Anemia:- Part 6 – Sickle Cell Anemia Summary

Sample

1. The screening test can be done on the EDTA blood.
2. Electrophoresis is more accurate, should be done in the positive sickle cell screening test.

Purpose of the test (Indication)

• To diagnose sickle cell anemia (Sickle cell disease or trait).

Precautions

1. Please note if the patients have a blood transfusion in the last 3 to 4 days, which will give the negative test.
2. Polycythemia gives the false-positive test.
3. Drugs that give false negative result is a phenothiazine.
4. Infants less than the age of 3 months may give a false negative result.

Pathophysiology

1. Definition: Sickle cell anemia is an autosomal disorder caused by the substitution of the amino acid Valine for Glutamine at the 6th position of the β-hemoglobin chain, which will give rise to the production of Hb-S.
2. Epidemiology: There are 8 to 10% of African Americans and fewer patients seen in the eastern Mediterranean, India, and Saudi Arabia.
   1. Sickle cell anemia represents the most common type of severe hemoglobinopathy, with an estimated prevalence in the united states of 1 in 375 African – American live births.
3. There are two major clinical features of Sickle cell anemia are:
   1. Chronic hemolysis.
   2. Acute episodic vaso-occlusive crises that may cause organ failure accounts for the mortality and morbidity associated with the disease.
      

      Sickle cells leading to ischemia

4. The abnormality of Hb causes sickle cell anemia.
   1. It is a group of hemoglobin disorders in which the sickle β-globin gene is inherited.
5. Sickle cell disease exists in two forms:
   1. Homologous HbS (HbSS), sickle cell anemia.
   2. Sickle cell trait, Hb S, varies from 25% to 45% of the total hemoglobin.
   3. Heterologous HbS (HbSC), sickle cell disease.
   4. Sickle β – thalassemia.
      

      Sickle cell formation mechanism

6. Sickle cell anemia is due to the genetic homozygous defect and is caused by the presence of HbS instead of HbA.
7. The protein that carries the oxygen from the lungs to the RBC Hb tissues is an abnormal Hb called Hemoglobin S (Hb S).
8. The life span of RBC with HbS is short than the normal RBCs.
9. These abnormal RBCs lost their normal shape and became rigid, deformed, and become sickle or crescent-like structures.

10. When HbS becomes deoxygenated, it tends to change them into the sickle shape, an agriculture tool with a semicircular shape.
    1. Sickle cells in the circulation increase the blood’s viscosity, which will slow the circulation, thereby increasing exposure to a hypoxic condition, particularly in the small vasculature of the spleen.
    2. Repeated sickling of the RBCs ultimately damages the membrane permanently.
    3. These cells can not squeeze freely through the capillaries that may block the blood vessel (microvascular tree).
    4. The area of that organs deprived of blood supply due to these sickle cells’ blockage damages the tissue or organs.
       

       HbS and role of O2

11. Homozygous sickle cell disease is associated with increased morbidity and mortality.
    1. Heterozygous sickle cell disease has less mortality.
12. Routine peripheral blood smears do not show these sickle-shaped cells unless there is hypoxemia.

Interpretation

1. Normal  = Negative
   1. There is 80 to 100% of Hb S (Homozygous Sickle cell disease) in Sickle disease.
   2. 20 to 40% Hb S present in the sickle cell trait.
   3. A definite diagnosis of Sickle cell disease is confirmed by electrophoresis.

Signs and symptoms:

1. These patients have severe hemolytic anemia.
   1. There is chronic hemolytic anemia and vaso-occlusion, resulting in ischemic tissue injury.
2. There may be vascular occlusive crises due to blockage of the blood vessels.
   1. All the organs are at risk for injury due to vascular obstruction, and the organs at greater risk are the spleen, kidney, and bone marrow.
   2. The eye and head of the femur are the targets for ischemic injury because of the limited blood supply to these organs.
3. There may be cerebral vascular accidents.
4. In older patients, organ damage is evident.
5. There may acute chest syndrome.
   
6. HbSS patients present with severe chronic hemolytic anemia in early life, and the Hb level is in the range of 6 to 8 g/dL.
   1. These patients may be mildly jaundiced.
   2. These patients may develop sickle cell crises:
      1. Aplastic crises may be due to infections like parvoviruses, which cause temporary suppression of erythropoiesis.
      2. Hemolytic crises reflect the acute attack resulting in the fall in Hb, Hct, increased retics, and jaundice.
      3. Pain (vaso-occlusive) is the hallmark of sickle cell anemia. The pain is very severe and is caused due to the occlusion of the blood vessel. The painful crises may last for 4 to 6 days and sometimes persists for weeks.
         

         Sickle cell manifestation

Sickle cell trait (Hb A/S).

1. Hb A = 60 to 80%.
2. Hb S = 20 to 40%.
3. Hb F = small amount (normal)
4. HbA2 = varies
   1. The molecular structure = α2 β^A β^S
5. These patients do not show signs but can inherit the gene for children.
6. Sickle cell trait may lead to :
   • Renal papillary necrosis.
   • Increased risk of pulmonary embolism.
   • Hematuria.
7. Signs and symptoms: 
   • This patient can develop severe hematuria.
   • It can have a fixed urine-specific gravity of 1.010 due to renal damage.
   • Some of the patients are normal.
8. Diagnosis :
   • Normal RBCs.
   • Normal Hb.
   • Normal Hct.
   • Normal reticulocytes.
   • Sickle screening is positive.

Sickle cell anemia (Homozygous state, HbS disease):

1. HbA = absent. (0%)
2. HbS = 80 to 100%.
3. HbF = variable
4. HbA2 = variable
   1. The molecular structure = α2 β^S β^S 
5. There are all the clinical signs of the disease.
6. HbS, in combination with thalassemia or HbS with HbC, may be seen (S/C).

Diagnosis (Lab tests advised are):

1. The affected child shows limitation to exercise,  shortness of breath, tachycardia, frequent severe infection, and an episode of very painful dactylitis (painful inflammation of fingers or toes).
2. Complete blood count:
   1. Low RBC count.
   2. Low Hb.
   3. Low Hct.
   4. MCV is normal.
   5. MCHC is normal.
   6. WBCs and platelets are usually high.
3. Increased reticulocytes.
4. Peripheral blood smears show inclusion like Howel-Jolly bodies and Pappenheimer bodies.
5. Sickle cells test to see the sickle cells.
6. Hemoglobin electrophoresis.

7. Measurement of Hb A₂ and Hb F.
8. Bone marrow shows hyperplasia.

Method to find Sickle cell

1. Sickle cell preparation Add sodium metabisulphite 2% to EDTA blood.
   1. Examine the slide immediately and then after one hour.
   2. If there are 25% or more HbS is present in the RBCs, then (there are crescent or) sickle-like cells = Test is positive.
2. The second method is that put a drop of blood on the slide and seals it with paraffin.
   1. Leave the slide for some time and keep on seeing the slide for the presence of a sickle cell.
3. The third method Add dithionite to the blood.
   1. Hb S will precipitate.
   2. A negative test indicates that the patient has no or very little <10% HbS.
      

      Various forms of red blood cells

• Please see more details in part 5.

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