The urachus revisited: multimodal imaging of benign & malignant urachal pathology

Abstract

The urachus is a fibrous tube extending from the umbilicus to the anterosuperior bladder dome that usually obliterates at week 12 of gestation, becoming the median umbilical ligament. Urachal pathology occurs when there is incomplete obliteration of this channel during foetal development, resulting in the formation of a urachal cyst, patent urachus, urachal sinus or urachal diverticulum. Patients with persistent urachal remnants may be asymptomatic or present with lower abdominal or urinary tract symptoms and can develop complications. The purpose of this review is to describe imaging features of urachal remnant pathology and potential benign and malignant complications on ultrasound, CT, positron emission tomography CT and MRI.

Introduction

Urachal embryology

A urachal remnant occurs due to maldevelopment of the allantois and cloaca. The allantois arises from the posteroinferior yolk sac after the second week of embryonic development. The urinary bladder initially extends to the level of the umbilicus descending into the pelvis by month 5 of gestation. The apical portion of the descending bladder, derived from the cloaca, degenerates into an extraperitoneal fibrous cord within the Retzius space, eventually becoming the median umbilical ligament. This structure extends from the bladder dome to the umbilicus. The vestigial remnant of this is known as the urachus ^1,2 ( Figure 1 ).

Figure 1.

Schematic diagram (a) showing a normal urachus. Saggital (b) and axial (c) CT images following intravenous contrast demonstrating a normal urachus extending into Retzius space.

Epidemiology of urachal anomalies

A persistent urachus has an incidence of 1/5000 in the adult population, occurring more frequently in males. The most common urachal anomalies are urachal cysts, urachal sinus, patent urachus and vesicourachal diverticulum ( Figure 2 ). Infection is the most common benign complication of urachal remnant pathology occurring in children and adults whereas malignant complications have been described exclusively in adults to date. ^1–3

Figure 2.

Schematic diagrams showing common urachal remnant pathology. (a) Urachal cyst; (b) Urachal sinus; (c) Patent urachus; (d) Urachal diverticulum.

Imaging of urachal remnants

Urachal cysts account for up to 54% of urachal remnant anomalies, arising from the lower third of the urachus and are often asymptomatic and incidentally discovered. Sonography can demonstrate an extraluminal midline fluid-filled structure that does not communicate with the umbilicus. CT following intravenous contrast usually reveal a fluid-attenuation supravesical lesion ( Figure 3 ). On MRI, urachal cysts usually demonstrate a thin, non-enhancing wall with intrinsic T1 hypointense and T2 hyperintense signal. ^1–4

Figure 3.

38-year-old male with a lower urinary tract symptoms. Axial (a) and saggital (b) CT images after intravenous contrast showing a supravesical cystic structure without definite communication with the bladder or umbilicus confirmed as a urachal cyst following image-guided aspiration.

A urachal sinus is a tubular dilatation of the umbilical portion of the urachus that occurs three times more frequently in children than adults. Clinical presentation is most commonly with periumbilical inflammation and purulent discharge. On ultrasound, a urachal sinus may appear tubular and hypoechoic communicating with the umbilicus only. Fluoroscopic sinography can show a tube-like cul-de-sac. CT may better delineate infective complications. MRI can better evaluate for superimposed infection or developing malignancy. ^1,3

The patent urachus is a fistulous communication occuring between the urinary bladder and umbilicus, most commonly presenting with umbilical urine leak. Ultrasound may demonstrate a tube-shaped anechoic structure communicating with the antero superior bladder and umbilicus. Diagnosis can be confirmed on voiding cystourography or fluoroscopic sinogram with instilled contrast media defining the umbilical-vesical tract. CT and MRI can better demonstrate air, fluid or calculi within the patent urachus, and evaluate for potential infective complications on post-contrast imaging. ^1–3

A urachal diverticulum is a rare clinical entity accounting for 3–5% of urachal remnant pathology occurring when the urachus communicates with the bladder dome only. Sonographic evaluation of a urachal diverticulum shows a blind-ending supravesical hypoechogenic structure arising from the bladder dome. CT can demonstrate an out-pouching from the superior aspect of the bladder (Figure 4) without umbilical communication, and may demonstrate intraluminal calcification, present in up to 50% of cases. In children, VCUG can show contrast reflux into the diverticulum and help delineate the extent of the involvement of the urachus. On MRI, a protruding midline soft-tissue tract can be seen, demonstrating variable signal intensity and post-contrast enhancement. ^1–3

Figure 4.

70-year-old male with back pain. Saggital (a) and axial (b) CT images following intravenous contrast showing a urachal diverticulum (arrows) detected as an incidental finding.

