An accessory spleen is a small mass of splenic tissue that is separated from a patient's primary spleen. Around one in ten people has one, and less than one percent of the population has multiple accessory spleens. For the most part, these spleens are not harmful and they may, in fact, go undiagnosed for a patient's entire life unless medical imaging studies of the abdomen are 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 where organs move around and reposition themselves, sometimes setting up situations where extra organs can develop in the process. Other people develop extra spleens as a result of trauma. The lump of tissue is usually very small and close to the spleen itself, although sometimes it can be located at a distance in the abdomen.
In some cases, the accessory spleen retains some spleen function 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 not necessary harmful, it also does not serve any function in the body. Sometimes, the growth may only be discovered after death during an autopsy, illustrating the fact that secondary spleens usually do not cause health problems.
There are some situations when an accessory spleen can become a problem. Patients who undergo a therapeutic splenectomy to address medical problems related to the spleen will not experience a resolution of these problems if the secondary spleen is not removed at the same time. Sometimes, the tissue mass can become deprived of blood supply and develop necrosis, or tissue death. This can cause abdominal pain and other symptoms. Torsion of an accessory spleen, a condition where the spleen moves out of position and cuts off its own blood supply, is another problem that can develop.
Another issue that can arise for people with an accessory spleen is that medical imaging studies may be misread. The mass of tissue could look like a tumor, leading a radiologist to recommend surgery to remove it. This may lead to unnecessary surgical procedures. Patients who know that they have an accessory spleen should make sure that this is noted in their medical charts and that imaging studies are kept on file, so new studies can be compared with previous ones in order to monitor changes in the location or appearance of the secondary spleen.