Information on the following diseases can be found in the Related Disorders section of this report:

• Caudal Regression Syndrome (Caudal Dysplasia Sequence)

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. Accompanying malformations of the spine and skeletal system, with vertebrae either absent or defective commonly occur. The internal and external sex organs, rectum, kidneys and/or bladder may also be missing or underdeveloped. The rectal opening (anus) may be completely closed (imperforate), and other abnormalities of the lower gastrointestinal tract are often present.
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The cause of Sirenomelia Sequence is unknown. It is believed to result from irregularities in early development of the blood circulating system (a disruptive vascular defect of the development of the vascular system) within the embryo. Individuals with Sirenomelia Sequence have been found to have a single large artery arising from high in the abdominal cavity without the usual two umbilical arteries that normally branch out of the lower part of the aorta and carry blood to the rearward tail (caudal) end of the embryo. The single artery present (called a "steal" vessel since it essentially steals blood from the lower portion of the embryo) diverts the flow of blood which normally circulates from the aorta (vitelline artery) through the umbilical arteries to lower points of the embryo, throughout the yolk sac and on to the placenta. Instead, the ‘steal’ vessel redirects the blood flow to the placenta without ever reaching the tail (caudal) end of the embryo. As a result of this rerouted blood flow, the steal vessel also diverts nutrients away from the blood-deprived portion of the embryo. Arteries in this caudal area are underdeveloped and tissues dependent upon them for nutrient supply fail to develop, are malformed, or arrest their growth in some incomplete stage. In individuals with Sirenomelia, the lower limb bud of the embryo fails to divide into two legs.
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Sirenomelia Sequence occurs once in every 60,000 to 100,000 births.

Symptoms of the following disorder can be similar to those of Sirenomelia Sequence. Comparisons may be useful for a differential diagnosis:

Caudal Regression Syndrome, also called Caudal Dysplasia Sequence, is characterized by abnormal development of the tail (caudal or distal) end region of the embryo. Abnormalities associated with Caudal Regression Syndrome may include incomplete development of the vertebrae, flattening of the buttocks, disruption of the distal spinal cord resulting in neurological impairment and an inability to control urination and bowel movement (incontinence). There may be extreme lack of growth in the caudal region. Less common abnormalities may include absence of kidneys, cleft lip, cleft palate, microcephaly and closed (imperforate) anus.

Sirenomelia Sequence can be detected in a fetus during the second trimester of pregnancy by ultrasound. Recently, surgery has been successful in separating joined legs. In preparation for surgery, balloon-like tissue expanders are inserted under the skin. When they are filled with a salt solution over a period of time, the balloons expand making the skin stretch and grow. The excess skin is then used to cover the legs once they are separated.

Other treatment is symptomatic and supportive. Prognosis depends on the extent of involvement of the gastrointestinal system, vertebrae and other structural deformities.

References
Jones KL, ed. Smith's Recognizable Patterns of Human Malformation, 4th ed: Philadelphia, PA. W.B. Saunders Company, 1998:634.

Journal Articles
Das BB, Rajegowda BK, Bainbridge R, et al. Caudal regression syndrome versus sirenomelia: a case report. J Perinatal. 2002;22:168-70.

Carbillon L, Seince N, Largilliere C, et al. First-trimester diagnosis of sirenomelia. A case report. Fetal Diagn Ther. 2001;16:284-88.

Blaicher W, Lee A, Deutinger J, et al. Sirenomelia: early prenatal diagnosis with combined two- and three-dimensional sonography. Ultrasound Obstet Gynecol. 2001;17:542-43.

Devitt AT, O’Donnell T, Fogarty EE, et al. Tibial hemimelia of a different class. J Pediatr Orthop. 2000;20:616-22.

Wiegner S, Donders J. Predictors of parental distress after congenital disabilities. J Dev Behav Pediatr. 2000;21:271-77.

Valenzano M, Paoletti R, Rossi A, et al. Sirenomelia. Pathological features, antenatal ultrasonographic clues, and a review of current embryogenic theoies. Hum Reprod Update. 1999;5:82-86.