Imaging of benign urachal pathology

Infection of a persistent urachus is the most common benign complication occuring in patients with a urachal remnant, usually due to Staphylococcus aureus. Patients with infected urachal cysts may be asymptomatic with leukocytosis or present with recurrent urinary tract infections, abdominal pain or fever. ^1,2,4

Infected urachal cysts may be difficult to differentiate from non-infected cysts or other pelvic pathology on initial sonographic evaluation in the absence of clinical signs and symptoms. ⁵ Ultrasound is often the first-line modality for imaging a child presenting with symptoms of abdominal pathology. Greyscale ultrasound reveals a midline extraperitoneal structure extending from the bladder dome to the umbilicus containing internal echoes (Figure 5) and may mimic other pelvic pathology, such as haematoma. On CT, infected urachal cysts can appear thick-walled and irregular with peripheral enhancment and underlying bladder wall thickening ( Figure 6 ). On MRI, infected urachal cysts can appear as irregular thick-walled supravesical structures demonstrating variable signal intensity and heterogenous contrast enhancement. MRI may delineate the infected urachal cyst from adjacent pelvic structures and identify possible rupture and abscess formation. ^1,3–5

Figure 5.

5-year-old child with fever and urinary retention. Longitudinal (a) and transverse (b) greyscale ultrasound images demonstrating a lesion containing internal echoes (arrows) anterosuperior to the urinary bladder (*). Surgical excision following intravenous antibiotics confirmed diagnosis of an infected urachal cyst.

Figure 6.

49-year-old male with recurrent urinary tract infections. Saggital (a) and axial (b) CT images following intravenous contrast demonstrating an irregular cystic structure anterosuperior to the urinary bladder demonstrating peripheral enhancement, confirmed as an infected urachal cyst following surgical excision.

Urachal abscess can develop if an infected cyst is left untreated. Infected urachal cysts usually discharge into the umbilicus or urinary bladder but may rarely rupture intraperitoneally from their extraperitoneal location causing sepsis and peritonitis. Given the lack of direct communication with either bladder or umbilicus, clinical asessment may result in misdiagnosis of more commonly encountered caused of acute abdomen, such as appendicitis or bowel perforation. ^5,6 Ultrasound may show a complex cystic mass in the midline above the urinary bladder. CT following intravenous contrast can demonstrate a supravesical heterogenous collection with a thick enhancing irregular wall and central non-enhancing low attenuation (Figure 7), that may abut or communicate with the abdominal wall. ^1,3,5,6 On MRI, a urachal abscess can appear as a thick-walled, multiloculated predominantly T2 hyperintense mass extending into Retzius space demonstrating heterogenous enhancement on post-contrast imaging. (Figure 8)

Figure 7.

21-year-old male with 2-week history of fever and tender suprapubic mass. Axial CT image after intravenous contrast showing an irregular, mixed attenuation lesion anterosuperior to the bladder dome (arrow) with surrounding inflammatory change. Image-guided aspiration yielded purulent material. Subsequent surgical excision confirmed diagnosis of a urachal abscess.

Figure 8.

28-year-old female with pelvic pain and fever. Axial (a) and sagittal (b) T₂ weighted images showing a multiloculated heterogenously T2 hyperintense collection anterosuperior to the bladder (*), confirmed as a urachal abscess following surgical intervention.

Benign urachal tumours are rare clinical and radiological entities and include fibroadenomatous lesions, lipoatous tumours and hamartomas. ^1,3

Imaging of malignant urachal pathology

Malignant transformation of a urachal remnant is rare, accounting for <0.5% of bladder cancer and develops twice as often in males, most commonly between the ages of 46 and 56. Adenocarcinoma accounts for 80–90% of cases with up to 75% producing mucin. Sarcomatous neoplasms, squamous cell and transitional cell urachal cancers have also been reported. ^1,3,7,8

Clinical presentation is variable and patients may be asymptomatic until local spread or development of metastases. Haematuria is the most common symptom, present in up to 80% of cases. Mucinuria can occur in up to 25% of patients, a rare symptom that should raise suspicion and prompt further investigation for a urachal cancer. ^3,7–9

On ultrasound, urachal malignancy can appear as a hyperechogenic soft-tissue lesion between the urinary bladder and anterior abdominal wall demonstrating internal vascularity on colour doppler imaging. Urachal cancer can demonstrate a variable appearance on cross-sectional imaging. On CT urachal malignancy can appear as a solid-cystic mass extending toward the umbilicus with an irregular, enhancing wall ( Figure 9 ). Up to 90% of cases arise from the juxtavesical urachus, extending cranially into the space of Retzius. Urachal cancer may also appear as a predominantly cystic mass (Figure 10) or as a midline or paramidline enhancing nodule arising from the anterosuperior aspect of the urinary bladder ( Figure 11 ). Urachal cancer appears low attenuation in 60% of cases due to the presence of mucin or necrosis. CT can also identify central or peripheral calcification, present in almost 75% of urachal cancers. ^1,3,8–10 Urachal malignancies may be predominantly cystic demonstrating high T2 signal (Figure 12) or multilobulated with solid and cystic components demonstrating intermediate and high T2 signal representing mucin, cystic change and/or necrosis ( Figure 13 ). On post-contrast T ₁ weighted imaging, urachal cancers may demonstrate inhomogenous enhancement. MRI may provide addional value by defining local invasion of adjacent soft-tissue and visceral structures, as well as identifying local nodal metastases. ^1,11,12

Figure 9.

