Extramedullary hematopoiesis (EMH)
By Rita Agarwala, MD
The CT scan of the chest revealed well-defined bilateral paraspinal
masses in the lower thoracic regions with no calcification (Figure
1). Focal areas of low density were also seen in the masses. There
were no erosive or extrinsic pressure changes on the adjacent
vertebrae or ribs. The vertebrae, however, showed increased
trabeculation (Figure 2). There was hepatosplenomegaly. A large
hypodense lesion was seen in the spleen (Figure 3). The diagnosis
of EMH was confirmed by fine-needle biopsy of the paraspinal
Extramedullary hematopoiesis is the production of blood elements
outside the bone marrow cavity. Until 20 weeks of prenatal life,
blood-cell production occurs in the yolk sac and in the
reticuloendothelial (RE) system. Thereafter, the bone marrow is the
primary site of hematopoiesis. Normal blood production is
maintained by the pluripotent stem cells.1
Extramedullary hematopoiesis occurs in severe chronic anemia, which
can arise from deficiency disorders such as vitamin B12
or folate deficiency2
or from disorders affecting
pluripotent stem cells. The latter are classified as: a) stem cell
dysplasia, as in thalassemia, sickle cell hemoglobinopathy, and
hereditary spherocytosis; b) stem cell failure resulting in
aplastic anemia; c) uncontrolled stem cell proliferation, as in
polycythemia vera; and d) malignant transformation and replacement,
as in leukemia, lymphoma, multiple myeloma, and
Extramedullary hematopoiesis mainly
involves the RE system (liver, spleen, and lymph nodes) but is also
known to occur in every organ of the body, including the thyroid,
prostate, pericardium, kidney, and lungs.3
it is microscopic and asymptomatic, but it can sometimes manifest
as organomegaly and tumor-like masses.2
Rarely, it can
cause cord compression, pleural effusion, massive hemothorax, and
Intrathoracic EMH is a rare cause
of paraspinal mass5
and should be differentiated from
other common causes, such as neurogenic tumors, lymphoma,
metastasis, paravertebral abscess, and lateral meningocele.
Bone changes are due to marrow expansion and reconversion. The
latter is best seen by magnetic resonance imaging examination.
There is thinning of the cortices, underconstriction of long bones,
and widening of ribs. The trabeculae are resorbed, giving the bones
a coarsened appearance (Figure 2). The tumor-like masses are
usually slow-growing and do not cause bone erosion or pressure
changes. Older, inactive masses may reveal iron deposits and/or
fatty replacement (Figure 1)5 and do not calcify.
Radionucleide imaging with technetium-99m sulfur colloid may reveal
extrahepatosplenic uptake of colloid. Fine-needle biopsy can
confirm the diagnosis.
Extramedullary hematopoiesis occurs as a compensatory mechanism
for abnormal hematopoiesis when normal red marrow is unable to
function because of deficiency disorders or because of various
pluripotent stem cell disorders. Most often, it is microscopic and
asymptomatic but can sometimes produce organomegaly, tumor-like
masses and even life-threatening symptoms. Extramedullary
hematopoiesis should be considered in the differential diagnosis in
a patient with chronic severe anemia and a paraspinal mass.
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- Steiner RM, Mitchell DG, Rao VM, et al. Magnetic resonance
imaging of bone marrow: Diagnostic value in diffuse hematologic
disorders. Magn Reson Q.1990;6:17-34.
- Bolaman Z, Polatli M, Cildag O, et al. Intrathoracic
extramedullary hematopoiesis resembling posterior mediastinal
tumor. Am J Med.2002;112:739-741.
- Yusen RD, Kollef MH. Acute respiratory failure due to
- Turgut B, Pamuk GE, Vural O, et al. An interesting presentation
of intrathoracic extramedullary hematopoiesis In a patient with
thalassemia intermedia. Clin Lab Haematol. 2003;25:409-412.
- Lall C, Payne DK. A patient with anemia and a paraspinal chest
Extramedullary hematopoiesis (EMH).
April 20, 2006