Extra-medullary haematopoiesis: a pictorial review of its typical and atypical locations.

Extra-medullary haematopoiesis (EMH) occurs in many haematological disorders and is secondary to insufficient bone marrow function or ineffective erythropoiesis. It usually presents as haematopoietic masses in several typical and atypical body locations. This pictorial review briefly discusses the common EMH locations encountered in clinical practice, including the liver, spleen, lymph nodes and paravertebral regions. Unusual presentation as focal hepatic and splenic masses is also discussed. Some atypical EMH locations are then presented together with their pathophysiology and relevant illustrations. These include EMH in the intra-spinal canal, pre-sacral region, nasopharynx and paranasal sinuses. Intra-spinal EMH can cause cord compression and present with neurological symptoms. In these cases, urgent treatment may be required. Haematopoietic masses in the other atypical locations can present with obstructive symptoms or may be diagnosed incidentally on imaging. EMH in unusual locations need to be monitored with follow-up imaging to ensure their stability. In some circumstances, tissue biopsy is appropriate to confirm the diagnosis and exclude other sinister pathology (e.g. malignancy). Treatment is only required where symptoms are present. Management options include blood transfusion, radiotherapy, hydroxyurea or surgical resection in selected cases.