Clinincally, it is a spleen palpable in the abdomen. It is not the same as hypersplenism which is where the spleen goes into overdrive, rapidly and prematurely destroying blood cells.
This very much depends on the cause. The spleen is basically a giant lymph gland and lymph glands are involved in infection, blood cells and blood flow. If you get certain infections, the spleen, like any lymph gland, fills with white blood cells and therefor gets bigger.
In terms of blood cell abnormality, you can get blood or haematological cancer where proliferation of cancerous blood cells cause the spleen to get bigger. You can also have an autoimmune response which can cause an enlarged spleen - how depends on the autoimmune condition. Increased RBC removal or "hypersplenism" also results in splenomegaly.
Finally, physical obstruction of the blood flow which cause lymph nodes to enlarge anywhere and this is true of the spleen.
Clinical and Associated Features
Usually felt in the left hypochondrium of the abdomen, when palpating for an enlarged spleen you should start from the right iliac fossa.
Broadly speaking, there are five things that an enlarged spleen is likely to be caused by: infection, haematological cancer, abnormal blood flow, increased RBC removal, or autoimmune. (More rarely, blood disorders resulting in increased RBC removal, such as haemolytic anaemias, will result in an increase in spleen size.)
- Viral - this is usually infectious mononucleosis, CMV, malaria, HIV or hepatitis.
- Bacterial - endocarditis or some forms of TB.
- This is caused by some form of blockage or other backflow. Liver cirrhosis, right heart failure, sickle cell anaemia (can cause small spleen due to infarct) and schistosomiasis.
- Essentially, any of the haematological cancers can cause an enlarged spleen. These are leukaemia and lymphoma by and large.
Increased RBC removal
- The spleen is involved in the breakdown of red blood cells. In a condition where lots are being broke down (haemolysis), the spleen is part of that breakdown process. It gets bigger in order to deal with the extra RBCs it's breaking down.
- Any connective tissue disorder which is autoimmune: rheumatoid arthritis, SLE or sarcoidosis are all examples.
Essentially, you're investigations, as always, should be used to confirm you clinical diagnosis.
- Infection - do a full blood count and test specifically for hepatitis or HIV if suspected.
- Blood flow - this very much depends on the cause again. LFTs should be done for liver cirrhosis but the others won't be done with baseline investigations.
- Haematological cancer - do a full blood count but also do a biopsy
- Autoimmune - if any sort of autoimmune condition is suspected, then a whole other battery of tests needs to be done.