Mermaid syndrome is the common name for Sirenomelia, a rare and severe congenital condition. This developmental anomaly affects the lower half of the body, characterized by the fusion of the lower limbs, giving the distinctive appearance of a mermaid’s tail. Sirenomelia is an uncommon birth defect, occurring in approximately one in 60,000 to 100,000 births. It is considered a multisystemic disorder due to associated malformations of internal organs, arising from a disturbance very early in embryonic development.
Defining Physical Characteristics
The defining feature of Sirenomelia is the fusion of the lower extremities, ranging from a partial joining of soft tissues to a complete bony fusion of the legs into a single limb. The severity of the limb fusion is classified into seven types based on the presence or absence of bones within the lower limb structure. In the most severe cases, only a single femur bone may be present; in milder forms, all bones may be present but encased in a single sheath of skin.
The external appearance is accompanied by significant internal anomalies, which often determine survival. Severe malformation or complete absence of the kidneys, known as renal agenesis, is a common finding. This defect prevents the kidneys from producing urine, which is necessary to form amniotic fluid in the womb, leading to a severe reduction of amniotic fluid called oligohydramnios.
The lack of amniotic fluid restricts lung development, resulting in pulmonary hypoplasia (underdeveloped lungs), a condition incompatible with life. Beyond the renal system, infants with Sirenomelia often have severe defects in the lower spine and pelvis, including the absence of the sacrum and an imperforate anus. External genitalia are often absent or ambiguous. Gastrointestinal malformations, such as a blind-ending colon, are also frequently observed.
Identifying Underlying Causes
The precise cause of Sirenomelia remains under study, but the prevailing hypothesis centers on a disruption of the vascular system early in gestation. This is known as the “vascular steal” theory, which suggests an abnormal blood vessel, often a single large vitelline artery, arises high in the fetal abdominal aorta. This aberrant vessel diverts blood flow and nutrients away from the lower part of the developing embryo toward the placenta.
This localized lack of adequate blood supply (ischemia) to the caudal mesoderm inhibits the proper development of these structures. The caudal mesoderm is the tissue that forms the lower spine, limbs, and urogenital system. The majority of Sirenomelia cases are sporadic, meaning they occur randomly without a clear genetic inheritance pattern. However, the condition is significantly more frequent in one of monozygotic (identical) twins, suggesting a possible shared early developmental insult.
A risk factor associated with the condition is poorly controlled maternal diabetes, particularly gestational diabetes. This association is controversial for Sirenomelia compared to other caudal defects. When maternal diabetes is a factor, the metabolic derangement may affect the migration of mesodermal cells during the primitive streak period of development. Other potential environmental factors include exposure to certain toxins or teratogenic drugs during the first weeks of pregnancy.
Diagnosis and Expected Outcomes
Sirenomelia can often be identified prenatally, sometimes as early as the first trimester, using high-resolution ultrasound imaging. Sonography detects the fused lower limbs, the absence or malformation of the kidneys, and the resulting severe lack of amniotic fluid. This combination of findings strongly suggests the diagnosis. Specialized vascular examinations, such as color Doppler sonography, can further confirm the presence of the single umbilical artery characteristic of the condition.
The prognosis for infants diagnosed with Sirenomelia is poor, as the condition is often lethal due to the severity of internal malformations. The high mortality rate is linked to the absence of functioning kidneys, which results in pulmonary hypoplasia and is incompatible with life. Many pregnancies with an affected fetus end in spontaneous miscarriage or stillbirth. Those born alive typically survive for only a few days.
Survival beyond the neonatal period hinges on the presence of at least one functioning kidney to maintain body fluid balance and enable lung development. Only a handful of cases have been reported worldwide where the infant survived to childhood or beyond. These survivors typically have less severe skeletal fusion and some degree of renal function. For families receiving a prenatal diagnosis, multidisciplinary consultation is provided to discuss the poor prognosis and potential options.
Treatment and Long-Term Care
Treatment for infants who survive the initial neonatal period requires an individualized and multidisciplinary approach involving surgical and medical specialties. The initial focus is on supportive medical care to manage immediate life-threatening issues, such as stabilizing cardiorespiratory function and addressing gastrointestinal defects like an imperforate anus. Surgical interventions, such as a colostomy, are often necessary to manage digestive issues shortly after birth.
Surgical separation of the fused lower limbs is a complex procedure considered only in select cases. This is typically done when the fusion is limited to soft tissues (Type I Sirenomelia) and the child is medically stable. Surgeons may use skin expanders to stretch the skin before separation to ensure adequate coverage for the newly separated limbs. The goal of limb separation is to improve lower extremity function and potentially allow for independent ambulation using orthotics or assistive devices.
The long-term care for surviving individuals focuses on managing urological and renal complications. If the kidneys are severely damaged but not completely absent, treatment may involve lifelong dialysis or, eventually, a kidney transplant. Orthopedic issues related to the malformed pelvis and lower spine require ongoing physical therapy and surgical correction to maximize mobility and quality of life.