Splenomegaly

Splenomegaly is a spleen that is larger than normal. The spleen is an organ that blood passes through to be filtered, and it helps keep the cells that travel in the blood in good shape: red blood cells, white blood cells, and platelets. It works a lot like a pool filter: the water runs through it and comes out clean.

That same work with the blood is what can make it grow. In hereditary spherocytosis, an inherited red blood cell disorder, some red blood cells are stiff and spherical, round as marbles. The spleen destroys them, and they build up inside it, like a filter that keeps filling up. Primary myelofibrosis, a bone marrow disease, shows a second way. There, blood cell formation in the bone marrow, the tissue inside the bones, is disrupted, and the spleen may start making blood cells itself. This is called extramedullary hematopoiesis, which means making blood outside the marrow.

What may come as a surprise is that an enlarged spleen usually causes no symptoms. When it does, it can cause pain or a sense of fullness below the ribs on the left side.