Urachal Remnants

16.1Introduction

•The urachus is a fibrous remnant of the allantois.

•The allantois is a canal that drains the urinary bladder of the fetus and runs within the umbilical cord.

•The urachus is a band of fibrous tissue extending from the dome of the bladder to the umbilical cord.

•By 32 weeks, the urachus is obliterated and becomes a vestigial structure known as the median umbilical ligament (not to be confused with the medial umbilical ligament, which is a separate structure that lies laterally to the median umbilical ligament).

•The urachus remnant lies in the space of Retzius, between the transversalis fascia anteriorly and the peritoneum posteriorly.

•In 0.1–2.0 % of the population, the urachus remains patent and may result in urine leaking from the umbilicus or infected urachal cysts.

•Persistence of urachal remnant can give rise to various clinical problems:

–Urachal cyst

–Urachal fistula

–Urachal diverticulum (Vesicourachal diverticulum)

–Urachal sinus

•Because urachal remnants are uncommon and manifest with nonspecific abdominal or urinary signs and symptoms, definitive presurgical diagnosis is not easily made.

•The urachus is also known to undergo malignant transformation and adenocarcinoma is the commonest.

•A rare urachal tumor has been reported as a manifestation of IgG4-related disease.

16.2Embryology

•Embryologically, the amniotic cavity bulges over the embryo and attaches to the yolk sac and the connecting stalk to form the umbilical cord.

•Further caudal folding of the embryo incorporates the proximal yolk sac into the hindgut and the allantois which is a diverticulum of the yolk sac into the urogenital sinus.

•The yolk and body stalks fuse to become the umbilical cord.

•In the third week of gestation, the allantois, which grows into the body stalk is formed as a diverticulum from the yolk sac.

•The allantois appears on about day 16 as a tiny, finger-like outpouching from the caudal wall of the yolk sac.

•The bladder develops from the ventral portion of the expanded terminal part of the hindgut, the cloaca, which is contiguous with the allantois ventrally.

•The cranioventral end of the bladder opens into the allantois at the level of the umbilicus; thus, the bladder initially extends all the way to the umbilicus.

•By the fourth or fifth month of gestation, the bladder descends into the pelvis and its apical portion progressively narrows to a small, epithelialized fibromuscular strand, the urachus (Figs. 16.1, 16.2, and 16.3).

•In late embryonic and fetal life and early postnatal life, the urachal portion, which is still microscopic, fails to grow; thus, its lumen

remains narrow and is usually obliterated by fibrous proliferation.

•In one-third of adults, it may be visible at microscopic examination as a structure communicating with the lumen of the bladder; however, in terms of function it can be considered closed by the latter half of fetal life.

•The urachus varies from 3 to 10 cm in length and from 8 to 10 mm in diameter.

•It is a three-layered tubular structure, the innermost layer being lined with transitional epithelium in 70 % of cases and with columnar epithelium in 30 %.

•The structure is surrounded by connective tissue and an outermost muscular layer in continuity with the detrusor muscle.

•Occasionally, the urachus may merge with one or both of the obliterated umbilical arteries, and there may be a slight deviation to the right or left of the midline.

•As the distal hindgut and the urogenital sinus separate, the developing bladder remains connected to the allantois through a connection called the urachus.

•Persistence of this communication leads to urachal remnants.

•The urachal remnant anomalies include:

–Patent urachus

–Urachal sinus

–Urachal cyst

–Urachal diverticulum

•Subsequently, the yolk and body stalks fuse to become the umbilical cord.

•Development of the abdominal wall narrows the umbilical ring, which should close before birth.

•Persistence of the umbilical ring results in an umbilical hernia.

16.3Classification

•There are four types of congenital urachal remnant anomalies.

•They are:

–Patent urachuss(50 %)

–Urachal cyst (30 %)

–Urachal-umbilical sinus (15 %)

–Vesicourachal diverticulum (5 %)

•Urachal cyst:

–A urachal cyst is a cyst which occurs in a

–It presents as an extraperitoneal mass in the umbilical region.

–Urachal cysts are usually silent clinically until infection, calculi or adenocarcinoma develop

–It is characterized by:

•Abdominal pain, and fever if infected

•It may rupture, leading to peritonitis

•It may drain through the umbilicus.

