Abscess, Psoas

Abscess, Psoas

Basics

Psoas abscess is a rare and potentially life-threatening suppurative myositis of the iliopsoas compartment (1).

Description

The iliopsoas compartment is an anatomic space comprised of the psoas major, psoas minor, and iliacus muscles, which mediate hip flexion (2). Commonly referred to as iliopsoas, this muscle group originates from the lateral borders of the 12th thoracic to 5th lumbar vertebrae, passes posterior to the inguinal ligament and anterior to the hip joint, and inserts on the lesser trochanter of the femur. Abscesses of the iliopsoas compartment are considered primary or secondary depending on infectious etiology.

Epidemiology

• Mycobacterium tuberculosis (TB) with secondary vertebral osteomyelitis is a common cause of psoas abscess in countries where TB is prevalent (3). About 5% of patients with vertebral tuberculosis develop psoas abscess (4). In developed countries such as the United States, nearly 3/4 of psoas abscesses are due to hematogenous spread of infection (5). About 57% of psoas abscesses occur on the right side, 40% on the left side, and 3% bilaterally (6).
• Primary psoas abscess is most common in developing and tropical countries (4). About 83% of primary abscesses occur in patients younger than 30 years old (1). In contrast, secondary psoas abscess is more common in Europe and the United States. Older patients are more predisposed to secondary psoas abscess due to increased incidence of age-related diseases such as diverticulitis (7). A male preponderance of 3:1 for psoas abscess has been reported (6).

Incidence

• 3.9 cases per year before 1985 to about 12 cases per year in the 1990s (2)
• Rising incidence from increased prevalence of risk factors (immunosuppressant therapy, diabetes, etc.)
• Increased detection from improvement in quality and interpretation of cross-sectional imaging (8,9)

Risk Factors

• Primary psoas abscess: diabetes, HIV/AIDS, and other immunodeficiency states (1)
  • Other factors include trauma to the lumbar spinous processes, incurring psoas muscle damage, and hematoma formation such as in the setting of hematologic disorders (4).
• Secondary psoas abscess: Crohn's disease, ulcerative colitis, appendicitis, and vertebral osteomyelitis (1,10)
  • Complications of hip arthroplasty, spinal surgery, aortic surgery, and kidney transplantation (10,11,12,13)
  • Paraspinal abscess, anastomotic leak, and hematoma set the stage for postoperative infection and abscess formation.

Pathophysiology

• Primary psoas abscess results from hematogenous or lymphatic spread of infection from a distant source (1). Primary abscesses are often monomicrobial, with 80 percent of infections due to Staphylococcus aureus bacteremia or sepsis. Pseudomonas aeruginosa, Haemophilus aphrophilus, and Proteus mirabilis infections have also been reported (7).
• Secondary psoas abscess results from contiguous spread of infection from nearby structures of the musculoskeletal system, gastrointestinal tract, genitourinary tract, and vasculature. Secondary psoas abscesses are often polymicrobial, most commonly involving Escherichia coli and other enteric organisms such as Salmonella enteritidis, enterobacter species, and Methicillin resistant Staphylococcus aureus (MRSA) (14).

Commonly Associated Conditions

• Primary psoas abscess: Most common associated conditions include intravenous drug use, HIV/AIDS and other immunocompromised states, renal failure (9).
• Secondary psoas abscess: Most common associated conditions include inflammatory bowel disease, appendicitis, diverticulitis, osteomyelitis. Infrequently, secondary psoas abscess has been associated with septic arthritis, pancreatitis, and Henoch-Schönlein Purpura.

Diagnosis

Diagnosis of psoas abscess can be difficult and involves a thorough history, physical examination, and appropriate imaging studies (2). Computed tomography scan (CT) is the preferred imaging modality for identification of psoas abscess, and definitive diagnosis is made by image-guided drainage and microbial culture (6).

History

• Reported symptoms are often nonspecific indicators of generalized infection:
  • Fever, flank or abdominal pain with or without radiation to the anterior hip and thigh (1)
• Additional symptoms:
  • Limp, nausea, anorexia, malaise, and weight loss
  • The classic symptom triad described by Myntner in 1881 of fever, abdominal or flank pain, and limp is present in less than 8 percent of cases.
• Most patients will present with pain, while persistent low-grade fever is found in more than 75% of cases (14).

Physical Exam

• Maneuvers that stretch or contract the inflamed psoas compartment result in considerable pain on the affected side (1,6).
• Pain with extension and internal rotation of the hip (the “psoas sign”) is the most common physical exam finding.
  • A positive psoas sign may be elicited by the examiner by placing one hand proximal to the patient's knee on the affected side and asking the patient to raise the ipsilateral thigh against resistance.
  • A second psoas test is performed by asking the patient to lay on the unaffected side and hyperextend the contralateral hip, eliciting pain with stretching of the affected psoas muscle.
  • These maneuvers may also be positive in the setting of other conditions causing iliopsoas inflammation, including retrocecal appendicitis. Sensitivity and specificity of the psoas sign have been reported to be 16% and 95% respectively (15).
• Rectal examination may aid in differentiating psoas abscess from retrocecal appendicitis, with pain on palpation of the retrovesical pouch more consistent with appendicitis. While patients with psoas abscess may find relief from pain with full hip flexion, patients with hip pathology experience persistent or heightened pain. Distal extension of a psoas abscess may yield a painful or painless mass palpable below the inguinal ligament (8). About 50% of patients report abdominal tenderness, but guarding and rebound are uncommon (6).

