Urachus

1. Overview

The urachus is a fibrous remnant of the allantois, a canal that connects the fetal bladder to the umbilicus during embryonic development. In adults, it typically becomes a non-functional ligament called the median umbilical ligament. Though functionless after birth, it can be the site of several congenital or acquired anomalies that may require medical attention.

2. Location

The urachus is located in the midline of the anterior abdominal wall, extending:

• From the apex of the urinary bladder (anterior dome)

• To the umbilicus (belly button)

It lies in the retropubic space (space of Retzius), posterior to the anterior abdominal wall and anterior to the peritoneum, sandwiched between the transversalis fascia and peritoneum.

3. Structure

The urachus is a narrow fibrous cord, typically 3–10 cm long, with the following features:

• Fibrous tissue core: Replaces the epithelial lining after fetal obliteration.

• Embryological origin: Derives from the allantoic duct, part of the fetal cloaca.

• Postnatal transformation: Becomes the median umbilical ligament, distinct from the paired medial umbilical ligaments (remnants of the umbilical arteries).

Although usually obliterated by birth, the urachus may retain a lumen or partial canal in some individuals, predisposing them to urachal anomalies.

4. Function

In the embryo, the urachus:

• Connects the bladder to the allantois: Allowing urine to drain from the fetal bladder to the yolk sac via the umbilical cord.

After birth, the urachus has no known physiological function. It becomes a fibrous vestige (median umbilical ligament) involved only structurally in the abdominal wall.

5. Physiological Role(s)

While non-functional in postnatal life, the urachus is significant in:

• Embryologic urinary drainage: Contributing to early fetal waste elimination.

• Developmental anatomy: Serving as a key landmark in bladder and umbilical development.

• Surgical and diagnostic relevance: Due to its potential persistence or transformation into pathological structures.

6. Clinical Significance

Urachal Anomalies

Persistence of the urachal lumen may result in congenital abnormalities, including:

• Patent urachus: Complete failure to close, resulting in urine leakage from the umbilicus.

• Urachal cyst: Central portion remains patent, forming a fluid-filled cavity prone to infection.

• Urachal sinus: Open near the umbilicus but closed near the bladder.

• Urachal diverticulum: Open near the bladder but closed near the umbilicus.

Symptoms may include umbilical discharge, recurrent infections, lower abdominal pain, and swelling.

Urachal Cyst Infection

Most common in children and young adults, infected urachal cysts may present with:

• Midline lower abdominal tenderness

• Purulent umbilical discharge

• Low-grade fever

Treatment includes antibiotics and surgical excision.

Urachal Carcinoma

A rare but aggressive adenocarcinoma arising from urachal remnants, typically found in:

• Adults aged 40–70 years

• More common in men

Presents with hematuria, suprapubic mass, and mucusuria. CT or MRI helps with staging. Treatment involves partial or radical cystectomy with en bloc urachal and umbilical excision.

Surgical Relevance

During procedures like laparoscopic hernia repair, bladder surgery, or umbilical excision, the urachus may be encountered. It serves as a surgical landmark and should be preserved or removed depending on pathology.

Imaging Features

Urachal remnants can be evaluated using:

• Ultrasound: First-line for cysts or fluid collections.

• CT scan: Excellent for detecting infection, calcification, or malignancy.

• MRI: Used in preoperative planning or staging tumors.