The retropsoas recess. Demonstration using computed tomography.

The retropsoas recess, a previously undescribed normal anatomic variant, is reported. In most individuals, the space posterior to the belly of the psoas muscle contains fat within the posterior pararenal space. However, a retrospective review of 250 consecutive CT examinations of the abdomen revealed 11 cases (4.4%) in which bowel loops extended at least partially posterior to the psoas muscle. Unilateral left-sided occurrence was present in seven, with four bilateral. Colon was invaginated in five, small intestine in four, and both colon and small intestine in one. There was no age or sex predominance. A paucity of retroperitoneal fat is the primary predisposing factor for development of a retropsoas recess. Although recognition of this normal variant is generally not difficult, the CT appearance may mimic a retroperitoneal mass or abscess in less-than-ideal examinations or targeted studies. The potential presence of bowel loops in this location should be considered during interventional procedures.