What is Mermaid Syndrome?
Mermaid syndrome, also known as sirenomelia is an extremely rare genetic disorder. This condition is characterized by anomalies of the lower spine and lower limbs- those affected are born with fused legs. Patients can also have genital, urinary, gastrointestinal anomalies, as well as anomalies of pelvis and lumbar and sacral part of the spine. In some cases there is absence of both kidneys.
Other conditions associated with mermaid syndrome are spina bifida, imperforate anus and cardiac malformations. The incidence of this syndrome is 0.8-1 case in 100 000 births. Male to female ratio is 3:1 [1,2]. Read about other syndromes that cause fusion of body parts: Klippel-Feil syndrome, Apert syndrome.
The exact causes of sirenomelia are not known. Most cases occur sporadically with no family history. In some literature sirenomelia is classified as the most severe form of caudal regression syndrome. This syndrome has multiple causes- mutations in Hb9 gene, pregnancy diabetes, and environmental factors such as tobacco, heavy metals and retinoic acid [2,3].
There are two main hypothesis of pathogenesis:
- Vascular steal hypothesis- fusion of the limbs results from deficiency of blood supply to the caudal mesoderm- a part of the embryo from which lower body parts develop.
- Defective blastogenesis-primary defect in development of caudal mesoderm is due to teratogenic even while the formation of a three-layer structure that later forms in an embryo develops .
Mermaid syndrome can already be diagnosed during pregnancy. Malformations and their severity vary greatly between individuals. The most characteristic sign of mermaid syndrome is the fusion of legs. Legs can be partly or completely fused. Patients might have:
- Only one thigh bone or two thigh bones with one shaft of the skin
- One foot
- Both feet
- No feet
- Feet can be rotated with the back facing forwad
- Urogenital abnormalities
- Absence of one or both kidneys
- Cystic kidney malformation
- Absent bladder
- Urethral atresia
- Spinal malformations
- Lordosis- abnormal front to back curvature
- Other anomalies
- Imperforate anus
- Lack of external genitalia
- Absence of spleen
- Absence of gallbladder
- Abdominal wall malformations- omphalocele (also present in Beckwith-Wiedemann syndrome)
- Heart defects
- Lung defects, such as pulmonary hypoplasia
- Typical facial abnormality- Potter´s facies
Mermaid syndrome has been classified into three types, depending on the number of bones in the lower part of the body:
- Sirenomelia apus- absent feet, one tibia, one femur
- Sirenomelia unipus- one foot, two femurs, two tibia, two fibula
- Sirenomelia dipus- two feet and two legs with all the bones fused together 
Sirenomelia is usually diagnosed during the second trimester of pregnancy. In antenatal ultrasound the typical signs are:
- Fusion of the lower limbs
- Varying degree of sacral agenesis
- Single umbilical artery (normally there are two)
- Oligohydroamnios- small amount of amniotic fluid
After birth, physical examination is enough to make a diagnosis of mermaid syndrome. Further investigations can be done to determine the extent of organ system involvement:
- X-ray imaging- to assess bone structures in the lower parts of body as well as lungs
- CT and MRI scans- to assess both bone and soft tissue structures
- Ultrasound- to assess abdominal organs [3,5]
There is no treatment for mermaid syndrome, but some therapies can be done to prolong lifespan.
Depending on the severity of symptoms that can be detectable during pregnancy, in some cases carrying on with the pregnancy is not advised.
- Surgical separation of lower extremities, if both thigh bones are present and the patient is strong enough for surgery
- Amputation of lower extremities
- Kidney transplant in case of absent or malformed kidneys
- Reconstructive surgeries for proper function on the body
- Colostomy- in case of absent parts of large intestine 
Usually the patients have life-threatening complications and rarely survive for more than 1 year. In most cases patients die a few hours after birth. Usually the cause for death is due to absence or severe malformations of the kidneys. Cases have been reported, when the patients have survived for longer. One of most famous cases is Shiloh Pepin- a girl who survived until 10 years of age. The longest living patient with mermaid syndrome is Tiffany Yorks who is 27 years old.
Prevention of this syndrome mostly includes screening and treating gestational diabetes, which is associated with this syndrome [3,7].
- Detailed information about the syndrome: https://rarediseases.org/rare-diseases/sirenomelia/
- Case reports: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4367057/
- Caudal regression syndrome: https://rarediseases.org/rare-diseases/caudal-regression-syndrome/
- Types of sirenomelia: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4420302/
- Ultrasound imaging: https://radiopaedia.org/articles/sirenomelia
- Treatment: http://www.dovemed.com/diseases-conditions/sirenomelia/
- Survivors: http://2paragraphs.com/2015/09/mermaid-syndrome-shiloh-pepins-death-leaves-two-survivors/