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Just as the lymph nodes are specialized to fight pathogens in the tissues, the spleen is the lymphoid organ specialized to fight bloodborne pathogens. It has no afferent lymphatics. The spleen has specialized areas like the lymph node for making antibodies (follicles) and amplifying antigen-specific T cells (periarteriolar lymphatic sheath, or PALS). In addition, it has a well-developed reticuloendothelial system for removing particles and antibody-coated bacteria. The flow of blood through the spleen permits it to filter pathogens from the blood and to maintain quality control over erythrocytes (RBCs)—those that are old and nondeformable are destroyed, and intracellular inclusions (sometimes including pathogens such as Babesia and Plasmodium) are culled from the cells in a process called pitting. Under certain conditions, the spleen can generate hematopoietic cells in place of the marrow.
The normal spleen is about 12 cm in length and 7 cm in width and is not normally palpable. Dullness from the spleen can be percussed between the ninth and eleventh ribs with the pt lying on the right side. Palpation is best performed with the pt supine with knees flexed. The spleen may be felt because it descends when the pt inspires. Physical diagnosis is not sensitive. CT or ultrasound is a superior test.
Spleen enlargement occurs by three basic mechanisms: (1) hyperplasia or hypertrophy due to an increase in demand for splenic function (e.g., hereditary spherocytosis where demand for removal of defective RBCs is high or immune hyperplasia in response to systemic infection or immune diseases); (2) passive vascular congestion due to portal hypertension; and (3) infiltration with malignant cells, lipid- or glycoprotein-laden macrophages, or amyloid (Table 44-2). Massive enlargement, with spleen palpable >8 cm below the left costal margin, usually signifies a lymphoproliferative or myeloproliferative disorder.
|Enlargement Due to Increased Demand for Splenic Function|
Reticuloendothelial system hyperplasia (for removal of defective erythrocytes)
Early sickle cell anemia
Paroxysmal nocturnal hemoglobinuria
Response to infection (viral, bacterial, fungal, parasitic)
Subacute bacterial endocarditis
Rheumatoid arthritis (Felty’s syndrome)
Systemic lupus erythematosus
Collagen vascular diseases
Immune hemolytic anemias
Thyrotoxicosis (benign lymphoid hypertrophy)
Marrow damage by toxins, radiation, strontium
Marrow infiltration by tumors, leukemias, Gaucher’s disease
|Enlargement Due to Abnormal Splenic or Portal Blood Flow|
Hepatic vein obstruction
Portal vein obstruction, intrahepatic or extrahepatic
Cavernous transformation of the portal vein
Splenic vein obstruction
Splenic artery aneurysm
Congestive heart failure
Portal hypertension (any cause including the above): “Banti’s disease”
|Infiltration of the Spleen|
Intracellular or extracellular depositions
Hurler’s syndrome and other mucopolysaccharidoses
Benign and malignant cellular infiltrations
Leukemias (acute, chronic, lymphoid, myeloid, monocytic, hairy cell)
Myeloproliferative syndromes (e.g., polycythemia vera, essential thrombocytosis)
Metastatic tumors (melanoma is most common)
Histiocytic and dendritic cell neoplasms (Histiocytosis X)
Hemangiomas, fibromas, lymphangiomas
Peripheral blood RBC count, white blood cell count, and platelet count may be normal, decreased, or increased depending on the underlying disorder. Decreases in one or more cell lineages could indicate hypersplenism, increased destruction. In cases with hypersplenism, the spleen is removed and the cytopenia is generally reversed. In the absence of hypersplenism, most causes of splenomegaly are diagnosed on the basis of signs and symptoms and laboratory abnormalities associated with the underlying disorder. Splenectomy is rarely performed for diagnostic purposes.
Individuals who have had splenectomy are at increased risk of sepsis from a variety of organisms including pneumococcus and Haemophilus influenzae. Vaccines for these agents should be given before splenectomy is performed. Splenectomy compromises the immune response to T-independent polysaccharide antigens. Newer vaccine formulations are T dependent and are more effective in splenectomized persons.