[MIM 600 145]

Extremely rare: < 1/100.000 fetus. Affects 2.7 boys for 1 girl. Severe anomalies of the lower part of the embryo. Clinically, only one single lower limb is present: a single lower limb with a single femur or two merged limbs  with fused feet (siren sympodia, sympus monopus) or a single leg (siren sympodia, sympus monopus) or without any foot (siren ectromelia, sympus apus). In some cases, there is only a rudimentary bottom end of the body without any bone structure.

Several subtypes are defined (Stocker and Heifetz):

-        I: 2 femurs, 2 tibias and 2 fibulas are present

-        II: 1 single fibula

-        III: absence of fibula

-        IV: partially merged tibias and femurs

-        V: partially fused femurs

-        VI:a single femur and single tibia

-        VII: 1 single femur, no tibia

Renal agenesis is often associated which then makes the defect incompatible with life. Other malformations can be combined such as: oligohydramnios and Potter facies, absence of anus, VACTERL (see this term), heart disease, spina bifida

Often considered as an extreme form of caudal regression (see this term) but the pathogenesis is different: the persistence of the vitelline artery leads to poor development of the abdominal aorta and vascular steal to the placenta. Toxic external factors could be involved in some cases (cfr "epidemic" in Colombia in 2004-5)

Anesthetic implications:

evaluation of associated malformations: kidneys, heart etc...

References:

-        Lakshmi H, D, Loget Herv‚ P, Paul P.
Diagnostic anténatal d'une sirénomélie..
J Gynecol Obstet Biol Reprod 2006; 35: 181 - 5.

-        Gerard M, Leigh V, T, Roumazeilles Costa Y et al.
Sirenomelia and caudal abnormalities in two families.
Am J Med Genet Part A , 158 A: 1801-7.

Updated: September 2018