Corpus: Accessory spleen

A supernumerary spleen is an anatomical variety that results from a dispersion of cells during the organogenesis of the spleen in the embryonic period. This results in the formation of clusters of spleen tissue that lie outside the spleen capsule in the abdominal cavity.

Splenosis, which is the result of a traumatic dispersion of spleen tissue following a rupture of the capsule, is distinguished from splenomegaly.

Secondary spleens are a relatively common finding. In autopsy studies, the incidence is given as 10-15%.

The exact mechanisms that lead to the formation of a secondary spleen are unknown. It may be a small separation from the splenic anlage in the dorsal mesogastrium caused by the displacement and rotation of the stomach.

In the context of trisomy 13, there is a frequent occurrence of intrapancreatic secondary spleens and other developmental anomalies of the spleen. The rare splenogonadal fusion results in an abnormal connection between the spleen and the gonads. Spleen tissue can then be carried over into the pelvis and finally into the testicles as part of testicular descent.

In around 80% of cases, the secondary spleen is found in the form of one or more small nodules near the splenic hilus. They are usually cherry-sized, 1 to 1.5 cm in diameter, dark red in colour and surrounded by a thin capsule. Rarely, they can grow up to 4 cm in size. Other possible localisations for a secondary spleen are the gastrosplenic ligament, the splenorenal ligament, the greater omentum, the tail of the pancreas (1-2% of cases) and the stomach wall. Histologically, the structure of a secondary spleen is identical to that of the spleen.

A secondary spleen is usually an incidental finding that is asymptomatic and does not require treatment. In imaging procedures, secondary spleens can simulate a tumour or lymph node metastases - especially if the tissue is localised in the tail of the pancreas.^[1]

In diseases where splenectomy is necessary (e.g. hypersplenism or hereditary spherocytosis), the secondary spleen must also be removed. Spleen scintigraphy can be used to detect and localise a secondary spleen.