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<< Sickle Cell Disease And Other Hemoglobinopathies: Approaches To Emergency Diagnosis And Treatment

Pathophysiology

 When deoxygenated, the sickle hemoglobin forms polymers. This polymerization damages the erythrocyte cell membrane, causes premature cell death, and ultimately leads to accelerated removal of the red blood cell from the circulation.9 Vaso-occlusion is the hallmark of SCD and is responsible for most of its significant complications. Due to the altered cell membrane and reduced deformability, the sickled cells coalesce and react with other blood cells and the vascular endothelium. This causes the characteristic "sludging" in the end-arterioles, resulting in microvascular occlusion and subsequent ischemia and acidosis. The acid environment promotes sickling of additional red cells and incites a worsening spiral. Other hemoglobins, specifically HbF, interfere with the polymerization of sickled cells, which helps terminate the cycle.6

Pain occurs when the inflexible sickled erythrocytes become trapped in the capillaries. This slows blood flow, which leads to hypoxemia and more sickling, creating a vicious cycle of infarction, tissue necrosis, and inflammation. 10 Infants younger than 6 months are typically spared because fetal hemoglobin (HbF) prevents the polymerization of HbS. When the level of HbF decreases (by 6 months of age), clinical complications begin.10

The other hematologic complications seen in SCD include acute splenic sequestration, chronic anemia, hyperhemolytic crisis, and aplastic crisis. The anemia of SCD is typically due to chronic hemolysis. The erythrocytes usually have a normal volume and hemoglobin concentration (normocytic and normochromic), but the plasma hemoglobin concentration is low (usually 7-8 g/dL). Most patients tolerate this degree of anemia well.

The immune defect in SCD appears to be specific for encapsulated organisms. This may be secondary to the loss of specific antibodies that normally are produced by the spleen.11 Children with sickle cell have functional asplenia (> 90% by age 5) and can easily succumb to septicemia from encapsulated organisms such as Streptococcus pneumoniae. As the person ages, the cause of bacterial infections changes from pneumococcus to gramnegative organisms such as Escherichia coli, Klebsiella sp., and Salmonella sp.12

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Last Modified: 07/23/2017
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