Sirenomelia. Pathological features, antenatal ultrasonographic clues, and a review of current embryogenic theories

We aimed to discuss the prenatal diagnosis and pathological features of sir enomelia, and to review current embryogenic theories. We observed two siren omelia fetuses that were at the 19th and 16th gestational week respectively , In the former, transvaginal ultrasound revealed severe oligohydramnios an d internal abortion, whereas bilateral renal agenesis, absence of a normall y tapered lumbosacral spine, and a single dysmorphic lower limb were detect ed in the latter. In both cases, X-ray and autoptic examination allowed cat egorization on the basis of the skeletal deformity, Subtotal sacro-coccygea l agenesis was present in both cases. Agenesis of the urinary apparatus and external genitalia and anorectal atresia were also found. Classification o f sirenomelia separately from caudal regression syndrome is still debated. Recent advances in the understanding of axial mesoderm patterning during ea rly embryonic development suggest that sirenomelia represents the most seve re end of the caudal regression spectrum. Third-trimester ultrasonographic diagnosis is usually impaired by severe oligohydramnios related to bilatera l renal agenesis, whereas during the early second trimester the amount of a mniotic fluid may be sufficient to allow diagnosis. Early antenatal sonogra phic diagnosis is important in view of the dismal prognosis, and allows for earlier, less traumatic termination of pregnancy.