Showing posts with label Hematology. Show all posts
Showing posts with label Hematology. Show all posts

Tuesday, April 21, 2015

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Hyperhomocysteinemia

Introduction:
Homocysteine is a sulphur containing amino acid that is produced during the conversion of methionine to cysteine.

Hyperhomocysteinemia results when there is an abnormality in the homocysteine metabolism.
It is an independent risk factor for stroke, MI, peripheral arterial disease and venous thrombotic disease.

Even mild to moderate hyperhomocysteinemia is a significant risk factor for vascular disease.



Pathophysiology:

The amino acid homocysteine is normally metabolized via the transsulfuration pathway by the enzyme cystathionine-β-synthase (CBS), which requires vitamin B6 as co-factor and via the
remethylation pathway by the enzymes methylenetetrahydrofolate reductase (MTHFR),
which is folate dependent and methionine synthase, which requires vitamin B12 as co-factor.


1 - Methylenetetrahydrofolate reductase
2 - Methionine synthase










Hyperhomocysteinemia can be either:
1) Inherited or
2) Acquired.

Inherited
Inherited severe hyperhomocysteinemia (plasma level >100 µmol/L), as seen in classic homocystinuria, may result from homozygous MTHFR and CBS deficiencies and more rarely from inherited errors of cobalamin metabolism. Classic symptoms for homozygous patients include premature vascular disease and thrombosis, mental retardation, ectopic lens and skeletal abnormalities.

Inherited mild to moderate hyperhomocysteinemia (plasma level >15 to 100 µmol/L) may result from heterozygous MTHFR and CBS deficiencies, but most commonly results from the thermolabile variant of MTHFR (tlMTHFR) that is encoded by the C677T gene polymorphism. Heterozygous carriers of the tlMTHFR mutation have normal plasma homocysteine levels unless folate levels are
reduced.

Acquired
Acquired hyperhomocysteinemia may be caused by folate deficiency, vitamin B 6 or B 12 deficiency, renal insufficiency, hypothyroidism, type II diabetes mellitus, pernicious anemia, inflammatory bowel disease, advanced age, climacteric state, carcinoma (particularly involving breast, ovaries or pancreas) and acute lymphoblastic leukemia, as well as methotrexate, theophylline and phenytoin therapy.

VTE risk is most closely related to elevated fasting plasma homocysteine levels, regardless of etiology. Hyperhomocysteinemia (plasma level >18.5 µmol/L) has been associated with a two- to fourfold increased VTE risk.

The precise mechanisms underlying the thrombogenicity of homocysteine remain unclear. Several diverse mechanisms have been proposed, including endothelial cell desquamation, low-density lipoprotein (LDL) oxidation, promotion of monocyte adhesion to endothelium and factor V activation and promotion of thrombin generation.
Homocysteine also enhances platelet aggregation and adhesiveness as well as turnover, presumably as a result of endothelial cell injury.

Laboratory Diagnosis:
The initial step in the evaluation of the patient with suspected hyperhomocysteinemia involves measurement of fasting total plasma homocysteine (the sum of nonprotein-bound and proteinbound).
A normal value in the nonfasting setting does not normally require repeating.

Standardized methionine loading test
Testing 2 to 8 hours after an oral methionine load (100 mg/kg) increases the sensitivity of detecting occult vitamin B6 deficiency and obligate heterozygotes for CBS deficiency, but methionine loading is not routinely recommended.
Vitamin B12 and folate deficiency do not affect post-methionine loading homocysteine values.
After 4-6 hours the level of homocysteine is measured again.
A level 5 times that of the fasting one or an increase by 40 µmol/L is considered a positive test for hyperhomocysteinemia.
In patients found to have elevated levels of homocysteine, testing for vitamin B12 deficiency is advocated to avoid missing subclinical deficiency before
beginning oral folic acid therapy.

Treatment:
1) Folic acid supplementation is the mainstay therapy. The usual recommended dose is 0.4 to 1.0 mg daily. This causes a 25% decrease in the homocysteine level.
2) Because patients with subclinical vitamin B12 deficiency may be prone to developing
peripheral neuropathy if they receive folic acid supplementation alone, additional treatment with 0.5 mg/day of oral vitamin B12 has been advocated. An additional 7% reduction of homocysteine levels was noted with vitamin B12 supplementation.
Vitamin B12 administration results in normalization of homocysteine levels in B12-deficient individuals. In these patients, a monthly intramuscular injection of 200 to 1,000 µg of vitamin B12 is considered adequate replacement.
3) Vitamin B6 supplementation did not appear to have any effect on homocysteine levels.
4) Thrombotic events in hyperhomocysteinemic patients should be treated accordingly.

