Definition: Congenital anomaly, caused by a disruptive vascular defect, characterized by fusion of the lower extremities, associated with renal agenesis, absence of the sacrum, rectum, and bladder. It was considered in the past to represent a severe form of caudal regression syndrome.

Synonyms: Mermaid syndrome, sirenomelia sequence.

Etiology: Although the etiology is unknown, sirenomelia is not thought to be hereditary.

Recurrence risk: Unknown.

Diagnosis: The diagnosis of sirenomelia is based on the presence of fusion of the lower extremities (fig. 1), associated with other skeletal and lumbar spine deformities, bilateral renal agenesis (which leads to severe oligohydramnios and lungs hypoplasia), heart and abdominal wall defects,[2]. The defect varies from simple cutaneous fusion of the limbs to absence of all long bones but one femur. The defect of the feet is proportional with the defect of the long bones, with cutaneous defect commonly presenting with a double fused foot with 10 toes, and more severe defect presenting with a more rudimentary foot and ectromelia. Since the legs are fused the rotation of the legs does not occur and they remain in their fetal position. Thus the fibulae when present are between the tibia and the sole of the foot is oriented "ventrally†instead of "dorsallyâ€.

Figure 1: The fused extremity and the flipper-like deformity of the foot are characteristic of sirenomelia

Pathogenesis: An alteration in early vascular development, leads to a "vitelline arterial stealâ€, in which blood flow is diverted from the caudal region of the embryo to the placenta, resulting in multiple defects of the lower extremities[3]. Many of these fetuses have an aberrant vasculature with the umbilical arteries connected to the old viteline arteries (the superior mesenteric arteries).

Genetic anomaly: Unknown.

Associated anomalies: Cardiac, renal, abdominal wall, chest, and lower spine defects are frequently seen. Single umbilical artery, imperforate anus, and absence of the genitals are commonly found.

Differential diagnosis: Caudal regression syndrome is the main differential diagnosis, and it usually presents with milder deformities compared to sirenomelia and normal amniotic fluid volume. Due to the bilateral renal agenesis, fetuses affected by sirenomelia frequently have Potter facies. The fusion of the lower extremities, present in sirenomelia, makes the differential diagnosis. Other conditions that should be excluded are Fraser and Vater syndromes1.

Prognosis: This is a lethal condition, due to the associated renal agenesis and its complications. Exceptional cases present without renal agenesis and may survive.

Management: Termination of pregnancy can be offered before viability. Standard prenatal care is not altered throughout the pregnancy if continuation is opted for.

Reference:

[1] Benacerraf BR. Caudal regression syndrome and sirenomelia in ultrasound of fetal syndromes -  Churchill Livingstone – New York – 1998, 250-254.

[2] Jones KL. Sirenomelia sequence in Smith’s recognizable patterns of human malformation. WB Saunders Company – Philadelphia – 1998, 634.

[3] Stevenson R E, et al. Vascular steal: the pathogenic mechanism producing sirenomelia and associated defects of the viscera and soft tissues. Pediatrics, 1986, 78: 451.