Extramedullary Hematopoiesis
Diane C. Farhi
Extramedullary hematopoiesis (EMH) is found in numerous settings (Table 3.1). Common underlying causes include increased hematopoiesis, as seen in constitutional and acquired hematopoietic disorders, and compensation for bone marrow replacement, as seen in marrow fibrosis. EMH may be diagnosed at any age.
Clinically, EMH ranges from an incidental finding to life-threatening mass effect or hemorrhage, particularly when EMH grows or bleeds into a confined space. The diagnosis may be made by fine-needle aspiration cytology or following surgical excision of a lesion. Grossly, EMH may present as a well-defined mass or diffuse organ enlargement, or it may be undetectable. Microscopically, EMH consists of immature erythroid and’or granulocytic cells, indicative of local reproductive capability, and, in many cases, megakaryocytes. In some instances, only erythroid precursors or megakaryocytes are found. EMH may show intimate involvement with dystrophic calcification or nonhematopoietic neoplasms.
Neoplastic hematopoietic diseases, including myeloproliferative disorders, myelodysplastic syndromes, and acute leukemia, often cause EMH; however, this chapter is concerned only with nonneoplastic causes of EMH.
INCREASED HEMATOPOIESIS
In conditions of increased hematopoiesis, blood-forming cells fill and expand the bone marrow cavity and eventually extend beyond the bone to involve adjacent areas, commonly the paravertebral and epidural spaces. The liver, spleen, and lymph nodes are also often involved.
Constitutional disorders causing excessive hematopoiesis and EMH include α- and β-thalassemia, hemoglobin C disease, hemoglobin S disease, and combinations of these disorders; congenital dyserythropoietic and microcytic anemia; hereditary spherocytosis; and pyruvate kinase deficiency (Fig. 3.1) (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13).
Acquired disorders producing excessive hematopoiesis, some of which present in the neonatal period, include immune-mediated hemolytic anemia, ABO-incompatibility, cyanotic congenital cardiac disease, cardiac failure, severe folate and vitamin B12 deficiency, and alcohol-related macrocytic anemia (14, 15, 16, 17, 18, 19, 20).
BONE MARROW REPLACEMENT
Replacement of bone marrow by nonhematopoietic tissue apparently forces the redistribution of hematopoiesis to other sites active during fetal hematopoiesis, commonly the liver and the spleen. Lymph nodes are also frequently affected.
Constitutional disorders causing bone marrow replacement and EMH include gray platelet syndrome, osteopetrosis, oxalosis, Gaucher disease, pyknodysostosis, and infantile cortical hyperostosis (Caffey’s disease) (24, 25, 26, 27, 28, 29, 30, 31, 32, 33).
Acquired disorders include marrow fibrosis, as seen in idiopathic myelofibrosis and tumor-related myelofibrosis; radiation-induced myelitis; increased and abnormal marrow bone formation, as seen in Paget disease of bone, renal osteodystrophy, hypertrophic osteoarthropathy, and rickets; marrow replacement by granulomas, as in histoplasmosis, sarcoidosis, and tuberculosis; and marrow replacement by tumor, as in multiple myeloma, metastatic carcinoma, and osteosarcoma (34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46).
The peripheral blood smear often shows leukoerythroblastosis in this type of EMH. The bone marrow itself may be occupied by histiocytes, granulomas, fibrous tissue, bone, and’or malignant cells.
DYSTROPHIC AND INFLAMMATION-RELATED HEMATOPOIESIS
Foci of disordered growth, inflammation, repair, and age-related ossification may eventually develop EMH. This type of EMH has been reported in numerous sites (Fig. 3.2) (47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69). These foci may also show fibrosis, vascular proliferation, and new bone formation.
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