Pictorial reviewExtramedullary haematopoiesis: radiological imaging features
Introduction
Extramedullary haematopoiesis (EMH) is the production of blood elements outside of the normal location. EMH occurs when there is either insufficient production or poor quality of the blood elements produced. Insufficient production of blood elements occurs when there is replacement of the bone marrow, most commonly caused by myelofibrosis (the most common cause of EMH overall), diffuse metastatic disease, and leukaemia.1 Examples of insufficient quality of blood products include haemoglobinopathies, most commonly sickle cell anaemia and thalassemia.1
The location of EMH in the adult can often be explained by the sites of haematopoietic production during fetal life and in the early stages of life1 (Fig. 1). In the fetus, the yolk sac is the primary site of haematopoiesis, followed by the liver, spleen, and bone marrow in the later gestational period.1 After birth, haematopoiesis should only occur in the bone marrow. Any extramedullary site of haematopoiesis is abnormal.2 In children and young adults, the marrow of the vertebral bodies, sternum, ribs, femora, and tibiae serve as the major sites of haematopoiesis. In adults, the ribs, femora, and tibiae become less important.1 Importantly, EMH does not always occur in the setting of anaemia.3
The computed tomography (CT) imaging appearance of EMH typically consists of heterogeneous, hypovascular soft-tissue masses, often interspersed with areas of fat attenuation. The presence of calcification is atypical for EMH.1, 2 On ultrasound (US), EMH can present as a distinct solid mass with internal vascularity. The magnetic resonance imaging (MRI) appearance of EMH is typically a heterogeneous mass with variable T1- and T2-weighted imaging appearance. Lipid components are typical, and enhancement is variable. Identification of intra-voxel/microscopic fat with chemical shift imaging can be a very helpful aid to diagnosis. Technetium-99m (Tc-99m) colloid imaging can be used to confirm the presence of bone marrow elements within a mass that is suspected to represent EMH.
Section snippets
Thoracic manifestations
The most common manifestation of EMH in the chest is paraspinal masses, which are typically discovered incidentally4 (Fig. 2). Paraspinal masses are the most common site of EMH after hepatosplenomegaly and the most common site to present as a mass. Thoracic masses in the chest are frequently associated with thalassemia.1 The thoracic paraspinal masses of EMH are typically bilateral, smooth-surfaced, soft-tissue masses that contain areas of fat attenuation and do not calcify.2 The presence of
Abdominal and pelvic manifestations
The liver and spleen are the two most common abdominal locations of EMH, most commonly manifesting as organomegaly without a discrete mass (Fig. 3c). Focal masses within the liver and spleen are much less common (Figs. 4).4 Periportal/peribiliary hepatic involvement, in addition to focal hepatic masses, has been observed (Fig. 5). EMH has also been found in the splenic resection bed in patients following splenectomy for EMH-related splenomegaly (Fig. 6).
Perirenal involvement is a common
Central nervous system (CNS) manifestations
EMH in the CNS typically manifests as multiple epidural soft-tissue masses (Fig. 10), which can occur either in the brain or spine.4, 7 Reported complications of CNS involvement include seizures, hydrocephalus, and cord compression.7 MRI is the preferred diagnostic tool for intraspinal and intracranial masses. The MRI appearance and enhancement pattern of EMH is variable. The presence of a peripheral fat intensity rim around the mass on MRI has been suggested as a way of differentiating EMH
Clinical presentation, biopsy considerations, and management
The majority of patients (63%) found to have non-hepatosplenic EMH present with site-specific symptoms.8 The remainder of patients present with either generalised symptoms such as fatigue (15%) or are asymptomatic (22%).8
Aside from hepatosplenomegaly, bilateral, heterogeneous, fat-containing paravertebral masses are the most common manifestation of EMH. In this scenario, biopsy is usually not necessary. The presence of a focal soft-tissue mass in additional locations, such as the liver, spleen,
Conclusion
EMH occurs in a predictable patient population, most commonly in patients with myelofibrosis. Manifestations include hepatosplenomegaly, fat-containing paravertebral soft-tissue masses in the chest, and soft-tissue masses in multiple sites. EMH has a non-specific appearance on most imaging modalities, but involvement of multiple locations and the presence of macroscopic or microscopic fat can be helpful, particularly in patients with known risk factors. Biopsy is often required for definitive
Acknowledgements
PJP has a “financial relationship” with VirtuoCTC, Cellectar Biosciences, and SHINE. The authors would like to thank Vicki Friedman for her assistance in creating Figure 1.
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2022, Radiology Case ReportsCitation Excerpt :EMH’s paraspinal thoracic masses are typically bilateral and have a smooth surface. They are much more common in the thorax than in the abdomen or pelvis [2]. Intraspinal EMH has only been documented in a few cases, and it is usually asymptomatic until there is spinal cord compression [7].