Sickle cell Anemia is hemoglobinopathie , where substitution of valine for glutamic acid at the sixth position of the β-chain produces HbS.
Autosomal recessive pattern of inheritance – most commonly found in African American – natural selection. Most common cause of familial hemolytic anemia in Asian countries.
Sickle cell exist is 2 forms in population – sickle cell disease – HbS and sickle cell trait HbS + HbA.
Patients with disease are homozygous for the mutation, where as with trait are heterozygous.
So under normal conditions sickle cell trait is asymptomatic. As it has nearly 40 – 50% of Hb A , so the RBC of heterozygote have little tendency to sickle.
Another trait is HbSC – here there is presence of HB C, this HB has greater tendency to aggregate than HbA. So this patient lies in spectrum between trait and disease.Hb F has protective effect as it inhibits polymerisation of Hb S. Donot manifests in newborn till 5 to 6 months.
Clinical consequences of Sickling:
Increased RBC destruction – severe hemolytic anemia.
Erytropoesis is activated in bone marrow - Erythroid hyperplasia in bone marrow ,bone resorption and appositional new bone formation giving crew –cut appearance on radiograph.
Extramedullary erythropoiesis – splenomegaly and hepatomegaly
Pigmented gallstones formation.
The sickling of RBCs produces widespread microvascular obstructions and capillary stasis resulting ischemia and infarction.
Obstruction of small capillaries of hand and foot leads to ischemic damage to hand and foot, such patients presents with swelling of hand and foot – this is referred to as hand foot syndrome.
Microvascular ischemia is responsible for vasooclusive crisis and autosplenectomy.