Teaching Case

A 78-year-old male followed up for 5 years after a low anterior resection of a stage 0 middle third rectal adenocarcinoma (TisN0M0, without mucinous component or surgical margins involvement), clinically asymptomatic, showed successive blood tests revealing a progressive increase in carcinoembryonic antigen (CEA) with maximum elevation of 20 ng/mL.

Contrast-enhanced computed tomography (CECT) showed a low-density circumferential perirectal collection at the level of the staple line of the colorectal anastomosis. This lesion had grown with respect to the previous CECT scan, on which contrast enema ruled out an anastomotic leak.

Magnetic resonance (MR) confirmed a 12-centimetre-sized mass around the rectum, whose lumen was compressed. It exhibited a well-defined wall and high signal intensity content on T2-weighted images, without clear areas of diffusion restriction nor enhancing nodules on gadolinium-enhanced images.

Whole-body positron emission tomography–computed tomography (PET-CT) didn´t show significant 18-fluorodeoxyglucose uptake either in the lesion or at distant sites.

Transrectal endoscopic ultrasound (TREUS) demonstrated a bilaminar wall and heterogeneous content without blood flow. No communication was observed between the lesion and the rectal mucosa, which was unremarkable except for the underlying mass effect. Fine needle aspiration (FNA) revealed mucus, macrophages and intestinal epithelial cells without atypia.

Rectal mucocele is a rare, benign entity consisting of a mass of mucinous content lined by a well-formed wall with normal intestinal epithelium [1]. The colorectal epithelium produces mucin that is usually excreted. However, it can accumulate, forming either luminal mucoceles when excretion is not possible because of a distal obstruction, as previously described complicating subtotal colectomy in patients with inflammatory bowel disease [2-5], Hartmann's procedures [6, 7] and abdominoperineal resection [8], or extraluminal mucoceles. The latter do not communicate with the rectal lumen, and have been described following plication of rectal mucosa in stapled hemorrhoidopexy [9-12] or after oversewing a bowel segment during an anastomosis [1], resulting in sequestration of intestinal mucosa in an isolated rectal segment secreting mucin and producing a blind cystic collection of mucus.

Clinical presentation is varied and nonspecific, and patients may report abdominopelvic discomfort, abdominal growing mass, bloating, evacuatory difficulty, anal spasm, tenesmus or dysuria. Obstructive urinary retention may also occur. Mucoceles have been associated with mild elevation of blood CEA and increased uptake on PET-CT [13].

Imaging plays an important role in the diagnosis of extraluminal rectal mucoceles, showing well-defined thin-walled cystic masses located in anastomotic or stapled sites not communicating with the rectal lumen. The wall has the typical stratification of rectal layers [9] and it can show linear or irregular calcifications [5, 8]. The “onion skin” sign has been proposed on TREUS [14]. Imaging methods can also be used to guide tissue sampling. TREUS-guided FNA was used in our case.

Although imaging-guided drainage is a treatment option, simple drainage is not definitive, and the use of sclerosing agents has been proposed to destroy mucosal lining and prevent recurrences. Surgical excision or marsupialization seems to be the treatment of choice, avoiding risk of infection, pressure effects or rupture and subsequent pseudomyxoma peritonei [1, 11]. Although more studies are needed, treated rectal mucoceles should have a good long-term prognosis. As our patient is completely asymptomatic, he is scheduled for a follow-up MR and then deciding the definitive treatment, probably a transanal surgical incision and drainage, as a transperitoneal excision seems to be difficult and anticipates a probable definitive colostomy.

As teaching points from this case, colorectal surgery can be complicated by the development of an extraluminal rectal mucocele, and imaging techniques play a key role in the differential diagnosis, sampling guidance and patient management.

Written informed patient consent for publication has been obtained.