Sirenomelia syndrome is a very rare congenital disorder characterized by severe malformations, most notably the partial or complete fusion of the lower limbs, earning it the name “mermaid syndrome.” This complex condition involves anomalies of the lower spine and other organ systems, presenting significant challenges from birth.
Understanding Sirenomelia
The fusion of the lower extremities is the defining characteristic of sirenomelia, ranging from partial joining to complete fusion resembling a single limb. Infants may present with one foot, no feet, or two feet, which might be externally rotated. Beyond external appearance, sirenomelia is consistently associated with severe internal organ malformations, particularly affecting the urinary and gastrointestinal systems. These often include absent or underdeveloped kidneys, an absent bladder, and imperforate anus.
The condition can also involve malformations of the lumbosacral spine and pelvis, with the tailbone often absent and the sacrum partially or completely missing. Cardiac malformations and spina bifida may also occur. Sirenomelia affects approximately 0.8 to 1 in 100,000 live births, with male babies affected about three times more often than females.
Causes and Risk Factors
The exact cause of sirenomelia is not fully understood, with most cases occurring sporadically. However, several theories explain its development. One hypothesis is the “vascular steal” phenomenon, where an abnormal artery, often a single umbilical artery, diverts blood flow from the embryo’s lower region to the placenta. This insufficient blood supply to the developing caudal mesoderm can result in malformation or absence of lower body tissues, including kidneys, lower intestine, sacrum, and lower extremities.
Another theory is defective blastogenesis, attributing the defect to a teratogenic event during gastrulation. This interferes with notochord formation, leading to abnormal development of caudal structures. While once classified as a severe form of caudal regression syndrome, some researchers now consider sirenomelia a distinct disorder, despite shared features.
Maternal diabetes mellitus is the most significant risk factor, with a relative risk estimated between 1:200 to 1:250; up to 22% of affected fetuses have diabetic mothers. Increased free oxygen radical production in poorly controlled maternal diabetes may exert a teratogenic effect. Other less common factors include maternal age (under 20 or over 40), monozygotic twinning, and exposure to certain teratogens like retinoic acid, cadmium, cyclophosphamide, or cocaine.
Diagnosis and Medical Management
Sirenomelia is typically diagnosed during pregnancy, primarily through ultrasound scans, with early detection possible as early as 13 weeks of gestation. Specific features indicating the condition include fused lower limbs, a single umbilical artery, and oligohydramnios (low amniotic fluid volume) due to renal agenesis. Fetal MRI may also be used for a more detailed assessment.
Immediate postnatal challenges for infants with sirenomelia are substantial. These focus on providing respiratory and renal support due to severe organ malformations.
Surgical interventions, such as limb separation and reconstruction of affected organ systems, are complex and often limited by malformation severity. While successful lower extremity separation and perineum reconstruction have been reported, these procedures are challenging and require a multidisciplinary approach. Feasibility depends on the extent of bone fusion and the presence of functional internal organs.
Prognosis and Long-Term Considerations
The prognosis for individuals diagnosed with sirenomelia is generally poor, with a high mortality rate primarily due to severe renal agenesis and pulmonary hypoplasia. More than half of cases result in stillbirth, and those born alive typically survive for only a few hours or days. The absence of functional kidneys leads to a lack of amniotic fluid, necessary for lung development, resulting in underdeveloped lungs that cannot support breathing.
Survival factors largely depend on the extent of kidney development and other life-sustaining organ malformations. In rare instances, individuals with less severe renal anomalies have survived beyond infancy, sometimes into childhood or young adulthood. These survivors often face significant long-term medical challenges, including chronic kidney disease requiring dialysis or transplantation, bladder dysfunction, and mobility issues due to fused or malformed lower limbs.
Ongoing medical care and potential reconstructive surgeries can be extensive. Families facing such a severe diagnosis also require emotional and psychological support.