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. In children, psoas abscesses are more likely to be primary.
• Secondary – Bacterial propagation from adjacent structures. In adults, psoas abscesses are usually secondary to another infection or process.

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 inability 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 or other focal infectious processes that can present in a similar fashion. However, signs of nerve damage, such as sciatica or femoral nerve neuropathy, are far more likely to be seen in patients with psoas abscesses.

Prevalence: The prevalence of psoas abscess in adults has risen from 0.5 cases per 10 000 admissions in 1993 to 6.5 cases per 10 000 admissions in 2004 in a study from Johns Hopkins. The increase has been attributed to improved diagnostic modalities.

Risk factors: Risk factors include conditions commonly associated with an increased risk of infection such as diabetes mellitus, immunocompromised status, alcohol use disorder, intravenous (IV) drug use, and a history of primary infections that can be complicated by psoas abscess such as genitourinary and gastrointestinal (GI) infection or preceding spinal infection. A recent history of trauma or instrumentation of the site also increases probability.

Pathophysiology: Microbiologic etiology varies based on the inciting etiology of psoas abscess, with 1 large US study revealing that the majority of primary psoas abscesses were due to Staphylococcus spp (93%), whereas secondary psoas abscesses were more likely to be polymicrobial (27%) with Staphylococcus spp making up the majority of isolates (69%), followed by Escherichia coli (16%), Bacteroides spp (12%), Prevotella spp (10%) Fusobacterium spp (8%), Enterococcus spp (7%), Streptococcus spp (7%), Klebsiella spp (5%), and Salmonella spp (2%).