A Mimic of Hip Septic Arthritis
- Pus within the psoas/iliacus: primary (haematogenous, most often S. aureus including MRSA, in otherwise healthy children) or secondary (spread from spinal osteomyelitis/discitis, bowel disease, or renal/retroperitoneal infection), so the source must be sought
- It mimics hip septic arthritis (and sometimes appendicitis): febrile, toxic, hip held flexed, extension resisted (positive psoas sign), limp or refusal to weight-bear, flank/back/groin pain - a high index of suspicion is needed to avoid misdiagnosis
- Raised white cells and CRP, usually normal plain radiographs; MRI is the key diagnostic tool, defining the abscess, its extent and any spinal source; ultrasound and contrast CT are alternatives; blood and abscess cultures guide antibiotics
- Telling it from a septic hip changes the management - drainage of the psoas collection rather than arthrotomy of the hip - which is why imaging is decisive
- Antibiotics plus drainage: empirical cover for S. aureus/MRSA, then culture-directed; image-guided percutaneous drainage first-line, open retroperitoneal drainage for multiloculated pus or failed percutaneous drainage
- Prognosis is good with timely diagnosis and treatment, the child typically recovering full function; delay from mistaking it for transient synovitis risks ongoing sepsis
- “Pediatric psoas abscess = pus in psoas/iliacus; PRIMARY (haematogenous, S. aureus/MRSA) vs SECONDARY (spine/GI/renal source). MIMICS hip septic arthritis/appendicitis.
- “Presentation: fever + toxic child + HIP HELD FLEXED, painful on EXTENSION (positive PSOAS sign) + limp/refusal to weight-bear + flank/back pain. Raised CRP/WCC; plain films usually normal.
- “MRI is the KEY diagnostic tool (also defines a spinal source). Treat with antibiotics (cover MRSA) + DRAINAGE - IMAGE-GUIDED percutaneous is first-line, open retroperitoneal the fallback, never through the hip joint; find/treat any secondary source. High index of suspicion.
Definition and Aetiology
A psoas (iliopsoas) abscess is a collection of pus within the psoas or iliacus muscle. In a child it classically presents as something else, a septic hip and sometimes appendicitis, and the treatment differs.
Primary and secondary. A primary abscess arises haematogenously in an otherwise healthy child. The muscle is seeded from a transient bacteraemia, often from a skin or soft-tissue focus, and the organism is most often Staphylococcus aureus, including MRSA. A secondary abscess spreads into the muscle from an adjacent source (spinal osteomyelitis or discitis, bowel disease, or renal or retroperitoneal infection), so the source must be sought whenever the diagnosis is made.
Organisms by mechanism. The organism dictates the empirical antibiotic, which makes the aetiological spectrum high-yield.
- Typical organisms
- Staphylococcus aureus, including MRSA; occasionally Streptococcus
- Notes
- Empirical cover must include MRSA where prevalent.
- Typical organisms
- Polymicrobial enteric Gram-negatives (E. coli) plus anaerobes
- Notes
- Add Gram-negative and anaerobic cover; treat the bowel source.
- Typical organisms
- Gram-negatives (E. coli, Proteus)
- Notes
- Consider urinary-tract imaging and culture.
- Typical organisms
- Mycobacterium tuberculosis
- Notes
- Tracks down from spinal Pott's disease; needs anti-tuberculous therapy.
A pyogenic psoas abscess (commonly S. aureus/MRSA) is acute, febrile and toxic. The classic tuberculous ("cold") abscess is indolent and cool, with little warmth, erythema or systemic toxicity, and tracks down from spinal Pott's disease. Tuberculosis was historically the dominant cause; in TB-endemic regions, and in any atypical course, it must be actively excluded.
Excluding tuberculosis in practice. In the cited immunocompetent child (Keny 2025) the Mantoux was negative and culture grew MRSA. After open retroperitoneal drainage the child was treated successfully with linezolid, recovered fully and was walking by 6 weeks. The spinal disease itself is covered in Tuberculosis of the Spine (Pott's Disease).
