Keywords
prenatal diagnosis - persistent cloaca - monoamniotic twins - fetal lung development
- discordant
Cloaca is one of the most complex and severe degrees of anorectal malformations in
girls, in which the vagina, rectum, and urinary tract merge into a common channel
leading to a single external orifice. We report a case of cloaca prenatally diagnosed
in one of a set of monoamniotic twins. This observation underlines the difficulties
of making a precise prenatal diagnosis and confirms the impact of an adequate amniotic
fluid volume on fetal lung development.
Case Report
A 21-year-old mother, gravida 2, para 0, presenting with a spontaneous monochorionic
monoamniotic pregnancy was referred at 21 weeks because of a suspected urinary tract
anomaly in twin A. The couple had no personal or family history of congenital anomalies
. At successive ultrasound scans, the amnionicity and normality of the female twin
B were confirmed. Twin A presented several following anomalies ([Fig. 1]): a phallus-like structure measuring 15 mm, a pelvic midline cystic mass, an absence
of anal dimple, a flat perineum and renal anomalies combining left kidney agenesis,
and a multicystic small right kidney. Amniotic fluid volume was normal on repeat scans
(with a deepest vertical pocket at 21, 26, 32, and 34 weeks always less than 80 mm).
Fetal blood sampling performed at 22 weeks in twin A revealed a 46,XX karyotype. Fetal
serum β2-microglobulin assayed for renal function evaluation was normal (4.4 mg/L for a normal
value < 5 mg/L). Amniocentesis confirmed a 46,XX fetal karyotype. Amniotic fluid 17-hydroxyprogesterone
and digestive enzyme activities (gamma–glutamyl transpeptidase, aminopeptidase M,
and intestinal alkaline phosphatase) were normal,[1] thus excluding congenital adrenal hyperplasia and digestive tract anomalies, respectively.
Magnetic resonance imaging (MRI) of twin A showed an enlarged bladder associated with
a fluid structure between the bladder and the spine but no T1-weighted high signal
corresponding to the rectum ([Fig. 2]). Renal anomalies were confirmed. These findings were consistent with the diagnosis
of cloaca. After prenatal counselling, selective termination of pregnancy was considered
but not chosen by the parents.
Fig. 1 Three-dimensional sonogram at 29 weeks of gestation shows a phallus-like structure,
absence of anal dimple, and a flat perineum.
Fig. 2 Twin A magnetic resonance imaging: Absence of the T1-weighted high signal corresponding
to the rectum.
A cesarean section was performed at 36 weeks. The female twin B (2,010 g) had no malformation.
Twin A (2,310 g) presented a phallus-like structure measuring 11.5 mm with a single
orifice at its extremity, a flat perineum with an imperforate anus, and no vaginal
opening ([Fig. 3]). Left preaxial hexadactyly was also noted. Retrograde opacification achieved through
the single orifice demonstrated a communication between the “urethra” and a pelvic
cavity. The neonate died at day 1 due to renal failure. Postmortem examination ([Fig. 4]) revealed that the urinary tract, the genital canal, and the rectum converged into
a common channel, therefore confirming the diagnosis of cloaca. The bladder was distended
by meconium. The consequent kidney abnormalities were confirmed. A sacral vertebral
defect was discovered. These anomalies were consistent with caudal dysgenesis association.
Interestingly, the lung volumes were normal.
Fig. 3 Gross examination of the affected fetus: phallus-like structure, absence of anal
dimple, and flat perineum.
Fig. 4 Macroscopic examination of the affected fetus: the vagina, rectum, and urinary tract
converge into a common channel leading to a single external orifice.
Comment
Persistent cloaca[2]
[3] (1 per 50,000 births),[4]
[5] in which the genital canal, rectum, and urinary tract converge into a common channel
leading to a single external orifice[2]
[6] is one of the most complex and severe degrees of anorectal malformation. Hydrocolpos
is present in 50% of these patients. Furthermore, cloaca is often associated with
other anomalies, especially spinal and renal defects.
