Sirenomelia, also known as “mermaid syndrome,” has been noted since the Greco-Roman period. Initially described in the medical literature by Rocheus in 1542 (Murphy et al., 1992), the condition is characterized by a single lower extremity, with the concomitant presence of severe anomalies of the urogenital and gastrointestinal system. Although historical and mythological accounts portray sirens and mermaids as females, the majority of patients with sirenomelia are male (deJonge et al., 1984). In all likelihood, the confusion originated as a result of the fact that most patients with sirenomelia had no obvious external genitalia.

It is often stated that sirenomelia is characterized by apparent fusion of the lower limbs. This terminology is embryologically incorrect because fusion refers to two processes joining after breakdown of intervening epithelia. Merging is the more correct term, because it does not imply intervening breakdown of epithelium. Sirenomelia, therefore, is a syndrome of merging, malrotation, and dysgenesis of the lower extremities (Kapur et al., 1991).

In 1865, Förster classified sirenomelia into three groups according to the number of feet present (van Zalen-Sprock et al., 1995). In symelia apus, the most common of the three conditions, both legs are merged completely into a single lower extremity. Both feet are absent or rudimentary. On radiographic studies, only one femur is present; there are no fibulae and one or two tibulae. In symelia unipus, 1 foot is present and up to 10 toes also may be seen (see Figure 87-1). In this type of sirenomelia, two femora, two tibia, and two fibulae are present. In symelia dipus, two distinct feet are present, although generally they are malrotated, and often give the appearance of fins. Stocker and Heifetz (1987) further classified sirenomelia according to seven types. In type I, all thigh and leg bones are present. In type II, there is a single fibula. In type III, there is an absent fibula. In type IV, the femurs are partially fused and the fibulae are fused. In type V, the femurs are partially fused. In type VI, there is a single femur and a single tibia. In type VII, there is a single femur and an absent tibia.

In 1961, Duhamel coined the term caudal regression syndrome and first recognized an association between caudal regression and lower extremity abnormalities. He noted the presence of additional associated anomalies in sirenomelia, including sacral agenesis, anorectal atresia, renal agenesis, single umbilical artery, and ambiguous genitalia (Duhamel, 1961).

The cause of sirenomelia is still debated. Most agree that the site of injury to the embryo is the caudal mesoderm (Murphy et al., 1992). Sirenomelia probably results from a localized insult to the caudal end of the developing embryo between days 13 and 22 of development (Hoyme, 1988). By 23 days of embryonic age, the prospective limb bud regions normally assume a lateral position, separated by allantoic structures. If the lower extremity remains unipodal, the insult must have occurred prior to this point in gestation (Stevenson et al., 1986). Two theories have been developed to explain the occurrence of sirenomelia: vascular and caudal injury. In 1927, Kampmeier reviewed 52 cases of sirenomelia and was the first to note the constant finding of a single umbilical artery. He postulated that the single umbilical artery was very important in the etiology of the sirenomelia and that a vascular abnormality would affect the allantoic circulation with subsequent abnormal development of caudal elements. More recently, Talamo et al. (1982) performed postmortem arteriography on an infant with sirenomelia and documented a persistent vitelline artery and a dorsal hypoplastic distal aorta. They demonstrated that the femoral arteries ran anteriorly and posteriorly along the single femur, rather than in the normal orientation of right and left. They thought that this demonstrated a failure of rotation during early embryonic life and supported the pathogenetic concept of limb bud merging and malrotation after damage to the posterior axis mesoderm. Stevenson et al. (1986) dissected the abdominal vasculature in 11 cases of sirenomelia and documented the common feature of a single large artery arising from high in the abdominal cavity, which they thought diverted nutrients from the caudal end of the embryo in a vascular “steal” phenomenon. The steal vessel was a derivative from the vitelline artery complex, an early embryonic vascular network important in supplying nutrients to the yolk sac. They showed that the fetal arteries below the level of this steal vessel were underdeveloped; the tissues dependent on the normal vasculature for supply of nutrients subsequently failed to develop. This study demonstrated that in sirenomelia, the celiac and superior mesenteric arteries are usually present and normal in size. The inferior mesenteric artery may be missing. If present, it is very small and supplies the distal colon, which usually ends blindly in sirenomelia. In all the cases described by Stevenson et al. (1986), the major abdominal artery was a derivative of the vitelline artery complex; this was the primary artery carrying blood from the fetus to the placenta. Of note, the abdominal aorta distal to the origin of the vitelline artery gave off no tributaries. However, there were renal or inferior mesenteric arteries present proximal to the aortic bifurcation into the iliac arteries. These authors stated that the presence of the vitelline artery alone did not predispose to sirenomelia; rather, it was the stealing of nutrients normally intended for the caudal structures of the body that increased the risk for development of sirenomelia. While single umbilical artery is quite common and occurs in 1% of all livebirths (see Chapter 109), it was the unique point of origin of this single umbilical artery just distal to the celiac axis that predisposed to the caudal steal (Stevenson et al., 1986).

Any hypothesis regarding sirenomelia must also explain the association between this condition and monozygotic twinning (Young et al., 1986). Vascular abnormalities are common in monozygotic twin gestations; thus, development of a vascular steal sequence could be related to abnormalities in development of the placenta occurring in early monozygotic twinning. Alternatively, an abnormal division of the embryo could potentially result in a deficiency of caudal determinants in one of the embryos (Kapur et al., 1991).

The incidence of sirenomelia is 1 in 60,000 livebirths (deJonge et al., 1984). A male predominance exists, with a male-to-female ratio of 2.7:1. A strong association between sirenomelia and all caudal regression defects has been noted with maternal diabetes (Kucera, 1971; van Zalen-Sprock et al., 1995). As previously noted, there is a 100- to 150-fold increase in the incidence of sirenomelia in monozygotic twins as opposed to singletons (Smith et al., 1976; Wright and Christopher, 1982; Di Lorenzo et al., 1991; McCoy et al., 1994). In all cases of sirenomelia, 8% to 15% are associated with monozygotic twinning (Stocker and Heifetz, 1987). The association might be even more common than suspected, as at least one case of sirenomelia with a vanishing twin has been reported (Kapur et al., 1991). One case has been described in association with an in vitro fertilization triplet gestation and early intrauterine death of one of the triplets (Drossou-Agakidou et al., 2004). Sarpong and Headings (1992) reported two cases of sirenomelia associated with a history of cocaine exposure during the first trimester, as well as an 18-fold higher than normal incidence of sirenomelia in a cocaine-exposed population. A case of partial sirenomelia has been reported in which the mother took large amounts of vitamins A and E during the periconceptual period (Von Lennep et al., 1985).

The diagnosis of sirenomelia may be easier to make in the first trimester as the amniotic fluid volume is relatively normal. The earliest reported diagnosis was made at 9 weeks of gestation in which an intra-abdominal cystic structure was observed (Schiesser et al., 2003). Subsequent scans at 11 and 13 weeks demonstrated fusion of the lower limbs. In another report, two cases of sirenomelia diagnosed in the first trimester were associated with increased nuchal translucency measurement (Monteagudo et al., 2002).