Iliopsoas Anatomy and the Basis of the Psoas Sign
The flexed hip, the pain on extension and the flank, groin and thigh pain all follow from iliopsoas anatomy, which also explains how the abscess spreads and why it is drained retroperitoneally.
The muscle. Psoas major arises from the bodies, intervertebral discs and transverse processes of T12-L5 and runs retroperitoneally within the psoas (fascial) sheath, anterior to the lumbar transverse processes. Iliacus arises from the iliac fossa and joins the lateral psoas; together they pass under the inguinal ligament to insert by a common tendon on the lesser trochanter. Iliopsoas is the primary hip flexor, and also a lateral rotator and trunk flexor.
Why the hip is held flexed. Flexion shortens and relaxes the inflamed psoas and eases tension on the abscess, so it is the position of comfort. Extension stretches the inflamed muscle and hurts, which is the psoas sign; resisted active hip flexion is also painful.
Why the pain is in the flank and groin. The psoas is retroperitoneal, so the abscess causes flank, back and groin pain. Pus within the sheath can extend cranially toward the spine, or arrive from it, and caudally along the muscle below the inguinal ligament toward the lesser trochanter. There it can present as a groin or thigh swelling in Scarpa's triangle, far from the spine.
The neighbours. The lumbar plexus and femoral nerve (L1-L3 contributions) lie within and on the muscle, which also lies close to the ureter, kidney and bowel. These relations are why a secondary source in the spine, bowel or kidney must be sought, and why the deep retroperitoneal collection is drained retroperitoneally or through an image-guided percutaneous catheter.
Clinical Presentation
The picture. A febrile, toxic, irritable child holds the hip flexed and resists extension, limps or refuses to weight-bear, and may have flank, back or groin pain. This closely mimics septic arthritis of the hip, and sometimes appendicitis, so a high index of suspicion is needed to avoid misdiagnosis.
Separating It From a Septic Hip at the Bedside
Imaging settles the question, but the examination should have raised it, and the discriminator follows from which structure each condition irritates.
- Psoas abscess
- Hip flexed - flexion shortens and relaxes the inflamed psoas
- Septic arthritis of the hip
- Hip flexed, ABDUCTED and externally rotated - the position of maximum capsular volume and therefore lowest intra-articular pressure
- Psoas abscess
- Relatively tolerated - rotation does not lengthen psoas much
- Septic arthritis of the hip
- Resisted and painful - rotation stresses the distended capsule. This is the single most useful bedside discriminator
- Psoas abscess
- The painful movement - it stretches the inflamed muscle (the psoas sign). Resisted active flexion also hurts
- Septic arthritis of the hip
- Painful, but not selectively more so than rotation
- Psoas abscess
- Flank, back, iliac fossa, groin; sometimes a tracking thigh swelling below the inguinal ligament
- Septic arthritis of the hip
- Anterior joint line and groin, with global irritability
- Psoas abscess
- Usually no effusion - but see the caveat below
- Septic arthritis of the hip
- Effusion present; aspiration confirms
The ultrasound caveat matters more than the rule. An absent hip effusion supports a psoas abscess, but it does not exclude a septic hip in the first hours. More importantly, a psoas abscess adjacent to the joint can produce a sympathetic effusion, and the two can coexist, because the abscess may decompress into the joint or the joint infection may track into the muscle. So an effusion does not exclude a psoas abscess either, and where the picture is not clean the answer is cross-sectional imaging of the whole region rather than a decision made on the ultrasound alone.
The hip-specific rules do not settle it. The Kocher criteria were derived to separate a septic hip from transient synovitis, so a psoas abscess can score fully on them and still not be a joint infection. It is the same limitation that lets pelvic infection be missed, set out in Paediatric Pelvic Osteomyelitis; the joint itself is covered in Septic Arthritis of the Paediatric Hip.
