Extramedullary hematopoiesis (EMH) is the production of hematopoietic cells outside the bone marrow — most commonly in the spleen and liver. During fetal life, the spleen and liver are primary hematopoietic organs; postnatally, this function transfers to the bone marrow. Chronic hemolytic anemias (sickle cell disease, thalassemia major), myeloproliferative neoplasms (myelofibrosis, polycythemia vera), and conditions causing bone marrow infiltration (metastasis, granulomatous disease) overwhelm bone marrow capacity and trigger compensatory EMH in the spleen. On imaging, marked splenomegaly and diffuse heterogeneous parenchyma are the most common findings. Focal EMH masses are rare and may be seen in solid organs or the paraspinal region. Diagnosis is established by the combination of clinical context (underlying hematologic disease) and imaging findings; biopsy is generally not required.
Age Range
20-70
Peak Age
50
Gender
Equal
Prevalence
Rare
When bone marrow capacity becomes insufficient, the spleen and liver reactivate their fetal hematopoietic capabilities. The splenic red pulp provides an ideal microenvironment for hematopoietic stem cell engraftment and proliferation through its sinusoidal vascular architecture. In conditions like myelofibrosis, where bone marrow is replaced by fibrosis, hematopoietic stem cells reach the spleen via portal circulation and settle in sinusoids. Erythroid, myeloid, and megakaryocytic lineages proliferate together. This intense cellular activity dramatically increases splenic size — splenomegaly is the most prominent clinical and radiological finding of EMH. Hematopoietic cells disrupt normal sinusoidal architecture and create parenchymal heterogeneity. In diffuse EMH, the entire parenchyma shows non-uniform contrast patterns, while focal EMH nodules can create solid mass-like appearances. When iron metabolism is disturbed, hemosiderin deposition in the splenic parenchyma increases — this causes T2 signal loss on MRI. Abnormal megakaryocyte proliferation can increase platelet consumption, and paradoxical thrombocytopenia may occur.
Both the spleen and liver are massively enlarged with diffuse T2 signal loss in both organs on MRI — combined effect of hemosiderin deposition and hematopoietic cell infiltration. Underlying myelofibrosis, thalassemia, or chronic hemolytic anemia is present clinically. This combination is nearly diagnostic for EMH.
In the arterial phase, the spleen is massively enlarged (usually >20 cm, sometimes >30 cm) with heterogeneous parenchymal enhancement. The normal zebra pattern (arcuate enhancement) of the spleen is distorted or absent. Active hematopoietic areas show slightly different enhancement intensity compared to surrounding parenchyma. If focal EMH nodules are present, they appear as isodense-to-mildly hypodense nodules.
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The spleen is massively enlarged showing heterogeneous enhancement in the arterial phase; the normal arcuate enhancement pattern is absent; consistent with extramedullary hematopoiesis.
In the portal venous phase, the splenic parenchyma becomes more homogeneous but mildly hyperdense or hypodense focal areas relative to surrounding parenchyma may persist. Concurrent hepatomegaly is frequently observed — heterogeneous enhancement in the liver parenchyma reflects hepatic EMH. Portal hypertension findings (portosystemic collaterals, varices, ascites) may accompany.
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Persistent parenchymal heterogeneity in the spleen on portal venous phase and concurrent hepatomegaly are observed, suggesting extramedullary hematopoiesis in the setting of hepatosplenomegaly.
On T1-weighted sequences, the massively enlarged splenic parenchyma shows isointense-to-mildly hypointense signal relative to normal parenchyma. When hemosiderin deposition is prominent, T1 signal decrease is more marked. Concurrent hepatomegaly and T1 signal decrease in liver parenchyma (hemosiderin/iron deposition) reflects the systemic nature of EMH.
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Massive splenomegaly is observed on T1-weighted sequences with the splenic parenchyma showing isointense-to-mildly hypointense signal.
