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At a glance

Congenital disorder characterized by a single median lower extremity with the knees facing backward and the heel forward. Commonly observed renal and bladder agenesis, undifferentiated external genitals as well as vertebral column anomalies (sacral and coccyx agenesis, abnormal closeness of iliac bones).


Mermaid Malformation.


Sirenomelia is either classified into three types:

  • Simpus Apus: Absence of feet, one tibia, one femur

  • Simpus Unipus: One foot, two femurs, two tibiae, two fibulae

  • Simpus Dipus: Two feet and two fused legs (flipper-like), which is called a “mermaid.”

Or according to Stockert and Heifetz Classification, there are seven types listed as follow:

  • Type I: All bones present: two femurs, two tibiae, and two fibulae

  • Type II: Single fibula

  • Type III: Fibula absent

  • Type IV: Partially fused tibiae and femurs, fibulae fused

  • Type V: Partially fused femurs

  • Type VI: A single femur and a single tibia

  • Type VII: Single femur, absent tibia


1/60,000 births. The male-to-female ratio is 3:1.

Genetic inheritance

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


Etiology is unknown, with a male:female ratio of 2.7:1. An aberrant vascular supply, leading to “vitelline artery steal” has been suggested (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: The anomaly in this newborn with a single, fused, lower extremity is called sirenomelia.


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.

Clinical aspects

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.

Anesthetic considerations

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.

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

Other ...

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