AssessmentsEssential thrombocythemia (NORD)
USMLE® Step 1 style questions USMLE
A 51-year-old female comes to the emergency department because of shortness of breath that began an hour ago. Three days ago, the patient returned from a business trip in Europe. Past medical history is notable for hypertension. The patient has been taking oral estrogen for management of menopausal symptoms. Her temperature is 37.8°C (100.0°F), blood pressure is 148/89, pulse is 130/min, and respiratory rate is 27/min. Oxygen saturation is 86% on room air. Physical examination reveals splenomegaly. Coagulation studies show a prothrombin time (PT) of 12 seconds and partial thromboplastin time (PTT) of 34 seconds. A peripheral blood smear shows the following:
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Computed tomography subsequently confirms the presence of a pulmonary embolus. Which of the following best explains the pathophysiology of this patient presentation?
Content Reviewers:Rishi Desai, MD, MPH
Now, the vast majority of bone marrow is made of hematopoietic cells which are the early progenitor cells that can differentiate into other cell types.
In the case of platelets, progenitor cells differentiate into megakaryocytes, which are responsible for creating platelets.
In essential thrombocythemia there’s a genetic mutation that occurs in the Janus Kinase 2 gene, also called JAK2, or somewhere along this pathway of cell signalling.
Normally, the liver and kidneys produce a a tiny hormone called thrombopoietin which binds to hematopoietic cell receptors.
When it binds, those cells activate the JAK2 gene which makes them divide and mature into megakaryocytes and platelets.
Cells can also develop mutations in the thrombopoietin receptor, MPL, or in the chaperone protein, Calreticulin or CalR.
Now, when there’s a genetic mutation in CalR, the signalling pathway remains active all the time, and that means that platelets keep getting produced even in the absence of thrombopoietin.
Although there are lots of platelets that are made, many of them end up being misshapen - they’re large and irregularly shaped.
Now, all of these excess platelets end up causing an increased risk of blood clots in the deep veins of the legs, lungs, and even sites where clots don’t usually form, like the abdomen.
Now, if the number of platelets is extremely high, over 1.5 million, then there’s an increased risk of bleeding.