A splenule, also known as an accessory spleen, is a small, separate nodule of tissue located outside the main spleen. The main spleen functions as both a blood filter and a component of the immune system. It removes old or damaged red blood cells and produces white blood cells and antibodies. A splenule is essentially a miniature version of this organ, performing the same functions. It is often discovered unintentionally during medical imaging for an unrelated issue.
Defining the Accessory Spleen and Its Origin
A splenule is a congenital anomaly, meaning it is present from birth, and is histologically identical to the main splenic tissue. This small mass is composed of the same red pulp (for blood filtration) and white pulp (for immune response) as the primary organ. Splenules are common, found in an estimated 10% to 30% of the general population.
The formation of a splenule occurs during early embryonic development, specifically around the fifth to eighth week of gestation. The spleen develops from multiple small buds of mesenchymal tissue. A splenule arises when one or more of these buds fail to fuse completely with the main mass of tissue that forms the spleen.
This failure results in separate masses of splenic tissue that maintain their own blood supply, typically from the splenic artery. Splenules are not considered a growth or tumor, but a structural variation of normal anatomy. They usually measure about 1 centimeter in diameter, though they can range up to several centimeters.
Common Locations and Incidental Discovery
The location of a splenule is typically along the path the spleen takes during development. The vast majority are found in two primary areas: the splenic hilum (75%), where blood vessels enter the main spleen, and near the tail of the pancreas (20%).
Less common locations include the gastrosplenic ligament, the splenorenal ligament, the greater omentum, or the wall of the stomach or intestine. A splenule is generally asymptomatic and causes no health issues, which is why its discovery is most often incidental. Patients undergoing imaging tests (ultrasound, CT, or MRI) for unrelated abdominal complaints may inadvertently reveal its presence.
Radiologists must differentiate a splenule from other abdominal masses, such as enlarged lymph nodes or tumors, which can look similar on initial scans. Splenules typically appear as rounded, well-marginated masses that enhance with contrast identically to the main spleen, sharing the same blood supply. If the diagnosis is unclear, a specialized nuclear medicine scan using a technetium-99m labeled compound can confirm the finding, as functional splenic tissue selectively absorbs the material.
When a Splenule Requires Medical Attention
For most people, a splenule is a harmless anatomical finding requiring no specific treatment or follow-up. Since the tissue is functional, it contributes a small amount to the body’s immune and filtering processes. Standard management involves observation, and patients are reassured that the confirmed splenule is not a concerning growth.
A splenule can become clinically significant under specific, rare circumstances requiring medical intervention. The most serious complication is torsion, where the splenule twists around its vascular stalk, cutting off blood flow. This causes acute, severe abdominal pain and may lead to tissue death (infarction), requiring emergency surgical removal.
The presence of a splenule is a concern in patients with certain hematologic diseases, such as Immune Thrombocytopenia (ITP) or hemolytic anemia. In these conditions, the spleen actively destroys blood cells or platelets, and surgical removal of the main spleen (splenectomy) is sometimes performed. If the splenule is not removed along with the main organ, the accessory tissue can continue the destructive process, leading to disease recurrence.
When a splenectomy is planned for a hematologic disorder, imaging identifies all splenules, which are then removed in a procedure called a splenule-ectomy to achieve a complete cure. Surgical removal is only performed when symptoms like torsion or bleeding are present, or when the tissue is implicated in a disease process.