What is a Splenule (Accessory Spleen)?
A splenule, or accessory spleen, is a congenital nodule of normal splenic tissue located separately from the main spleen, occurring in approximately 10-30% of the population 1, 2.
Anatomical Characteristics
Accessory spleens are typically small, well-marginated, round masses that are usually less than 2 cm in diameter 2. They most commonly appear in the following locations:
- Most frequent location (22%): Posteromedial to the spleen, anterolateral to the upper pole of the left kidney, and lateral/posterior/superior to the pancreatic tail 2
- Splenic hilum: The second most common location 1
- Intrapancreatic: Nearly 20% occur in or near the pancreatic tail 3
- Other sites: Greater omentum, gastric wall (rare), and other peritoneal locations 4, 5
The dimensions typically range from 2-3.5 cm in length and 0.5-2.5 cm in width 1.
Imaging Features
On contrast-enhanced CT, accessory spleens demonstrate characteristic features that help distinguish them from pathology:
- Enhancement pattern: Homogeneous enhancement identical to the main spleen 2
- Attenuation: May appear lower than the spleen when smaller than 1 cm due to partial volume effects 2
- Morphology: Well-defined, round masses with smooth margins 2
Clinical Significance
Benign Nature with Important Caveats
While accessory spleens are benign congenital anomalies, they carry several clinically important implications:
Diagnostic confusion: They can be misidentified as:
- Pancreatic tumors (especially islet cell tumors when intrapancreatic) 3
- Adrenal tumors 1
- Reactive lymph nodes 4
- Metastatic disease 3
- Gastrointestinal submucosal tumors 4
Post-splenectomy considerations: Accessory spleens have critical importance after splenectomy:
- May cause hyperplasia and assume splenic function 1
- Can be responsible for recurrence of hematological disorders that were the original indication for splenectomy 1
- Should be considered when submucosal gastric lesions are found in patients with prior splenectomy 4
Diagnostic Approach
When an accessory spleen is suspected, particularly in the pancreatic tail, a combination of imaging modalities can confirm the diagnosis and prevent unnecessary biopsy or surgery 3:
- Contrast-enhanced CT showing enhancement identical to the spleen
- Nuclear medicine studies (technetium-99m sulfur colloid scan) to confirm splenic tissue
- MRI with characteristic enhancement patterns
- Endoscopic ultrasound (for gastric wall lesions) 4
The key is maintaining a high index of suspicion based on the classic location and typical imaging features, which can definitively establish the diagnosis non-invasively 3.