TABLE 1332 Comparison of Findings in Sequestration Aplastic and Hemolytic Crises in Sickle Cell Disease

ACUTE SPLENIC SEQUESTRATION This occurs primarily in infants and young children and is the second most common cause of death in children with SCD under the age of 5. The spleen will enlarge to beyond its baseline size, accompanied by a decline in hemoglobin concentration; hence patients present with splenomegaly and hypovolemic shock. These symptoms may occur suddenly or insidiously, and repeated episodes are common. It is a result of sickled cells blocking splenic outflow and drainage, thus causing pooling of both peripheral blood and sickled cells in the spleen. These crises are often preceded by a viral infection, with parvovirus B19; rhinoviruses and echoviruses have also recently been implicated.

These splenic sequestration crises are often divided into major and minor types. In major sequestration, the spleen enlarges rapidly and the serum hemoglobin levels drop to less than 6 g/dL—or to 3 g/dL from that particular patients' baseline. A minor episode is more insidious and leads to progressive splenomegaly, with hemoglobin levels generally greater than 6 g/dL. Reticulocyte counts tend to be higher than normal, reflecting the compensatory increase in bone marrow activity.

Management includes transfusion of packed RBCs and exchange transfusions. Ultimately splenectomy may become necessary.

APLASTIC CRISIS Among the most life-threatening of all complications related to SCD, this complication occurs when bone marrow erythropoiesis slows or completely ceases, resulting in erythroid aplasia. It is characterized by severe anemia with hematocrit levels of 10 percent or lower, hemoglobin levels of 1 to 3 g/dL or less, and reticulocyte counts of as low as 0.5 percent. White blood cell (WBC) counts and platelet counts tend to remain stable despite the lack of erythropoiesis. It is now known that these crises can be precipitated primarily by viral infections (most notably parvovirus B19), but folic acid deficiency and bone marrow suppressive/toxic drugs such as phenylbutazone have also been implicated. Fortunately these crises tend to occur only once in the lifetime of a patient with SCD and are usually self-limiting. RBC transfusions are usually necessary secondary to the severe anemia. This helps to avoid any secondary cardiopulmonary complications.

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