What is an accessory spleen?

What is an Accessory Spleen

An accessory spleen is a congenital focus of normal splenic tissue that exists separately from the main spleen, resulting from incomplete fusion of splenic tissue during embryonic development.

Embryologic Origin and Pathophysiology

• Accessory spleens form during embryonic life from the left side of the dorsal mesogastrium when separate splenic masses fail to fuse completely 1
• This represents a congenital developmental anomaly rather than a pathologic process 2, 3
• The tissue is histologically identical to normal splenic parenchyma, containing the same cellular components and immunohistochemical profile (CD20, CD3, CD23, α-SMA expression) 3

Epidemiology and Location

• Accessory spleens are observed in approximately 15% of patients during primary splenectomy when the abdomen is systematically inspected 4
• Most commonly located near the splenic hilum or in the tail of the pancreas 2, 1
• Less common locations include the greater omentum 3, retroperitoneum 5, and rarely the pelvis 1
• The spleen shows intense accumulation on radionuclide imaging, making accessory spleens identifiable when present 4

Clinical Significance

In Asymptomatic Patients

• Usually discovered incidentally during imaging studies or surgical procedures 2, 3, 6
• Generally has no clinical significance in most patients 1
• Typical size is small (rarely exceeds 4 cm), though exceptional cases up to 7 cm have been reported 5

In Post-Splenectomy Patients

• Critical importance in patients with immune thrombocytopenic purpura (ITP): Among patients who fail or relapse after splenectomy, 12% are found to have an accessory spleen by radionuclide imaging 4
• Accessory splenectomy (removal of the accessory spleen) increases platelet counts in approximately 50% of post-splenectomy ITP failures, with 10-30% achieving sustained normal platelet counts 4
• No studies have demonstrated that accessory splenectomy reduces morbidity or mortality, and the procedure carries similar risks to primary splenectomy 4

Potential Complications

• Torsion of an accessory spleen can occur, presenting as acute abdominal pain and mimicking an acute abdomen 6
• May be misdiagnosed as pancreatic neoplasm (solid pseudopapillary tumor or nonfunctioning islet cell tumor) when located in the pancreatic tail 2
• Can mimic adnexal masses when located in the pelvis, requiring differentiation from ovarian tumors, fibroids, or tuboovarian abscess 1

Diagnostic Approach

• Radionuclide imaging is the gold standard for detecting accessory spleens in post-splenectomy patients with treatment failure 4
• Ultrasound with contrast-enhanced ultrasound (CEUS) can demonstrate the parenchymal nature and vascularization of the tissue 6
• CT and MRI may identify the lesion but often cannot definitively distinguish it from neoplastic processes preoperatively 2, 5
• Definitive diagnosis typically requires histopathological examination 2, 3

Management Considerations

• In asymptomatic patients, no intervention is required 1
• During primary splenectomy, the abdomen should be systematically inspected for accessory spleens, which should be removed when identified 4
• For post-splenectomy ITP patients with persistent thrombocytopenia, radionuclide scanning should be performed to identify accessory spleens before considering accessory splenectomy 4
• Accessory splenectomy should only be considered after radioisotope scanning confirms the presence of an accessory spleen 4
• In trauma cases with an undiagnosed retroperitoneal mass near the spleen, careful management is essential to avoid unnecessary removal, as preservation allows normal immune function post-splenectomy 5