INTRODUCTION

The spleen is a hematopoietic organ capable of supporting elements of the erythroid, myeloid, megakaryocytic, lymphoid, and monocyte-macrophage (ie, reticuloendothelial) systems [1]. As such, it is important in the following situations:

• The spleen participates in cellular and humoral immunity through its lymphoid elements. (See "The adaptive cellular immune response" and "The humoral immune response".)
• The spleen is involved with the removal of senescent red blood cells, bacteria, and other particulates from the circulation through elements of the monocyte-macrophage system. An increase in this function (ie, hypersplenism) may be associated with varying degrees of cytopenia, while removal of the spleen may render the patient susceptible to bacterial sepsis, especially with encapsulated organisms [2]. (See "Extrinsic nonimmune hemolytic anemia due to mechanical damage: Fragmentation hemolysis and hypersplenism", section on 'Extravascular nonimmune hemolysis due to hypersplenism' and "Clinical features and management of sepsis in the asplenic patient".)
• Splenectomy in patients with various hematologic disorders (eg, polycythemia vera, essential thrombocythemia, thalassemia, stomatocytosis) has been associated with an increased incidence of vascular complications, including venous and arterial thrombosis and pulmonary hypertension [2,3].
• Normally, about one-third of circulating platelets are sequestered in the spleen, where they are in equilibrium with circulating platelets. (See "Approach to the adult patient with thrombocytopenia", section on 'Dilutional thrombocytopenia' and "Approach to the adult patient with thrombocytopenia", section on 'Distributional thrombocytopenia caused by splenomegaly'.)
• Under abnormal circumstances (eg, primary myelofibrosis), the spleen may become the site of extramedullary hematopoiesis and contain developing erythroid, myeloid, and megakaryocytic precursors. (See "Clinical manifestations and diagnosis of primary myelofibrosis", section on 'Extramedullary hematopoiesis'.)

This topic review will discuss the approach to a patient whose spleen is enlarged on physical examination (ie, splenomegaly) or is more than minimally enlarged on ultrasound, x-ray, nuclear medicine liver-spleen colloid study, CT scan, or magnetic resonance imaging. However, it should be remembered that the diagnostic criteria for various disease states have historically been built on long experience with the physical examination, while scanning procedures have been available for a shorter period of time. Thus, the clinical or diagnostic significance of a spleen that is modestly enlarged on scan but is not palpable (ie, "scanomegaly") is uncertain.

As an example, in polycythemia vera, palpable splenomegaly was a major diagnostic criterion according to the Polycythemia Vera Study Group. However, a spleen enlarged on a scanning procedure has been considered to be only a minor criterion using other diagnostic algorithms. (See "Diagnostic approach to the patient with suspected polycythemia vera".)

The approach to the child with splenomegaly is discussed separately. (See "Approach to the child with an enlarged spleen".)