Perinatal/Neonatal Case Presentation

Journal of Perinatology (2004) 24, 329–331. doi:10.1038/

Sirenomelia with an Angiomatous Lumbosacral Myelocystocele in a Full-term Infant

Marybeth Browne MD1, Philip Fitchev MD2, Brian Adley MD2 and Susan E Crawford MD2

  1. 1Department of Surgery, Children's Memorial Hospital, USA
  2. 2Department of Pathology, Feinberg School of Medicine, Northwestern University, Chicago, IL, USA.

Correspondence: Susan Crawford, MD, Feinberg School of Medicine, Northwestern University, Department of Pathology, 303 E Chicago Ave, w127, Chicago, IL 60611, USA.



Sirenomelia, also known as the mermaid syndrome, is a rare congenital malformation of uncertain etiology. It is characterized by fusion of the lower limbs and commonly associated with severe urogenital and gastrointestinal malformations. In this report, we describe the first case of an infant with sirenomelia and a massive angiomatous lumbosacral myelocystocele.



Sirenomelia, also known as the mermaid syndrome, is a rare congenital anomaly characterized by lower limb fusion, and associated with severe urogenital and gastrointestinal malformations. With its physical resemblance to the mystical mermaid, the term "sirenomelia" was derived from the sirens of Greek and Roman mythology.1 Although the time of insult is known to be between 28 and 32 days of gestation, the pathogenesis of this malformation remains unclear, with some studies implicating caudal regression or vascular steal phenomenon.1,2,3 We will describe the first report of a fetus with sirenomelia and a massive angiomatous lumbosacral myelocystcele.



A 40-week, 1760-g term female infant was born to a 22-year-old primigravida mother with no significant past medical history. During the prenatal ultrasound, oligohydramnios was noted as well as multiple congenital anomalies including an absent lower limb, absent bladder and kidneys, a malformed pelvis, scoliosis, a large cystic mass in the lumbosacral region of unknown etiology, and a two-vessel umbilical cord. Cytogenetic analysis at that time revealed a normal female fetus (46, XX). The infant was delivered at full term with Apgar scores of 0,0. A complete autopsy was performed.

Gross examination of the infant displayed multiple external deformities including a single lower extremity with a single digit and absence of external genitalia or anus. Additionally, there was an obvious right-sided hypoplasia of the iliac bone, sacral agenesis, multiple rib deformities, Potter's facies, left-sided ear appendage, and a large cystic lesion occupying the lumbosacral region. The cyst was retrotoneal, unilocular, and measured 17 times 7 times 2 cm in its greatest dimensions (Figure 1). It extended cephalad through the soft tissues of the back and had a smooth lining filled with serous fluid. Internally, the infant was noted to have severe bilateral lung hypoplasia, blind termination of the colon with proximal dilatation, and absence of the bladder, ureters, and bilateral kidneys.

Figure 1.
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Infant with sirenomelia and lumbosacral myelocystocele. Note the single lower extremity and the large lumbosacral cystic structure, which extends cephalad through the soft tissues of the back.

Full figure and legend (118K)

Radiographic pictures revealed a single distal leg with two ossification centers consistent with a Stocker Type VI abnormality and several rib abnormalities (Figure 2). Histological evaluation of the cystic mass revealed a skin-covered lesion lined by a thin layer of neural tissue. A mixture of neural elements was identified including neurons, peripheral nerves, and ependymal tissue. Sections of the spinal cord revealed cystic dilatation around the cord with contiguous extension to the lumbosacral cyst. Marked angiogenesis and vascular congestion was apparent in adjacent fibromuscular tissue (Figure 3). No additional elements were identified excluding the possibility of a sacroccygeal teratoma.

Figure 2.
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X-ray examination of infant with sirenomelia. Note the multiple rib abnormalities, sacral agenesis, iliac hypoplasia, and single femur with two ossification centers.

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Figure 3.
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Histopathology of myelocystocele. Cystic wall demonstrates neural tissue with an ependymal lining. Note the cystic dilatation around the spinal cord and the marked angiogenesis and vascular congestion in the adjacent fibromuscular tissue (H&E, 10 times).

