Urachal Remnants




(1)
Pediatric Surgery, AlSadik Hospital, Qatif, Saudi Arabia

 




16.1 Introduction






  • 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.


16.2 Embryology






  • 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).

    A429541_1_En_16_Fig1a_HTML.gifA429541_1_En_16_Fig1b_HTML.gif


    Figs. 16.1, 16.2, and 16.3
    Diagrammatic representations of the embryo at 3, 4 and 5 weeks of intra-uterine life. Note the developing allantosis and omphalomesenteric duct


  • 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.


16.3 Classification




Jul 10, 2017 | Posted by in UROLOGY | Comments Off on Urachal Remnants
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