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Primary Psoas Abscess
1 Infectious Diseases Clinic II,
2 Laboratory Department V.Babes Hospital Timisoara,
3 Surgical Clinic II, Victor Babes University of Medicine and Pharmacy, Timisoara
Background: Psoas abscess is an uncommon clinical entity and presents diagnostic and therapeutic challenges. Clinical diagnosis is difficult because of non-specific symptoms and signs. Materials and Methods: The authors present the case of a 50 years old female patient admitted for fever of unknown origin with repeated sterile blood, urine and stool cultures. Results: The outcome was good with no relapse at one year follow-up. Conclusions: Routine laboratory evaluation is seldom useful for the diagnosis of primary pyogenic psoas abscess. Conventional radiological techniques are often unhelpful. Computed tomography and magnetic resonance imaging are more useful than ultrasonography.

Psoas abscess may be classified as primary or secondary, depending on the presence or absence of underlying disease. Primary psoas abscess is a rare clinical entity with subtle and non specific symptoms, most commonly seen in immunosuppressed patients or those predisposed to infections.1-3 It has no definite etiology. The psoas muscle has a rich vascular supply that is believed to predispose it to hematogenous spread from sites of occult infection.4 Psoas abscess can also be secondary to gastrointestinal or renal pathology through direct infection of adjacent structures. The most common causes are appendicitis, diverticulitis, Crohn’s disease and carcinoma.
The organisms responsible for infection are gram-negative germs (Escherichia coli, Klebsiella spp., Pseudomonas aeruginosa, Proteus mirabillis Enterobacter spp.,) and gram-positive cocci (Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus agalactiae, a-hemolytic streptococci, especially Streptococcus mitis).5-7,10 It can also be of tuberculous etiology and associated with cold abscesses of lower thoracic and upper lumbar vertebral bodies, as the psoas is attached to these vertebrae. Percutaneous drainage and antibiotics provide an effective and safe alternative to more invasive surgical drainage in most patients.8,9,11 Surgical drainage is associated with shorter hospital stay.11 The mortality in undrained psoas abscesses is high with a mortality rate ranging between 45 and 100%.12-13
The aim of this report is to present the diagnostic pitfalls and an unusual etiology of primary pyogenic psoas abscess – cause of fever of unknown origin.

Case presentation

A 50 years-old Caucasian female was admitted on 3.07.2003 with the following diagnosis: prolonged fever of unknown origin. She presented high fever, chills, rigor, myalgia, arthralgias, nausea, vomiting, and hypotension.
The illness began 3 weeks before admission with fever, chills, myalgia, arthralgia, abdominal cramps and watery diarrhea. She was treated with ciprofloxacin and diosmectite (SmectaR) for 5 days. Fever vanished and stools became normal. After another 4-5 days fever reappeared and the patient developed abdominal pains. She was treated with norfloxacin. The persistence of fever and the worsening of her general condition determined the admition.
When she was admitted the patient was pale, anxious, sweating and complaining of nausea. The body temperature was 38oC. Physical examination revealed: blood pressure 90/60 mmHg, pulse 116/min, white coated tongue, and tenderness on palpation in the right upper quadrant and epigastric region and an enlarged liver- 2 cm bellow the right costal margin on the midclavicular line. The psoas sign and the classical sign of limping were absent.
Laboratory findings: 3.07.2003: Hgb 11,2 g%; HCT 32,8%; WBC 17.600/mm3 Ne 93% Ly 4% Mo 2% Eo 1%; ESR 125/132 mm; C-reactive protein positive. 7.07.2003: Hgb 11,6 g%; HCT 34%; WBC 13.900/mm3 Ne 78% Ly 16% Mo 4% Eo 2% ESR 130/150 mm.
All the blood cultures (processed on BACTEC Plus Aerobic/F and Plus Anaerobic/F media), throat cultures, urine cultures and stool cultures were sterile.
The chest X ray was normal.
The ultrasound exam revealed a homogenous, hiperechogenic, slightly enlarged liver without other changes in the abdomen.
The case was considered an undiagnosed bacterial infection, a ”culture negative” sepsis. Empirical antibiotic therapy was started (pefloxacin 2x400 mg/day + gentamicin 3x80 mg/day, as the patient was allergic to ampicilin and cephalosporins) with no clinical improvement.
The magnetic resonance imaging on 7.07.2003 showed a heterogenous hypodense collection without enhancement of 5/6/8 cm in the right psoas .Figure 1, figure 2.
Fig. 1. Right paravertebral liquid collection.
Fig. 2. Hypodense collection in the right psoas mass.
On 8 .07 the patient was transferred to the Surgery Clinic and on 9 .07 she was operated. After a median laparotomy in the peritoneal cavity an abscess of 15/10/8 cm in the right psoas was observed. Debridement and drainage of the abscess were performed via an anterior retroperitoneal approach and about 200 ml of sticky pus were evacuated. The culture obtained from the pus revealed a b-hemolytic Streptococcus pyogenes (group A streptococcus). In-vitro susceptibility testing showed the following: penicillin 30 mm, imipenem 30 mm, cefepime 30 mm, ceftriaxone 30 mm, cefotaxime 30 mm, amoxicillin 26 mm, ertapenem 25 mm, rifampicine 25 mm, clarithromycine 23 mm, oxacillin 22 mm.
The patient was treated with antibiotics for 10 days (pefloxacin and gentamicin), had an uneventful recovery and left the hospital on 21.07. She recovered completely and was symptom free at one year follow-up.


Primary psoas abscess is an uncommon condition, a rare affection and is usually associated with a predisposing factor or immunosuppresion (they were absent in our patient). Early diagnosis and appropriate management are challenging aspects for physicians.14-15
The present case demonstrates the importance of imagistic methods, especially magnetic resonance. Before the advent of these technics the psoas abscess was an incidental finding at operation and, not seldom, at autopsy. In the present case the classical signs of psoas abscess (lower back pain, thigh pain, limping and psoas sign) were absent. The clinical image was dominated by peritoneal inflammation expressed by nausea, vomiting and watery stools.
Furthermore despite all the efforts, the cultures remained sterile, probably because the pacient had taken antibiotics prior to admission. Cultures of appropriate clinical specimens (urine, sputum, blood) should be obtained prior to starting empiric therapy to provide bacterial isolates for in-vitro susceptibility testing.16-17 When antibiotic therapy has been started empirically, the question of how long to continue it when the origin of infection has not been defined is often a problem. Discontinuing antibiotic therapy too early can lead to clinical deterioration. Antimicrobial therapy of primary psoas abscess must continue 7 days after drainage.
In any case of sepsis it is very important to detect any sites of collection that must be solved by the surgeon under antibiotic protection. In this case the more invasive surgical procedure was preferred because the origin of abscess was not clear.
Generally, psoas abscess is caused by gram-positive cocci (especially Staphylococcus aureus) or gram-negative bacteria. The present case points out a unique cause of psoas abscess and also an important process of pathomorphose of diseases caused by b-hemolytic streptococci – there are fewer cases of scarlet fever during the last years but new illnesses appear (necrotizing fasciitis).


Awareness of this disease entity, careful physical examination and appropriate imaging studies such as ultrasonography, computed tomography and magnetic resonance imaging are the key to make a correct diagnosis. Although Streptococcus pyogenes is not the main cause of primary psoas abscess, it must be kept in mind as a reemerging infectious agent.

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