•Patent urachus:

–This is the commonest congenital malformation of the urachus.

–In patent urachus, the whole urachus fails to obliterateandthereisapatentcommunication between the urinary bladder and the umbilicus.

•Umbilical sinus:

–In umbilical sinus, the distal part of the urachus remains open to the umbilicus.

–The usual presentation is persistent umbilical discharge.

•Vesico-urachal diverticulum:

–In this, there is a wide patent urachal opening to the urinary bladder

•Inflammation and malignancy are the commonest complications of urachal remnants.

•Inflammation occurs more frequently in children and young adults.

–Inflammation may be complicated by the development of an abscess which can remain clinically unrecognized or it can present as acute surgical abdomen.

–This must be kept in mind as the diagnosis often is confused with other diseases such as Meckel’s diverticulum, acute appendicitis, recurrent urinary infections, or abdominal colicky pain of unknown origin.

•The most common infecting pathogens are E. coli and Proteus, but a variety of other pathogens can also be found, including Staphylococcus aureus, Bacteroides, Fusobacterium, and Streptococcus iridans.

•Rarely it is secondary to Actinomycosis,

Aspergillus or Tinea corporis.

•Occasionally, a chronic inflammatory process can result in the unusual form of a xanthogranulomatous urachitis

–Patients may also complain of urinary symptoms such as suprapubic pain, dysuria, and/ or intermittent episodes of urinary retention.

–Infection of a urachal sinus and the cord stump must be taken seriously because of its potential sequelae such as cellulitis, necrotizing fasciitis, peritonitis, multiple hepatic abscess, septicaemia, and possible retroperitoneal abscess.

PATENT

URACHUS URINARY BLADDER

Fig. 16.4 Diagrammatic representation of a patent urachus

•Malignant degeneration of urachal remnants occurs more frequently in middle-aged and older people.

16.4Clinical Features

•Congenital urachal anomalies are rare and occur more common in males than females (M:F is 2:1).

•The majority of patients with urachal abnormalities (except those with a patent urachus) are asymptomatic.

•However, they may become symptomatic if these abnormalities are infected.

•Patent urachus (Figs. 16.4, 16.5, and 16.6):

–If a persistent communication remains between the bladder lumen and the umbilicus, urine leakage is usually noted during the neonatal period.

–In about one-third of cases, this condition is associated with posterior urethral valves or urethral atresia.

–Some patients with patent urachus are asymptomatic, and sometimes an acquired obstructive lesion of the lower urinary tract may result in umbilical-urinary fistulas.

–A definitive diagnosis can be made with sinography or cystography.

–Patent urachus can also be demonstrated at longitudinal US and occasionally at CT performed in infants during the bladderfilling stage.

Fig. 16.7 Diagrammatic representation of a urachal sinus

•Umbilical-urachal sinus (Figs. 16.7):

–Umbilical-urachal sinus consists of blind dilatation of the urachus at the umbilical end.

–A small opening into the umbilicus is generally present.

–This may result in periodic umbilical discharge.

•Vesicourachal diverticulum mayVesicourachal diverticulum (Fig. 16.8):

–In vesicourachal diverticulum, the urachus communicates only with the bladder dome.

–This condition results when the vesical end of the urachus fails to close.

–Vesicourachal diverticulum is asymptomatic in most cases.

–It tends to be found in patients with chronic bladder outlet obstruction.

–be complicated by:

• Urinary tract infection

Fig. 16.8 Diagrammatic representation of a vesicourachal diverticulum

•Intraurachal stone formation

•Increased prevalence of carcinoma after puberty

–It is usually discovered incidentally at axial CT performed for unrelated reasons. It appears as a midline cystic lesion just above the anterosuperior aspect of the bladder.

–On US, it appears as an extraluminally protruding, fluid-filled sac that does not communicate with the umbilicus.

–In infants, vesicourachal diverticulum is commonly accompanied by prune-belly syndrome.

•Urachal cyst (Figs. 16.9, 16.10, 16.11, 16.12, and 16.13):

–An urachal cyst develops if the urachus closes at both the umbilicus and the bladder but remains patent between these two endpoints.