Splenunculus

Splenunculus, also called an accessory spleen or splenule, is a small island of splenic tissue that is separate from the main spleen. It results from embryologic persistence of splenic tissue that fails to fuse with the developing organ.

During early embryogenesis, multiple splenic nodules form in the dorsal mesogastrium. Most coalesce to become a single spleen, but one or more nodules may persist as accessory spleens. Accessory spleens are relatively common, present in an estimated 10–30% of people, and are usually asymptomatic.

Location and size vary. Accessory spleens are most often found near the splenic hilum but can occur along the splenic vessels, near the tail of the pancreas, stomach, or greater omentum. They are typically small, commonly around 1–2 cm, but can enlarge to several centimeters.

Clinical significance is usually limited to incidental detection on imaging, autopsy, or surgery. Imaging features resemble those of the native spleen: on ultrasound they are typically isoechoic to splenic tissue; on contrast-enhanced CT or MRI they enhance in the same pattern as the spleen. Nuclear medicine scans using technetium-99m–labeled damaged red blood cells or sulfur colloid show uptake by the accessory splenic tissue, helping confirm the diagnosis. They can be mistaken for lymph nodes or small tumors.

Management is generally conservative unless the patient is undergoing splenectomy for a hematologic condition. In that setting, all accessory spleens should be identified and removed to avoid persistent disease activity. Splenosis, a related but distinct condition, refers to autotransplanted splenic tissue after rupture, not congenital splenunculi.