Septic ischial bursitis in systemic lupus erythematosus presenting as a perirectal mass.

We describe a patient with systemic lupus erythematosus with dissection of a large ischial bursal cyst which presented as a perirectal mass. Dissecting bursal and articular synovial cysts are known to cause multiple complications including a number of "pseudosyndromes" such as pseudothrombophlebitis. The cyst in this case was initially thought to represent a perirectal abscess and we propose the term "pseudoperirectal abscess" to describe this condition. This cyst was found to be infected and required an extended course of antibiotics and bursectomy.