2020, Volume 36, Number 3, Page(s) 256-260
Sirenomelia (Mermaid Syndrome): A Case Report
Şirin KÜÇÜK1, İzzet Göker KÜÇÜK2
1Department of Pathology, Faculty of Medicine, Usak University, UŞAK, TURKEY
2Spec. Dr., Kemal Öz Family Health Center, UŞAK, TURKEY
Keywords: Sirenomelia, Fusion of lower extremities, Caudal regression syndrome
Sirenomelia, which is also known as mermaid syndrome and characterized by the fusion of the lower extremities, is the most severe form of
caudal regression syndrome and one of the rare and lethal congenital malformations. The anomalies that might be seen in this syndrome include
pelvic-sacral dysplasia, genital anomalies, bilateral pelvic renal fusion accompanied by renal dysplasia, colon atresia, unilateral umbilical artery,
and imperforated anus. The incidence of sirenomelia is 0.8-1 cases in 60,000-100,000 deliveries and the male/female ratio is 2.7-3:1. The case
reported in the present study was a 13-week-old male fetus 30 g in weight with a macerated appearance. The upper extremities had a relatively
normal appearance but the lower extremities were conjoined and there was a single lower extremity consisting of conjoined feet and toes. In
the face, the nasal bridge was sunken, the ears had a low position, and there were cleft palate and cleft lip. Examination of the external genital
organs revealed that the penile part was in the anal region. There was no anus opening. The crown-rump length was 8.5cm, the heel-toe length
was approx. 1cm, and the rump-heel length was approx. 3.7cm. There were none of the two kidneys, ureter, bladder, urethra, or rectum. In the
umbilical cord, there were 2 venous structures, one of which was the artery. Perivillous congestion and hyperemia, perivillous calcification,
deciduitis, and focal infarct regions were observed in placental tissues. This report aims to discuss this very rare case together with the literature.
Sirenomelia is a rarely seen and fatal disease with severe
malformation in lower extremities, in which the legs are
attached to each other and there are various anomalies related
to the visceral organs. Some of the most frequently affected
organs is the genitourinary system and the gastrointestinal
. Its worldwide incidence ranges between 1.1
and 4.2 per 60,000-100,000 deliveries 1
. These fetuses
generally die either during the pregnancy or immediately
after the delivery. The syndrome is more frequently seen
in one of the monozygotic twins 3,18
. Although its
etiology is not exactly known, maternal diabetes, genetic
susceptibility, and vascular hyper-perfusion are considered
among the causes. Although a possible mechanism is that
the extremity bud cannot separate from the primordial
cells due to the lack of the posterior mesoderm axis and
is unable to rotate during the organogenesis period, it
has then been shown that the defect develops in the early
period of vascular development. Two main causes were
claimed for its pathology: a developmental anomaly in
the veins feeding the lower extremities and an anomaly in
mesodermal cell migration 1,3,10-12,14,16-19
. Based on
the type of fusion, sirenomelia is classified as sympus dipus
or symmelia, sympus monopus or uromelia, and sympus
apus or sirenomelia 1-5
. In the literature, surviving cases
have been reported very rarely 2,11
. The present case is discussed together with the literature since the condition
A 30-year-old pregnant women at the 13th
week (G2, P1, A0, L1) presented at the emergency service
with complaints of vaginal bleeding, inguinal pain, and
nausea. On physical examination, vaginal bleeding and
sensitivity at the suprapubic region were detected. The
other examination findings were normal and there was
nothing characteristic in the patients history. Obstetric
ultrasonography (USG) was performed. CRL was found
to be 60 mm, which is compatible with 12 weeks and 3
days. There was no cardiac or fetal activity, the amniotic
fluid was significantly decreased, and the internal os was
closed. No characteristic findings were observed in whole
blood, biochemistry, hormone, TORCH, and ELISA
tests. Among the coagulation tests, aPTT was found to be
slightly prolonged. The blood type was 0 Rh (+). Based
on these findings, the patient was diagnosed with in utero
mort fetalis (I.U.M.F.) and hospitalized at the obstetrics
and gynecology clinic. Revision-curettage (R/C) was
performed. The curetted material was sent to pathology.
No bleeding or complication was observed during the
follow-up period. In the macroscopic examination,
the fetus was found to have a completely macerated
appearance, 30 g of weight, a hydropathic head, umbilical cord rotating around the neck several times, relatively
normal upper extremities, conjoined lower extremities,
and a single lower extremity with conjoined toes and feet.
