Synomyms: Mermaid syndrome, symmelia, uromelia

Prevalence: 0.01 - 0.16/10,000 live births, M3:F1
Definition: Fusion of the lower extremities
Etiology: Unknown, probably part of the caudal regression syndrome.
Pathogenesis: Probably ischemia of the lower part of the body.
Associated anomalies: Potter"s facies, single umbilical artery, imperforate anus, bilateral renal age­nesis, absent or ambiguous external genitalias, vertebral defects, cardiac anomalies, abdominal wall defects, chest deformities, anhydramnios.
Differential diagnosis: None for the limb fusion.
Prognosis: Fatal

Recurrence risk: Not increased.
Management: As for disorders with fatal outcome.

MESH Ectromelia-diagnosis; -etiology BDE 3191 POS 3399 ICD9 755.3 CDC 755.360

* Address correspondence to Danilo ­Dordoni, MD 2nd Department of Obstetrics Gynecology, Brescia University School of Medicine, Spedali Civili 25100 Brescia, Italy (§) Department of Radiology and Radiological Sciences, Ultrasound Section, Vanderbilt University Medical Center, 21st and Garland Nashville, Tennessee 37232-2675 USA

Introduction

Sirenomelia is a very rare malformation characterized by a complete fusion of the lower limbs commonly associated with gastro­intestinal and urogenital malformations. The incidence is 0.01 - 0.16/10,000 live births¹. Because of associated anomalies, sireno­melia is usually incompatible with postnatal life. Approximately 300 cases have been reported in the literature, with a male to female ratio of 3:1.

Case report

A 27-year-old gravida 3 para 1 was referred to our department for evaluation of a 22-week gestation. Her past medical history included one miscarriage early in the pregnancy and one premature birth who is alive but blind. The patient also had a history positive for dia­betes mellitus for the past several years which had been poorly controlled.

A sonogram performed elsewhere revealed severe oligo­hydr­amnios, bilateral small fetal kidneys, absence of fetal legs, and lack of fetal movement. Our examination also demonstrated no fetal movement over 25 minutes and IUGR with parameters corresponding to 17-18 weeks. The anatomic study of the fetus de­monstrated a fusion of the lower extremities with two femurs, two tibias, and a single fibula with two feet (fig. 1).

Fig. 1: The lower extremities demonstrate closely apposed femurs, two tibias, and a single fibula projecting over the distal right tibia. Note that the fibula is medial because of the failure of rotation of the legs. (RF = right femur, LF = left femur, RT = right tibia, LT = left tibia, F = common fibula).

The legs were completely extended in a pike position over the head. The diagnosis of sireno­melia was made. At the level of the kidneys, bilaterally enlarged polycystic structures were identified (fig. 2).

Fig. 2: Left: At the level of the kidneys, bilaterally enlarged hypoechoic structures were identified. They were thought to represent enlarged adrenal glands (in view of the common association with bilateral renal agenesis) but represented polycystic kidneys (right).

These were misdiagnosed as adrenal glands because of the high incidence of renal agenesis associated with sireno­melia. Because of the usual incompatibility with postnatal life, termination of pregnancy was decided upon by the parents. The gross appear­ance of the fetus is shown in fig. 3, and the unusual presence of external genitalia can be recognized in fig. 4.

Fig. 3: Gross appearance of the fetus. Note the supination of the fused legs and the single foot.

Fig. 4: Male genitalia.

The autopsy findings are listed in Table 1.

Table 1: Associated anomalies in propositus

• sirenomelia sympodia (one fused foot)
• small omphalocele containing only loops of bowel
• two-vessel  umbilical cord
• umbilical artery arising from the superior mesenteric artery
• cord cyst (allantoic or omphalomesenteric)
• male fetus (penis and testicles)
• imperforate anus
• partial duplication of the small bowel
• short colon
• Potter facies with low set misshapen ears
• lung hypoplasia
• bilateral multicystic kidneys

Discussion

"Symelia" is the fusion of the lower extremities,² and it has been classified into three types: 1) Apus- no feet, only one tibia and one femur, 2) Unipus- one foot, two femora, two tibiae, two fibulae, and 3) Dipus- two feet and two fused legs (giving the appearance of a flipper). This latter type is also called "mermaid syndromeâ€.

Etiology

The precise etiology of sireno­melia is not well understood. Many theories have been proposed, but none of these are considered conclusive. Sirenomelia has been reported as a result of teratogenic agents⁶. Hibelink et al.⁶ have demonstrated that an intravenous administration of cadmium and lead can produce sirenomelia in the golden hamster. In experimental studies this malformation has been induced in the chicken embryo by irradiation of the axial portion of the caudal region⁷ and in hamsters treated by retinoic acid, the acid form of vitamin A.⁸ VonLennep et al.⁸ described a case of partial si­re­no­melia and discus­sed a possible teratogenic effect of vitamin A.

