CONGENITAL URACHAL ANOMALIES (Fig.5) occur in 1/5000 individuals, with 2/1 male-to-female ratio.

Patent urachus or urachal fistula (Fig.6), secondary to complete failure of allantoic duct regression, is the most frequent (50%). It can be associated with omphalocele, posterior urethral valves (1/3) and prune belly syndrome. Clinically, there is umbilical urine leakage with or without infection, although it can remain asymptomatic until adulthood. A tubular hollow structure is seen between the bladder dome and the umbilicus. If complete patency is in doubt, fluoroscopy or CEUS voiding cystourethrography or umbilical fistulography can demonstrate it.

Urachal sinus (Fig.7) (15%) is a consequence of partial cranial absence of regression of allantois. A fusiform duct communicates with the umbilicus, but not with the bladder lumen. The patient usually present with umbilical pain and intermittent umbilical drainage ("wet navel"). Fluoroscopic or CEUS sinography can demonstrate blind ending. An infected urachal cranial cyst may also develop umbilical fistulizaton, simulating urachal sinus.

Urachal or vesicourachal diverticulum (Fig.8) (3-5%) results when caudal absence of regression of allantois occurs. It appears as an oval diverticulum of the bladder dome, with urinary stasis that predisposes to infection and stone formation, although it is frequently asymptomatic. Mild bladder filling and pseudodiverticulum in the context of bladder trabeculation (Fig.9) can mimic urachal diverticulum.

Urachal cysts (Figs.10,11) (30%) are the result of intermediate absence of regression of allantois, with persistence of urothelial lining, filled with fluid or mucin. They can appear anywhere along the urachus course from the navel to the bladder, although the distal third is the most frequent site, sometimes abutting the bladder dome. It is usually an incidental-imaging finding, but it can be noticed as a palpable mass (if umbilical, large and/or superficial) or become symptomatical after infection. It may develop dystrophic wall calcifications or luminal lithiasis.

URACHAL INFECTION

Any urachal anomaly can superinfect. Patent urachal abnormalities become infected because of urine and/or skin exposure (Figs.6,7), while urachal cyst infection (Fig.12) is a consequence of desquamated epithelial layer accumulation. The most frequently involved organism is Staphylococcus aureus. Infection should be clinically suspected in case of recurrent urinary tract infections, local or systemic signs or symptoms of inflammation and changing or frankly purulent umbilical discharge.

At imaging, infected urachal congenital anomalies present with thick-enhancing walls, ill-defined borders, peripheral inflammatory changes in the perivesical fat and complex contents: heterogeneous, mildly echogenic, slightly attenuating on CT, slightly or markedly hypointense on T2 and eventually (if purulent) with restriction of water diffusion on MRI (Figs.6,7,12,13,14). Abscesses may be extraperitoneal (Fig.12) or subcutaneous (Fig.13). Extraurachal sinus tracts or fistulas may develop secondary to infection. An inflammatory chronic mass might be hard to distinguish from neoplasm.

Image-guided percutaneous drainage may be attempted, although it frequently requires surgical excision in order to eliminate promoting epithelium.

Interestingly, acquired urachal fistula and infection can arise due to extension of a nearby inflammatory process such as Crohn disease (Fig.14), colonic diverticulitis or cholecystitis, either with or without prior urachal anomaly. Characteristic findings of the primary bowel disease may suggest the diagnosis. This involvement usually requires surgical resection.

NON-INFECTED NON-NEOPLASIC URACHAL COMPLICATIONS

• Stones rarely develop, especially in urachal diverticulum (Fig.15), secondary to urinary stasis and facilitating infection.
• Endometriosis is also rare (0,1-0,5% of urachal abnormalities). It may occur at the bladder dome (Fig.16) or at the umbilicus (Fig.17) (Villar nodule, clinically manifesting as cyclical umbilical pain and/or local inflammation), usually coexisting with other manifestations of deep endometriosis. Endometriotic nodules appear with ill-defined infiltrating margins, hypoechoic with posterior shadow, soft-tissue attenuation, and low heterogeneous signal on T2-weighted MRI images. They commonly do not exhibit Doppler signal, although progressive enhancement is usually seen with iodinated or paramagnetic contrast.
• Hematoma of the urachus is infrequent, after trauma or surgery (typically at laparoscopic port access).

URACHAL NEOPLASMS

• Benign urachal neoplasms are rare.
  • Approximately 15 cases of mucinous cystoadenomas are reported in the literature. They have malignant potential and are hard to distinguish from cysts and malignant mucinous tumors, which are more common. They present as uni or multilocular cystic lesions with various signals on T2-weighted images that denote mucin content. Curvilinear wall calcifications may appear. Malignancy should be suspected in presence of solid nodules or papillary projections.
  • Adenomas of the urachus, resembling colonic adenomas, are also very uncommon, resulting from chronic epithelial inflammation. They are also considered premalignant.
• Malignant urachal neoplasms are less infrequent.
  • Urachal glandular carcinoma represent approximately 90% of primary urachal malignancies, mostly after TP53 and/or KRAS mutation. Most involve the yuxtavesical segment of the urachus, frequently invading the wall of the bladder and raising the question of organ-dependency. In the absence of AJCC specific consideration, a TNM staging system (Fig.18), not yet validated, has recently been proposed with specific therapeutic recommendations for each situation, based on three prior systems (Mayo, Sheldon and Ontario). Urachal adenocarcinomas represent <1% of bladder tumors, but they are a significant proportion of bladder adenocarcinomas (34%). Non-cystic adenocarcinomas (Fig.19) include different subtypes similar to colonic adenocarcinomas. They appear as solid hypoechoic masses with soft-tissue attenuation next to the bladder dome, usually with irregular infiltrating edges. Cystic adenocarcinomas are more frequent, mucinous cystoadenocarcinomas (Fig.20) being the most common (42%). They appear as mixed solid-cystic masses with mucinous contents, characteristically with calcifications and in the same situation. Urachal urothelial carcinomas (Fig.21) may be primary or contiguous from bladder carcinoma. They sometimes mix with adenocarcinoma and are difficult to distinguish from other urachal malignancies and, if inferior, from bladder urothelial carcinoma, which is at increased risk of synchronous or metachronous appearance.
  • Urachal non-glandular primary neoplasms (squamous, neuroendocrine, mixed-type) are exceedingly rare, often indistinguishable from glandular carcinomas.

Urachal metastasis are more frequent than primary malignancies, most commonly resulting from peritoneal carcinomatosis (which is usually obvious at imaging) at the bladder dome insertion.