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Vitamin B12 Deficiency
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Vitamin B12 Deficiency
Vitamin B12 Deficiency
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1
Question
What role does methylcobalamin play in folic acid metabolism?
Answer
Methylcobalamin acts as a cofactor for the enzyme homocysteine-methionine methyltransferase, catalyzing the demethylation of dietary N5-methyltetrahydrofolate to form tetrahydrofolate, which is needed for dTMP synthesis. It transfers a methyl group to homocysteine to form methionine, helping reduce homocysteine levels.
2
Question
What is the function of deoxycobalamin in the body?
Answer
Deoxycobalamin serves as a cofactor for methylmalonyl coA mutase, which catalyzes the isomerization of methylmalonyl coA to succinyl coA. This reaction helps incorporate odd-chain fatty acids, some amino acids, cholesterol, and thymine into the Krebs cycle and may be important in myelin formation.
3
Question
What is the major hematological consequence of vitamin B12 deficiency?
Answer
Vitamin B12 deficiency causes megaloblastic erythropoiesis characterized by defective DNA synthesis leading to slowly maturing large erythroblasts called megaloblasts with large, immature nuclei and dispersed chromatin.
4
Question
How does vitamin B12 deficiency affect DNA synthesis and cell division?
Answer
Vitamin B12 deficiency slows ribonucleotide reduction and DNA synthesis, causing delayed nuclear maturation in cells. This leads to megaloblast formation where nuclear development lags behind cytoplasmic maturation, impairing effective cell division.
5
Question
What is ineffective erythropoiesis and its relevance in vitamin B12 deficiency?
Answer
Ineffective erythropoiesis is the process where red blood cell precursors fail to mature properly and undergo apoptosis before entering circulation. In vitamin B12 deficiency, this leads to reduced red cell output despite intense bone marrow activity.
6
Question
Why is reticulocyte count reduced in vitamin B12 deficiency despite marrow hyperplasia?
Answer
Although the bone marrow shows intense erythroid hyperplasia due to increased production effort, many erythroblasts die prematurely by apoptosis, leading to low reticulocyte counts in peripheral blood.
7
Question
How does vitamin B12 deficiency affect megakaryocytes and platelet production?
Answer
Megakaryocytes increase in number but have diminished ploidy, limiting their ability to produce adequate platelets, potentially leading to thrombocytopenia.
8
Question
Why can symptoms of vitamin B12 deficiency take years to develop after dietary deficiency begins?
Answer
The body stores 2-5 mg of vitamin B12 but only uses 1-5 µg per day, so substantial stores can maintain sufficiency for years even without intake, delaying symptom onset.
9
Question
List key causes of vitamin B12 deficiency.
Answer
Key causes include low dietary intake (common in vegans/vegetarians), pernicious anemia (autoimmune destruction of intrinsic factor or parietal cells), previous gastric or ileal surgery, pancreatic insufficiency, Crohn’s disease, use of certain drugs (PPIs, methotrexate), infection with fish tapeworm, malabsorption syndromes like tropical sprue and coeliac disease, and small intestine bacterial overgrowth.
10
Question
What is pernicious anemia and how does it cause vitamin B12 deficiency?
Answer
Pernicious anemia is an autoimmune disease where antibodies attack intrinsic factor or parietal cells, impairing vitamin B12 absorption in the stomach leading to deficiency.
11
Question
How do anti-parietal cell antibodies contribute to pernicious anemia?
Answer
Anti-parietal cell antibodies attack gastric parietal cells, causing gastric mucosa atrophy and decreased secretion of intrinsic factor, which impairs vitamin B12 absorption.
12
Question
Why are intrinsic factor blocking and binding antibodies significant in pernicious anemia?
Answer
Blocking antibodies prevent intrinsic factor from binding vitamin B12, while binding antibodies interfere with the absorption of the intrinsic factor-vitamin B12 complex in the gut, both impairing vitamin B12 uptake.
13
Question
What are the neurological manifestations of vitamin B12 deficiency?
Answer
Neurological symptoms include neuropsychiatric disorders (irritability, depression, confusion, delirium, dementia), rare cranial nerve involvement (optic atrophy, retinal hemorrhage), subacute combined degeneration of the spinal cord (loss of proprioception and vibration, weakness, stiffness, characteristic reflex changes), and peripheral neuropathy (numbness, tingling, sensory loss).
