Overview Iliopsoas abscess (IPA) is a rare condition defined by a collection of purulence in the iliopsoas compartment. IPA is classified as primary when a causative organism from a distant occult site spreads to the iliopsoas compartment by a hematogenous or lymphatic route, or secondary when a contiguous extension of an intra-abdominal infectious or inflammatory process causes the condition. Diagnosis and treatment is often delayed secondary to nonspecific features at time of presentation and outcomes from this disease are potentially poor or fatal without early and effective clinical management. Literature on this topic is limited to case reports and relatively small case series. IPA has classically been associated with Mycobacterium tuberculosis infection, but this etiology is increasingly rare. In current practice, Staphylococcus aureus is the most common organism isolated from primary IPA, with an increasing incidence of methicillin-resistant S. aureus (MRSA). In addition to S. aureus, enteric organisms are commonly isolated from secondary IPA. Anatomy The iliacus muscle and psoas major muscle comprise the retroperitoneal muscle group referred to as the iliopsoas, which functions as the primary flexor of the hip. An accessory psoas minor muscle may be present in 10% to 65% of patients. The psoas major arises from T12 and the five lumbar vertebrae, passes along the posterior abdominal wall under the inguinal ligament, and inserts on the lesser trochanter of the femur. The iliacus arises from the superior portion of the iliac fossa and passes under the inguinal ligament to insert on the lesser trochanter as well as the femoral shaft. The muscles are often referred to as a single muscle, the iliopsoas, since both muscles contribute fully to the tendinous insertion at the femur. The psoas fascia invests the muscle group and runs from the lumbar vertebrae to the iliopubic eminence.
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