What is a Urachal Cyst?
A urachal cyst is a congenital anomaly that results from incomplete obliteration of the urachus—the embryologic remnant connecting the bladder dome to the umbilicus—leaving a fluid-filled sac typically located in the midline between these two structures 1.
Embryologic Origin
The urachus is a vestigial remnant of the allantois that normally degenerates and obliterates during fetal development to become the median umbilical ligament 1, 2. When this obliteration fails, various urachal anomalies can occur:
- Urachal cyst (most common type—a closed fluid-filled sac)
- Patent urachus (complete tubular connection from bladder to umbilicus)
- Urachal sinus (opening at the umbilical end)
- Vesicourachal diverticulum (outpouching from the bladder dome)
Clinical Presentation
Most urachal cysts remain asymptomatic and are discovered incidentally on imaging 1. When symptoms do occur, they vary by age:
In Infants
In Older Children and Adults
- Abdominal pain 1
- Recurrent urinary tract infections 1
- Lower urinary tract symptoms 4
- Hematuria (in adults) 5
Infection is the most common complication requiring clinical intervention, with abscess formation occurring in approximately 75.5% of infected cases 3.
Diagnosis
Ultrasound is the primary diagnostic modality and can usually confirm the diagnosis 1, 6. The cyst appears as a midline fluid-filled sac, most frequently located near the bladder dome 6.
Diagnostic Algorithm:
- Initial evaluation: Careful history and physical examination focusing on umbilical findings and abdominal pain 1
- First-line imaging: Ultrasound to visualize the cyst and its relationship to the bladder 1
- Voiding cystography: Only indicated when the child presents with UTI or ultrasound shows upper tract abnormalities 1
- CT imaging: May be used in adults or complex cases, showing thick-walled cystic or mixed attenuation when infected 5
Important Caveat:
Large urachal cysts can cause falsely elevated post-void residual volumes on bladder scanners, as the device may misinterpret the cyst as retained urine 4.
Prevalence and Clinical Significance
Urachal cysts occur in approximately 1 in 5,000 live births but are only clinically relevant in about 1 in 150,000 of the population 5. The reported prevalence is highly variable, likely due to different definitions and the fact that most remain asymptomatic 1.
Malignant transformation in children has not been observed 1. The risk of urachal adenocarcinoma is extraordinarily low—5,721 urachal remnants would need to be excised to prevent a single case of malignancy 1.