Examining the external genital organs, it was observed
that the penis was in the anal region. There was no anus
opening (imperforated anus). In the face, the nasal bridge
was sunken and the ears were low in position, besides the
cleft palate and cleft lip (Figure 1A,B
). The crown-rump
length was measured as 8.5 cm, the heel-toe length approx.
1 cm, and the rump-heel length approx. 3.7 cm. There were
none of two kidneys, ureter, bladder, urethra, or rectum.
There was a single testis (Figure 2
). The organs were found
to be immature. There were several hemorrhagic foci on the placenta. On microscopic examination, necrosis and
widespread cytolysis were observed in the liver, intestine,
bladder, the single testicle, and the heart in identifiable
regions. There were two venous structures in the umbilical
cord. One of them was the artery and the other one was
the vein (Figure 3
). Perivillous congestion and hyperemia,
perivillous calcification, deciduitis, and focal infarct
regions were observed in the placental tissues. According
to POTTERs 2007, the fetus was found to correspond to
the 12th-13th gestational week. Based on the microscopic
and macroscopic findings, the case was diagnosed as
Click Here to Zoom
|Figure 1: A-B) Sirenomelic fetus having lower extremities attached as single extremity, fused fingers and feet, flat nasal base, low-set
ear, cleft palate, and cleft lip.
Click Here to Zoom
|Figure 3: Umbilical cord consisting of two venous structures, an
artery and a vein (H&E; x20)
The informed consent form was obtained from parents.
Sirenomelia is a rare congenital malformation, which is
seen in approx. 1/60,000-1,000,000 deliveries 1-15,16-. The number of cases reported to date is approx.
300 18,21. According to the results obtained from the
studies carried out on mice and aiming to discover how
this defect develops (the genetic basis), it was claimed that
this defect develops as a result of excessive secretion of
retinoic acid (RA) (disruption of Cyp26a1 enzyme causing
a gain of RA function) and the lack of signalization of
bone morphogenetic protein (Bmp) to the lower part of
the body among humans. Reduction of Bmp signalling
in the ventral caudal embryonic mesoderm leads to
the occurrence of several caudal defects, severe cardiac
malformations and absence of major vessels. RA and Bmp
thus regulate each other. The genetic reasons also include
balanced translocation including chromosome 16 and
triploid mosaic (16-21). In our case, no genetic analysis
was performed. There are two common hypotheses for
sirenomelia. According to the first hypothesis, the lack of
blood supply in the lower extremities develops as a result of
the abdominal and umbilical venous structure. According
to the second hypothesis, a lack of caudal mesoderm
develops as a result of a defect in blastogenesis and the
malformations in the lower portion of body develop due
to a caudal embryologic development disorder in the early
Several risk factors related with the sirenomelia
malformation are maternal diabetes, retinoic acid,
exposure to heavy metals and teratogens, genetic factors,
monozygotic twins, male gender, and mothers age <20
years or >40 years 1,3-12,18. There is a strong relationship
between sirenomelia and maternal diabetes; the relative
risk ratio is 1/200-250, and this ratio was found to be 22%
among the fetuses with anomaly 8,10,11,16-18. The male/
female ratio is 2.7-3 5,6,16-18. In the present case, none of
the known risk factors except for male gender was detected.
The most characteristic finding of sirenomelia is the fusion
of lower extremities 1-14,18. The disease has a wide
diversity ranging from the mildest form that is fusion,
in which all the lower extremity bones exist, to the most
severe type, in which there is only rudimentary bone.
Significant efforts have been made in order to classify
sirenomelia. However, the classification that is most
widely used nowadays is the one developed by Stocker and
Heifetz, which has been used since 1987 (Table I). This
classification was developed based on the presence of the
skeletal elements in the thigh and leg. In Type I, which is
the mildest form, all the bones are present and fusion is
observed only in superficial tissues. In Type VII, which is
the most severe form, there is a single bone and the legs and feet are not present 1,7,8. Although there was no
radiological finding available, the present case was found
to correspond to Type IV or sympos monopus in terms of
The visceral organ malformations seen in sirenomelia
include various grades of renal and urethral dysplasia.
Absence of one or both kidneys (frequently the total renal
agenesis), cystic malformations in kidneys, absence of the
bladder, and urethral atresia can be seen 1,6-8,12,13,16,18.
