Mermaid Syndrome, or Sirenomelia

This is a rare and fatal congenital malformation characterized by the partial or complete fusion of the lower limb structures forming a single lower limb instead of  two.

Originally described by Rocheus in 1542 and Palfyn in 1953 and named after the mythical Greek Goddesses "Sirens".
Sirenomelia or Mermaid syndrome was initially claimed by some authorities to be the most severe form of "Caudal Regression syndrome" which was first  described by Duhamel  (1961), Until recently when Stevenson et al described it as a result of "Vascular Steal".

Mermaid Syndrome / Sirenomelia (Picture Credit ; Scialert )

The prevalence has been estimated at about 0.8-1 in 100,000 birth and as at now about 300 cases reported in the literature.
It has a male : female ratio of 2.7-3 : 1, and is more common in one of identical twins.

Features of Mermaid syndrome include:

• A single lower extremity,
• No feet (ectromelic mermaid), one foot (monopode mermaid) or both feet (sympode mermaid), which may be rotated externally.
• Sacrum is partially or completely absent,
• The femur may be Single (most commonly) or partially divided,
• Single umbilical artery,
• Absent or ambiguous external genitalia,
• Absent or imperforate anus,
• Renal anomalies.

Risk Factors
Maternal diabetes (16 – 22%)
Monozygotic Twin (15%)
Genetic factors;  as in autosomal dominant Currarino syndrome, also known as hereditary sacral agenesis (HSA).
Extremes of temperature,
Radiations,
Drugs (lithium, sulfamides)
Organic solvents (xylene, trichloroethylene, methylchloride and acetone)

Pathogenesis
In Mermaid Syndrome, the umbilical cord forms a single artery instead of two arteries, leading to insufficient blood supply to the fetus; (A vascular steal phenomenon causing severe ischemia to the caudal portion of the fetus). Suffering malnutrition, the fetus fails to develop two separate limbs.


Types
Tocker and Heifetz classified the leg abnormalities into Types I through VII.
Type I is the least severe with all bones present; and Type VII, most severe, consisting of a lower extremity with a fused femur and absent tibia and fibula.

Diagnosis
Prenatal ultrasonography as early as 13 weeks of pregnancy can detect gross structural anomalies, presence of a single umbilical artery, presence of oligohydramnios etc.
If legalized in your country, following a prenatal ultrasound Diagnosis, termination of pregnancy could be considered in appropriate situations .

Differentials
Caudal Regression Syndrome
VATER association

Complications
Urogenital complications (renal agenesis, absent or ambiguous external genitalia )
Gastrointestinal complications ( tracheo-esophageal atresia, imperforate anus, etc )
Cardiopathy,
Musculoskeletal complications.

Prognosis
Being often fatal, the condition is incompatible with life; some are stillbirth while others rarely survive longer than a few days.
However, The some without renal agenesis may survive past infancy. The oldest surviving patient with mermaid syndrome is Tiffany Yorks (1988- date) after series of surgery.

Mermaid Syndrome (Picture Credit; pixshark)

NOTES

1. I.D.Young,MD,MRCP, K. M. O’Reilly,MB,ChB, MRC Path, C. H. Kendall, MB,ChB, I986,  ETIOLOGICAL HETEROGENEITY IN SIRENOMELIA,  Pediatric Pathology, 5:31-43, Copyright 1986 by Hemisphere Publishing Corporation (HINARI)

2. Jocelyn H. Bruce, Rita L. Romaguera, Maria M. Rodriguez, Víctor H. González-Quintero, and E. Michel Azouz ,2009,CAUDAL DYSPLASIA SYNDROME AND SIRENOMELIA: ARE THEY PART OF A SPECTRUM? Informa Healthcare Journal Vol. 28, No. 3 , Pages 109-131 (doi:10.1080/15513810902772383)  (HINARI)

3. Marybeth Browne MD1, Philip Fitchev MD2, Brian Adley MD2 and Susan E Crawford MD, 2004 , Sirenomelia with an Angiomatous Lumbosacral Myelocystocele in a Full-term Infant, Journal of Perinatology (2004) 24, 329–331. 

4.  Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier

5. Reddy KR, Srinivas S, Kumar S, Reddy S, Hariprasad, Irfan GM. Sirenomelia a rare presentation. Journal of Neonatal Surgery. 2012;1:7.

6. Stevenson RE, Jones KL, Phelan MC, Jones MC, Barr M, Jr, Clericuzio C. Vascular steal: The pathogenetic mechanism producing sirenomelia and associated defects of the viscera and soft tissues. Pediatrics. 1986;78:451–7

7. Vinayak Y. Kshirsagar, Minhajuddin Ahmed, and Sylvia M. Colaco, 2012 , Sirenomelia Apus: A Rare Deformity Journal of Clinical Neonatology. 2012 Jul-Sep; 1(3): 146–148.

8.  Z.Kokrdova  Caudal regression syndrome, Journal Of Obstetrics and Gynecology (1983) 4,101-102   DOI: 10.3109/01443615.2012.743508 (HINARI)