Psoas abscess is a rare intramuscular infection of the psoas, or iliopsoas muscle, caused typically by Mycobacterium tuberculosis or Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA). It may be attributed to the spread of bacteria by the circulatory (hematogenous) route from infected sites anywhere in the body (primary abscess), or from contact with adjacent organs (secondary abscess).

Primary psoas muscle abscess is more common in persons under age 30 (83% of cases), and is more common in males. Although the etiology of primary psoas abscess is often uncertain, the majority of blood samples (88%) test positive for S. aureus. Immunocompromised patients with human immunodeficiency virus (HIV), renal disease, diabetes, intravenous drug use, or alcohol use disorder are at risk for developing psoas abscess.

Secondary psoas muscle abscess is more likely to occur in adults with concurrent illness. The digestive disorders most often associated with secondary abscess are perforated appendicitis, perforated diverticulitis, Crohn disease, colorectal cancer, and pancreatic abscess. Other associated conditions include septic arthritis, suppurative adenitis, endocarditis, urinary tract infection, tuberculous spondylitis, osteomyelitis of the vertebrae, infectious sacroiliitis, and lumbar spondylodiscitis.

Psoas abscess has been reported during pregnancy.

Psoas abscess is characterized by pus collection and inflammation within the muscle compartment, presenting the hallmark findings of lower back pain, limp, and spiking fever. Other signs and symptoms may include flank pain, abdominal pain, anorexia, nausea, weight loss, malaise, and inguinal mass. Pain may radiate to the hip and upper thigh.

Because of similarities to other back pain disorders (lumbar disk hernia, trauma, or spinal compression), the diagnosis of psoas abscess can be overlooked and correct treatment delayed. Psoas abscess may be associated with primary malignancy; care must be taken to accurately diagnose and manage both the primary malignancy (such as colorectal carcinoma or metastatic squamous cell carcinoma) and secondary psoas abscess.

Management of psoas abscess involves abscess drainage with broad spectrum antibiotic therapy. Once the infectious organism is identified, antibiotic therapy can be geared appropriately. Abscess drainage can include percutaneous drainage guided by CT or ultrasound or surgical drainage.