In addition to the renal agenesis, the present case also had
absence of the ureter, urethra, and rectum. Ectopic renal
tissues may be seen at various points of pelvis because of
the anomalies in the migration of metanephritic tissue. The
most important genital anomalies are the external genital
organ malformations. Either the external genital structures
are absent or they are located at a different position and the
gonads are generally not affected. In the present case, the
penis was in the anal region and not at its normal position.
However, contrary to the literature, there was a single
testis. Gastrointestinal anomalies are frequently seen and
some of these are the blind-ended colon, rectal atresia, and
imperforate anus 1,5-7,12,18. An imperforate anus was
observed in the present case. The venous malformations
seen in this disorder include a dual venous structure (an
umbilical artery and an umbilical vein) instead of a triple
venous structure (two umbilical arteries and an umbilical
vein) 1,5-8,10,12,13,18. In the present case, there was a
dual venous structure consisting of an umbilical artery and
an umbilical vein.
The differential diagnosis includes bilateral renal agenesis,
kidney malformations, single umbilical artery, megacystis,
VACTERL/VATER (vertebral defects, anal atresia, cardiac
defects, tracheo-esophageal fistula, renal anomalies and
limb abnormalities) association, and caudal regression
syndrome (CRS). Although sirenomelia may be regarded
as a severe form of the latter, the two entities represent
distinct nosologic conditions and should receive individual
genetic counseling 21.
CRS is a broad term that refers to a heterogenous
constellation of congenital caudal anomalies affecting the
caudal spine and spinal cord, the hindgut, the urogenital
system, and the lower limbs 16,17,19-21. In fact, while
CRS is hypothesized to arise from a primary defect of the
caudal mesoderm that interferes with the formation of the
notochord, the primary mechanism leading to sirenomelia
is the diversion of blood flow away from the caudal portion
of the embryo through a single vitelline artery 21. As a
result, fetuses with caudal regression show two umbilical
arteries, two hypoplastic lower extremities, and non-fatal
kidney findings with imperforate or normal anus, while
sirenomelic fetuses have renal agenesis or dysgenesis, a
single aberrant umbilical artery, fused sub-extremities, and
imperforate anus 20-21. We diagnosed our patient with
sirenomelia due to the presence of single umbilical artery,
a single testis, imperforate anus, fused lower extremities,
cleft palate, cleft lip, total renal agenesis, and absence of the
bladder, ureter, urethra and rectum.
The other anomalies that might be seen in sirenomelia
include sacral agenesis, absence of gallbladder and spleen,
omphalocele, lordosis, malformed vertebra-hemivertebra,
and central nervous system (CNS) anomalies, as well as
less frequently observed ones such as cleft palate, upper
thoracic and cervical vertebra anomalies, pulmonary
hypoplasia, and cardiac anomalies 1,6-8,12,13,16-19.
In the literature, adrenomegaly has been detected in the
case of a baby with a diabetic mother 15. In the present
case, there were sunken nasal bridge, fallen ears, cleft
palate, and cleft lip. Ultrasonography (USG) performed
in the antenatal period (end of the first trimester) is very
important in the diagnosis of sirenomelia. If possible, 3D
USG should be preferred over 2D USG. This is because the
skeletal anomalies can be better detected using 3D USG
3,13. The absence of fetus movements; adorsal, lumbal, or
sacral vertebral gap; short craniocaudal length, and lower
extremities with an abnormal appearance can be detected
with USG. Oligohydramnios is the finding that is easiest
to detect 3,16. In the present case, the pregnant woman
was not followed-up on a regular basis and she applied to
the emergency unit with complaints of vaginal bleeding,
inguinal pain, and nausea. No cardiac or fetal activity was
observed, the amniotic fluid volume was significantly low
(oligohydramnios), and the internal os was closed. Revision
curettage was performed. It was therefore not possible
to make an early diagnosis using USG images and the
diagnosis was made based on the pathological examination.
In surviving sirenomelia cases, treatment can be provided
with a multidisciplinary approach. The conjoined legs can be surgically separated. During the preparation for surgery,
subcutaneous tissue expanders such as balloons can be
placed and the skin stretched and prepared for enlargement
by filling the balloons with saltwater. The enlarged skin is
used during surgical separation 6,8,11.
In conclusion, the diagnosis of sirenomelia can be made
using ultrasonography performed at the end of the
first trimester. Among the ultrasonographic findings,
oligohydramnios and lower extremity defects can be useful
in the diagnosis. The pregnancy may be terminated in cases
of a fetus with a severe anomaly that is inconsistent with
CONFLICT of INTEREST
The authors report no conflicts of interest.
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