|ECR 2019 / C-2770|
|Imaging of Urachal remnant diseases|
Findings and procedure details
The terms urachus, urachal cancer and urachal adenocarcinoma were searched using picture archiving and communications software (PACS) over a 10 year period from March 2008 -2018. There were 217 patients who had an imaging study (ultrasound, CT or MRI) of which 2 (1%) patients had a urachal adenocarcinoma. 127 (59%) patients had a urachal anomaly and 71 (33%) were normal. 18 (7%) patients had an alternative diagnosis.
Embryology development and anatomy
The urachus is a ductal remnant that arises embryologically, originating from the involution of the allantois and cloaca and extending between the bladder dome and the umbilicus.
The allantois is an extraembryonic diverticulum that appears early in the 3rd week of gestational life located on the posterior aspect of the yolk sac. The cloaca is an endoderm-lined structure, which is divided by the urorectal septum between the 4th and 7th weeks of gestation, creating the urogenital sinus ventrally and the rectum dorsally. The inferior portion of the urogenital sinus later becomes the pelvic and penile urethra in males and the pelvic urethra and vaginal vestibule in females. The superior portion of the ventral urogenital sinus later becomes the bladder and its apex, which subsequently becomes the bladder dome, which is contiguous with the allantois (1-7).
Around the 4th to 5th gestational months, involution of the allantois and urogenital sinus occurs as the bladder descends into the pelvis, forming the urachus, which elongates and becomes a fibromuscular cordlike structure connecting the apex of the bladder to the umbilicus. Urachal remnants are present in almost 100% of infants at birth and regress due to fibrosis with age, eventually becoming the median umbilical ligament. Two-thirds of adolescents and one-third of 35-year-old adults show remnant urachal tissue, but it is seen in only 3% of the general population at autopsy (8-11). These remnants are rarely seen and are not considered pathologic, unless they fail to obliterate, retaining some degree of luminal patency (11).
The length of the urachus ranges from 3 to 10 cm and it generally has an approximate diameter of 8–10 mm (9)
Congenital urachal anomalies result from a failure of the developing urachus to completely obliterate its lumen, and represent uncommon and often underdiagnosed entities that may be found incidentally at imaging or manifest clinically with the development of mostly nonspecific abdominal or urinary symptoms. Persistence of undetected urachal anomalies may cause abdominal and/or urinary complications in both paediatric and adult populations as a result of chronic urinary stasis, inflammation, and infection, predisposing the patient to potential morbidity and malignancy. In addition, urachal anomalies can be confused with other abdominal and pelvic entities unless one is familiar with the embryologic development, anatomy, and imaging features of the different urachal anomalies and their potential complications.
The reported incidence of urachal anomalies is approximately one in 5000 population for adults, with a significantly lower rate of one in 150 000 population among infants (1). There is a higher prevalence in men than women (1). Most anomalies of the urachus are unexpected, being detected incidentally and more frequently with the increased use of cross-sectional imaging.
Classification and imaging of urachal anomalies.
Congenital urachal anomalies.
Four types of urachal anomalies have been described, on the basis of the location of the abnormal residual patency along the urachal tract: patent urachus (sometimes referred to as urachal fistula), urachal cyst, umbilical-urachal sinus, and vesicourachal diverticulum.
The most common type of urachal anomaly is a patent urachus (accounting for 47%), followed by urachal cyst (30%), sinus (18%), and, least commonly, vesicourachal diverticulum (3%) (1, 15-18). (figure 1 shows a pictoral representation)
1. Patent urachus is characterized by the presence of a free and persistent communication between the bladder and the umbilicus. CT shows a tubular structure extending from the umbilicus to the bladder with no enhancement (figure 2).
2. Urachal cysts develop when both the umbilical and bladder ends of the urachus are obliterated, but a focal segment remains patent somewhere along the course of the urachus, more commonly at the lower third of the urachal tract.
At imaging, the diagnosis of an uncomplicated cyst can be made by demonstration of a midline homogeneous fluid-filled structure anywhere along the theoretical course of the urachus between the bladder and the umbilicus (figure 3,4).
3. Umbilical-urachal sinus occurs when the umbilical end of the urachus fails to obliterate and persists as a fusiform outpouching structure just below the navel (1). This urachal anomaly represents a potential space in which the accumulation of cellular debris can favor the development of potential complications such as infection and less commonly stone formation (figure 5).
