What is an accessory spleen?

An accessory spleen is a small mass of splenic tissue that separates from a patient's primary spleen. About one in ten people has an accessory spleen, and less than one percent of the population has multiple accessory spleens. For the most part, accessory spleens are not harmful and, in fact, may go undiagnosed for a lifetime unless medical imaging of the abdomen is performed for unrelated reasons.

In some patients, the accessory spleen is the result of developmental variations. When a fetus develops, there are numerous stages of development in which organs move and reposition themselves, sometimes creating situations where additional organs can develop in the process. Other people develop accessory spleens as a result of trauma. The lump of tissue is usually very small and close to the spleen, although it can sometimes be located some distance away in the abdomen.

In some cases, the accessory spleen retains some of the function of the spleen and processes red blood cells along with the primary spleen. In others, the mass of tissue is simply a benign growth in the abdomen. While it's not necessarily harmful, it also doesn't serve any function in the body. Sometimes the growth can only be discovered after death during an autopsy, illustrating the fact that secondary spleens do not usually cause health problems.

There are some situations where an accessory spleen can become a problem. Patients who undergo therapeutic splenectomy to treat medical problems related to the spleen will not experience resolution of these problems if the secondary spleen is not removed at the same time. Sometimes the mass of tissue can be deprived of its blood supply and develop necrosis or tissue death. This can cause abdominal pain and other symptoms. Accessory spleen torsion, a condition where the spleen moves out of position and cuts off its own blood supply, is another problem that can develop.

Another problem that can arise for people with an accessory spleen is that medical imaging studies can be misread. The mass of tissue could look like a tumor, leading a radiologist to recommend surgery to remove it. This can lead to unnecessary surgical procedures. Patients who know they have an accessory spleen should ensure that this is noted in their medical records and that imaging studies are kept on file so that new studies can be compared with old ones to monitor for changes in location or the appearance of the secondary spleen

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