Learning Center Articles

Sickle Cell Disease

Sickle Cell Disease

  • Group of inherited, autosomal recessive disorders
    • Characterized by presence of an abnormal form of Hgb in RBC
    • Genetic disorder usually identified during routine neonatal screening
    • Incurable and often fatal by middle age

Etiology and pathophysiology

  • Abnormal hemoglobin, HgbS, causes the RBC to stiffen and elongate
    • Erythrocytes take on a sickle shape in response to decrease O2 levels
    • Substitution of valine for glutamic acid on the β-globin chain of hemoglobin
  • Types of SCD
    • Sickle cell anemia
      • Most severe
      • Homozygous for hemoglobin S (HgbSS)
    • Sickle cell thalassemia
    • Sickle cell HgbC disease
    • Sickle cell trait (HgbAS)
  • Sickle cell hemoglobin aggregates and alters shape of RBC

Normal red blood cells

Sickled red blood cells

Normal hemoglobin

Sickle cell hemoglobin forms long and inflexible chains

Normal RBCs are compact and flexible, enabling them to squeeze through small capillaries

Sickled RBCs are stiff and angular, causing them to become stuck in small capillaries

  • Sickling episodes
    • The major pathophysiologic event of this disease
      • Triggered by low O2 tension in blood
      • Infection is most common precipitating factor
      • Initially, sickling is reversible with re-oxygenation
    • Sickling episodes are most commonly triggered by low oxygen tension in the blood.
    • Hypoxia or deoxygenation of the RBCs can be caused by viral or bacterial infection, high altitude, emotional or physical stress, surgery, and blood loss.
    • Infection is the most common precipitating factor. Other events that can trigger or sustain a sickling episode include dehydration, increased hydrogen ion concentration (acidosis), increased plasma osmolality, decreased plasma volume, and low body temperature. A sickling episode can also occur without an obvious cause.
    • Initially the sickling of cells is reversible with reoxygenation, but it eventually becomes irreversible because of cell membrane damage from recurrent sickling.
    • Thus vaso-occlusive phenomena and hemolysis are the clinical hallmarks of sickle cell disease.
  • Sickle cell crisis
    • Severe, painful, acute exacerbation of sickling causes a vaso-occlusive crisis
    • Severe capillary hypoxia eventually leads to tissue necrosis
    • Life-threatening shock is a result of severe O2 depletion of the tissues and a reduction of the circulating fluid volume