4. Vesicourachal diverticulum on CT demonstrates a midline focal protruding extension of the bladder arising from its dome with no communication to the umbilicus or the rest of the urachal tract (figure 6).
Acquired urachal disease.
These include infection and neoplasm (benign and malignant).
Infective urachal disorders represent the most common complication of urachal anomalies. These often present with symptoms such as discharge form the umbilicus, redness around the umbilicus, abdominal pain or UTI. In the setting of an acute clinical presentation, imaging findings that should prompt consideration of infection would include heterogeneous attenuation of the urachal anomaly together with variable increased contrast enhancement at CT (figure 7,8).
2. Benign tumours
Benign tumors are extremely rare and can arise from any portion of the urachal tract, usually mimicking urachal malignancy. The most commonly described benign tumors of the urachus are adenomas and cystadenomas, followed by fibromas, fibromyomas, fibroadenomas, and hamartomas (8). Because they mimic malignancy, the diagnosis is usually made at histopathologic analysis after resection.
3. Malignant tumours
Malignant urachal neoplasms are also rare, accounting for less than 1% of all bladder cancers. 90% develop in the urachus adjacent to the bladder and grow in a cranial direction (1). They usually remain undiscovered for a long period of time and may be found incidentally at imaging or at an advanced stage when symptoms of local invasion or systemic spread have developed (11).
Although urachal remnants are lined by urothelium, 80% of urachal cancers are adenocarcinomas, including mucin-producing adenocarcinomas (69%) and mucin-negative adenocarcinomas (15%). The remaining 20% of urachal cancers are urothelial, squamous, and sarcomatoid neoplasms (1). This differs from the most common type of bladder cancer, which is typically urothelial.
At CT, urachal carcinoma can be cystic, solid, or mixed. It usually appears as a midline mass adjacent to the bladder dome with extension into the space of Retzius, demonstrating heterogeneous low-attenuation components, which represent mucin content (figure 9,10). Calcification is present in up to 70% of cases (1).
MR imaging will typically manifest as a midline mass with focal areas of heterogeneous high signal intensity on T2-weighted images that are suspicious for mucin content. Solid components of the tumor are isointense to soft tissue on T1-weighted images and enhance following the administration of intravenous gadolinium contrast material (figure 11,12,13)
Ultrasound is often the first line investigation in these patients and useful for assessment of a bladder cyst/diverticulum. However in adults MRI is often used to define these anomalies in more detail. CT has a role for staging or assessing other complications.
Our proposed protocol for MRI imaging would include:
Sagittal T2 ( to include umbilicus) large FOV
Axial T1/T2 (from umbilicus to perineum) large FOV
Ax T2 small FOV
Axial T1 fat saturated pre and post contrast.
Additional sequences : Axial / Sagittal STIR
A voiding cystourethrogram or cystoscopy may also be helpful in aiding with the diagnosis and managment.
Management of congenital urachal anomalies
The approach to patients with urachal anomalies has evolved over time and remains controversial(1). There are no clear guidelines for the managemnt of these patients.
Most urachal remnants spontaneously resolve with age.
Symptomatic infants are now usually treated medically, and only in cases of recurrent symptoms or failure of spontaneous resolution are patients under 1 year of age treated surgically. These patients would be followed up with Ultrasound to confirm egression of the remnant.
In older children, surgical excision of asymptomatic urachal anomalies has been proposed to prevent complications, mostly to prevent the development of malignancy (1).
In adults, if the diverticulum/cyst appears simple in nature on CT/MR then interval imaging would be advised. If there is any cause for concern on the imaging i.e. focal thickening of the wall, abnormal enhancement or restricted diffusion on DWI then surgical excision would be considered.
Management of acquired urachal anomalies.
Infection is initially treated with antiobitics. Once infection is under control then surgical excision can be considered.
Depending on staging, a malignant lesion can be treated with surgical excision, chemotherapy or radiotherapy (1).
The prognosis of urachal carcinoma is poor. The 5 year survival rate is reported as 49% after treatment (1). Local recurrence is common. Metastatic disease has an extremely poor prognosis. Metastases are seen in the bladder, lungs, bowel, liver, brain and bones.