Pyogenic Baker cyst in a patient with psoriatic arthritis during etanercept therapy

Patients with psoriatic arthritis (PsA) are at increased risk of infection due to disease course or treatment-related immunodeficiency. We describe a patient with a 10-year history of PsA, with arthritis of the right knee and pain and edema in the right calf, treated with the TNFα inhibitor etanercept for 6 months. Ultrasound showed accumulation of hypoechoic fluid, which was aspirated and was positive for staphylococcus and aspergillus. The patient recovered after surgical drainage and four weeks of antibiotic and antifungal therapy.