FLEX
Hook:FLEX: Flexed hip, Lengthening hurts, Examination - rotation tolerated, eXclude the hip but beware coexistence.
Investigations and Differential Diagnosis
Bloods and plain films. White cells and inflammatory markers, including CRP, are raised, and plain radiographs are usually normal. Blood and abscess cultures guide the antibiotics.
MRI is the key diagnostic tool. It defines the abscess and its extent, detects any spinal source and avoids radiation in a child. It also shows the distinction between phlegmon and abscess, which changes whether there is anything to drain.
Ultrasound and contrast CT are the alternatives, and CT can also demonstrate the abscess and its extent.
Differential diagnosis. Principally septic arthritis of the hip, but also:
- Transient synovitis
- Osteomyelitis of the femur, pelvis or spine
- Discitis
- Appendicitis
- Pyomyositis
Why the distinction matters. A psoas abscess and a septic hip are treated differently, by drainage of the psoas collection rather than arthrotomy of the hip, and it is imaging, with MRI the key modality, that makes the distinction.
A febrile, toxic child with a flexed, irritable hip that is painful on extension and refuses to weight-bear is exactly the picture that prompts a hip ultrasound and possible arthrotomy. The two errors to avoid are dismissing the child as transient synovitis, which delays treatment of a true abscess and risks ongoing sepsis, and overlooking a secondary source.
Management
The principle. Antibiotics plus drainage, with any secondary source (spinal osteomyelitis or discitis, GI or renal infection) identified and treated. The response is monitored clinically and by inflammatory markers, watching for progression.
Empirical antibiotics. Begin intravenous cover for S. aureus/MRSA: an anti-staphylococcal agent plus an MRSA-active drug such as vancomycin, clindamycin or linezolid. Broaden to enteric Gram-negative and anaerobic cover if a GI source is suspected, then narrow to culture-directed therapy. Anti-tuberculous therapy is given if TB is confirmed.
Drainage. Image-guided percutaneous drainage is first-line for a drainable collection, by aspiration or catheter placement under ultrasound or CT guidance. Open retroperitoneal drainage is the fallback, not the default. Which collections actually need draining, and by what route, is not simply a question of large versus small.
- Approach
- Antibiotics alone, with imaging repeated if the response stalls
- Why
- Not every collection needs a needle; phlegmon without organised pus will not yield
- Approach
- IMAGE-GUIDED percutaneous drainage - ultrasound or CT guided, catheter left for larger cavities
- Why
- Avoids an open retroperitoneal approach and its morbidity, and yields the organism, which is often the only positive culture
- Approach
- OPEN retroperitoneal drainage
- Why
- Septations defeat a single catheter
- Approach
- Address the source, with the abscess drained at the same time
- Why
- Draining the collection while leaving the source guarantees recurrence
- Approach
- NOT through the hip joint
- Why
- The collection is retroperitoneal; crossing the joint contaminates a clean space
Duration. The rules are those of osteomyelitis elsewhere, so one set of figures applies: intravenous until the child is afebrile with a falling CRP, typically 3 to 5 days, then oral to complete a total of about 3 to 6 weeks, with CRP tracked weekly. Associated vertebral osteomyelitis or discitis lengthens the course; see Paediatric Acute Osteomyelitis and Pyogenic Discitis and Osteomyelitis.
When the CRP will not fall. A CRP that plateaus or climbs means an undrained or re-accumulating collection, and should prompt repeat imaging rather than a longer prescription. A collection that persists or re-expands means an undrained locule, the wrong antibiotic or an untreated spinal source.
Look for a vein thrombosis in the sick child. The abscess sits directly on the iliac vessels, and severe staphylococcal infection, particularly with PVL-positive strains, is associated with septic thrombophlebitis and pulmonary emboli. In a child who is toxic, has a very high CRP or develops chest signs, image the veins.
Outcome. With prompt imaging, appropriate antibiotics covering S. aureus/MRSA and timely drainage, the prognosis is good and children typically recover full function.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A febrile child holds the hip flexed and screams on extension, refusing to walk. The hip ultrasound shows no effusion. What are you thinking, and how do you proceed?”
