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Congenital disorder characterized by a single lower extremity. Only one case of a living child described.


The anomaly in this newborn with a single, fused, lower extremity is called sirenomelia.


This whole-body radiograph of the same baby shows partial fusion of the femora in the proximal third, whereas the bones of the lower leg are fused in the distal third.


The pelvis and the lower extremity of the same patient. Note the absence of the external genitalia.

Mermaid Malformation.

Sirenomelia is classified into three types:

  • -Simpus Apus: absence of feet, one tibia, one femur
  • -Simpus Unipus: One foot, two femur, two tibia, two fibula
  • -Simpus Dipus: two feet and two fused legs (flipper-like), which is called a “mermaid”.

Sirenomelia has a prevalence of two to three cases per 100,000 births. The male-to-female ratio is 3:1. About 300 cases have been reported in the world literature, eight in India.

Genetic mapping is not known. Incidence is increased in monozygotic twins, making a genetic mechanism possible to discuss.

Etiology is unknown, with a male:female ratio of 2.7:1. An aberrant vascular supply, leading to “vitelline artery steal” has been evoked (persistence of the vitelline artery leads to abnormal aortic development with curtailment of blood flow to the lower extremities). Some other theories include posterior axial mesodermal defect, teratogenic effect, axial mesodermal dysplasia sequence, and midline development at field defect. Some of the manifestations have been related to oligohydramnios and vascular insufficiency.

Sirenomelia sequence is a birth defect in which affected infants are born with a single lower extremity or with two legs that are fused together.

In a case of live birth, the treatment is supportive and the prognosis is very poor, with the baby succumbing in the early neonatal period. Common associated malformations include absent external genitalia, imperforate anus, renal agenesis, and lumbosacral, vertebral, and pelvic abnormalities. Sirenomelia with craniorachischisis totalis has also been described.

Only one debatable case of a living child described. Probably evaluate renal function and ensure proper intraoperative fluid regimen. Renal elimination of administered drugs can be severely affected. Avoid perimedullar blockade.

Caudal Regression Syndrome (Sacral Agenesis Syndrome): Characterized by an abnormal development of the caudal part of the spine. Clinical features include absence or underdevelopment of the lower vertebrae, pelvis, and coccyx, paralysis or paresis of the legs, anal and urinary problems, hip dislocation and/or fixation, muscle atrophy, clubfoot, polycystic kidneys, and hypospadias in the male. Other less frequent anomalies ...

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