A psoas abscess is a painful collection of pus in the psoas muscle of the spine (in the lower lumbar region). The psoas muscle extends from the lumbar spine through the pelvis to the femur.

Causes / typical injury mechanism: Abscesses in the psoas muscle are typically caused by bacterial infections.

• Primary – Hematogenous dissemination, suppurative lymphadenitis, or direct colonization of bacteria.
• Secondary – Bacterial propagation from adjacent structures.

Classic history and presentation: There is variation in presentation, but the typical patient may present with:

• Inability to extend the hip or severe pain when extending the hip
• Fever
• Limp or refusal to bear weight
• Hip pain
• Abdominal pain
• Lumbar pain

Signs from physical examinations and blood tests are unreliable in distinguishing between psoas abscesses and septic arthritis of the hip. However, signs of nerve damage such as scoliosis, sciatica, or femoral nerve neuropathy are far more likely to be seen in patients with psoas abscesses.

Prevalence: This is slightly more prevalent in boys.

Risk factors: Risk factors include previous traumatisms in the general area, such as spinal tuberculosis, appendicitis, and osteomyelitis of the pelvis or lumbar vertebrae, and immunocompromised status.

Pathophysiology: Psoas abscesses are most commonly caused by Staphylococcus aureus; however, they may also be caused by Staphylococcus epidermidis, Salmonella spp, or Mycobacterium tuberculosis.