The MastAttack 107: The Layperson’s Guide to Understanding Mast Cell Diseases, Part 72

86. What is the role of the spleen in systemic mastocytosis? (Part One)

  • The spleen is basically a big filter for the blood. It is supposed to catch certain types of infections in the blood that your immune system has a hard time fighting in other ways.
  • When the spleen is swollen but still functions pretty well, it is called splenomegaly.
  • Swelling of the spleen is not uncommon in systemic mastocytosis. Splenomegaly is most often seen in patients with smoldering systemic mastocytosis, aggressive systemic mastocytosis, and mast cell leukemia, but sometimes patients with indolent systemic mastocytosis have swelling of the spleen.
  • When the spleen swells, the pathway for the blood going through the filter gets pinched. Blood goes in but has to pass through a narrow exit route to get out of the spleen. The more swollen the spleen is, the narrower the pathway for the blood to get through the spleen. This means that cells can be damaged or broken open if the spleen is swollen.
  • How much this happens depends upon how swollen the spleen is. If it is only a little swollen, the change in blood cell counts can be minimal.
  • For systemic mastocytosis, a swollen spleen that works well (splenomegaly) is what is called a B finding. A B finding is a way to tell if a patient’s indolent systemic mastocytosis is moving to a more serious form, like smoldering systemic mastocytosis or aggressive systemic mastocytosis. If a patient has a B finding, they are monitored more closely to look for other clues that could mean the disease is progressing.
  • Please note that the B finding MUST be caused by the mastocytosis to count. For example, if an SM patient falls off their bike and injures their spleen, causing it to swell, this is not a B finding. If the mastocytosis didn’t cause the problem, it doesn’t count.
  • Mast cell patients who have a spleen that is swollen but works correctly don’t damage too many blood cells. This means blood counts are often normal in this situation. If blood cell counts are not normal, the spleen is not the cause.
  • Some patients with aggressive systemic mastocytosis and mast cell leukemia develop a condition called hypersplenism. Hypersplenism basically means the spleen is working way too hard. Hypersplenism is a C finding, a marker that indicates that a patient’s mastocytosis has become very aggressive. If a patient has a C finding, they are diagnosed with aggressive systemic mastocytosis (ASM).
  • Sometimes patients with mast cell leukemia have hypersplenism. However, there are stringent criteria for diagnosing mast cell leukemia. Just having a C finding isn’t enough for a diagnosis of mast cell leukemia, while just having a C finding IS enough for a diagnosis of aggressive systemic mastocytosis.
  • Having a C finding is not a defining feature of mast cell leukemia the way it is for aggressive systemic mastocytosis.
  • Some patients with systemic mastocytosis have another blood disorder that causes the bone marrow to make too many cells. This is cleverly named systemic mastocytosis with associated hematologic disorder (SM-AHD). People with SM-AHD can have any stage of systemic mastocytosis. If they have another blood disorder, they are categorized as having SM-AHD even if they have aggressive systemic mastocytosis or smoldering systemic mastocytosis. So a person with SM-AHD can have any type of systemic mastocytosis, including aggressive systemic mastocytosis.
  • Sometimes patients with systemic mastocytosis alongside another blood disorder (called SM-AHD) have hypersplenism. Here, the hypersplenism could be caused by one of two conditions: systemic mastocytosis, or the other blood disorder. If the mastocytosis causes the spleen issue, the patient gets a diagnosis of aggressive systemic mastocytosis just like any systemic mastocytosis patient. If the other blood disorder is what causes the hypersplenism, the patient does not get a diagnosis of aggressive systemic mastocytosis.
  • If the mastocytosis causes the spleen issue, then we know that this is a C finding, a marker for aggressive systemic mastocytosis. If the other blood disorder is what causes the hypersplenism, it is not a C finding and does not indicate aggressive systemic mastocytosis.
  • Please note that having a C finding is not a defining feature of SM-AHD the way it is for aggressive systemic mastocytosis.
  • Hypersplenism sometimes occurs in patients with SM-AHD. It could be caused by either the systemic mastocytosis or the other blood disorder. It can be trickier to figure out exactly what is causing the splenic issues.
  • If the mastocytosis causes the spleen issue, then we know that this is a C finding, a marker for aggressive systemic mastocytosis. If the other blood disorder is what causes the hypersplenism, it is not a C finding and does not indicate aggressive systemic mastocytosis.
  • Please note that having a C finding is not a defining feature of SM-AHD the way it is for aggressive systemic mastocytosis.
  • You can tell that a person has hypersplenism by looking at four things:
    1. Low counts of certain blood cells in the blood. Red blood cells, platelets, and some white blood cells can be low because of hypersplenism. The white blood cells that are low when a person is hypersplenic are eosinophils, neutrophils, and basophils. These cells all have granules full of chemicals like mast cells do.
    2. The bone marrow trying to make extra blood cells to make up for the ones that being destroyed by the spleen.
    3. Swelling of the spleen.
    4. The expectation that if the spleen is removed, the blood cell counts will go up and the bone marrow will start making normal amounts of blood cells again.

This question was answered in two parts. Please see the following post for more information.

For additional reading, please visit the following posts:

The Provider Primer Series: Diagnosis and natural history of systemic mastocytosis (ISM, SSM, ASM)

The Provider Primer Series: Natural history of SM-AHD, MCL and MCS

Mast cell disease and the spleen