Extramedullary Hematopoiesis and Splenic Vein Thrombosis, a Unique Presentation Of Pre-Clinical Essential Thrombocythemia
Authors: Yacoub Abdulraheem , Brockman Abigail 
Published: October 2019

Abstract
Essential thrombocythemia (ET) is of the BCR-ABL-Negative myeloproliferative neoplasms (MPN). The incidence of ET is approximately 2.5 in every 100,000 person per year. However, given the good prognosis, associated long life expectancy, and increasing detection in younger populations,ET is associated with a higher prevalence rate estimated to be 24 in every 100,000 person per year. ET is characterized by thrombocytosis, vasomotor symptoms, and a variable but increased risk of thrombosis and bleeding. Half of all ET patients will have a positive JAK2 and/or MPL mutation(s).
Extramedullary hematopoiesis (EMH) is not a common finding in ET. Nonetheless, ET and other MPNs are associated with the mobilization of CD34+ cells into the peripheral blood. This process can ultimately lead to the seeding of extramedullary sites with primitive hematopoietic capacity, resulting in EMH within the spleen and liver, as well as a variety of other organs. Herein we describe a case that presented with life-threatening thrombosis and was found to have hepatic EMH several months prior to a clinical and pathologic diagnosis of ET.

Case description
A 22 year-old woman presented 10 days post Cesarean section with abdominal pain and hematemesis. Abdominal imaging showed hepatomegaly, splenomegaly, along with splenic and portal vein thrombosis. The patient underwent an emergency surgical splenectomy due to severe portal hypertension and endoscopic evidence of gastric variceal bleeding. A random liver biopsy was also performed intra-operatively. The splenectomy resulted in resolution of the GI bleeding and the varices normalized on follow up. Her platelet count was normal at the time of operation, but post-splenectomy her platelet count peaked at 1,217 K/ µL. Extensive testing did not unravel any identifiable inherited and/or acquired hypercoaguable factors. Subsequently anticoagulation therapy was recommended for 6 months.
On pathology review, the spleen histology showed congestion, but otherwise no diagnostic abnormalities were noted. The liver biopsy showed evidence of EMH but did not identify any liver parenchymal disease.
On subsequent follow up, the patient had persistent and marked thrombocytosis for over a year. A bone marrow biopsy was performed which showed a hypercellular bone marrow and megakaryocytic hyperplasia with a few large forms. There was no dysplasia or significant reticulin fibrosis. JAK2 mutation and BCR-ABL translocation were negative. Hydroxyurea and aspirin were started due to high risk of thrombosis.

Discussion
We report this unique case in which there was evidence of extramedullary hematopoiesis, along with pathologic and life threatening visceral thrombosis several months before the patient met criteria for diagnosis of ET. This supports the notion that neoplastic cells can mobilize and seed other organs early in the course of MPNs, including ET. Thrombotic risk in MPNs can also occur in the preclinical phase of MPNs as has been suggested in other reports. We also conclude that the demonstration of EMH in individuals with no preexisting hematologic neoplasm should warrant close follow up and assessment.

Disclosures:
No relevant conflicts of interest to declare.