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Splenunculi

Splenunculi (also known as supernumerary or accessory spleens or splenules) are small nodules of spleen that are detached from the rest of the organ. They are common, seen in up to 16% of CTs of the abdomen and up to 30% of autopsies 2. They are benign and asymptomatic, their importance mainly related to the need to distinguish them from more sinister pathology.  

Pathology

Accessory spleens are congenital nodules, composed of normal splenic tissue. The spleen forms from multiple smaller components during embryogenesis, and failure of this fusion can lead to one or more nodules remaining separate 2. Each component is extraperitoneal, as are splenunculi. They should not be confused from splenosis (see below) which is acquired and intraperitoneal. 

Radiographic features

Splenunculi are typically a few centimetres in diameter when identified, well circumscribed rounded or ovoid nodules. Although most are located near the spleen, they have been identified elsewhere in the abdominal cavity including 2,3.

CT

They have density and enhancing characteristics similar to the rest of the spleen. Small (<1cm) may appear hypodense c.f. to spleen, most likely as an artefact of partial voluming 2.

The converse of that is that if a mass is seen at the splenic hilum, which does not enhance in the same fashion as the adjacent spleen, then alternative diagnoses should be entertained.

Scintigraphy
Tc 99m sulphur colloid scan

In rare instances the diagnosis can be confirmed with Tc99m sulphur colloid scan which will demontrate increased uptake as long as the splenunculus is at least 2cm in diameter.

Differential diagnosis

General imaging differential considerations include

Other conditions with abnormal location or morphology of splenic tissue include:

  • lobulations or clefts - represent incomplete but partial fusion and are very common 2
  • polysplenia - rarely multiple splenules are seen without a 'parent' spleen; this is refered to as polysplenia and is seen associated with situs abnormalities.
  • splenosis - splenic tissue deposits following abdominal trauma or surgery (e.g. splenectomy), and do not have splenic arterial supply
  • wandering spleen - spleen located in an unusual position within the abdominal cavity

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