An unusual cause of lower abdominal pain

Clinical History

Presented with lower abdominal pain and progressive weight loss of 2 months duration.

Imaging Findings

The patient, previously fit and well, presented with lower abdominal pain and progressive weight loss of 2 months duration. Physical examination was unremarkable. Initial workup with chest radiograph and routine blood tests were normal.
The patient was then subjected to ultrasound examination of the abdomen. The ultrasound scan showed a well-defined 2.7 x 2.8 cm diameter mass in the suprapubic region close to the midline(Fig 1). On longitudinal plane, the mass was seen just beneath the anterior abdominal wall, indenting the apex of the bladder(Fig 2). The lesion was predominantly hypo echoic with no evidence of calcification. No other lesions were identified either in the abdomen or in the pelvis.
Axial CT scan confirmed the ultrasound findings. The lesion was hypo dense and showed peripheral enhancement with intravenous contrast(Fig 3). On sagittal reconstruction, the lesion was seen as a tubular structure extending from bladder apex to umbilicus(Fig 4). No local or distant metastasis was seen.
The patient underwent surgery and the mass was removed in Toto. Histologically, the lesion was proved to be of mucinous adenocarcinoma. The postoperative period was uneventful. The patient regained weight within few months following surgery.

Discussion

The urachus is a remnant of the allantois, which is an embryonic structure. It is a fibro muscular tubular structure, which connects the apex of the urinary bladder to the umbilicus. It can persist into adult life, although urachal remnants are more commonly seen in children. In adults it lies in the space of Retzius, approximately 5 cms long, between the peritoneum and the transversalis fascia. It can be divided into three parts: supravesical, intramural and intramucosal .
Carcinoma of the urachus usually arises outside the urinary bladder with secondary infiltration into the bladder dome. They therefore present with non-specific abdominal pain or painless haematuria on bladder invasion similar to bladder carcinoma (1). They can also present with suprapubic mass, urinary frequency, dysuria or discharge of pus, blood or mucus from the umbilicus. The average age of presentation is in the 5th-7th decade (2), though individual cases have been reported in as young as 15 years old patients. Urachal carcinoma is not pedunculated on invasion of the urinary bladder and the mucosa overlying it may still be intact . The prognosis of these patients is poor as they present late owing to the lack of early symptoms (3).
Histologically, mucin-secreting adenocarcinoma is the most frequent type of urachal carcinoma. The vast majority of adenocarcinomas arise from foci of cystitis cystica in the bladder mucosal lining, secondary to chronic inflammation of the bladder wall. The criteria for diagnosing an adenocarcinoma arising from the urachus are as follows: tumour at the apex or anterior wall of the urinary bladder with an intramural or supravesical mass, normal bladder mucosa adjacent to it, no evidence of adenocarcinoma elsewhere, absence of cystitis cystica . Cystitis cystica may however coexist with a primary urachal adenocarcinoma and its presence is not necessarily an exclusion factor . The other histological types reported are transitional cell carcinoma, squamous cell carcinoma and sarcoma. Single case reports of neuroblastoma arising from the urachus and pseudomyxoma peritonei secondary to urachal adenocarcinoma are also found in the literature.
Radiology is having an ever-increasing role in the management of these patients. A plain abdominal x-ray and an IVU series may be normal, unless calcified. An ultrasound examination is usually the first imaging modality used to make a diagnosis. On ultrasound, a well-defined solid hypo echoic mass in the supravesical region with areas of heterogeneity is seen (4). The mass may be distinct from the urinary bladder or seen to invade the dome of the bladder. The mass may lie quite close to the anterior abdominal wall.
A CT scan would show the mass in its characteristic anterior, midline position. Differentiation from a bladder carcinoma may not be easy on axial images. Reconstruction in sagittal plane may be of diagnostic value. A mass confirming to the site and shape of the urachus is very important in making the diagnosis. A low attenuation mass with mural calcification within it is suggestive of an urachal carcinoma (4). Other lesions like ovarian dermoid, uterine fibroids, ovarian carcinoma, and metastatic adenocarcinoma may however have a similar appearance. Bladder wall invasion, pelvic and inguinal lymphadenopathy can also be demonstrated on CT.
An MR scan is probably the best investigation due to its capability of multiplanar imaging and not using ionising radiation. Scanning in the sagittal plane is particularly useful in demonstrating the shape and size of the mass with its relations to the urinary bladder, the umbilicus and the rectus muscles. The diagnosis of an urachal carcinoma is considered when a mass is tubular and extending along the urachus towards the umbilicus. The mass would be best demonstrated on T1 weighted sequences in which the mass would be of low signal intensity and clearly visible against high signal from surrounding fat. A heterogeneous increase in signal intensity in a mass lying in the supravesical region on T2 weighted images with distortion of the bladder dome is also seen (5). As with CT scans, local invasion, lymphadenopathy and distant spread can be assessed.
The treatment options are either partial or radical cystectomy depending on the extent of bladder invasion. This has been combined with chemotherapy and radiotherapy with some response reported(3). Accurate and early diagnosis with multimodality imaging is required to treat this malignancy with an otherwise not so good prognosis.

Differential Diagnosis List

Final Diagnosis

Urachal carcinoma

URL:	https://www.eurorad.org/case/2476
DOI:	10.1594/EURORAD/CASE.2476
ISSN:	1563-4086