Psoas Abscess

A collection of pus within the psoas muscle compartment.

It arises either as a primary infection through haematogenous spread from a distant source. This is particularly common in younger patients without an obvious contiguous source and historically strongly associated with staphylococcal bacteraemia.

Or, more commonly in older patients, it is secondary to direct extension from an adjacent infected structure.

The adjacent structure is most often the spine (vertebral osteomyelitis or discitis), gastrointestinal tract (Crohn disease, appendicitis, diverticulitis), or genitourinary tract.

The distinction between primary and secondary disease carries important implications for the required extent of investigation and source treatment.

Pathophysiology and anatomy

• The psoas muscle, extending from the lumbar vertebrae to the lesser trochanter of the femur, lies in close anatomical proximity to the spine, kidneys, colon and appendix. This explains the range of recognised contiguous sources for secondary infection and the characteristic pattern of pain and hip flexion that reflects the muscle's function and irritation.
• Primary psoas abscess, more common in children and young adults, typically reflects haematogenous seeding, with Staphylococcus aureus the predominant causative organism.
• Secondary psoas abscess, more common in older adults, reflects the microbiology of the underlying contiguous source:
  • frequently polymicrobial with enteric Gram negative organisms and anaerobes where the source is gastrointestinal
  • typical vertebral osteomyelitis pathogens (Staphylococcus aureus, and, particularly relevant in endemic regions or specific risk groups, Mycobacterium tuberculosis) where the source is spinal

• The classical triad of fever, back or flank pain, and a limp or difficulty with hip extension is present in only a minority of patients. Presentation is frequently more insidious and non specific, particularly in secondary disease where symptoms of the underlying contiguous source may predominate over those directly attributable to the psoas involvement itself.
• A positive psoas sign reflects irritation of the psoas muscle and supports the diagnosis. It is present when passive extension of the hip with the patient lying on the unaffected side reproduces pain, or active hip flexion against resistance elicits pain.
• Patients may adopt a flexed, externally rotated position of the hip to minimise discomfort.
• Constitutional symptoms including fever, malaise and weight loss accompany the more specific musculoskeletal features. A palpable mass is present in some cases, particularly with a large collection tracking toward the groin along the course of the muscle.
• Symptoms and signs of the underlying contiguous source should be actively sought where relevant, including back pain and tenderness with vertebral involvement, or gastrointestinal symptoms with an abdominal source. They are relevant to both diagnosis and the required scope of subsequent management.

• Contrast enhanced computed tomography of the abdomen and pelvis is the investigation of choice, both confirming the diagnosis and, critically, identifying any underlying contiguous source requiring specific additional treatment.
• Magnetic resonance imaging offers superior detail where spinal involvement (vertebral osteomyelitis or discitis) is suspected as the underlying cause.
• Blood cultures should be obtained in every case. Culture of aspirated abscess material, obtained at the time of drainage, provides definitive microbiological diagnosis and guides targeted antibiotic therapy.
• Specific mycobacterial culture and testing is pursued where tuberculous infection is a relevant consideration based on epidemiological context or a compatible clinical and radiological picture.
• Full blood count, inflammatory markers and, where spinal involvement is identified or suspected, further assessment as detailed under spinal infections, complete the diagnostic evaluation.
• A careful search for the underlying source, guided by the imaging findings and clinical context, is an essential component of full evaluation. It includes colonoscopy or further gastrointestinal assessment where a bowel source is suspected, given the direct relevance to definitive management of secondary disease.

• Percutaneous image guided drainage is the primary intervention for the majority of psoas abscesses, providing both source control and microbiological diagnosis. Catheter drainage is generally preferred over simple aspiration to allow continued drainage over subsequent days.
• Empirical broad spectrum intravenous antibiotic therapy is initiated alongside drainage, with the specific regimen guided by whether primary or secondary disease is suspected:
  • for likely primary (staphylococcal) disease, flucloxacillin 2 g intravenously four times daily, or vancomycin where methicillin resistant Staphylococcus aureus is a concern
  • for secondary disease with a gastrointestinal source, broader cover with piperacillin with tazobactam 4.5 g intravenously three times daily, or a combination of a third generation cephalosporin with metronidazole, reflecting the likely polymicrobial, enteric organism profile
• Antibiotic therapy is narrowed once culture results from the drained material are available. Total duration is generally prolonged, typically four to six weeks, guided by clinical, biochemical and radiological response, and extended further where associated vertebral osteomyelitis is present given the specific treatment duration requirements of that condition.
• Surgical drainage is reserved for cases where:
  • percutaneous drainage has failed
  • the collection is not amenable to a percutaneous approach given its location or complexity
  • there is a specific indication for surgical management of the underlying source (such as surgical resection for a perforated diverticular or appendiceal source)
• Definitive treatment of any identified underlying contiguous source, whether spinal, gastrointestinal or genitourinary, is essential alongside treatment of the abscess itself, following the specific management principles relevant to that underlying condition. There is a risk of recurrence if the source is not adequately addressed.
• Close clinical and radiological follow up, with repeat imaging to confirm resolution, is an important component of ensuring adequate treatment of this condition. This is particularly so given the frequently prolonged treatment course required and the potential for the underlying source to require separate, sometimes staged, definitive management.