32-year-old female with abdominal swelling and lower urinary tract symptoms. Axial CT image following intravenous contrast showing a solid-cystic mass (arrow) containing a focus of curvlinear calcification (arrowhead) anterosuperior to the urinary bladder (*). Histopathology confirmed the diagnosis of urachal adenocarcinoma, following surgical excision.

Figure 10.

39-year-old female with abdominal bloating and mucosuria. Axial CT image demonstrating a well-defined cystic structure with mural thickening and peripheral calcification (arrowhead) anterosuperior to the urinary bladder (*). Image-guided aspiration and histopathology confirmed mucinous urachal adenocarcinoma.

Figure 11.

41-year-old male with haematuria. Axial CT image showing an enhancing soft-tissue nodule (arrow) arising from left anterosuperior wall of the bladder (*). Histopathology following cystoscopy and biopsy confirmed diagnosis of urachal adenocarcinoma.

Figure 12.

39-year-old female with abdominal bloating. Saggital (a) and coronal (b) T₂ weighted magnetic resonance images show a paramidline cystic mass arising from the dome of the bladder (*). Histopathology confirmed mucinous urachal adenocarcinoma.

Figure 13.

45-year-old female with haemturia and abdominal bloating. Axial (a) and sagittal (b) T₂ weighted images demonstrate a complex, multilobulated mass involving the dome of the urinary bladder (*) with intravesical extension (arrowhead). Histopathology confirmed mucinous urachal adenocarcinoma.

Assessment of urachal pathology: the role of imaging

Accurate diagnosis of benign from malignant urachal pathology on imaging is imprecise. Meeks et al found that pre-operative imaging with CT had a low specificity and negative predictive value of 21 and 43% respectively. ¹³ In addition, although the majority of malignant urachal tumours are solid, up to 27% can demonstrate a cystic component. ² Similarly, although intrinsic calcification is considered 'pathognomic' of urachal cancer seen in up to 70% of cases on CT, 'egg-shell' calcification can also be seen in ~20% of urachal cysts. ^2,3 Fludeoxyglucose positron emission tomography (PET) CT (Figure 14, ^14,15 ) has been demonstrated as useful in the assessment of urachal cancer ¹⁴ However, due to lack of avid fludeoxyglucose uptake in over half of mucinous tumours, diagnostic performance of PET/CT may be unreliable in excluding urachal malignancy, given that up to three quarters urachal tumours produce mucin. Imaging may identify a urachal remnant complication but ultimately cystoscopy and biopsy may be required to confirm the diagnosis of a benign or malignant lesion. ^2,7,8,15

Figure 14.

47-year-old female with haematuria. Axial (a) and sagittal (b) 18-fluorine FDG PET CT showing an FDG-avid supravesical mass (arrows) containg punctate calcifications demonstrating a maximum SUV of 6.8. Biopsy and subsequent resection confirmed the diagnosis of urachal adenocarconoma. FDG, fludeoxyglucose; PET, positron emission tomography; SUV, standardized uptake value.

Treatment of urachal pathology

Controversy regarding optimal treatment of urachal remnants has become more apparent in recent years. Management strategies also differ between adult and paediatric patients. While surgery is required for most symptomatic patients, spontaneous regression of urachal anomalies has also been reported. Current series demonstrate a 10-year survival rate of only 49% with surgery for urachal cancer while salvage chemotherapy is effective in less than 10% of patients with metastases, suggesting that the risk of delaying the urachal remnant resection in older adults may exceed the risk of surgery. ^2,7,8

Whereas earlier recommendations favoured surgical intervention for in the majority of paediatric patients with a urachal remnant (due to the risk of recurrent infection and urachal cancer in later life), more recent reports have suggested that between 50 and 80% resolve non-operatively. In addition, almost 15% of paediatric patients develop post-operative complications including iatrogenic bladder injury requiring reoperation. ^2,13,16

Conclusion

Urachal pathology, although rare, can cause significant morbidity and mortality. Non-specific pelvic symptomatology of urachal disease can potentially result in delayed diagnosis and treatment. Multimodal imaging with ultrasound, CT, PET/CT and MRI can identify and characterise potential complications related to the urachal remnant, however differentiating benign from malignant urachal complications is imprecise with imaging alone and histopathological analysis is usually required to confidently diagnose urachal cancer from an infectious/inflammatory process.