Full figure and legend (161K)



Sirenomelia is a rare congenital malformation, which occurs in 1.05 out of 100,000 births.4 It is described as being three times more common in males, having a higher incidence in identical twins, and having a slight increase in risk with maternal age of less than 20 years and older than 40 years.4,5 This syndrome was once thought to be uniformly fatal; however, since 1989, there have been four reports of children surviving with sirenomelia.6 The abnormalities found in these surviving children were variable and did not include renal agenesis; thus, demonstrating the unpredictable heterogeneity of the associated anomalies.6,7

Duhamel was the first to describe the associated anomalies of sirenomelia. He included sacral agenesis, imperforate anus, colonic or rectal atresia, renal agenesis, absent bladder, and absent internal genitalia.2 As a result of the renal agenesis, the description has come to include Potter's syndrome, which consists of Potter's facies (large low-set ears, epicanthal folds, hypertelorism, flattened nose, and receding chin), oligohydramnios, and pulmonary hypoplasia.4,8 In most cases, sirenomelia is found to have a single umbilical artery, considered a true artery with an abnormal course. The unique point of origin for the single umbilical artery from the dorsal aortic trunk distinguishes this condition from others.1

There is great variability in the extent of the malformations within sirenomelia. Stocker and Heifetz1 classified the leg abnormalities into Types I through VII. Type I is the least severe with all bones present; and Type VII, most severe, consisting of a lower extremity with a fused femur and absent tibia and fibula.1 Our infant had a Stocker Type VI leg abnormality, the associated urogenital and gastrointestinal anomalies, and a lumbosacral myelocystocele. There have been three sirenomelia infants with meningomyeloceles; two with Arnold–Chiari's malformation and one with significant hydrocephalus.1,3 This fetus had an unusual variant of a terminal or lumbosacral myelocystocele whereby the massive cyst was associated with sacral agenesis and spinal dysraphia with cystic dilatation around the spinal cord.9

There have been many theories about the etiology of the mermaid syndrome. In ancient times, many postulated about supernatural events, to consorting with a merman or an evil spirit, or to blaming a maternal experience or observation during gestation.3 Currently, there are two major theories. First, Duhamel described the caudal regression syndrome to explain various congenital anomalies, sirenomelia being the most severe form. He postulated that a small localized lesion would lead to anal imperforation and mild vertebral anomalies; larger lesions would lead to urinary tract and gastrointestinal malformations; and lastly, extreme lesions would cause lower limb fusion and anomalies associated with sirenomelia.2 This theory was popular for many years until Stevenson described an alternate theory of vascular steal that has recently taken favor over caudal regression for the mermaid syndrome. Stevenson et al.3 explain that in the mermaid syndrome, blood is diverted from the caudal region of the embryo to the placenta producing a nutritional deprivation and abnormal development of the caudal structures. The site at which the steal occurs determines the severity of the anomalies.3 The mechanism of this discrepancy is not fully understood. Both theories may be oversimplifications of the true etiology of sirenomelia, since neither explains the noncaudal anomalies, which have been seen in some instances.7 Few have noted drug use in mothers of sirenomelia children; however, no substance has ever been proven to be the teratogenic cause of sirenomelia.1,6,8 Maternal diabetes is 200 times more likely in children with caudal regression syndrome; however, Stocker has failed to confirm this association in his series, where maternal diabetes was rarely seen in connection with sirenomelia.1,10

Early antenatal diagnosis should be made prior to the dramatic decrease in amniotic fluid.5 This is achieved by sonographic demonstration of constant simultaneous movement of the lower limbs, skeletal abnormalies in the lower extremities, and oligohydramnios related to kidney agenesis.5,11 The progressive oligohydramnios, noted in the second trimester, can be the first sign of this malformation; however, with severe oligohydramnios, a prenatal diagnosis may be difficult.5,11 This is the first reported case of a sirenomelia with a massive angiomatous lumbosacral myelocystocele. Despite a cystic structure being noted on prenatal ultrasound, the size of the mass obscured the ability to render a definitive diagnosis until postmortem evaluation.



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