Sirenomelia is a rare lethal malformation characterized by severe anomalies of the caudal part of the fetus. The prevalence has been estimated at about 1 in 100,000 births. Malformations include a single lower limb, with various degrees of involvement ranging from single to separate femurs in the same skin shaft, presence of two feet (sympode mermaid) or one foot (monopode mermaid), to absence of both feet (ectromelic mermaid). Urogenital anomalies are also present and include bilateral renal agenesis, absence of outflow tract and absence of external genitalia. Imperforate anus and sacro-coccygeal agenesis have also been reported. Together these malformations comprise the extreme form of the caudal regression sequence. Sirenomelia is usually a sporadic condition but rare familial recurrences have been described. Differential diagnosis includes VATER association as the malformations comprising this syndrome (radial aplasia, tracheo-esophageal atresia, cardiopathy, and spina bifida)are among the anomalies described in sirenomelia.
Last update: August 2005