What it is
- Pus within psoas/iliacus muscle
- Primary: haematogenous (often S. aureus/MRSA), healthy child
- Secondary: from spine (osteomyelitis/discitis), bowel, kidney
Presentation (mimics septic hip)
- Fever, toxic child; hip held flexed, painful on extension (positive psoas sign)
- Limp/refusal to weight-bear; flank/back/groin pain
- Raised WCC/CRP; plain radiographs usually normal
Diagnosis & management
- MRI = key diagnostic tool (defines abscess + any spinal source); US/CT alternatives; cultures
- Antibiotics (cover MRSA), then culture-directed; IV until afebrile + falling CRP (~3-5 days), then oral; total ~3-6 weeks; CRP weekly
- Drainage: image-guided percutaneous FIRST-LINE; open retroperitoneal if multiloculated or percutaneous fails; NEVER through the hip joint
- Rotation of the flexed hip relatively spared (vs septic hip); an absent effusion does not exclude either, and they can coexist
- Find/treat secondary source; image the veins in the toxic child (septic thrombophlebitis)
Evidence & Key Studies
Primary pyogenic abscess in an immunocompetent child: an Indian perspective
- Pyogenic psoas abscess is a rare but severe condition; once linked mainly to tuberculosis, it now has diverse causes, and can present in healthy (immunocompetent) children, initially suspected to be a hip problem.
- The child was febrile and toxic with the hip held flexed and painful/restricted movement, raised white cells and inflammatory markers, and normal lumbosacral radiographs; MRI demonstrated a large psoas/iliacus abscess, and culture grew MRSA.
- Open retroperitoneal drainage plus antibiotics led to full recovery; the case emphasises a high index of suspicion (it mimics septic arthritis) and the importance of timely imaging and drainage-plus-antibiotic treatment.
Iliopsoas abscess in children: report on five patients with a literature review
- Five children (2 girls, 3 boys, ages 10-15) hospitalised with iliopsoas abscess over two years; symptom duration ranged from five days to one year.
- The primary symptoms were fever and difficulty in walking; one patient presented in septic shock after trauma.
- Retrospective review with a literature summary - reinforcing that presentation is subacute and easily missed.
Evaluation of pediatric patients diagnosed with psoas abscess over 20 years in a tertiary care hospital
- 28 children over 20 years (equal sex ratio; 32.1% under 5 years); an underlying condition in 64.3%.
- Hip pain was present in ALL primary cases; fever was commoner in secondary cases (54.5% vs 16.7%).
- Gram-positive pathogens, particularly Staphylococcus, dominated; median stay 38 days; surgical intervention in 67.9%; no mortality.
The presentation of a paediatric pyogenic psoas abscess as a mimic of hip pathology (febrile, toxic child with a flexed, restricted hip), the normal plain radiographs with MRI demonstrating the abscess, the MRSA aetiology, and the management by drainage (open retroperitoneal) plus antibiotics with full recovery come from the cited Keny report. The primary (haematogenous) vs secondary (spinal/GI/renal) classification, the psoas sign, the differential with septic arthritis/transient synovitis/osteomyelitis, and image-guided versus open drainage are standard, well-established teaching. The hip-pain-in-every-primary-case observation and the two-thirds intervention rate come from the cited Cetin 20-year series, and the smouldering symptom course from the cited Karli series. Note that the cited case was drained openly, which is a single report and not an argument for open drainage as the default - image-guided percutaneous drainage is first-line where the collection is accessible. The antibiotic durations, the IV-to-oral criterion and the CRP monitoring follow Paediatric Acute Osteomyelitis, so the same figures apply wherever the infection sits. (See also Septic Arthritis of the Paediatric Hip, Paediatric Pelvic Osteomyelitis, Pyogenic Discitis and Osteomyelitis, Pyomyositis and Tuberculosis of the Spine (Pott's Disease).)