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What is Mermaid Syndrome?

written by: •edited by: Paul Arnold•updated: 11/28/2009

Mermaid syndrome, alternatively known as sirenomelia is a rare disorder where an infant is born with fused legs, giving the appearance of a mermaid's tail. It is believed to occur in 1 in every 100,000 births. The term "sirenomelia" comes from "siren" or "mermaid".

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    Mermaid Syndrome

    Malformations include a single lower limb, the presence of two feet, or one foot, or both feet could be missing. Foetuses also have one umbilical artery instead of the usual two. More than half of the babies with sirenomelia are stillborn and of the rest the condition is usually fatal within a few days of birth because of complications associated with the kidneys, bladder, genitals, heart deformities, and in some cases the lungs. It is not the fused limbs that cause death, it is the fact that key organs do not form as they should.

    Only a very small handful of patients survive beyond the first few days after birth. The three most famous cases are Milagros Cerran who was born in 2004, Shiloh Pepin who was born in 1999 and died in October 2009, and Tiffany Yorks who was born in 1988 and is the longest surviving sirenomelia patient to date.

    An indication of the severity of the condition is that Shiloh was born without a uterus, rectum, or genitals, and had dozens of operations during her short life.

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    Mermaid Syndrome Genetic

    Mermaid syndrome develops early in foetal development when blood is diverted from cartilage, bones, muscles, nerves and blood vessels (the caudal structures) to the placenta. It's this lack of blood flow that causes the limbs to fuse, as the lower extremities are starved of oxygen and nutrients. Why this happens is still a mystery.

    At the time of writing (November 2009) it is not known what causes Mermaid syndrome. Several ideas have been put forward and these include poor health of an expectant mother, exposure to large amounts of radiation that creates genetic mutations, and/or a major illness suffered by the expectant mother.

    There may well be a series of genetic mutations, but if there are, they have not been identified. However, it has been established that sirenomelia can have a genetic basis in mice. Sirens have been observed when some strains have been crossed and as a consequence of mutations that boost the levels of retinoic acid. Researchers have also reported that sirenomelia has been caused by a loss of bone morphogenic protein 7 (Bmp 7) in combination with a complete loss or half dose of another Bmp protein - called twisted gastrulation (Tsg). Bone morphogenic proteins are multi-functional growth factors that play crucial roles in bone and cartilage development.

    All incidences of Mermaid syndrome have occurred as isolated cases in families. It could be that each case represents a new spontaneous mutation.

    There is also a possibility that sirenomelia does not have a genetic component; some doctors postulate that an environmental trigger could cause the malfunctions with the blood circulatory system that result in organs not developing correctly.

    Another idea is that it could be caused by direct damage to the caudal region of the embryo by excessive distention of the neural tube.

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    The condition is usually fatal, but for those that survive surgery is the only option to separate limbs. Other treatments depend on the extent of the birth defects.

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    Kyung A Lee et al. Prenatal diagnosis of sirenomelia with a single umbilical artery: A case report. Journal of Women’s Medicine Vol. 1 No. 1 June 2008

    Lise Zakin et al. Sirenomelia in Bmp7 and Tsg compound mutant mice: requirement for Bmp signaling in the development of ventral posterior mesoderm. Development 132, 2489-2499 (2005)