Anomalies of cloacal development involve the primitive hindgut and the caudal mesenchyme.
Cloacal septation and rotation of the cloacal membrane, which swings down from a vertical
to a horizontal position, involve two major processes[7]: active growth of the urorectal mesenchyme and fusion of the epithelial walls of
the cloaca. These steps lead to a significant caudal lengthening of the so-called
urorectal septum and produce an apparent septation of the cloaca. Later, regression
of both the tailgut and the dorsal cloaca, and ventral protrusion of the urogenital
sinus are the other major changes in the cloaca configuration. A defective cloacal
anlage could result in a missing or misplaced anal orifice and abnormal communication
between the rectum and the urogenital tract.[8] It has been suggested[9] that abnormal notochord development may induce neural tube defects and/or anorectal
malformations.
To our knowledge, our case is the third occurrence of cloaca prenatally diagnosed
in one of monozygotic twins.[10]
[11] In addition, we observed that the affected twin lacked functional renal tissue but
had a normal lung volume, therefore demonstrating that a sufficient amount of amniotic
fluid produced by a monoamniotic twin environment could prevent pulmonary hypoplasia.
This case report, such as the previous ones, provides an opportunity to highlight
the importance of adequate amniotic fluid,[12] among several other factors, for normal lung development. The normal fetal serum
β2-microglobulin we observed in the affected fetus is explained by the placental vascular
anastomoses. Similarly, in this case, amniotic fluid digestive enzymes were paradoxically
normal despite the presence of an urodigestive fistula in one of the two fetuses.[13] This might be due to dilution and swallowing by the normal co-twin, which could
have clarified the amniotic fluid. An another hypothesis could be the narrowness of
the long and thin “urethra.”
Anorectal malformations are difficult to diagnose prenatally. In a large series of
50 cases,[4] cloaca was prenatally diagnosed at ultrasound scan in only three cases. The most
common finding is a midline pelvic cystic mass located behind the bladder.[3]
[4]
[14] If the cystic mass is septated, conically shaped and/or contains sediments or calcifications,
the likelihood of a cloaca increases. Other sonographic abnormalities[3] are highly suggestive of cloaca: bowel dilatations or calcifications, genitourinary
(renal cyst, absent kidney, hydronephrosis, and ambiguous genitalia), and lumbosacral
anomalies. In addition to these findings, the failure to see the anorectal tract by
fetal sonography or MRI is very suggestive of cloaca. However, confounding diagnoses
for a female fetus must be ruled out based on ultrasound scan, and/or MRI: ovarian
cyst (usually off midline), bladder obstruction (no debris within the cystic mass),
intestinal duplication (thick echogenic wall and peristalsis), megacystis microcolon
intestinal hypoperistaltsis syndrome (megacystis and absence of meconium in the rectum),
isolated hydrocolpos, or urogenital sinus (meconium in the rectum). Amniotic fluid
digestive enzymes and cyst fluid biochemistry[13] could be of help to differentiate these diagnoses.
Congenital anomalies, concordant, or discordant, occur in approximately 10% of monozygotic
twins. Several postfertilization[15]
[16] mechanisms have been proposed to explain discordant malformations in monozygotic
twins such as chromosomal mosaicism, skewed X-inactivation, imprinting, postzygotic
dominant or recessive gene mutation, and other epigenetic processes. For discordant
cloacal dysgenesis, it has been hypothesized that there is an unequal allocation of
stem cells to each twin, related to the twinning process itself, leading to an unequal
potential of development and/or abnormal determination of laterality gradients.[15]
In conclusion, this case of cloaca in discordant monoamniotic twins underlines the
difficulties of making a precise prenatal diagnosis and of analyzing biochemical results
because of the vascular and amniotic communications between the twins. This case confirms
the impact of an adequate amniotic fluid volume on fetal lung development.