Diagnostic Tests & Interpretation

Lab

• WBC >10,000/mL
• Anemia (hemoglobin <11g/L)
• Thrombocytosis (1,14)
• Elevations in blood urea nitrogen (BUN), sedimentation rate, and C-reactive protein (CRP) have been reported (10). CRP level has been shown to correspond with the severity of infection (3).

Imaging

• Computed tomography (CT) is the gold standard imaging modality:
  • Sensitivity rate between 88% and 100% (2)
  • Typically reveals a focal hypodense lesion consistent with an abscess
  • May demonstrate:
    • enlargement of the iliopsoas muscle
    • gas or air fluid levels within the muscle and fat stranding.
  • Constrast CT may show rim enhancement of the abscess wall (6).
• While not integral to the diagnosis of psoas abscess, magnetic resonance imaging (MRI) and technetium 99 m scintigraphy may elucidate local infectious sources, such as vertebral osteomyelitis (2).
• Plain abdominal radiographs occasionally reveal the outline of an inflammatory mass.
• Chest plain films may identify scant pleural effusions or raised hemidiaphragm (6).
• Ultrasound may disclose evidence of an inflammatory mass and is often diagnostic in cases of pediatric psoas abscess (16).
• An intravenous pyelogram may show deviation of the kidney and ureter, and barium studies may reveal bowel loop displacement and associated GI disease.

Diagnostic Procedures/Surgery

• Gram stain and culture of blood and aspirated abscess fluid confirm the diagnosis and guide antimicrobial treatment (1).
• AFB stain and mycobacterial culture if tuberculosis infection suspected

Differential Diagnosis

Differential diagnosis may include retrocecal appendicitis, bacterial infection or avascular necrosis of the hip, renal colic and pyelonephritis, arthritis, hip joint infection, S1 disc herniation, inflammatory bowel disease, epidural abscess, vertebral osteomyelitis, pelvic inflammatory disease (1,6).

Treatment

Most cases of psoas abscess will require percutaneous or surgical drainage as well as parental antibiotic treatment (1,14).

CT-guided percutaneous drainage is the initial procedure of choice, leading to successful decompression in the majority of cases (17).

Open drainage may be indicated:

• For large, complex, and multiloculated abscesses.
• If imaging shows gross involvment of adjacent structures or when percutaneous drainage fails (4,14).

Psoas abscess associated with conditions such as inflammatory bowel disease may be effectively managed with open drainage and surgical treatment of the underlying disease process.

Medication

• Broad-spectrum empiric antibiotics targeting staphylococcal and enteric organisms are indicated in most cases (14).
• Intravenous monotherapy with Unasyn, Zosyn, or a carbapenem is effective as initial treatment.
• Dual therapy with a third-generation cephalosporin such as Ceftriaxone with Metronidazole is also adequate.
• Suspected MRSA infection is best treated with Vancomycin, and alternatively, Linezolid or Daptomycin.
• Coverage should be determined by culture sensitivity results from aspirated fluid.
• While most psoas abscesses require a drainage procedure, abscesses 3 cm or less in size have been successfully managed by antibiotics alone.

Ongoing Care

Following drainage, pigtail catheter placement permits continued decompression of the abscess cavity and monitoring of purulent output for improvement (2).

Follow-Up Recommendations

Duration of antibiotic treatment should be tailored to the patient and may be continued for two to six weeks (3,14).

Patient Monitoring

Patients should be assessed for symptomatic relief, defervescence, and resolving white blood cell count (3). Follow-up imaging may be warranted to verify adequate drainage of the fluid collection.

Diet

Patients should be kept NPO for percutaneous and surgical drainage procedures.

Prognosis

A high index of suspicion for abscess in the iliopsoas compartment is crucial as early intervention is associated with favorable outcomes. Delays in diagnosis can lead to serious complications including septic shock, reported in up to 20% of cases (14). Mortality is reported to range from 2.4% to 19% for treated patients with primary and secondary abscesses respectively (7,14). Untreated cases face a near 100% mortality rate (10).

Complications

Risk factors for unfavorable outcomes include delays in treatment, bacteremia, and inadequate treatment (percutaneous drainage or antibiotics only) in patients with advanced age (4,14). About 40% of patients require more than one drainage procedure for full recovery. Incomplete drainage or suboptimal antibiotic treatment may lead to relapses of psoas abscess up to a year after initial presentation in 15–36% of cases (3,14).