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Primary psoas abscess is a rare infection with an often vague and non-specific clinical presentation, especially in children. In Asia and Africa 99.5% of all psoas abscesses are primary, compared with 61% in the United States and Canada and 18.7% in Europe.1 2 Approximately 70% of psoas abscesses occur in patients younger than 20 years of age, with a male preponderance of 3:1.1 Fifty seven per cent of psoas abscesses occur on the right side, 40% on the left side, and 3% bilaterally.3We present the following case and show the magnetic resonance imaging to emphasise the presenting signs, symptoms, and findings of this unusual infection.
A 13 year old white girl was in excellent health until she developed a dull ache in the superior posterior thigh without radiation. She denied any direct trauma or excessive strenuous activity. Over five days she developed progressively severe, dull pain, localised to the posterior hip in association with fever to 38.9°C, nausea, vomiting, and diarrhoea. She walked with a limp. Her past medical history was non-contributory; she denied smoking, alcohol, drug use, or sexual activity. The girl was 1.52 m tall and weighed 70 kg. Vital signs were normal; temperature rose to 38.9°C within 24 hours of admission. A detailed general physical examination was normal. Abdominal and pelvic examinations were benign without organomegaly or peritoneal signs. Stool for occult blood was negative. Musculoskeletal examination was normal, with the exception of the left hip which showed pain on active and passive motion, particularly abduction and medial rotation. The range of motion of the hip was normal; there was no localised warmth or palpation tenderness. The gait was antalgic for the left leg.
Laboratory examination showed a white blood cell count of 15.2 × 109/l (77% neutrophils/14% lymphocytes/8% monocytes) and platelets were 415 × 109/l. An erythrocyte sedimentation rate was 115 mm/1st h (normal <20 mm/1st h). Urine analysis disclosed trace blood and protein; the remainder of the laboratory tests were within normal limits. Blood and cervical cultures were negative. Posteroanterior radiographic examination of the left hip was normal. Bone scan was normal. Magnetic resonance imaging (MRI) of the abdomen and pelvis showed grossly abnormal signal intensities of the left psoas muscle (figs 1 and 2). Although a discrete abscess was identified, fine needle aspiration under imaging guidance yielded no pathological material. Vancomycin was started empirically, based on likely causative organisms. The patient defervesced, became ambulatory within one week, and was discharged to complete an outpatient antibiotic course.
This case demonstrates the manifestations of psoas abscess formation. The classical presenting symptoms of psoas abscess are pain, limp, fever, and psoas spasm.2-4 Pain is most commonly localised to the ipsilateral hip, but occasionally radiates to the abdominal wall, back, thigh, inguinal area, flank, knee, and calf.1 3 Patients may also present with a chronic illness and generalised symptoms of systemic infection, such as prolonged fever, malaise, anorexia, weight loss, and anaemia.2 3
Pain with flexion and external rotation of the affected hip is the most common physical finding.1-3 A tender palpable mass may be found in the iliac fossa and inguinal area.2 3 Fifty per cent of patients have abdominal tenderness, but guarding and rebound tenderness are uncommon.3 Because of the non-specific pain location, the diagnosis of psoas abscess may be delayed or missed. Differentiation between psoas abscess and hip pathology can be difficult; however, prudent physical examination of the hip can be useful. With psoas abscesses there usually is no discomfort on full flexion of the hip, whereas the presence of hip pathology typically elicits pain.3 5 Laboratory studies are non-specific and typically show leucocytosis, anaemia, a raised erythrocyte sedimentation rate, and, usually, normal urine analysis.2 3 6
Plain abdominal radiographs occasionally define an outline of the inflammatory mass. Chest radiographs may disclose minimal pleural effusion or raised hemidiaphragm. An intravenous pyelogram may show deviation of the kidney and ureter. Barium studies may disclose bowel loop displacement and associated gastrointestinal diseases.2 3 5 However, the most accurate diagnostic imaging is computed tomography scan (CT) or MRI, which typically show a low density lesion of the psoas muscle and gas within the muscle itself.5 7 There may be rim enhancement of the abscess wall with contrast medium injection. Definitive diagnosis is made by fine needle aspiration under imaging guidance, and microbial culture of the causative organism.5 7 If abdominal CT or MRI is unavailable, ultrasonography may demonstrate the inflammatory mass.8 Gallium-67 scanning may be useful in the diagnosis of psoas abscesses and detection of concomitant infectious foci.9 Differential diagnoses of psoas abscess include bacterial infection of the hip, avascular necrosis of the hip, irritable hip, necrotising fasciitis of the psoas muscle, pyelonephritis, pelvic inflammatory disease, retrocaecal appendicitis, S1 herniated disc, avascular necrosis, vertebral or pelvic osteomyelitis, and epidural abscess.3 4 These entities should be distinguishable upon the correlation of history, physical examination, laboratory tests, and imaging studies.
The cause of primary psoas abscess remains uncertain. Proposed mechanisms of psoas abscess formation include haematogenous spread from primary infectious foci or local trauma with intramuscular haematoma formation predisposing to abscess development.6 In secondary psoas abscess the most commonly associated disorder is Crohn's disease; other disorders include appendicitis, colonic inflammation or neoplasm, disc infections, and a variety of intra-abdominal or retroperitoneal infections.1-4 Primary psoas abscesses are caused by a single organism in 87.5% of cases: primarily Staphylococcus aureus (88.4%), streptococci (4.9%), and Escherichia coli(2.8%).1 Blood cultures are positive in 41.7%, usually for Staphylococcus aureus.1 In the past decade the majority of patients with a primary psoas abscess were intravenous drug users (86%) infected with the human immunodeficiency virus (57%).7
Treatment for primary psoas abscess includes percutaneous drainage combined with systemic antibiotic administration.10Surgical drainage is preferred for the patients in whom the psoas abscess is associated with underlying bowel disease.10With appropriate treatment, psoas abscess rarely results in death (2.5%).1 Death from psoas abscess is associated more commonly with inadequate or delayed drainage, or both.1Our patient responded well to antibiotic treatment and recovered completely.
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