Some authors consider this an­omaly as one of the clinical mani­festations of the "caudal regression syndrome†that is a consequence of an abnormal development of structures derived from the caudal mesodermal axis of the embryo before the fourth week of gestation, and extended to various craniocaudal levels.⁹ This leads to an absence of genitalia (except for gonads) and renal agenesis when paramesonephric and meso­ne­phric ducts are involved. According to Barr,¹⁰ if the mesonephric ducts progress enough to reach the ureteric buds and penetrate the metanephric blastema, the kidneys can be formed. Sometimes the meta­ne­phric part of the intermediate meso­derm is damaged or defective; in this case, hypoplastic or abnormal kidneys may be present such as in our case.

Three pathogenic theories have been proposed to explain this malformation, including: 1) a pressure theory³, 2) primary failure in the development of caudal somities that leads to defective development of the lower parts of the embryo³, and 3) a lack of nutritional support³ to the caudal region of the body"s embryo. Stevenson et al.³ proposed a "vascular steal†theory to explain the various abnormalities present in sirenomelia fetuses. Only one umbilical artery is present in those fetuses, and anomalies in the vascular system of the lower part can lead to abnormal development. Blood is shunted from the caudal portion of the embryo to the placenta and, in consequence to the lack of perfusion, the lower somites do not develop normally.

Associated anomalies

Anomalies associated with sireno­melia in order of frequency are: Facial anomalies (Potter, face), single umbilical artery (usually the right), imperforate anus, genitourinary system agenesis, no external genitalia or ambiguity, lower sacral/vertebral defects, cardiac anomalies, abdominal wall defects, malformed thorax, and usually severe oligo- or anhydramnios. In another report³ in which 11 fetuses are described, the authors found, in two cases, cystic kidneys and in two other cases, one "phallus†was present in each. Sireno­melia has also been described in association with maternal diabetes⁴ and monozygotic twins¹ (the incidence is 150 times greater than in singleton). Anencephaly and sireno­melia have been report­ed also in cases of dizygotic twins⁵.

Differential diagnosis

When unexplained oligo-anhydramnios is observed in the second trimester of pregnancy, in the absence of PROM, the diagnosis of sireno­melia can be considered together with the other main causes such as Potter"s syndrome, Meckel-Gruber syndrome, polycystic kidneys, obstructive uro­pathies and severe IUGR.

The differential diagnosis may be difficult when a "mermaid syn­drome†(the most rare) with cystic kidneys is present. Because oligo-an­hydramnios is almost always present, the injection of saline solution in the amniotic cavity may improve the visualization of the fetal anatomy. Sirenomelia is differentiated from these conditions by the anomaly of the leg: reduced numbers of long bones, abnormal proximity of the femurs, and absence of motion of the legs.

Obstetrical Management

Cases reported in the medical literature have always been associated with neonatal death. The only reference we found of a survival was in a non-medical journal¹¹. Presumably, this fetus survived because of a normal renal function and therefore normal amniotic fluid level. Except for this case this anomaly is usually not compatible with a postnatal life as a consequence of the renal agenesis and consequential lungs hypoplasia; for these reasons, termination of pregnancy should be considered.

Acknowledgements

We appreciate the help with this case from Philippe Jeanty, MD, Ph.D.

References

1. Smith DW, Jones KL: Recognizable patterns of human malformation. Genetic, Embryologic and Clinical Aspects. 2nd Ed. Saunders, Philadelphia, London, Toronto 486-487, 1976.

2.Forster A: Die missbildungen des meuschen, nebst einem atlas. Jena, Friedrich Manke, 1861, 1865.

3. Stevenson RE, Jones KL, Phelan MC et al: Vascular steal: the pathogenetic mechanism producing sirenomelia and associated defects of the viscera and soft tissues. Pediatrics 78:451-7, 1986.

4. Schwaibold H, Ochler U, Helpap B and Böhm N. Sirenomelia and anencephaly in one of dizygotic twins. Teratology 34:243-247, 1986.

5. Pfeiffer RA, Becker V. Teratology (letter) 38:497-498, 1988.

6. Hilbelink DR, Kaplan: Sirenomelia: Analysis in the cadmium and lead-treated golden hamster. Teratology Carcinog 6:431-440, 1986.

7. Sirtori M, Ghidini A, Romero R, Hobbins JC: Prenatal diagnosis of sirenomelia. J Ultrasound Med 8:83-88, 1989.

8.VonLennep E, Elkhazen , DePierreux G, et al: A case of partial sirenomelia and possible vitamin A teratogenesis. Prenatal Diagnosis 5:35-40, 1985.

9. Kallen B, Winberg J: Caudal mesoderm pattern of anomalies: Transrenal agenesis to sirenomelia. Teratology 9:99, 1973.

10. Barr M. Teratology (letter) 38:487-488, 1988.

11. Weekly World News, Sept 12 1989, p3.