Teaching Case

A 61-year-old woman with recurrent episodes of hematuria. The patient had no medical history of interest, except for being a smoker.

CT scan of the abdomen and pelvis was performed with intravenous contrast (Fig 1). No lesions in bladder or urinary tract were seen, but as an incidental finding an oval and hypodense lesion was observed in right retrorectal space without affecting soft tissue or bone remodeling.

To complete the study, MRI with intravenous contrast administration was performed, characterizing the lesion as multiloculated cystic (despite presenting a predominant loculus), with intermediate signal intensity in axial T2WI (Fig 2 and 3) with restriction on DWI (Fig 4a) and signal drop on ADC map (Fig 4b). The wall of the lesion was smooth and thin with no contrast enhancement (Fig 2b). Two adjacent blind fistulous tracts had developed, one towards intergluteal fold (Fig 5a) and the other towards ischio-anal fossa (Fig 5b). Neither soft tissue involvement nor any dependence on bone, nerve or rectal structures was observed.

Retrorectal space is limited at posterior side by sacrococcygeal bone and nerve structures from the sacral plexus. Anteriorly, it contacts the posterior wall of rectum, while above and below there are the peritoneal reflection and the anal elevator muscles, respectively. Laterally, the iliac vessels and the ureters are found. The content of this space is mainly connective tissue and fat [1]. 

The most frequent cystic lesions in retrorectal space are developmental cysts [1-6], occurring mostly in middle-aged women, which include tailgut cysts (the most frequent) [1-2], epidermoid, dermoid and rectal duplication cysts.

Clinical perspective

Retrorectal space cystic lesions are often asymptomatic, usually discovered incidentally after a pelvic examination. Symptoms usually include palpable masses or clinical compression of structures [3-6]. There may also be other symptoms related to complications such as infection, fistula or bleeding.

These are underdiagnosed lesions that, due to the possibility of complications, must be actively sought out by the physician in ordinary studies.

Imaging perspective

Although ultrasound and CT can be useful tools in the identification of cystic lesions in the retrorectal space, MRI is the technique of choice for studying these lesions [1,2,6]. 

Typically, cystic lesions are hypointense on T1-WI, although they can be hyperintense. Usually, the uniloculated lesions are more frequently epidermoid or dermoid cysts, while lymphangioma or tailgut cysts tend to be multiloculated [3]. Fat presence inside the lesion is common in dermoid cysts and teratomas. Rectal duplication cysts usually communicate with anorectal lumen, and anterior sacral meningocele usually associates with sacral bone defects [3].

In our patient, a cystic lesion with smooth walls and without the involvement of adjacent soft tissues indicated the benign origin [3]. Restriction in DWI with ADC signal drop is a characteristic sign of epidermoid cyst [3], which was the final diagnosis.

Even with these imaging findings, the final diagnosis of retrorectal space lesions is histological.

Outcome

Management of retrorectal cystic lesions is usually surgical resection [3], to prevent possible associated complications. Imaging studies help delineate the extent and relationship to pelvic structures and are also used for follow-up to prevent recurrences that may occur locally [2,4,6].

Teaching points

Cystic lesions in the retrorectal space are underdiagnosed entities that should be actively sought out in ordinary studies by the physician. The most frequent are developmental cysts. There are imaging findings that allow differentiation of some of them, although the final diagnosis is histological.