[Pyogenic sacroiliitis complicated by iliopsoas muscle abscess].

Both pyogenic sacroiliitis and iliopsoas muscle abscess are uncommon infectious entities, and their coexinstence has been reported in very few patients. We present here the case of a woman who developed a large iliopsoas abscess as a consequence of a pyogenic sacroiliitis, initially misdiagnosed as a common sciatica and treated with corticosteroids. The patient was cured by the surgical drainage of the abscess and a long-lasting antibiotic treatment. We discuss diagnostic difficulties linked to the two infectious entities, their possible pathogenic connections, the role of imaging procedures, and therapeutic options. We conclude that pyogenic sacroiliitis and the potential evolution to an iliopsoas abscess must be taken into consideration in the differential diagnosis of lower back pain, especially if fever is a concomitant sign.