The spleen filters the blood, and as a very vascular organ it contains a lot of it too. About 5% of our red cells, 30% of our platelets and up to 50% of our marginating neutrophils find themselves somewhere within the spleen at any given moment. This isn't usually a problem (our bodies compensate for this), but if the spleen should get significantly bigger than normal, things can change. It doesn't really matter what the cause of the enlarged spleen is, if it's big enough it will trap enough of the blood components to cause their circulating levels to fall. A big spleen can cause anaemia, thrombocytopenia and/or leukopenia just by being that massive.
There are traditionally four components that make up hypersplenism:
- Large spleen (obviously!)
- Cytopenia(s) (at least one of: anaemia, thrombocytopenia, leukopenia)
- Normal bone marrow (which should therefore be hyperplastic in compensation for the cytopenia(s))
- Correction of the cytopenias following splenectomy
Obviously, you don't just whip out the spleen to see if things improve; you use the other criteria as well as a fair dollop of clinical acumen before embarking on any butchery. Patients tend not to like unnecessary operations.
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