Primary iliac muscle abscess due to Staphylococcus aureus.

A 55-year-old man presented with a 3-day history of lower back pain and right thigh pain. A diagnosis of discogenic pain had been made at two other hospitals. He had been admitted to a medical center for acute hepatitis 5 months prior to this admission. Large doses of parenteral hydrocortisone were used for 13 days to treat acute hepatitis. At the present admission, he was unable to stand and refused to move his right leg. There was mild tenderness in the right lower abdomen on deep palpation. Passive flexion and rotation of the right hip produced mild pain, while passive extension of the right hip produced severe pain and resistance. The Patrick test was positive and the psoas sign was present on the right side. The erythrocyte sedimentation rate (ESR) was 66/hr. The C-reactive protein (CRP) level was 0.161 g/L. Abdominal sonography showed a lobulated mass in the right iliac fossa. Magnetic resonance imaging showed severe swelling of the right iliac muscle with a central heterogeneous mass. Debridement, drainage of the abscess, and application of a septopal chain were performed via an anterior retroperitoneal approach, and parenteral cephazolin and gentamicin were administered. A culture of the abscess grew Staphylococcus aureus. The ESR and CRP concentrations decreased to within the normal ranges 3 weeks later. Awareness of this disease entity, careful physical examination, and appropriate imaging studies such as ultrasonography and magnetic resonance imaging are key to making a correct diagnosis.