ESSENTIALS OF DIAGNOSIS
Recurrent pain episodes.
Positive family history and lifelong history of hemolytic anemia.
Irreversibly sickled cells on peripheral blood smear.
Hemoglobin S is the major hemoglobin seen on electrophoresis.
Sickle cell anemia is an autosomal recessive disorder in which an abnormal hemoglobin leads to chronic hemolytic anemia with numerous clinical consequences. A single DNA base change leads to an amino acid substitution of valine for glutamine in the sixth position on the beta-globin chain. The abnormal beta chain is designated betas and the tetramer of alpha-2betas-2 is designated hemoglobin S (eFigure 13–13). Hemoglobin S is unstable and polymerizes in the setting of various stressors, including hypoxemia and acidosis, leading to the formation of sickled red blood cells. Sickled cells result in hemolysis and the release of ATP, which is converted to adenosine. Adenosine binds to its receptor (A2B), resulting in the production of 2,3-biphosphoglycerate and the induction of more sickling, and to its receptor (A2A) on natural killer cells, resulting in pulmonary inflammation. The free hemoglobin from hemolysis scavenges nitric oxide causing endothelial dysfunction, vascular injury, and pulmonary hypertension.
Hemoglobin SS disease. (Peripheral blood, 50 ×.) Multiple sickled forms are the consequence of hemoglobin S polymerization. Also present are target cells—a manifestation of either sickle cell hepatopathy or concomitant sickle-beta thalassemia. (Used, with permission, from L Damon.)
The rate of sickling is influenced by the intracellular concentration of hemoglobin S and by the presence of other hemoglobins within the cell. Hemoglobin F cannot participate in polymer formation, and its presence markedly retards sickling. Factors that increase sickling are red blood cell dehydration and factors that lead to formation of deoxyhemoglobin S (eg, acidosis and hypoxemia) either systemic or local in tissues. Hemolytic crises may be related to splenic sequestration of sickled cells (primarily in childhood before the spleen has been infarcted as a result of repeated sickling) or with coexistent disorders such as G6PD deficiency.
The betaS gene is carried in 8% of American blacks, and 1 of 400 American black children will be born with sickle cell anemia; prenatal diagnosis is available when sickle cell anemia is suspected. Genetic counseling should be made available to patients.
The disorder has its onset during the first year of life, when hemoglobin F levels fall as a signal is sent to switch from production of gamma-globin to beta-globin. Chronic hemolytic anemia produces jaundice, pigment (calcium bilirubinate) gallstones, splenomegaly (early in life), and poorly healing ulcers over the lower tibia. Life-threatening severe anemia can occur during hemolytic or aplastic crises, the latter generally associated with viral or other infection or by folic acid deficiency causing reduced erythropoiesis, or infection caused by immunoincompetence from ...