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
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.
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
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
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, ...