First Trimester Diagnosis of Sirenomelia

Abstract

Sirenomelia is a rare congenital anomaly characterized by a single lower extremity which is associated with abnormalities in other organ systems, commonly affecting the gastrointestinal and the urogenital systems. It is sporadic with no increased risk in subsequent pregnancies. In almost all the cases of sirenomelia, a single umbilical artery (SUA) is present which arises from the abdominal aorta. Sirenomelia can be confidently diagnosed in the first trimester while the diagnosis in the second and third trimesters is rendered difficult due to the lack of amniotic fluid in the later gestation. Transvaginal ultrasound and color Doppler have a key role in making the diagnosis and 3D ultrasound helps in better demonstrating the abnormality.

Sirenomelia, the Mermaid syndrome: a case report

“…3,4 Sirenomelia is a sporadic condition, with normal fetal karyotyping without an increased risk of recurrence in subsequent pregnancies. 5 Genetic as well as environmental factors (nutritional and toxic) like retinoic acid, ochratoxin A, cadmium, lead and cocaine have been reported as causes for sirenomelia. 1,4 Sirenomelia was named after the creatures with the head of a woman and body of a bird from the wings downwards called 'sirens' described in Greek mythology, who narcotized the sailors with their enchanting music and voice to later kill them.…”

“…The commonest abnormality seen is single umbilical artery (SUA) of abnormal origin branching off high after the celiac branch from abdominal aorta which then abnormally narrows down and lacks a considerable number of branches resulting in impaired perfusion to the lower extremities, kidneys, urogenital tract and large intestine. 1,5,6 The pathognomonic feature of sirenomelia is this SUA of abnormal origin referred to as the persistent vitelline artery (PVA), a possible derivation from the vitelline plexus thus distinguishing it from other cases of SUA, found in about 1% of newborns, having a normal origin not related to other malformations. 1,6 The 2 main pathogenic hypotheses based on various clinical studies explaining the possible causal mechanism of sirenomelia are the vascular steal phenomena and the blastogenetic hypothesis.…”

“…This leads to the agenesis of lower midline structures which along with the subsequent abnormal approximation of both lower limb fields is responsible for the variable phenotype seen in sirenomelia. 1,2,5,6 According to the defective blastogenesis hypothesis, sirenomelia is believed to be secondary to the maldevelopment of the caudal somites and tail bud, due to primary defect in blastogenesis, occurring in the final stages of gastrulation (at about the 3rd week of gestation) in humans, concomitantly affecting the organ and vessel development. 1,5 The phenotypic variability depends on the intensity, time of initiation and duration of the underlying event.…”

“…1,2,5,6 According to the defective blastogenesis hypothesis, sirenomelia is believed to be secondary to the maldevelopment of the caudal somites and tail bud, due to primary defect in blastogenesis, occurring in the final stages of gastrulation (at about the 3rd week of gestation) in humans, concomitantly affecting the organ and vessel development. 1,5 The phenotypic variability depends on the intensity, time of initiation and duration of the underlying event. 1,5 A cascade of events occurs during gastrulation in vertebrates as the initial 2-layered blastocyst develops into a 3-layered embryo with formation of end mesoderm in a rostro caudal sequence.…”

“…1,5 The phenotypic variability depends on the intensity, time of initiation and duration of the underlying event. 1,5 A cascade of events occurs during gastrulation in vertebrates as the initial 2-layered blastocyst develops into a 3-layered embryo with formation of end mesoderm in a rostro caudal sequence. At late gastrulation stage, the remnant of the regressing primitive peak forms the caudal eminence (tail bud).…”