A splenule, also known as an accessory spleen or splenunculus, is a small, separate piece of splenic tissue located outside the main spleen. These structures are often harmless and are frequently discovered incidentally during medical imaging for unrelated reasons. Many individuals may have a splenule without ever being aware of its presence. It is a common finding, occurring in an estimated 10% to 30% of the population.
What Exactly is a Splenule?
A splenule is a small, additional collection of tissue that is structurally and functionally similar to a normal spleen. These formations typically measure around 1 centimeter, though sizes can range from a few millimeters to several centimeters. They usually appear as well-circumscribed, rounded or ovoid nodules.
While a splenule can be found in various locations within the abdomen, it is most commonly situated near the main spleen, particularly in the splenic hilum, which is the indentation where blood vessels enter and exit the spleen. Other frequent locations include the tail of the pancreas, within the gastrosplenic or splenorenal ligaments, or in the greater omentum. This accessory tissue contains the same specialized components as the main spleen, such as red and white pulp, allowing it to filter blood, remove old red blood cells, and contribute to immune responses.
How and Why Splenules Form
Splenules can form through two main processes: congenitally during development or acquired later in life. The most common way they form is during embryonic development. During the fifth week of fetal life, if the spleen’s multiple small components do not fully fuse into a single organ, small, separate splenic nodules can remain. These are the congenital splenules.
Another way splenic tissue can appear in separate locations is through an acquired condition known as splenosis. This occurs after trauma to the spleen, such as a rupture, or following surgical removal (splenectomy). In these situations, fragments of splenic tissue can break off and implant themselves in other parts of the abdominal cavity. These implanted fragments can then develop their own blood supply and grow into functional splenic tissue.
Detecting and Understanding the Impact of Splenules
Splenules are most often discovered incidentally when medical imaging, such as an ultrasound, CT scan, or MRI, is performed for other health concerns. During imaging, a splenule appears similar to the main spleen, enhancing in the same way after contrast material is administered, which helps distinguish it from other abdominal masses. In some cases, specialized nuclear medicine scans, like a Technetium-99m sulfur colloid scan, can confirm the tissue is functional splenic tissue.
For the vast majority of individuals, splenules are benign and pose no health concerns. They rarely cause any symptoms or complications. In very rare instances, a splenule might cause issues such as torsion (where it twists on its blood supply) or rupture. In specific medical scenarios, particularly for individuals undergoing splenectomy for certain blood disorders, an unremoved splenule could potentially lead to a recurrence of symptoms.
Despite these rare possibilities, splenules do not require treatment or ongoing monitoring. Their incidental discovery leads to reassurance rather than intervention. Medical professionals determine that the potential risks of surgical removal outweigh the benefits, given their harmless nature.