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A 24-year-old man was investigated because of a 3-day history of epigastric pain that was accompanied by nausea and vomiting. Blood tests revealed an elevated white cell count (13.2 × 109/l) and a mild elevation of erythrocyte sedimentation rate (27 mm/hr) and C-reactive protein (18 mg/dl). An ultrasound (US) study revealed a homogeneous hypoechoic mass in the left upper abdomen with an internal tubular anechoic structure. The tubular structure was thought to be a blood vessel but there was no blood flow on a Doppler study. A contrast-enhanced computed tomography (CT) scan showed an ovoid mass, 8 cm in diameter, in the left upper abdomen that was adjacent to the anterior aspect of the distal transverse colon (Figure 1). The mass was clearly separate from a normal spleen, left kidney and left adrenal gland. The mass did not show contrast-enhancement apart from a small wedge-shaped area in the left lateral portion. There was also a whirled tubular structure adjacent to the mass (arrow). At laparotomy, he had a violet, oval tumor under the omentum. The tumor had a twisted vascular pedicle attached to the splenic pedicle. The resected specimen is shown in Figure 2 and pathologic examination confirmed torsion and hemorrhagic infarction of an accessory spleen.

In the developing embryo, the spleen arises from several small mesodermal buds in the dorsal mesogastrium. These normally coalesce to form a single organ. However, failure of coalescence can result in the formation of one or more accessory spleens. These can be identified on an US or CT scan in approximately 3%–4% of patients. However, at postmortem examination, the frequency of accessory spleens ranges from 10%–30%. The most common sites for accessory spleens are the splenic hilum, the gastrosplenic ligament and the tail of the pancreas. Most accessory spleens are asymptomatic. However, torsion may occur as described above and hyperplasia can develop after splenectomy. The latter may be clinically significant if splenectomy is performed for hematological disorders such as hereditary spherocytosis and idiopathic thrombocytopenic purpura. On contrast-enhanced CT, the differential diagnosis of torsion of an accessory spleen can include hemorrhagic complications of cysts such as mesenteric cysts, intestinal duplication and pancreatic pseudocysts.