First Reports: first described by Mynter in 1881 as “Psoitis”
Age Predominance
Age 44-58 y/o: peak age group in developed countries
Children/Younger Adults: peak age group in developing countries
Geographic Distribution: psoas abscess is more common in tropical and developing countries
Asia and Africa: 99% of cases are due to primary hematogenous/lymphatic mechanism of spread to the psoas muscle
Europe and North America: 17-61% of cases are due to primary hematogenous/lymphatic mechanism of spread to the psoas muscle
Sex Predominance: more common in males than females
Anatomic Distribution: equal prevalence on right and left
Anatomy
Muscles Involved in Hip Flexion: the iliacus and psoas muscles are the main hip flexors (together, they are referred to as the iliopsoas)
They are located in the retroperitoneal iliopsoas compartment
Psoas Muscle: originates from the transverse processes and the lateral aspects of the T12-L5 vertebrae
The psoas continues downward across the pelvic brim, posterior to the inguinal ligament and anterior to the hip joint capsule -> the psoas ultimately forms a tendon that inserts into the lesser trochanter of the femur
The iliacus muscle joins the psoas to form this tendon
The iliopsoas bursa separates the iliopsoas from the hip capsule: however, in 15% of patients, this bursa communicates with the hip capsule, allowing the spread of infection between the hip joint and the iliopsoas
Nearby Anatomic Structures: the psoas is located adjacent to the vertebrae, abdominal aorta, sigmoid colon, appendix, hip joint, and the iliac lymph nodes
Infection may spread beween the psoas and these structures
Pathogenesis
Definition: psoas abscess is a collection of pus in the iliopsoas muscle compartment
Mechanisms of Psoas Muscle Infection: in some cases, the exact mechanism of psoas infection may be impossible to determine
Contiguous Spread to Psoas Muscle
Hematogenous/Lymphatic Spread to Psoas Muscle: may be an obvious or occult source
Etiology
Hematogenous/Lymphatic Spread to Psoas Muscle (Primary)
Crohn’s Disease (see Crohn’s Disease, [[Crohns Disease]]): may especially occur in patients with severe ileocolitis
Incidence of Psoas Abscess in Crohn’s Disease: 0.4-4.3% [MEDLINE]
Ulcerative Colitis (UC) (see Ulcerative Colitis, [[Ulcerative Colitis]])
Pancreatic Abscess
Genitourinary (GU) Tract Disease
Extracorporeal Shock Wave Lithotripsy
Renal Abscess/Urinary Tract Infection (UTI) (see Renal Abscess, [[Renal Abscess]])
Renal Surgery/Nephrectomy
Xanthogranulomatous Pyelonephritis
Hip Joint/Arthroplasty Infection
One series reported that 12% of their hip arthroplasty infection cases had an accompanying psoas abscess [MEDLINE]: in these cases, hematogenous infection and history of neoplasm were risk factors for the development of psoas abscess
Klebsiella Pneumoniae (see Klebsiella Pneumoniae, [[Klebsiella Pneumoniae]]): common in Taiwan (especially in diabetics and in association with urinary tract infection)
Septic Shock (see Sepsis, [[Sepsis]]): may occur in up to 20% of cases
Treatment
Antibiotics: usually required for approximately 3-6 weeks after drainage has been achieved (or 6 months in cases of tuberculous abscess)
Antibiotic therapy alone has been reported to have 78% success rate in some series [MEDLINE]: however, antibiotics alone is typically not the approach of choice
Percutaneous Catheter Drainage (usually CT-guided): effective in the majority of cases
Laparoscopic or Open Surgical Drainage: may be required in cases that fail percutaneous catheter drainage, cases with difficult to drain multiloculated collections, cases with abscess secondary to bowel disease (in Crohn’s disease), or in cases with significant involvement of adjacent structures (requiring additional resection, etc)
