Myeloproliferative Neoplasms: Why Is My Spleen Enlarged?

A Review of Spleen Function and Role in Myeloproliferative Neoplasms

Physician and Patient Discussing Treatment
Physician and Patient Discussing Treatment. Hero Images/Creative RF/Getty Images

If your physician told you that you have a blood disorder you might wonder, Then why is my spleen so big? What does my spleen have to do with my blood? Or even What is the spleen? These are all excellent questions. Let's start with the function of the spleen, then discuss why the spleen enlarges in myeloproliferative neoplasms, and finally review treatment options for splenomegaly (enlargement of the spleen).

What Is the Spleen?

The spleen is a relatively small organ (about the size of your fist) that resides on the left side of the abdomen underneath the ribcage. The spleen is made of two types of tissue (1) the red pulp which filters the red blood cells and (2) the white pulp which is part of the immune system. In the red pulp, old or misshaped red blood cells (such as sickle cells) are removed from the circulation. The red pulp also helps the body filter out infections, particularly certain bacterial infections. The white pulp helps produce lymphocytes, a type of white blood cell, that helps make antibodies to infections or in response to immunizations. Although these are the major functions of the spleen once you are born, prior to birth, the spleen is the major producer of blood cells (hematopoiesis). Towards the end of pregnancy and after birth, the bone marrow takes over this production.

Why Does the Spleen Become Enlarged?

In polycythemia vera an excessive number of red blood cells are produced, resulting in an increased number of red blood cells that need to be filtered by the spleen, leading to splenomegaly. In primary myelofibrosis, the bone marrow is damaged by fibrosis, making it more difficult to produce blood cells.

In this case, the spleen may enlarge to support blood cell production outside of the bone marrow. This can also occur in the liver usually to a smaller degree.

What Are the Symptoms of Having an Enlarged Spleen?

Many people who have an enlarged spleen may not know, especially if the spleen is only slightly enlarged. Others might report a "fullness" in the abdomen. When the spleen is significantly enlarged, it can press on the stomach, which may give you the sensation of feeling full, like you just ate a full meal when you only ate a small amount.

If My Spleen Can Make Blood Cells, Why Does It Matter That It Is Big?

The spleen is fragile and normally protected by the ribcage. As it enlarges it is no longer protected and prone to injury, particularly trauma from a car accident or contact sport (like football or hockey). Trauma to an enlarged spleen can cause massive bleeding.

So How Can Splenomegaly Be Treated?

If you spleen is only mildly enlarged, treatment may not be needed. Your physician will likely monitor your blood counts and the size of your spleen closely.

If treatment is needed there are three big categories: medical therapy, splenectomy, and radiotherapy.

The first is medical therapy. In general, these are medications aimed at reducing blood cell production. One of the most common medications used to reduce spleen size in the myeloproliferative neoplasms is hydroxyurea. Hydroxyurea is an oral medications taken daily. It is generally started at a low dose and escalated until desired effect (usually a specific level of hemoglobin, white blood cells, or platelets). Other first line therapies include busulfan, melphalan, alpha interferon, thalidomide, or lenalidomide. Prednisone might be given with thalidomide or lenalidomide. Second line therapies include cladribine (also called 2CDA), daunorubicin, decitabine, or 5-azacytidine. Your physician will choose your therapy based on your specific diagnosis, other medical problems and the side effects of treatment.

The second treatment choice is splenectomy or surgical removal of the spleen. The most significant risk of removing the spleen is the potential for serious life threatening infections. If you haven't already, you should receive specific vaccines to protect you from pneumococcal and meningococcal infections prior to splenectomy. Once the spleen is removed, you will likely be placed on penicillin twice daily to prevent these infections. Additionally, fever (greater than 100.4F) is an emergency that requires immediate medical attention.

A third treatment option is radiotherapy (also called radiation therapy). Radiation is aimed at the spleen which can help shrink it in size. These effects are temporary so splenic radiotherapy is considered palliative, treatment aimed at minimizing symptoms to improve quality of life. This may be a good option for people who are not great candidates for splenectomy.

There are many things to consider when deciding if or how to treat splenomegaly in polycythemia vera or primary myelofibrosis. Make sure to discuss with your physician the benefits and possible side effects of these treatments.


Mesa RA. How I treat symptomatic splenomegaly in patients with myelofibrosis. Blood. 2009;113:5394-5400. 

Vannucchi AM. How I treat polycythemia vera. Blood. 2014;124:3212-3220.

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