Tuesday, October 4, 2011

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Venous thrombo embolism / Pulmonary embolism - Anticoagulation

As soon as a diagnosis of VTE / PE is strongly suspected, anticoagulant therapy should be started unless there are contraindications. Parenteral drugs like unfractionated heparin (standard heparin) and low molecular weight heparin (lovenox) are started and therapy shifted to a long term stable vitamin K antagonist like warfarin.

Unfractionated heparin
The anticoagulant action is by binding to and accelerating the activity of antithrombin III. This inactivates thrombin, factor IXa and Xa and thus prevents further clot formation. The classical regimen for the dosage is a loading dose of 5000 - 10000 units followed by a continuous infusion of 1000 - 1500 units/hour. Unfortunately we all do not have the same weight. So, a more appropriate dosage is a loading dose of 80 units/kg and a continuous infusion of 18 units/kg/hr.

The aim is to achieve a target activated partial thromboplastin time (aPTT) aka partial thromboplastin time with kaolin (PTTK) of 2-3 times the normal laboratory values, the normal values being 30-40 seconds.

The PTTK is checked every 4-6 hours and the infusion is adjusted accordingly.

Low molecular weight heparin
Enoxaparin (lovenox) is given in a dosage of 1 mg/kg twice daily. The advantages over unfractionated heparin is that it binds less to plasma protein and endothelial cells. So the bioavailability is higher and the half life is longer. The dose response is more predictable. No repeated tests are required for monitoring but care must be taken to lower the dosage in patients with renal insufficiency.

Warfarin
This vitamin K antagonist prevents the gamma carboxylation activation of factors II,VII,IX and X, as well as the proteins C and S. The anticoagulant effects appear only after 5 days because factor II has a half life of 5 days.

The dosage to be used initially is between 5 - 10 mg/ day. We should aim for an INR between 2.0 - 3.0.

Warfarin is a difficult drug to dose and monitor as it has multiple drug-drug and drug-food interactions.

Warfarin should be started as soon as the PTTK is within the therapeutic range. The heparin should be continued until a therapeutic INR has overlapped with a therapeutic PTTK for 2 consecutive days. This usually occurs after a minimum of 5 days.

Thursday, June 16, 2011

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Beta thalassemia - target cells


This is a slide at 50X magnification of a beta thalassemia patient. This is also a type of microcytic hypochromic  anemia as shown in the slide.
The red blood cells have a diameter smaller than that of the nucleus of the lymphocyte.
They are also bizarre shaped.
Target cells aka codocytes or mexican hat cells are predominantly seen. They are characterised by a dark centre followed by a white ring and then a second rim of dark region.

Tuesday, June 14, 2011

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Iron deficiency anemia - Microcytic hypochromic cells


It is a typical peripheral blood smear for an iron deficiency anemia patient showing microcytic hypochromic red blood cells at magnification 50X.
In normal conditions, the size (diameter) of an RBC should roughly correspond to that of the nucleus of a lymphocyte. In this slide, it is smaller.
The numerous small dots refer to platelets and they are found in a larger number (thrombocytosis), a finding commonly seen in iron deficiency anemia.
Differential diagnosis includes:
1) anemia of chronic disorders,
2) thalassemia,
3) sideroblastic anemia.

Thursday, May 19, 2011

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Red blood cell formation


Under certain specific stimuli, pluripotent stem cells form CFU-E i.e Colony Forming Units - Erythrocytes. These cause production of cells that are the first to belong to the RBC series, proerythroblast. The next generation is called basophilic erythroblast because it stains with basic dyes. 


As the genesis continues, hemoglobin concentration increases while the nucleus condenses to a very small size. The remnant of the nucleus is either absorbed or extruded out of the cell. Endoplasmic reticulum is also absorbed. The cell is now called a reticulocyte


The reticulocyte has some basophilic remnants of Golgi apparatus, mitochondria and other organelles. During this reticulocyte stage, the cells pass from the bone marrow into the blood capillaries by diapedesis i.e squeezing through the pores of the capillary membrane.


The remaining basophilic material in the reticulocyte normally disappears within 1 to 2 days, and the
cell is then a mature erythrocyte.