Accessory spleen or a splenule is a congenital focus of healthy splenic tissue, that is separate from the main body of the spleen. The reported incidence is as high as 16% in patients undergoing contrast-enhanced abdominal CT. The most frequent location (22%) is posteromedial to the spleen; anterolateral to the upper pole of the left kidney; and lateral, posterior, and superior to the tail of the pancreas.

Why should we care about it?

For nephrology-related POCUS, it is important to be familiar with this entity because it may be confused with neoplastic growth of the kidney. Other important things to note about accessory spleen are: It can undergo hypertrophy after splenectomy, and can be responsible for the recurrence of hematological disorders for which the splenectomy has been performed if not removed during surgery. Occasionally, an accessory spleen may become symptomatic because of torsion, spontaneous rupture, hemorrhage or cyst formation and can cause an acute abdomen.

What does the image show?

The renal sonogram shown here demonstrates an approximately 1.5 cm hypoechoic well-circumscribed lesion outside the left kidney (not anchored to the kidney), which is an accessory spleen. It was confirmed by MRI.

The small renal mass here looks like a splenule. However, note that it is below the perirenal fat and attached to the renal parenchyma (always image in two planes to confirm).