14
Question
Describe subacute combined degeneration of the spinal cord caused by vitamin B12 deficiency.
Answer
It involves symmetrical degeneration of dorsal columns (causing loss of proprioception and vibration leading to ataxia) and corticospinal tracts (causing weakness and spasticity). Clinically, there is brisk knee jerk, absent ankle jerk, and extensor plantar response. Spinothalamic tracts and pain/temperature sensation are preserved.
15
Question
How does vitamin B12 deficiency cause peripheral neuropathy?
Answer
By damaging peripheral nerves, it causes numbness and tingling in fingers and toes and distal sensory loss, particularly of vibration and proprioception.
16
Question
What are common clinical signs of anemia in vitamin B12 deficiency?
Answer
Signs include pallor from anemia, jaundice from hemolysis, a beefy red tongue due to glossitis, and angular stomatitis.
17
Question
How does vitamin B12 deficiency affect bone marrow morphology?
Answer
Bone marrow shows hypercellularity with many megaloblasts (large erythroblasts with immature nuclei and fine dispersed chromatin) and giant metamyelocytes reflecting delayed nuclear maturation from defective DNA synthesis.
18
Question
What findings on peripheral blood smear are characteristic of vitamin B12 deficiency?
Answer
Macro-ovalocytes (large oval-shaped red cells), anisopoikilocytosis (variation in size and shape), and hypersegmented neutrophils (over 5% with more than 5 lobes) are characteristic.
19
Question
How can serum metabolites help distinguish vitamin B12 deficiency from folic acid deficiency?
Answer
Both deficiencies cause elevated homocysteine, but only vitamin B12 deficiency causes elevated methylmalonic acid, helping differentiate the two.
20
Question
What are the typical laboratory features of vitamin B12 deficiency anemia?
Answer
Macrocytic anemia (Hb <12.5g/dL, MCV >100 fL), increased RDW, pancytopenia in severe cases, reduced reticulocyte count, elevated serum iron, unconjugated bilirubin, and LDH, with low haptoglobin indicating hemolysis.
21
Question
What diagnostic tests are used to confirm pernicious anemia in the context of vitamin B12 deficiency?
Answer
Tests include anti-intrinsic factor antibodies (high specificity, moderate sensitivity), anti-parietal cell antibodies (high sensitivity, lower specificity), and the Schilling test which measures vitamin B12 absorption.
22
Question
How is the Schilling test performed and interpreted?
Answer
A radiolabeled oral dose of vitamin B12 is given, then an intramuscular flushing dose to clear circulating labeled B12. Urine excretion of labeled B12 is measured over 24 hours. Normal (>5%) excretion indicates good absorption (dietary deficiency). If low, a second stage uses oral intrinsic factor; normalization suggests pernicious anemia or gastrectomy; no improvement suggests other malabsorption causes.
23
Question
What are general principles for managing vitamin B12 deficiency?
Answer
Management involves treating the underlying cause if possible and replacing vitamin B12, usually by parenteral hydroxycobalamin or cyanocobalamin or high-dose oral supplementation.
24
Question
Describe the typical parenteral vitamin B12 replacement regimen.
Answer
1 mg hydroxycobalamin daily for 1 week, then weekly for 4 weeks; if cause is permanent, maintenance injections of 1 mg every 3 months lifelong.
25
Question
Why can oral vitamin B12 supplementation be effective even in absorption issues?
Answer
At very high oral doses (about 2 mg daily), 1-2% of vitamin B12 is absorbed by passive diffusion without intrinsic factor, making oral therapy feasible in some cases.
26
Question
What is the clinical timeline for improvement after vitamin B12 treatment?
Answer
Clinical improvement can begin within 48 hours, reticulocytosis appears in 2-3 days, and neurological improvements may take 6-12 months. However, longstanding spinal cord damage is mostly irreversible.
27
Question
What complications can occur during early treatment of vitamin B12 deficiency?
Answer
Iron deficiency may develop within months, requiring supplementation. Hypokalemia can occur due to rapid red cell production and potassium utilization, especially in severely anemic patients.