B) Veganism C) gluten sensitivity D) inflammatin E) pregnancy
B
Which one of the following is most commonly characterised by macrocytosis
- anaemia due to poor nutrition
Which of the following biochemical abnormalities is seen in severe vit B12 deficiency
- raised red cell folate concentration
- reduction in TCII
- MMA in urine
- reduced homocysteine in plasma
- excess deoxynucleotide production
- MMA in urine