On T2-weighted sequences, the splenic parenchyma shows diffuse signal loss — while normal spleen has high T2 signal, the parenchyma becomes markedly hypointense in the setting of EMH. This signal loss reflects the combined effect of hemosiderin deposition and hematopoietic cell infiltration. Signal loss is more dramatic on gradient-echo (GRE) T2* sequences — blooming artifact is prominent.
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Diffuse signal loss in the splenic parenchyma on T2-weighted sequences reflecting hemosiderin deposition and hematopoietic cell infiltration is observed; consistent with extramedullary hematopoiesis.
On DWI, the splenic parenchyma may show diffuse mildly hyperintense signal — dense packing of hematopoietic cells partially restricts free water diffusion. ADC values are mildly decreased compared to normal spleen (1.0-1.3 × 10⁻³ mm²/s) but not as markedly as in malignant tumors. Hemosiderin deposition can cause signal loss on DWI due to susceptibility artifact.
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Diffuse mild diffusion restriction is observed in the splenic parenchyma on DWI, consistent with hematopoietic cell infiltration.
On B-mode US, the spleen is massively enlarged (usually >20 cm bipolar length) with diffusely heterogeneous parenchymal echotexture. The normal homogeneous splenic echotexture is replaced by a granular or coarsely heterogeneous pattern. Concurrent hepatomegaly is common. Compression of the left kidney and pelvic extension may be seen. Portal hypertension findings may accompany.
Report Sentence
On B-mode US, the spleen is massively enlarged with diffusely heterogeneous parenchymal echotexture; hepatomegaly accompanies; consistent with extramedullary hematopoiesis.
Criteria
Diffusely infiltrates splenic parenchyma without forming distinct focal masses. Massive splenomegaly is dominant.
Distinct Features
Most common form. No focal mass; the primary finding is massive splenomegaly + diffuse parenchymal heterogeneity.
Criteria
One or more focal nodular EMH foci within the splenic parenchyma. Nodules are generally 1-5 cm in size.
Distinct Features
Rare form that can mimic lymphoma and metastasis. Clinical context and concurrent diffuse splenomegaly aid in differential diagnosis.
Criteria
Bilateral soft tissue masses in the paraspinal region accompany splenic EMH. Typically at thoracic vertebral level in the posterior mediastinum.
Distinct Features
Bilateral symmetric paraspinal masses are pathognomonic for EMH. Neurological compression may develop. Can mimic posterior mediastinal lymphoma but symmetry and hematologic disease history differentiate.
Distinguishing Feature
In lymphoma, splenic nodules form more prominent focal masses and T2 signal loss is cellularity-driven not hemosiderin-related. In EMH, diffuse heterogeneity dominates, T2 loss is hemosiderin-weighted, and hematologic disease history is key.
Distinguishing Feature
In sarcoidosis, multiple small (1-5 mm) hypodense nodules and hilar/mediastinal lymphadenopathy are typical. In EMH, focal nodules are rare, diffuse heterogeneity dominates, and hematologic disease history is present.
Distinguishing Feature
Hemangioma presents as a focal mass with peripheral nodular enhancement and high T2 signal. In EMH, diffuse parenchymal heterogeneity dominates, and hemosiderin-related T2 signal loss contrasts with hemangioma's high T2 signal.
Distinguishing Feature
In metastasis, known primary malignancy history is present and focal mass/nodules are dominant. In EMH, hematologic disease history is foremost and diffuse heterogeneity is more prominent than focal lesions.
Urgency
routineManagement
medicalBiopsy
Not NeededFollow-up
specialist-referralSplenic EMH is a compensatory response to underlying hematologic disease — primary treatment targets the underlying condition. JAK2 inhibitors for myelofibrosis, transfusion and iron chelation for thalassemia. Splenectomy considered for symptomatic massive splenomegaly but decision must be careful. Hematology follow-up is mandatory.
Splenic EMH requires treatment of the underlying hematological disease. Massive splenomegaly may lead to hypersplenism and spontaneous rupture. It may increase transfusion dependency.