Psoas Abscess - Postgraduate 10 Marks Answer

Definition

Anatomy - Why It Matters

• Superiorly with the posterior mediastinum (via aortic hiatus)
• Inferiorly with the femoral triangle (via subinguinal space)
• Medially with the vertebral column and adjacent retroperitoneal structures

Psoas abscess tracking from caries spine to the groin, lateral to the femoral artery and vein (S Das, Manual of Clinical Surgery)

Classification

Aetiology / Causes

Secondary Sources (most common today):

1. Spinal / vertebral column - Pott's disease (TB spondylitis, historically the #1 cause), pyogenic vertebral osteomyelitis, discitis
2. Gastrointestinal - Crohn's disease (most common GI cause today), appendicitis with perforation, diverticulitis, carcinoma colon, sigmoid abscess
3. Urinary - Perinephric abscess, renal stone with infection, pyelonephritis
4. Gynaecological - Pelvic inflammatory disease, tubo-ovarian abscess, septic abortion, infected caesarean haematoma
5. Vascular - Infected aortic aneurysm or graft, infected retroperitoneal haematoma

Microbiology:

• Primary abscess: S. aureus (most common, including MRSA)
• Secondary abscess (spinal/skeletal origin): S. aureus, Mycobacterium tuberculosis
• Secondary abscess (GI/GU origin): Mixed enteric organisms - aerobic and anaerobic gram-negative bacilli (E. coli, Bacteroides, Streptococcus milleri group, Klebsiella)

Pathogenesis

Clinical Features

Symptoms:

• Back/loin pain (most common) - lumbar or lower thoracic in location
• Fever - usually low-grade in TB (cold abscess); high spiking fever in pyogenic abscess
• Groin swelling - the pathognomonic feature when pus has tracked down
• Hip pain - referred to the hip or knee
• Weight loss, anorexia (especially TB)

Signs:

1. Flexion deformity of the hip - the patient lies with the thigh flexed to relax the psoas (Thomas's test positive)
2. Psoas sign - pain on passive extension of the hip (stretches the muscle over the abscess)
3. Fluctuant groin swelling - lateral to the femoral artery and vein (differentiates from femoral hernia, which is medial to femoral vein)
4. Reducible swelling with cough impulse in cold abscess
5. Cross-fluctuation between iliac fossa mass and groin swelling (when large)
6. Signs of primary pathology - spinal tenderness, kyphosis, restricted spinal movements (TB); abdominal mass/tenderness (Crohn's)

The "classic triad" of psoas abscess:

Back pain + Fever + Limp (hip flexion)

Differential Diagnosis of Groin Swelling (from psoas abscess)

• Inguinal hernia (medial to ASIS, above and medial to pubic tubercle)
• Femoral hernia (medial to femoral vein)
• Femoral artery aneurysm (pulsatile, expansile)
• Enlarged inguinal lymph nodes (multiple, firm)
• Saphena varix (impulse on coughing, empties on lying)
• Psoas bursa enlargement (Bailey & Love, 28th Ed)

Investigations

Bloods:

• FBC: leukocytosis (pyogenic), lymphocytosis (TB)
• ESR, CRP elevated
• Blood cultures (positive in ~50% of pyogenic cases)
• Mantoux/IGRA test (if TB suspected)

Imaging:

Coronal CT reconstruction showing a right-sided psoas abscess (Bailey & Love, 28th Ed)

Microbiological:

• Aspiration of pus for Gram stain, culture & sensitivity (aerobic + anaerobic)
• AFB smear and culture, PCR for TB
• Histopathology of tissue if TB or malignancy suspected

Treatment

1. Antibiotic Therapy

• Empirical antibiotics directed at likely organisms, then tailored to culture results
• Pyogenic (primary/S. aureus): Anti-staphylococcal antibiotics (flucloxacillin; vancomycin if MRSA suspected); IV initially, then oral for total 4-6 weeks
• GI source: Broad spectrum covering gram-negatives and anaerobes (piperacillin-tazobactam, or carbapenems in severe sepsis)
• TB: Standard anti-tuberculous therapy (HRZE for 2 months then HR for 4-7 months minimum)

2. Drainage of the Abscess (mandatory)

(a) Percutaneous CT/Ultrasound-Guided Drainage

• Preferred first-line approach for most patients
• CT-guided aspiration or drain placement
• Safe, minimally invasive, can be repeated
• Success rate ~80-90% for pyogenic abscesses

(b) Surgical Drainage - indicated when:

• Percutaneous drainage fails
• Multilocular abscess
• Underlying pathology requiring surgery (e.g., bowel resection for Crohn's, debridement for TB spondylitis)
• Abscess pointing superficially in the thigh

3. Treatment of the Primary Cause

• Spinal TB: anti-tuberculous therapy; surgery (decompression/stabilisation) if neurological deficit
• Crohn's disease: medical therapy, bowel resection if necessary
• GI perforation/diverticulitis: appropriate surgical management
• Renal source: treat underlying cause (nephrolithiasis, perinephric abscess)

Complications

• Untreated: septicaemia, multi-organ failure, death
• Hip joint infection (due to proximity of iliopsoas to the hip capsule)
• Femoral nerve palsy (pressure neuropathy - femoral neuropathy more common with iliacus abscess, as the iliacus fascia is less distensible)
• Flexion contracture of the hip
• Fistula formation (enterocutaneous, spino-cutaneous)
• Recurrence if underlying cause not treated

Prognosis

Summary Points for Exam