Extramedullary Hematopoiesis

Kichun Jason Lee, MD; William D. Boswell, MD


October 26, 2005


Extramedullary hematopoiesis is a well-recognized process in which the body attempts to maintain erythrogenesis in response to an alteration in the normal production of red blood cells.[1] It is observed in hemoglobinopathies, myeloproliferative disorders, neoplasms involving the bone marrow, and other conditions. Patients with hemoglobinopathies are more likely to have EMH in paraosseous locations, whereas patients with myeloproliferative disorders are more likely to have extraosseous masses.[2] More commonly observed areas of EMH include the paraspinal regions of the thorax, liver, and spleen, but it has been reported in other locations, including the adrenal gland, bowel, dura mater, and breast.[2,3,4] Only a few cases of perirenal and pelvicaliceal EMH have been reported in the literature.[5]

Extramedullary hematopoiesis in the abdomen most commonly involves the liver and spleen. Several different theories for the phenomenon have been proposed.[5,6] Recently, it has been postulated that hematogenous spread of multipotential stem cells occurs with eventual infiltration of various tissues and organs.[5]

Involvement of the kidneys with EMH is rare. It is more common in the parapelvic/pelvicalyceal areas (which are active during in utero erythrogenesis) than in the perirenal and pararenal locations. Perirenal and pararenal EMH can surround the kidneys without distorting their shape.[7] In distinct contrast, our ultrasound showed significant hydronephrosis in large, echogenic kidneys. Other sonographic findings may be confused with renal cell carcinoma, renal lymphoma, or other diffuse pathologies such as polycystic kidney disease.[8] Previous reports indicate that an enhancement pattern may be present in renal EMH .[7,9]

The signal characteristics of these lesions on MRI suggested the presence of blood/iron products. As paramagnetic substances, iron in ferrous and ferric states (deoxyhemoglobin, methemoglobin, and hemosiderin) can shorten T1 and T2. Higher concentrations of hemosiderin, however, can cause a disproportionately greater effect on T2. As a result, appearance of blood products can vary from slightly increased signal to strikingly low signal on T1-weighted imaging.[10] Decreasing T2 relaxation rates will result in low signal on T2-weighted imaging.


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